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AR88 -- help!
Hi all
I've struck an infuriatingly obscure problem with my AR88 and the AVC line. Are there any true experts here? If so, I'll post the details. Don't want to bore you all otherwise! |
AR88 -- help!
I have an early AR88 that developed a very elusive fault while I was coincidentally changing some capacitors and resistors. Of course I suspected I may have miswired something but I have checked and rechecked and it doesn't seem so. May be just a coincidence. This fault has defied many weeks of painstaking work. I wonder if members of the list have any ideas On the excellent test table provided in EMER 773 (on the VMARS website), under the specified test conditions (including AVC off) there should be voltages of - 1.2 V on the grids of V1, V2, V5, V6 and the anode (plate) and cathode of V8b, taken to the slider of RV3 (RF gain) when at max. This is of course the AVC line, and for the voltage as shown to the slider of RV3 there must be 1.2v across resistor R42 (390K). Well those readings used to be obtained, but after my work they are very slightly positive! (I emphasise that the readings are not to chassis but to the slider of RV3 as instructed. looking at the circuit diagram, to get that voltage between those points, there must be a very small current flowing through R42. It would have to be as a result of voltage from the bias dropper chain of R45, R44, and R43. These have all been checked OK and there is approx -30 volts to chassis at the end of the chain (total HT/B current for the set 110mA). With v8b shorted (per EMER 773) the other end of R42 goes via R47 (2.2 Meg) RV1 (66K) and R39 (33K). All those have been checked OK but no volts across R42! I have tried disconnecting the AVC main feed to V1 etc from SW22, but no difference so the problem is not in any of the circuitry before v8b. Next problem (connected?) There is mild AVC action *whether the AVC switch is on or off*! Any fresh ideas? BTW, to get -1.2 volts across R42 (390 K) would require a very small current of .003 mA. How is that obtained if there's a chain through R42, R47, RV1, R49? Wouldn't strain your mind with this, but you did ask! Cheers |
AR88 -- help!
"MoiInAust" wrote in message ... I have an early AR88 that developed a very elusive fault while I was coincidentally changing some capacitors and resistors. Of course I suspected I may have miswired something but I have checked and rechecked and it doesn't seem so. May be just a coincidence. This fault has defied many weeks of painstaking work. I wonder if members of the list have any ideas On the excellent test table provided in EMER 773 (on the VMARS website), under the specified test conditions (including AVC off) there should be voltages of - 1.2 V on the grids of V1, V2, V5, V6 and the anode (plate) and cathode of V8b, taken to the slider of RV3 (RF gain) when at max. This is of course the AVC line, and for the voltage as shown to the slider of RV3 there must be 1.2v across resistor R42 (390K). Well those readings used to be obtained, but after my work they are very slightly positive! (I emphasise that the readings are not to chassis but to the slider of RV3 as instructed. looking at the circuit diagram, to get that voltage between those points, there must be a very small current flowing through R42. It would have to be as a result of voltage from the bias dropper chain of R45, R44, and R43. These have all been checked OK and there is approx -30 volts to chassis at the end of the chain (total HT/B current for the set 110mA). With v8b shorted (per EMER 773) the other end of R42 goes via R47 (2.2 Meg) RV1 (66K) and R39 (33K). All those have been checked OK but no volts across R42! I have tried disconnecting the AVC main feed to V1 etc from SW22, but no difference so the problem is not in any of the circuitry before v8b. Next problem (connected?) There is mild AVC action *whether the AVC switch is on or off*! Any fresh ideas? BTW, to get -1.2 volts across R42 (390 K) would require a very small current of .003 mA. How is that obtained if there's a chain through R42, R47, RV1, R49? Wouldn't strain your mind with this, but you did ask! Cheers I'll follow along the schematic and see if I have any ideas. RCA's schematics can be confusing:-( -- Richard Knoppow Los Angeles WB6KBL |
AR88 -- help!
"Richard Knoppow" wrote in message m... "MoiInAust" wrote in message ... I have an early AR88 that developed a very elusive fault while I was coincidentally changing some capacitors and resistors. Of course I suspected I may have miswired something but I have checked and rechecked and it doesn't seem so. May be just a coincidence. This fault has defied many weeks of painstaking work. I wonder if members of the list have any ideas On the excellent test table provided in EMER 773 (on the VMARS website), under the specified test conditions (including AVC off) there should be voltages of - 1.2 V on the grids of V1, V2, V5, V6 and the anode (plate) and cathode of V8b, taken to the slider of RV3 (RF gain) when at max. This is of course the AVC line, and for the voltage as shown to the slider of RV3 there must be 1.2v across resistor R42 (390K). Well those readings used to be obtained, but after my work they are very slightly positive! (I emphasise that the readings are not to chassis but to the slider of RV3 as instructed. looking at the circuit diagram, to get that voltage between those points, there must be a very small current flowing through R42. It would have to be as a result of voltage from the bias dropper chain of R45, R44, and R43. These have all been checked OK and there is approx -30 volts to chassis at the end of the chain (total HT/B current for the set 110mA). With v8b shorted (per EMER 773) the other end of R42 goes via R47 (2.2 Meg) RV1 (66K) and R39 (33K). All those have been checked OK but no volts across R42! I have tried disconnecting the AVC main feed to V1 etc from SW22, but no difference so the problem is not in any of the circuitry before v8b. Next problem (connected?) There is mild AVC action *whether the AVC switch is on or off*! Any fresh ideas? BTW, to get -1.2 volts across R42 (390 K) would require a very small current of .003 mA. How is that obtained if there's a chain through R42, R47, RV1, R49? Wouldn't strain your mind with this, but you did ask! Cheers I'll follow along the schematic and see if I have any ideas. RCA's schematics can be confusing:-( Well I agree. Being a Pom, I used the Anlglicized version of the circuit which is on EMER772. I find the convention of having high voltage lines higher than low ones (and the negative AVC line below the chassis 0) easier to understand. The US diagram in the original handbook has the valves upside down to my mind (!) Having said that, I consider the AR88 (US design that it is) to have been decades ahead of any Pom effort (like the infamous 38 set!). 1940 or even earlier and it's still a beaut performer. |
AR88 -- help!
"MoiInAust" wrote in message ... "Richard Knoppow" wrote in message m... "MoiInAust" wrote in message ... I have an early AR88 that developed a very elusive fault while I was coincidentally changing some capacitors and resistors. Of course I suspected I may have miswired something but I have checked and rechecked and it doesn't seem so. May be just a coincidence. This fault has defied many weeks of painstaking work. I wonder if members of the list have any ideas On the excellent test table provided in EMER 773 (on the VMARS website), under the specified test conditions (including AVC off) there should be voltages of - 1.2 V on the grids of V1, V2, V5, V6 and the anode (plate) and cathode of V8b, taken to the slider of RV3 (RF gain) when at max. This is of course the AVC line, and for the voltage as shown to the slider of RV3 there must be 1.2v across resistor R42 (390K). Well those readings used to be obtained, but after my work they are very slightly positive! (I emphasise that the readings are not to chassis but to the slider of RV3 as instructed. looking at the circuit diagram, to get that voltage between those points, there must be a very small current flowing through R42. It would have to be as a result of voltage from the bias dropper chain of R45, R44, and R43. These have all been checked OK and there is approx -30 volts to chassis at the end of the chain (total HT/B current for the set 110mA). With v8b shorted (per EMER 773) the other end of R42 goes via R47 (2.2 Meg) RV1 (66K) and R39 (33K). All those have been checked OK but no volts across R42! I have tried disconnecting the AVC main feed to V1 etc from SW22, but no difference so the problem is not in any of the circuitry before v8b. Next problem (connected?) There is mild AVC action *whether the AVC switch is on or off*! Any fresh ideas? BTW, to get -1.2 volts across R42 (390 K) would require a very small current of .003 mA. How is that obtained if there's a chain through R42, R47, RV1, R49? Wouldn't strain your mind with this, but you did ask! Cheers I'll follow along the schematic and see if I have any ideas. RCA's schematics can be confusing:-( Well I agree. Being a Pom, I used the Anlglicized version of the circuit which is on EMER772. I find the convention of having high voltage lines higher than low ones (and the negative AVC line below the chassis 0) easier to understand. The US diagram in the original handbook has the valves upside down to my mind (!) Having said that, I consider the AR88 (US design that it is) to have been decades ahead of any Pom effort (like the infamous 38 set!). 1940 or even earlier and it's still a beaut performer. The RCA schematics are sort of combination schematic and wiring diagram with all the pins shown in actual order. A lot of schematics for home radios were drawn like this. Its confusing because there is no natural flow. My other favorite is Western Electric transmitter diagrams with all the filaments and ground wires shown so they look like a map of a large freight yard. BTW, I downloaded a bunch of stuff from the VMARS site, lots of R-390 and other books there and a military handbook on the older Super-Pro models. -- Richard Knoppow Los Angeles WB6KBL |
AR88 -- help!
"Richard Knoppow" wrote BTW, I downloaded a bunch of stuff from the VMARS site, lots of R-390 and other books there and a military handbook on the older Super-Pro models. Good site, isn't it! Now Richard, stop salivating over the R-390 stuff and solve my problem with the AR88! |
AR88 -- help!
MoiInAust ha scritto:
I have an early AR88 that developed a very elusive fault while I was coincidentally changing some capacitors and resistors. Of course I suspected I may have miswired something but I have checked and rechecked and it doesn't seem so. May be just a coincidence. This fault has defied many weeks of painstaking work. I wonder if members of the list have any ideas On the excellent test table provided in EMER 773 (on the VMARS website), under the specified test conditions (including AVC off) there should be voltages of - 1.2 V on the grids of V1, V2, V5, V6 and the anode (plate) and cathode of V8b, taken to the slider of RV3 (RF gain) when at max. This is of course the AVC line, and for the voltage as shown to the slider of RV3 there must be 1.2v across resistor R42 (390K). Well those readings used to be obtained, but after my work they are very slightly positive! (I emphasise that the readings are not to chassis but to the slider of RV3 as instructed. looking at the circuit diagram, to get that voltage between those points, there must be a very small current flowing through R42. It would have to be as a result of voltage from the bias dropper chain of R45, R44, and R43. These have all been checked OK and there is approx -30 volts to chassis at the end of the chain (total HT/B current for the set 110mA). With v8b shorted (per EMER 773) the other end of R42 goes via R47 (2.2 Meg) RV1 (66K) and R39 (33K). All those have been checked OK but no volts across R42! I have tried disconnecting the AVC main feed to V1 etc from SW22, but no difference so the problem is not in any of the circuitry before v8b. Next problem (connected?) There is mild AVC action *whether the AVC switch is on or off*! Any fresh ideas? BTW, to get -1.2 volts across R42 (390 K) would require a very small current of .003 mA. How is that obtained if there's a chain through R42, R47, RV1, R49? Wouldn't strain your mind with this, but you did ask! Cheers Hi, probably a gassy 6H6 ? Cheers, P. |
AR88 -- help!
"Piero Soldi" wrote in message ... Hi, probably a gassy 6H6 ? Cheers, P. Thanks. Yes, I thought that possible but according to my AVO 160, no gas on any valves! I'll try that again with some replacements... |
AR88 -- help!
"MoiInAust" wrote in message ... "Piero Soldi" wrote in message ... Hi, probably a gassy 6H6 ? Cheers, P. Thanks. Yes, I thought that possible but according to my AVO 160, no gas on any valves! I'll try that again with some replacements... Further to that Peiro, there is no provision on either of my valve testers to test diodes for gas! I always understood that was a problem if there was grid current but of course in a 6H6 there are no grids! |
AR88 -- help!
MoiInAust ha scritto:
"MoiInAust" wrote in message ... "Piero Soldi" wrote in message ... Hi, probably a gassy 6H6 ? Cheers, P. Thanks. Yes, I thought that possible but according to my AVO 160, no gas on any valves! I'll try that again with some replacements... Further to that Peiro, there is no provision on either of my valve testers to test diodes for gas! I always understood that was a problem if there was grid current but of course in a 6H6 there are no grids! Also a gassy diode can make havoc in a very high impedance circuit like the AVC of AR88 ( note resistor's values ! ) . If slightly ionized inside, the *reverse* resistance is much lower than normal, as some positive ions are bombing 6H6's cathode. Try exchanging it using the limiter tube, also a 6H6, and redo measurements, if something different, try another 6H6. Sometimes old *new* tubes need to be "formed" before use activating residual getter inside applying filament only, for a very long time, like 48 hours and after that some other time wth plate voltage on ! HTH, Piero. |
AR88 -- help!
"Piero Soldi" wrote in message ... MoiInAust ha scritto: "MoiInAust" wrote in message ... "Piero Soldi" wrote in message ... Hi, probably a gassy 6H6 ? Cheers, P. Thanks. Yes, I thought that possible but according to my AVO 160, no gas on any valves! I'll try that again with some replacements... Further to that Peiro, there is no provision on either of my valve testers to test diodes for gas! I always understood that was a problem if there was grid current but of course in a 6H6 there are no grids! Also a gassy diode can make havoc in a very high impedance circuit like the AVC of AR88 ( note resistor's values ! ) . If slightly ionized inside, the *reverse* resistance is much lower than normal, as some positive ions are bombing 6H6's cathode. Try exchanging it using the limiter tube, also a 6H6, and redo measurements, if something different, try another 6H6. Sometimes old *new* tubes need to be "formed" before use activating residual getter inside applying filament only, for a very long time, like 48 hours and after that some other time wth plate voltage on ! HTH, Piero. Well that's something new I learned! I've got a few and will try different ones! Thanks heaps! |
AR88 -- help!
"MoiInAust" wrote in message ... "Piero Soldi" wrote in message ... MoiInAust ha scritto: "MoiInAust" wrote in message ... "Piero Soldi" wrote in message ... Hi, probably a gassy 6H6 ? Cheers, P. Thanks. Yes, I thought that possible but according to my AVO 160, no gas on any valves! I'll try that again with some replacements... Further to that Peiro, there is no provision on either of my valve testers to test diodes for gas! I always understood that was a problem if there was grid current but of course in a 6H6 there are no grids! Also a gassy diode can make havoc in a very high impedance circuit like the AVC of AR88 ( note resistor's values ! ) . If slightly ionized inside, the *reverse* resistance is much lower than normal, as some positive ions are bombing 6H6's cathode. Try exchanging it using the limiter tube, also a 6H6, and redo measurements, if something different, try another 6H6. Sometimes old *new* tubes need to be "formed" before use activating residual getter inside applying filament only, for a very long time, like 48 hours and after that some other time wth plate voltage on ! HTH, Piero. Well that's something new I learned! I've got a few and will try different ones! Thanks heaps! Well, all 5 valves (tubes!) that I have -- 2 of them new -- gave the same results. So, still puzzled... |
AR88 -- help!
MoiInAust ha scritto:
"MoiInAust" wrote in message ... "Piero Soldi" wrote in message ... MoiInAust ha scritto: "MoiInAust" wrote in message ... "Piero Soldi" wrote in message ... Hi, probably a gassy 6H6 ? Cheers, P. Thanks. Yes, I thought that possible but according to my AVO 160, no gas on any valves! I'll try that again with some replacements... Further to that Peiro, there is no provision on either of my valve testers to test diodes for gas! I always understood that was a problem if there was grid current but of course in a 6H6 there are no grids! Also a gassy diode can make havoc in a very high impedance circuit like the AVC of AR88 ( note resistor's values ! ) . If slightly ionized inside, the *reverse* resistance is much lower than normal, as some positive ions are bombing 6H6's cathode. Try exchanging it using the limiter tube, also a 6H6, and redo measurements, if something different, try another 6H6. Sometimes old *new* tubes need to be "formed" before use activating residual getter inside applying filament only, for a very long time, like 48 hours and after that some other time wth plate voltage on ! HTH, Piero. Well that's something new I learned! I've got a few and will try different ones! Thanks heaps! Well, all 5 valves (tubes!) that I have -- 2 of them new -- gave the same results. So, still puzzled... OK, nothing useful. When you have restored the capacitors and resistors, substituted any valve ? If so, check for a *slightly* gassy one ! ( RF amplifiers, mixer, MF amplifiers ) Cleaned carefully where soldered new components ? Capacitors are of the *best* type ? ( low loss types ). Impedances are very high, a current of 3 *microAmp* is nothing with a grid resistance of 100 KOhm, but with a 5 MOhm, became noticeable. Wires are well insulated ? No cracks or degrading rubber ? Cheers, Piero. |
AR88 -- help!
Hi Piero
Many thnaks for your ongoing hep re this. I have interleaved comments in your posting *** "Piero Soldi" wrote in message ... MoiInAust ha scritto: "MoiInAust" wrote in message ... "Piero Soldi" wrote in message ... MoiInAust ha scritto: "MoiInAust" wrote in message ... "Piero Soldi" wrote in message ... Hi, probably a gassy 6H6 ? Cheers, P. Thanks. Yes, I thought that possible but according to my AVO 160, no gas on any valves! I'll try that again with some replacements... Further to that Peiro, there is no provision on either of my valve testers to test diodes for gas! I always understood that was a problem if there was grid current but of course in a 6H6 there are no grids! Also a gassy diode can make havoc in a very high impedance circuit like the AVC of AR88 ( note resistor's values ! ) . If slightly ionized inside, the *reverse* resistance is much lower than normal, as some positive ions are bombing 6H6's cathode. Try exchanging it using the limiter tube, also a 6H6, and redo measurements, if something different, try another 6H6. Sometimes old *new* tubes need to be "formed" before use activating residual getter inside applying filament only, for a very long time, like 48 hours and after that some other time wth plate voltage on ! HTH, Piero. Well that's something new I learned! I've got a few and will try different ones! Thanks heaps! Well, all 5 valves (tubes!) that I have -- 2 of them new -- gave the same results. So, still puzzled... OK, nothing useful. When you have restored the capacitors and resistors, substituted any valve ? If so, check for a *slightly* gassy one ! ( RF amplifiers, mixer, MF amplifiers ) *** Many new valves BUT I have disconnected the AVC line from V1,2,5,6 and there is absolutely NO current flow across that gap or chnage in the wrond voltage at V8, so I think we can rule out gassy valves? Cleaned carefully where soldered new components ? Capacitors are of the *best* type ? ( low loss types ). *** They are a darned sight better than the leaky (in both ways!) tubs that I removed. Impedances are very high, a current of 3 *microAmp* is nothing with a grid resistance of 100 KOhm, but with a 5 MOhm, became noticeable. Wires are well insulated ? No cracks or degrading rubber ? *** not to allow foreign contact Cheers, Piero. *** still scratching skull (hair out!) |
AR88 -- help!
MoiInAust ha scritto:
Hi Piero Many thnaks for your ongoing hep re this. I have interleaved comments in your posting . When you have restored the capacitors and resistors, substituted any valve ? If so, check for a *slightly* gassy one ! ( RF amplifiers, mixer, MF amplifiers ) *** Many new valves BUT I have disconnected the AVC line from V1,2,5,6 and there is absolutely NO current flow across that gap or chnage in the wrond voltage at V8, so I think we can rule out gassy valves? Cleaned carefully where soldered new components ? Capacitors are of the *best* type ? ( low loss types ). *** They are a darned sight better than the leaky (in both ways!) tubs that I removed. Impedances are very high, a current of 3 *microAmp* is nothing with a grid resistance of 100 KOhm, but with a 5 MOhm, became noticeable. Wires are well insulated ? No cracks or degrading rubber ? *** not to allow foreign contact Cheers, Piero. *** still scratching skull (hair out!) OK, make some measures. With a VTVM or other *high* impedance voltmeter, make a reading at juncton between R43 and C97 ( 4uF ) and chassis ground. Must be approximately *minus* 3-4 volt. Again, make a measure at junction between pot R46 pin 3 with R55 and chassis. Must be approximately *minus* 0,3-0,4 volt. After done, post readings. HTH, Piero. |
AR88 -- help!
Although I do not have a schematic of the AR-88, I just started reading the
thread, so I thought I would jump in more to kick you into changing your detective methods than to hand you an answer (which I do not know) If everything was ok before the surgery, and things are now bad, then something happened during the surgery. That is probably painstakingly obvious. But, it is important, because it probably rules out changes that might suddenly have occurred outside of your surgery - such as bad tubes. So, the question is what happened during the surgery that could cause the problem. It could be a defective capacitor, or an electrolytic instlled backwards (especially in negative line). Or, it could be a wire from one of those components sticking through a terminal and shorting something. Or, it could be that a resistor got moved and broke in two pieces, but is not visible. It might be useful to do a resistance check if the manual has one, or check individual components. Another possibility is, when soldering to a pot, that the internal connection was broken by the movement of removing and reconnecting the wires. Same thing on power resistors, especially adjustable power resistors. The AVC circuit is usually a long string that can be disconnected in sections. It sounds like you have been doing that. I think one problem I had like you are having was when a mica coupling cap in the if stage was bringing in a small positive leakage into the avc circuit. I kept addressing the avc circuit, but it was not there at all. Sort of a Trojan Horse. The good thing about this is that you will feel very good when you find the problem. Enjoy the old receiver. Good luck. 73, Colin K7FM |
AR88 -- help!
"MoiInAust" wrote in message ... "Richard Knoppow" wrote BTW, I downloaded a bunch of stuff from the VMARS site, lots of R-390 and other books there and a military handbook on the older Super-Pro models. Good site, isn't it! Now Richard, stop salivating over the R-390 stuff and solve my problem with the AR88! I redrew the circuit. The AVC circuit is fairly conventional. The AVC voltage comes from the detector and the second half of the 6H6 is used to generate a "delay" voltage to prevent the AVC from acting until the signal strength reaches a certain minimum level. This is done to maintain the RF and IF stages at full gain. There is a negative bias supply obtained from a series of resistors in the center-tap return of the rectifier. The RF gain voltage comes from the RF gain control which is in this string. About the only way I can see for a positive voltage to appear on the AVC-RF gain bus is if this whole string is being pulled positive. If that's so it should also show up on the audio tubes which are biased from this source. Unless you made a gross error in wiring I would be suspicious of C-96 and C97, the first two filter caps since they are returned to the top of the negative supply rather than to chassis ground. C-98, the last cap, goes to chassis ground so can't cause trouble. If one or both of the the filter caps is leaky it could be pulling the negative supply to a positive value. I wouldn't worry about gassy tubes. Positive voltage can come from one of the controlled tubes becoming gassy but they are pretty well isolated in this circuit. I can't account for the slight AVC action with the AVC switch in manual but if the negative supply has been pulled positive it might affect this. The AVC switch essentially shorts out the AVC and puts the controlled stages directly on the negative supply. However, the whole AVC bus floats on the negative supply so that the RF gain control works in both AVC and Manual. I certainly agree with those who suggest going over your work carefully. If you know another knowlegible person have them look also since its possible for someone to overlook the same thing over and over where someone else will spot it right away. But, check the filter caps first. -- Richard Knoppow Los Angeles WB6KBL |
AR88 -- help!
Thanks again Piero.
Interleaved as before "Piero Soldi" wrote in message ... OK, make some measures. With a VTVM or other *high* impedance voltmeter, make a reading at juncton between R43 and C97 ( 4uF ) and chassis ground. Must be approximately *minus* 3-4 volt. That's the foot of the AVC chain and surely 3-4 Volts is *much* too low. I get 27-30, which would be expected with a total current of 110 mA and a total AVC chain of 275 Ohms. Again, make a measure at junction between pot R46 pin 3 with R55 and chassis. Must be approximately *minus* 0,3-0,4 volt. There is no pot R46. Do you mean RV3. Which do you mean by 'pin 3' Cheers Alan |
AR88 -- help!
"MoiInAust" wrote in message ... Thanks again Piero. Interleaved as before "Piero Soldi" wrote in message ... OK, make some measures. With a VTVM or other *high* impedance voltmeter, make a reading at juncton between R43 and C97 ( 4uF ) and chassis ground. Must be approximately *minus* 3-4 volt. That's the foot of the AVC chain and surely 3-4 Volts is *much* too low. I get 27-30, which would be expected with a total current of 110 mA and a total AVC chain of 275 Ohms. Again, make a measure at junction between pot R46 pin 3 with R55 and chassis. Must be approximately *minus* 0,3-0,4 volt. There is no pot R46. Do you mean RV3. Which do you mean by 'pin 3' Cheers Alan The designations are different on the US and UK versions of the schematic. RV-3 and R-46 are the same, i.e., the RF gain pot. The voltage table specifies measuring to the pot but fails to say which contact. Since the pot is variable (doh) that makes a difference. There is a US version of the schematic at BAMA which will show the differences in component lables. I still can't see how the AVC bus could become positive unless there is something shorting it to the B+ (HT) or else you are measuring to the wrong reference. -- Richard Knoppow Los Angeles WB6KBL |
AR88 -- help!
"Richard Knoppow" wrote in message m... "MoiInAust" wrote in message ... Thanks again Piero. Interleaved as before "Piero Soldi" wrote in message ... OK, make some measures. With a VTVM or other *high* impedance voltmeter, make a reading at juncton between R43 and C97 ( 4uF ) and chassis ground. Must be approximately *minus* 3-4 volt. That's the foot of the AVC chain and surely 3-4 Volts is *much* too low. I get 27-30, which would be expected with a total current of 110 mA and a total AVC chain of 275 Ohms. Again, make a measure at junction between pot R46 pin 3 with R55 and chassis. Must be approximately *minus* 0,3-0,4 volt. There is no pot R46. Do you mean RV3. Which do you mean by 'pin 3' Cheers Alan The designations are different on the US and UK versions of the schematic. RV-3 and R-46 are the same, i.e., the RF gain pot. The voltage table specifies measuring to the pot but fails to say which contact. Since the pot is variable (doh) that makes a difference. There is a US version of the schematic at BAMA which will show the differences in component lables. I still can't see how the AVC bus could become positive unless there is something shorting it to the B+ (HT) or else you are measuring to the wrong reference. No, the AVC is definitely is NOT negative with reference to the chassis, but the EMER773 states reference must be to the slider of RV3, where is should be -1.2v. That is the reading that used to be OK and now is not. And I've checked and rechecked aagain all the changes I made. |
AR88 -- help!
"MoiInAust" wrote in message ... "Richard Knoppow" wrote in message m... "MoiInAust" wrote in message ... Thanks again Piero. Interleaved as before "Piero Soldi" wrote in message ... OK, make some measures. With a VTVM or other *high* impedance voltmeter, make a reading at juncton between R43 and C97 ( 4uF ) and chassis ground. Must be approximately *minus* 3-4 volt. That's the foot of the AVC chain and surely 3-4 Volts is *much* too low. I get 27-30, which would be expected with a total current of 110 mA and a total AVC chain of 275 Ohms. Again, make a measure at junction between pot R46 pin 3 with R55 and chassis. Must be approximately *minus* 0,3-0,4 volt. There is no pot R46. Do you mean RV3. Which do you mean by 'pin 3' Cheers Alan The designations are different on the US and UK versions of the schematic. RV-3 and R-46 are the same, i.e., the RF gain pot. The voltage table specifies measuring to the pot but fails to say which contact. Since the pot is variable (doh) that makes a difference. There is a US version of the schematic at BAMA which will show the differences in component lables. I still can't see how the AVC bus could become positive unless there is something shorting it to the B+ (HT) or else you are measuring to the wrong reference. No, the AVC is definitely is NOT negative with reference to the chassis, but the EMER773 states reference must be to the slider of RV3, where is should be -1.2v. That is the reading that used to be OK and now is not. And I've checked and rechecked aagain all the changes I made. I found somewhere on the web a manual for the CR-88. I no longer remember where but I suspect you can find it. If not I can send via e-mail, its about 3Mb. It has a different voltage chart. All voltages are measured to ground. For instance, using a VTVM the voltage at pin 4 of the first RF tube is -18 volts with the RF gain control at minimum and -2 volts wtih the control at maximum. I can't understand measuring to the slider of the RF gain pot because the voltage there will depend on the position of the pot. I suspect this is an error of some sort. Note that the whole RF gain/AVC sytem gets a negative voltage from a devider string in the center tap return of the power transformer. That is, the RF gain control is at a negative potential with regard to ground on both sides. Actually, there are two strings from the center tap, one consists of R-46 (RV-3) and a 6800 ohm resistor called R-55 on the CR-88 schematic. The other string consists of R-43, 44, 45 and the taps supply a fixed negative bias to the audio driver tube and audio output tube. If you are not getting a negative voltage on the grids of these tubes its a good indication that something is pulling this point positive. Almost the only thing there is the first filter cap which returns to the the top of the negative voltage deviders rather than to ground. If this is the case you will also see a positive voltage at both sides (and the slider) of the RF gain control. The lines from all of the negative supply points to their respective grids have rather high value resistors in them so I think its unlikely that something on that side could pull the entire chain positive. If you replaced the first filter cap check its wiring and check to see that it isn't presenting a low resistance path from positive to the devider chain. -- Richard Knoppow Los Angeles WB6KBL |
AR88 -- help!
Richard Knoppow ha scritto:
"MoiInAust" wrote in message ... Thanks again Piero. Interleaved as before "Piero Soldi" wrote in message ... OK, make some measures. With a VTVM or other *high* impedance voltmeter, make a reading at juncton between R43 and C97 ( 4uF ) and chassis ground. Must be approximately *minus* 3-4 volt. That's the foot of the AVC chain and surely 3-4 Volts is *much* too low. I get 27-30, which would be expected with a total current of 110 mA and a total AVC chain of 275 Ohms. Again, make a measure at junction between pot R46 pin 3 with R55 and chassis. Must be approximately *minus* 0,3-0,4 volt. There is no pot R46. Do you mean RV3. Which do you mean by 'pin 3' Cheers Alan The designations are different on the US and UK versions of the schematic. RV-3 and R-46 are the same, i.e., the RF gain pot. The voltage table specifies measuring to the pot but fails to say which contact. Since the pot is variable (doh) that makes a difference. There is a US version of the schematic at BAMA which will show the differences in component lables. I still can't see how the AVC bus could become positive unless there is something shorting it to the B+ (HT) or else you are measuring to the wrong reference. -- Richard Knoppow Los Angeles WB6KBL Hello Alan, Richard and Colin. Alan, yes, i've done a tenfold error ! Take in good advice what Richard say, about the AVC bus, he's absolutely correct, *if you are shure of wiring redone*. The Richard's subsequent reply is *perfect* to pin down the error(s) or defective component(s), and also the Colin's reply is of great help. I can't add anything to those replies. Good debugging, cheers, Piero. |
AR88 -- help!
Richard noted:
"Almost the only thing there is the first filter cap which returns to the the top of the negative voltage deviders rather than to ground." If that filter cap is a can, it would be insulated from ground, but it would be easy to connect to ground when replacing, rather than the top of the negative dividers. 73, Colin K7FM |
AR88 -- help!
I stupidly said (confusing the issue)...
No, the AVC is definitely is NOT negative with reference to the chassis, but the EMER773 states reference must be to the slider of RV3, where is should be -1.2v. That is the reading that used to be OK and now is not. And I've checked and rechecked aagain all the changes I made. ....and Richard said... I found somewhere on the web a manual for the CR-88. I no longer remember where but I suspect you can find it. If not I can send via e-mail, its about 3Mb. It has a different voltage chart. All voltages are measured to ground. For instance, using a VTVM the voltage at pin 4 of the first RF tube is -18 volts with the RF gain control at minimum and -2 volts wtih the control at maximum. I can't understand measuring to the slider of the RF gain pot because the voltage there will depend on the position of the pot. I suspect this is an error of some sort. Note that the whole RF gain/AVC sytem gets a negative voltage from a devider string in the center tap return of the power transformer. That is, the RF gain control is at a negative potential with regard to ground on both sides. Actually, there are two strings from the center tap, one consists of R-46 (RV-3) and a 6800 ohm resistor called R-55 on the CR-88 schematic. The other string consists of R-43, 44, 45 and the taps supply a fixed negative bias to the audio driver tube and audio output tube. If you are not getting a negative voltage on the grids of these tubes its a good indication that something is pulling this point positive. Almost the only thing there is the first filter cap which returns to the the top of the negative voltage deviders rather than to ground. If this is the case you will also see a positive voltage at both sides (and the slider) of the RF gain control. The lines from all of the negative supply points to their respective grids have rather high value resistors in them so I think its unlikely that something on that side could pull the entire chain positive. If you replaced the first filter cap check its wiring and check to see that it isn't presenting a low resistance path from positive to the devider chain. OK, time to bring a few things together, I feel, especially as I introduced a massive red herring in the first comment that I've reproduced in this thread... I meant of course that the AVC was not *positive* with respect to chassis, as indeed it shouldn't be. Measurements taken from chassis are OK for the negative grid bias that must exist. I've just measured and if the RF gain is at minimum I get minus 18.5 volts on the grid of V1 (pin4) and if the RF gain is at maximum I get minus 3 volts (using a DVM, so impedance similar to VTVM). All pretty much in line with Richard's figures for the CR88. BTW I also get minus 0.8 on the grid of V10 (1st AF) and minus 17 on the grid of V11 (output). The grid voltages are all the result of the chain R43, R44, R45, as has been said. The problem is rather more subtle. You really need to download the document EMER 773 from the VMARS website to understand it, but I'll quote the relevant bits. There is a list of test resistances and voltages round each valve on page 4 and 5. The relevant part is the grid voltage for valves 1, 2, 5, 6 (AVC line) (and incidentally the anode and cathode of v8, with the AVC off (ie V8b shorted), *against the slider of RV3*. Now, I quote, 'RF gain set to full gain'. That is why the measurement to the slider does not depend on its setting -- or rather the setting is specifed.... 'full gain'. The desired voltage is specified as minus 1.2 volts and is what I originally got and do not get now. Looking at the circuit, it can be seen that this measurement must be a result of the voltage drop across R42 (390K) and that can only be the result of a current flow (about 3 microamps). I first queried why there should be *any* flow as there should not be grid current, but someone kindly pointed out that there is a path to earth via R47 (2.2 Meg), RV1 (66K) and R49 (33K). I still can't make sense of this using Ohms Law (requires more volts than would exist at this point?), but it appears that the test to the Anode and Cathode of V8b is to verify all those resistors as well as the main AVC chain, plus the operation of the shorting part of the AVC switch. Testing minus 1.2 volts on the other grids also tests for continuity of the AVC chain and incidentally, if any valve was gassy there would be grid current and different readings just for that valve as the current would cause a drop on eg R2 (2.2 Meg). Incidentally, the filter caps (4 mfd) C96, 97, 98 don't come into this. I have not touched them. The caps I have replaced are the tubs. Summarizing: Normal grid bias apparently OK. Specified reading to slider (as fixed posotion) WAS OK (so not a mistake in the manual*) but now not. A final bizarre possibility exists. Suppose the manual WAS in error and the reason why I got the origina readings was a compensating error in that the tabbed capacitors were leaky? Then I replaced them with good ones and now the error in the manual showed up! That would explain why the application of Ohms Law to the relevant part of the circut does not produce the quoted desired results either! I think I'll have another coffee now... |
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