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#6
August 17th 09, 09:12 PM
posted to rec.radio.amateur.boatanchors
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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. |
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