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Isolating shorted PCB component ?
Sometime ago I think I recall someone posted or wrote an article on a neat
way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr |
When I worked as a circuit test engineer that produced fairly complex
multi-layer PCBs many years ago, there were three primary methods we used to find shorts across the power rails. 1. Use a calibrated micro-ohm meter, fixing one lead to the PCB ground and taking various resistance readings by moving the other lead from the board edge connector along the traces to locate the point where the meter provided the lowest resistance reading. You could then fix the lead to that point and then take readings by moving the other (previously fixed) lead along the grounds to again find the minimal reading. This usually led you to the area of the board where the short or defective component was located. If you had sensitive enough equipment and some good test leads, this procedure usually worked pretty well when there was an actual hard short. 2. Apply a current limited, voltage limited (lower than the nominal design voltage, for instance 5V DC) power source across the power rail at the PCB edge connector. Start with a fairly low current limit and increase this as needed, but keeping the current reasonable (you don't want a defective component to explode - been there done that). We then used either a thermal sensitive plastic sheet (material it contained was like the stuff used in "mood rings" from the 1970's) or an infra-red camera to find the "hot spot" on the PCB. This technique had the advantage of working for soft shorts, such as were created by defective components (transistors, other semiconductors, ICs, capacitors, etc.) or even resistive type shorts that were created by contamination from foreign materials (conductive growths, moisture, salt water contamination, etc.) We were even able to "see" internal shorts on 8 layer circuit boards using the camera. 3. Visual observation and path tracing coupled with selective unsoldering of legs of suspect components (assumes through hole mounting, not surface mounting) and use of isolation rings (small plastic rings that slid over the unsoldered leg of an IC isolating it from the multi-layer solder pad/circuit. This technique was generally used as a last resort and usually in combination with procedures 1 and 2 above, prior to scrapping "difficult", but costly product that had been diagnosed with a power rail short. The above techniques are what I used about 20+ years back when I had engineering responsibility in a large electronics factory. I'd imagine that there are likely better approaches today due to improvements in technology so would be interested to hear what others recommend. Good luck! Bob "Henry Kolesnik" wrote in message ... Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr |
Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? Bob Pease gives a schematic for a short-circuit detector on page 21 of his "Troubleshooting Analog Circuits". It uses an LM10 op amp and an LM331 voltage-to-frequency converter, plus one transistor and some passives. You feed some low-voltage, current-limited power into one end of the shorted trace, slide the probe along the PCB trace starting from the power injection point, and listen to the tone. When you go along portions of the trace which aren't carrying the short-circuit current to ground, the tone remains stable. When you go along portions which _are_ carrying short-circuit current to ground, the tone rises (lower voltage present on the trace) as you move towards the short, and falls as you move away from it. When you pass the shorted point, the tone rises to its highest frequency and stays there. Pease points out that you can use this same basic technique with nothing more than a current-limited voltage supply and a sensitive voltmeter (VTVM or FET-input DVM)... but that listening to tones is a good deal easier. At .4 ohms, if you feed in 100 milliamps you'll get around 40 millivolts at the injection point, falling to zero at the point of the short. A good 3.5-digit voltmeter with a 2-volt scale ought to give you enough resolution to get quite close to where the short circuit is. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Change all those little tear drop bypass caps connected to the shorted line.
Won't hurt to replace them all, might save a future short. Ken Henry Kolesnik wrote: Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr |
Right now it looks like 2 might be the problem marked AVX 103 and AVX 849.
I'm guessing that they're tantalums but not sure. The Wavetake was mfg in 1989. I need help on the values and voltages. There's too many to just start changing. tnx hank "kenneth l wright" wrote in message ... Change all those little tear drop bypass caps connected to the shorted line. Won't hurt to replace them all, might save a future short. Ken Henry Kolesnik wrote: Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr |
Henry Kolesnik wrote:
Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr Use an adjustable DC power supply to feed the shorted power rail. Use 16 gauge, or larger wire to minimize the voltage drop. Make sure to connect the meter negative to ground at the same point you connect the adjustable power supply to the bad board. Set the adjustable DC power supply to about a half amp, and use a DC voltmeter to read the voltage drops across the traces. You will find a point where they level off. Back up one part to the last linear voltage drop and you have found your bad part. I prefer to use a 4½ digit voltmeter, or better to read minor variations. Also, check the voltage on the ground buss if the board isn't bolted to a chassis to find which part of the board has the problem. I have fixed hundreds of shorted boards this way. -- We now return you to our normally scheduled programming. Take a look at this little cutie! ;-) http://home.earthlink.net/~mike.terrell/photos.html Michael A. Terrell Central Florida |
Power it up on a variac, spray the board with freeze spray. The shorted components will defrost first!
"Henry Kolesnik" wrote in message ... Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr |
Just change the ones across the B+ line, shouldn't be that many. Ken
Henry Kolesnik wrote: Right now it looks like 2 might be the problem marked AVX 103 and AVX 849. I'm guessing that they're tantalums but not sure. The Wavetake was mfg in 1989. I need help on the values and voltages. There's too many to just start changing. tnx hank "kenneth l wright" wrote in message ... Change all those little tear drop bypass caps connected to the shorted line. Won't hurt to replace them all, might save a future short. Ken Henry Kolesnik wrote: Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr |
On Thu, 15 Jan 2004 16:44:35 -0600, "Steve Nosko"
wrote: Seems to me there's a way to use a moderate current and sense the _magnetic_ field. Follow the field around the board along the runners. However, I don't remember what was used to sense the field. HP used to make a hall effect (IIRC) probe for current tracing. One of their Bench Briefs technotes described the probe and the process. |
I had good success with a variant of #2 on a very large digital board that had
a short between the +5V and ground -- both of which were planes due to the high current and multiple loads. I energized the +5V bus with a supply set for 1 V and current limited to 10 A. Then went around the board measuring between adjacent +5 and ground points [like across a bypass cap or an IC]. Where I found the lowest drop was the short. Dr. G. In article , "Bob Shuman" wrote: When I worked as a circuit test engineer that produced fairly complex multi-layer PCBs many years ago, there were three primary methods we used to find shorts across the power rails. 1. Use a calibrated micro-ohm meter, fixing one lead to the PCB ground and taking various resistance readings by moving the other lead from the board edge connector along the traces to locate the point where the meter provided the lowest resistance reading. You could then fix the lead to that point and then take readings by moving the other (previously fixed) lead along the grounds to again find the minimal reading. This usually led you to the area of the board where the short or defective component was located. If you had sensitive enough equipment and some good test leads, this procedure usually worked pretty well when there was an actual hard short. 2. Apply a current limited, voltage limited (lower than the nominal design voltage, for instance 5V DC) power source across the power rail at the PCB edge connector. Start with a fairly low current limit and increase this as needed, but keeping the current reasonable (you don't want a defective component to explode - been there done that). We then used either a thermal sensitive plastic sheet (material it contained was like the stuff used in "mood rings" from the 1970's) or an infra-red camera to find the "hot spot" on the PCB. This technique had the advantage of working for soft shorts, such as were created by defective components (transistors, other semiconductors, ICs, capacitors, etc.) or even resistive type shorts that were created by contamination from foreign materials (conductive growths, moisture, salt water contamination, etc.) We were even able to "see" internal shorts on 8 layer circuit boards using the camera. 3. Visual observation and path tracing coupled with selective unsoldering of legs of suspect components (assumes through hole mounting, not surface mounting) and use of isolation rings (small plastic rings that slid over the unsoldered leg of an IC isolating it from the multi-layer solder pad/circuit. This technique was generally used as a last resort and usually in combination with procedures 1 and 2 above, prior to scrapping "difficult", but costly product that had been diagnosed with a power rail short. The above techniques are what I used about 20+ years back when I had engineering responsibility in a large electronics factory. I'd imagine that there are likely better approaches today due to improvements in technology so would be interested to hear what others recommend. Good luck! Bob "Henry Kolesnik" wrote in message ... Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr |
"Henry Kolesnik" wrote in message ... Right now it looks like 2 might be the problem marked AVX 103 and AVX 849. I'm guessing that they're tantalums but not sure. The Wavetake was mfg in 1989. I need help on the values and voltages. There's too many to just start changing. tnx hank The tantalums in WaveTeks were the weak links. I was continually chasing shorted tantalum bypasses when I owned two WaveTek 3000 signal generators. Pete |
Wild Bill wrote:
I haven't tried one, but floppy disk drive heads are sensitive. I was reminded of a circuit in a magazine (many years ago) that used a floppy head for a pickup. Some questions. How are you going to position it against the trace accurately? What about double sided, or multilayer boards? The problems with using AC to find shorts is that you get false peaks and dips from the inductance of the traces, and the characteristics of the components. Another problem is that some parts self destruct with only a small reverse voltage so you can damage a lot of parts while troubleshooting the board. I used the DC voltage drop & sensitive digital meter method on boards that people couldn't fix with AC, then had to find the parts they damaged. The whole idea is to find and fix a problem quickly, and reliably. -- We now return you to our normally scheduled programming. Take a look at this little cutie! ;-) http://home.earthlink.net/~mike.terrell/photos.html Michael A. Terrell Central Florida |
I found the bad tant cap, it was a 22 ufd, 20 volt, not any AVX but a
yellowish tan blob with a big L. I replaced it with an electrolytic. I found it using my Fluke 87 that measures to the nearest tenth of an ohm and only the suspect cap would flicker between 0.3 and 0.2 ohms, all the rest were 0.4 or 0.3. Now I'm kicking myself for not buying an HP meter that could read 1/100ths maybe 1/1000ths because I could see no use for it. Now I can see a use and one is on my list but nevertheless my Fluke saved a lot of desoldering. The Wavetek still doesn't work as something else is keeping the voltage low and I think it might be a regulator. Now I wish I could find a schematic for the Wavetek 188-S-1257, as it would keep me from wasting so much time. Thanks for all the tips. guys. 73 hank wd5jfr "Henry Kolesnik" wrote in message ... Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr |
On Thu, 15 Jan 2004 16:54:48 -0500, kenneth l wright
put finger to keyboard and composed: Change all those little tear drop bypass caps connected to the shorted line. Won't hurt to replace them all, might save a future short. Ken I agree. I saw a *lot* of shorted tantalums when I was troubleshooting multilayer minicomputer PCBs during the 80s. Henry Kolesnik wrote: Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr - Franc Zabkar -- Please remove one 's' from my address when replying by email. |
Some quick, off hand comments imbedded below:
Steve N. "Michael A. Terrell" wrote in message ... Some questions. How are you going to position it against the trace accurately? First you do some controlled experiments where you put the test signal through known runners to characterize the given pickup. How to orientate it & where it picks up from. What about double sided, I don't see a problem. If you get it set up so it detects at sufficient distance (.070 or so). or multilayer boards? Should still work, but you'll have to follow the target trace by "braile" (sp) since you might not be able to see it. The problems with using AC to find shorts is that you get false peaks and dips from the inductance of the traces, and the characteristics of the components. At audio this can't be a problem. I'm convinced it will just be current defined by the generator. With a runner short you won't have much current in anything but the runners anyway, no? Another problem is that some parts self destruct with only a small reverse voltage so you can damage a lot of parts while troubleshooting the board. Also, there won't be much voltage (remember the DC method?). If you do have this, you're in risk of burning up runners. That's too much current. I used the DC voltage drop & sensitive digital meter method on boards that people couldn't fix with AC, then had to find the parts they damaged. The whole idea is to find and fix a problem quickly, and reliably. Hard to believe a 600 ohm audio gen will blow up anything, but it certainly is possible... of course you just don't go in there blazing away with power.. I don't mean to say the DC method is bad, or that AC is better. AC is just another option, especially if you don't have a good enough DVM. Seems to me I saw a commercial system which did use AC. My Fluke has problems under an ohm and at fractions of a volt even though it is a 5 digit. Should be good down there. May try it to see how DC works just for info. 73 -- Steve N, K,9;d, c. i My email has no u's. |
Henry, dont laugh at this method..it works....read it thru before nixing it.
I have had this happen before too. simply put about 5 volts at 500 mils on the b+ line, regulated at the 500 mils. Let it sit for a few minutes and then go looking for the part to be running warm. The part will be dissipating 5*.5= 2.5 watts of heat. sooner or later the bad part WILL get warm. It will NOT lift traces unless they are VERY small. If this approach fails, the next thing I do is go in with a new(sharp) razorblade and start as far away from the power supply and cut B+ traces one at a time until the short goes away. This isolates the short to a smaller area. Suspect Tantalum caps as they usually fail in this mode of low ohms shorts...let us know when you find it...Eddie "Henry Kolesnik" wrote in message ... Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr |
See my previous post about finding the culprit but I had another problem and
it was the 7815 3 term reg which I replaced with NOS and the Wavetek came to life but not great. I measured the 15 volt terminal and it was 23 volts so I may not have much left. I have question on your method. For a shorted component with very low resistance, 0.2 I can't see much heat created since P=I squared R. A short of 0.2 ohms wouldn't dissapate much power and all you'll do is heat the traces, other components and perhaps blow short if you're lucky! 73 hank wd5jfr "Eddie Haskel" wrote in message m... Henry, dont laugh at this method..it works....read it thru before nixing it. I have had this happen before too. simply put about 5 volts at 500 mils on the b+ line, regulated at the 500 mils. Let it sit for a few minutes and then go looking for the part to be running warm. The part will be dissipating 5*.5= 2.5 watts of heat. sooner or later the bad part WILL get warm. It will NOT lift traces unless they are VERY small. If this approach fails, the next thing I do is go in with a new(sharp) razorblade and start as far away from the power supply and cut B+ traces one at a time until the short goes away. This isolates the short to a smaller area. Suspect Tantalum caps as they usually fail in this mode of low ohms shorts...let us know when you find it...Eddie "Henry Kolesnik" wrote in message ... Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr |
On Fri, 16 Jan 2004 08:53:49 +0800 budgie wrote:
On Thu, 15 Jan 2004 16:44:35 -0600, "Steve Nosko" wrote: Seems to me there's a way to use a moderate current and sense the _magnetic_ field. Follow the field around the board along the runners. However, I don't remember what was used to sense the field. HP used to make a hall effect (IIRC) probe for current tracing. One of their Bench Briefs technotes described the probe and the process. It was the HP 547A Logic Current Tracer combined with the HP 546A Logic Pulser. These worked only with TTL or CMOS circuits. The pulser would drive current into any point you wanted and the tracer could be used to follow the current path. You would set the pulser to put out a continuous pulse train, and then adjust the sensitivity of the tracer so that it would just trigger on the current pulses sent out by the pulser. It would thus ignore other current that was flowing in the same trace as long as that current was either DC or consisted of pulses which were smaller than those put out by the logic pulser. I think the logic pulser could put out an amp or so, but they were short enough that they didn't damage anything. They work well, but the current tracers are rather hard to come by now and they are rather expensive when you find them. - ----------------------------------------------- Jim Adney Madison, WI 53711 USA ----------------------------------------------- |
The 7815 needs only 3 volts of headroom, so 23 volts is plenty. Any more
than that will be dissapated as heat. Your right that the traces will also get warm but it's still safe. The other post that advised the use of freeze spray was a good one too. I have used both...Eddie "Henry Kolesnik" wrote in message ... See my previous post about finding the culprit but I had another problem and it was the 7815 3 term reg which I replaced with NOS and the Wavetek came to life but not great. I measured the 15 volt terminal and it was 23 volts so I may not have much left. I have question on your method. For a shorted component with very low resistance, 0.2 I can't see much heat created since P=I squared R. A short of 0.2 ohms wouldn't dissapate much power and all you'll do is heat the traces, other components and perhaps blow short if you're lucky! 73 hank wd5jfr "Eddie Haskel" wrote in message m... Henry, dont laugh at this method..it works....read it thru before nixing it. I have had this happen before too. simply put about 5 volts at 500 mils on the b+ line, regulated at the 500 mils. Let it sit for a few minutes and then go looking for the part to be running warm. The part will be dissipating 5*.5= 2.5 watts of heat. sooner or later the bad part WILL get warm. It will NOT lift traces unless they are VERY small. If this approach fails, the next thing I do is go in with a new(sharp) razorblade and start as far away from the power supply and cut B+ traces one at a time until the short goes away. This isolates the short to a smaller area. Suspect Tantalum caps as they usually fail in this mode of low ohms shorts...let us know when you find it...Eddie "Henry Kolesnik" wrote in message ... Sometime ago I think I recall someone posted or wrote an article on a neat way to isolate a shorted component on a pcb using common test eqpt but I can't recall the methodology. I'm trying to find a shorted component on a Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and I'm not having much luck disconnecting componets. I don't have a schematic and my eyes ain't what they used to be for tracing and I want to minimize the unsoldering. Does anyone recall the article or have a good way? tnx hank wd5jfr |
There have been quite a few good suggestions, but there's a little I can
add. I've successfully found shorts between inner layers of multiple-layer PC boards due to misaligned layers. (When layers are misaligned, plated-through holes -- vias -- can contact layers they're supposed to miss, shorting them.) The method I used was to connect a high-amplitude signal generator (one that puts out a few volts into 50 ohms) of about 50 kHz between the shorted layers. Then, I used a small coil as a detector, connected to a scope. The coil was a few hundred turns on a ferrite bobbin (essentially a ferrite rod), about 1/4" diameter and 1" long. With it, I was able to trace the current from the "hot" signal generator terminal to the shorted via -- the signal drops off pretty quickly beyond the short. The via was drilled out to clear the short, and connections to the intended layers made manually. If the frequency is too low, the AC current spreads too much on a plane-type layer. If it's too high, it won't penetrate through layers which are over the conducting one. If you're not dealing with multiple, plane-type layers, you could use a higher frequency and it should be pretty simple. A toroid, as someone mentioned earlier, is a poor choice for a detector, unless you grind a slot (gap) in it -- half of a split toroid would work fine. I tried a floppy drive head, too, but found that it had very poor sensitivity. They're apparently intended to be driven by high current, and at quite a bit higher frequency. The resolution would be good, though, if you put an amplifier between it and the scope. I found the bobbin coil to be adequate. I used this method quite a number of times. Knowing how to find inner-layer shorts does have disadvantages, though. The very first prototype PC boards for the Tek 11400 series 'scope mainframes arrived at about 2:00 a.m. on the day they absolutely had to be built, and all had inner layer shorts. Guess who got called in to fix them. As I recall, I managed to fix something like two out of the four or five -- just enough for that day's build. The rest had too many shorts to salvage. The same technique can be used to find shorted components, although some of the other methods suggested might be easier depending on the circumstances and type of boards. Roy Lewallen, W7EL |
"Eddie Haskel" wrote in message .. . The 7815 needs only 3 volts of headroom, so 23 volts is plenty. Any more than that will be dissapated as heat. Your right that the traces will also get warm but it's still safe. The other post that advised the use of freeze spray was a good one too. I have used both...Eddie "Henry Kolesnik" wrote in message ... See my previous post about finding the culprit but I had another problem and it was the 7815 3 term reg which I replaced with NOS and the Wavetek came to life but not great. I measured the 15 volt terminal and it was 23 volts so I may not have much left. I have question on your method. For a shorted I think he was referring to a 15 volt rgeulator that has 23 volts on the output indicating it was bad and the overvoltage caused other components to go bad. |
On Fri, 16 Jan 2004 19:08:09 -0600, Jim Adney wrote:
On Fri, 16 Jan 2004 08:53:49 +0800 budgie wrote: On Thu, 15 Jan 2004 16:44:35 -0600, "Steve Nosko" wrote: Seems to me there's a way to use a moderate current and sense the _magnetic_ field. Follow the field around the board along the runners. However, I don't remember what was used to sense the field. HP used to make a hall effect (IIRC) probe for current tracing. One of their Bench Briefs technotes described the probe and the process. It was the HP 547A Logic Current Tracer combined with the HP 546A Logic Pulser. These worked only with TTL or CMOS circuits. The pulser would drive current into any point you wanted and the tracer could be used to follow the current path. That may have been the sensor, but the article I saw related to tracing shorts. To that extent, it was environment-independent. (snip) They work well, but the current tracers are rather hard to come by now and they are rather expensive when you find them. Most HP stuff I've seen works well. Being HP, I'm sure they were expensive way back then. |
23 voolts on output!
later "Ralph Mowery" wrote in message ... "Eddie Haskel" wrote in message .. . The 7815 needs only 3 volts of headroom, so 23 volts is plenty. Any more than that will be dissapated as heat. Your right that the traces will also get warm but it's still safe. The other post that advised the use of freeze spray was a good one too. I have used both...Eddie "Henry Kolesnik" wrote in message ... See my previous post about finding the culprit but I had another problem and it was the 7815 3 term reg which I replaced with NOS and the Wavetek came to life but not great. I measured the 15 volt terminal and it was 23 volts so I may not have much left. I have question on your method. For a shorted I think he was referring to a 15 volt rgeulator that has 23 volts on the output indicating it was bad and the overvoltage caused other components to go bad. |
I found a good 7815 and installed it and except for the frequency dial being
off the thing seems to work. If I find a manual I should be able to cal it or find out if the cal is off because of a failed componet because of being overstressed to 23 volts when it was designed to operate at 15 volts. The output remains flat from 0 to 4 MHz but the when the dial reads 4 MHz the counter shows 3.5 MHz. At 1MHz on the dial the counter reads 0.570MHz and similar readings on at audio frequencies. hank wd5jfr "Ralph Mowery" wrote in message ... "Eddie Haskel" wrote in message .. . The 7815 needs only 3 volts of headroom, so 23 volts is plenty. Any more than that will be dissapated as heat. Your right that the traces will also get warm but it's still safe. The other post that advised the use of freeze spray was a good one too. I have used both...Eddie "Henry Kolesnik" wrote in message ... See my previous post about finding the culprit but I had another problem and it was the 7815 3 term reg which I replaced with NOS and the Wavetek came to life but not great. I measured the 15 volt terminal and it was 23 volts so I may not have much left. I have question on your method. For a shorted I think he was referring to a 15 volt rgeulator that has 23 volts on the output indicating it was bad and the overvoltage caused other components to go bad. |
"Henry Kolesnik" wrote in message ... 23 voolts on output! later The way I read it , the 15 volt regulator had gone bad and was passing a much higher voltage as it was not regulating . That was giving 23 volts on the output instead of the 15 volts it was suspose to. |
On Sat, 17 Jan 2004 16:28:01 +0800 budgie wrote:
On Fri, 16 Jan 2004 19:08:09 -0600, Jim Adney wrote: On Fri, 16 Jan 2004 08:53:49 +0800 budgie wrote: On Thu, 15 Jan 2004 16:44:35 -0600, "Steve Nosko" wrote: Seems to me there's a way to use a moderate current and sense the _magnetic_ field. Follow the field around the board along the runners. However, I don't remember what was used to sense the field. HP used to make a hall effect (IIRC) probe for current tracing. One of their Bench Briefs technotes described the probe and the process. It was the HP 547A Logic Current Tracer combined with the HP 546A Logic Pulser. These worked only with TTL or CMOS circuits. The pulser would drive current into any point you wanted and the tracer could be used to follow the current path. That may have been the sensor, but the article I saw related to tracing shorts. To that extent, it was environment-independent. I'm sure you're right that it would work anywhere that you could use a +5 to +15V pulse. They work well, but the current tracers are rather hard to come by now and they are rather expensive when you find them. Most HP stuff I've seen works well. Being HP, I'm sure they were expensive way back then. The Logic probes and pulsers in this series are fairly easy to find today, but I had to look a long time before I found one of the tracers at a reasonable price. Yes, they cost more when they were new, but the tracers were only about 25% more than the probes. Used, they usually cost at least twice as much as a probe. - ----------------------------------------------- Jim Adney Madison, WI 53711 USA ----------------------------------------------- |
When I got the 188 the 7815 output was nearly zero and I assume the shorted
tantalum caused the low voltage. After I found the bad tantalum and replaced it the 7815 voltage was still very and the output pin to ground mesured 12 ohms. I had a NOS Radio Shack 7815 still sealed in its package so Iused it and that's when pin 3 went to 23 volts and that nearly made a grown man cry because I thought I fried addtional components. The PCB was running too hot. After getting a known good 7815 the B+ stayed at 15 volts and the unit runs cooler and seems to function except that the dial calibration is off. I'm trying to find a manual. 73 hank wd5jfr "Ralph Mowery" wrote in message ... "Henry Kolesnik" wrote in message ... 23 voolts on output! later The way I read it , the 15 volt regulator had gone bad and was passing a much higher voltage as it was not regulating . That was giving 23 volts on the output instead of the 15 volts it was suspose to. |
Thanks to all of you for posting to this thread. Man did I learn a lot!
Mike Burch K8MB Apache Junction AZ |
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