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I have a GMII that needs repair. Battery wiring was bad, coil feedline was bad, now it always emits a tone regardless of Tuner setting. Is there a schematic here that is similar to it that I can use as a reference for debugging? Any shotgunning advice? I see several electrolytic caps and CMOS ICs -do they tend to go after this long? The unit does seem to react to nearby iron but just never can be quieted. I have another one that works fine. Even if I get lucky and swap out a bad component, I'm clueless as to what the trimpots do.
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Electrolytic capacitors are really only reliable for up to 10 years, and that's only if it is used on a regular basis.
So depending on how old the machine is I would replace the electrolytic caps and restore any wiring.
That might just fix it. When they dry out, they pass DC where they shouldn't and cause all sorts of problems.
Swap the coil with the good tester too, to rule it out.
You can take measurements from the known good detector too, if needed.
Hopefully it works without having to play with the trimpots. -
Better yet, just remove the loop... should still tune to reasonable threshold. Also, make sure the SAT switch is set properly. Beyond that, could be 100 things. Comparative probing might be a Good Idea.Comment
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if I remove the coil connection the "good" one goes to a high pitch that can't be squelched. The "bad" one however can be squelched using either the tuner or sens controls. So now what?? Is the good one bad too? It quiets normally with the coil attached. The bad one seems sensitive to pressure on the pcb so maybe a marginal solder connection or pot. I haven't opened up the good one yet.
Man, these babies are loud - no room for a volume control I guess...
Oh, what's with the red led on the board?Comment
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I removed and tested all electrolytics with my LC102 cap tester and they are fine. I wrote down all the IC's used and will have to go comparison probing with my scope.
Looking at the various IC's used I'd first suspect anything CMOS. There is a LINCMOS quad opamp, CD4053, CD4011... the scope should tell all.
Might do something like Carl suggests - maybe toroidally couple a signal generator to the RX input on each and document IC signals.Comment
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I'm not getting very far. I have received IC replacements for all IC's in the design but still don't really have a good feel for where to start swapping chips. With both detectors, good and bad, the iron detection meter really doesn't work - is it supposed to? How? Tried a crowbar - nothing. I've read where others have the same problem with it. The bad detector will not maintain the threshold setting set by the Threshold pot. Instead after seconds of time the audio comes back up. Is there a White's patent that gives an overview of the GM design? All that I've seen look more complicated or more simple - and don't use the same IC's. Aggrivating that it looks like I have to send both units to Whites for repair.Comment
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Are all of the chips in sockets.
Sometimes the contacts go lazy, If you can pull them and reinsert them - you can temporarily clear a fault.Comment
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Nope, everything soldered in. Debating whether to use sockets on parts I replace - I have some nice gold plated ones.Comment
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I gave up on the repair. I could have just started to swap out chips but doing so would put the pcb traces at risk. I sent it and my "good" one both to Whites for repair/calibration. I'm wondering if the Iron meter is supposed to work on these. Doesn't seem to move upon detecting a crowbar. Does move for the battery level check on power up though. Thus I sent both detectors to get them both up to snuff and save on shipping.Comment
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