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 Post subject: Fun With An EICO 377
PostPosted: Nov Sat 09, 2019 1:15 am 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3847
Location: Florida
I have a closet full of old test equipment and every now and then I pull something out just to run it for a while and keep it happy. This time I pulled an old EICO 377 audio generator. It's one of the blue ones and came from a friends TV shop when he closed. It worked the last time I tried it, probably four or five years ago.

It was dead, no output of any sort. I opened it up and there was nothing obvious like an unlit tube. Trouble shooting found a leaky coupling cap and a screen resistor that had drifted high but nothing wildly off. Voltages were about right, except for a screen voltage and that was fixed by changing the resistor. No help. More troubleshooting gave some strange resistance readings around the feedback network. Disconnecting the variable cap eliminated these. Measuring across the variable cap showed about 50K Ohms in parallel even though it was completely disconnected.

Removing the variable cap from the chassis showed that the 50K reading was there when the plates were completely open, completely closed, or anywhere in between. There was no visible contamination of any sort found. Cleaning with alcohol, especially around the pressed-in standoffs that hold the stator plates in place had no effect. The 50K was still there.

Using my good old PS-3 I applied 200 volts across the cap. There was no smoke, no sparks, no current shown on the PS meter, nothing. After applying the voltage, the mystery resistance was gone and the 377 was happy after the variable was re-installed. The applied voltage had wiped out whatever was contaminating things.

So what was going on? My guess is that there are micro fissures in the ceramic insulation used to insulate the stand-offs that hold the stationary plates in place. This would allow moisture to creep in over time and gradually make a path across the entire cap. This particular cap has four sections connected as parallel pairs to give one large two-section variable cap. Leakage was present in both pairs. Applying voltage across the cap burned it out. This was a weird one.

RRM


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 Post subject: Re: Fun With An EICO 377
PostPosted: Nov Sat 09, 2019 6:23 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1398
It is possible that some time in it's history someone decided to 'clean' it and used a chemical that was less than ideal. For example, WD-40 is NOT for electrical equipment - ask me how I know.

Your treatment might have mitigated any remaining residue. Congrats on that, BTW. Very smart troubleshooting and repair.

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I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


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