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Not unlucky anymore, I guess....a tale of an audio oscilaltor. Thanks Brenda. - Printable Version

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RE: Not unlucky anymore, I guess....a tale of an audio oscilaltor. Thanks Brenda. - morzh - 01-14-2014

OK, the tubes:

12J5
12SA7
50L6
35Z5


Now, the interesting thing: I have just come home and put two halves of that same cap to keep reforming, installing two 50K resistors to each half. One of them has been reformed since yesterday evening and through the nigh.
The thing is, the second half behaves as if it were reformed too - it has the same more or less leakage current and the difference between the two voltage drops across the two resistors is about 1V - 3V vs 4V or so. Right now (after 20 minutes) it is 3.3V vs 2.7V.

there should be some peculiarity of the way the cap is made that resulted in it.

RE: Not unlucky anymore, I guess....a tale of an audio oscilaltor. Thanks Brenda. - morzh - 01-15-2014

OK, yesterday night I started reforming the second cap and it was reforming much faster, and in the morning I saw the residual voltages on the resistors about order of magnitude lower than on the other cap. Then again, this one is from 1953 and the previous cap was from 1948. Not sure of that should matter. But in the morning I saw the voltages of 0.2V and 0.15V avross 50K resistors whereas the otehr cap had about 1.5 and 3V respectively.

Long story short, tonight I soldered the caps in place (good thing I photographed them from above and from the solder side so there were no questions, plus the wires are rigid solid type so they kept at their places well. BTW the wire is enameled. Almost looks like a magnet wire in jacketing. I had to strip the enamel), used two wires to connect isolating transformer to the input and fired it up, putting the scope at the output.

It works just fine. Generates the full range in cycles and kilocycles.

One down.

Now, I got the Pushpull transformer for the 90, might as well finish it tonight.

RE: Not unlucky anymore, I guess....a tale of an audio oscilaltor. Thanks Brenda. - Mondial - 01-15-2014

That's great. Its always satisfying to work on a well engineered and made piece of equipment that's still working with all its original parts more than a half century later.

Its also interesting to note that while Chinese electrolytics are failing in modern equipment after only a few years, good US made caps can still test and operate as new 65 years later.

And it does illustrate than many of these old electrolytics can be returned to service without replacement by proper reforming. I would expect that the electrolytics in your HP 200CD can similarly be saved. You may even be able to reform them in place without desoldering them.

RE: Not unlucky anymore, I guess....a tale of an audio oscilaltor. Thanks Brenda. - morzh - 01-15-2014

Actually this is what I think I will start doing from now on.
If I see that a cap shows full or larger capacitance, I will try to re-form it rather than open it and re-stuff. I have done that to at least 3-4 caps and every time I felt bad. Luckily most of them caps from 30s are dry so it is an easy decision.