Eqp-1a Intermittent noise

[gato]

I have a turret board build of an Eqp-1a that I built years ago that has developed intermittent noise.
Here is a recording I made of the noise:
https://www.dropbox.com/scl/fi/z6ir...oise.mp3?rlkey=v1wjo6pbg4441w0koj94wd2k5&dl=0

I tried swapping tubes, and after going through a few a found a pair that had no noise at all, but the next time I turned the unit on the noise was back. I suspected it might be the cathode resistors, and tried gently poking around with a wooden skewer. I discovered that the 0.25mf caps that go from the cathodes of V1 to the grids of V2, as well as the 50uf capacitors that go from B+ to the cathodes of V2 all audibly respond to tapping. Could it be that all 4 capacitors are microphonic? How useful is this as a diagnostic tool?

Thanks for any suggestions!

 

[CJ]

could be plate resistors also,

are there any mica caps? i have had those make the same sound as a noisy tube,

 

[gato]

Thanks for your reply!
There is a 22pf mica cap across the plates of V1, I'm going to check right now if I have a replacement, if so I'm going to swap it out right away.
In general, does tapping with a wooden skewer necessarily tell you anything?

 

[musipol]

My 2 cents. Tapping can discover microphonic components, but.............................if the tube socket is not well isolated from the rest of the components(it's usually not) , you could be confusing tube microphonics with other component microphonics.

 

[gato]

Fixed! Thanks y'all for your help! It's whisper quiet again!

It ended up being a bad solder joint on one of the caps that went from the cathodes of V1 to the grid of V2. The wire was wrapped but there was no solder and it worked for about a decade and I guess time did its thing and my shoddy craftsmanship did its thing.

 

[Ike Zimbel]

Ah...that old chestnut, the "no-solder joint". I've seen many over the years, and not in DIY builds, but from many "name' manufacturers.

 

[rock soderstrom]

Ah...that old chestnut, the "no-solder joint". I've seen many over the years, and not in DIY builds, but from many "name' manufacturers.

+1