Troubleshooting Fosage Signature phono amp.

The Fosgate was the second of many phono stages I’ve owned since, and it is slightly microphonic in general. It does sound like the switch itself might need a cleaning (deoxit) or replacement. The fact that it’s vertically oriented enables dust and oils from fingers to seep in over the years.

All the switch should be doing is rerouting the signal path to go through the internal step up transformers or not. The fact that it comes back in means the transformers are ok. If a deoxit spray down doesn’t help, it might also be worth inspecting the board on the left side in your pic to see if any wires are crossing, maybe unscrewing that board to see the other side where the transformer wires are getting soldered in.

One odd thing that also might be causing it is that the filament of one tube may start to die out. 

But more than likely a deoxit cleaning will fix it. Fingers crossed. 

blisshifi-

The input is appreciated, and your reasoning is sound, but your comment " internal step up transformers or not. The fact that it comes back in means the transformers are ok."

I believe is incorrect. There ARE no "internal" step up transformers-MC gain is a product of active tube gain which made this unit different than others.

Hopefully someone familiar with this circuit can correct one of us on what’s going on with it that topology.

Those 2 cans on top ARE NOT step transformers covers, if anyone thinks so. Those are to be precise- a random part that is the EXACT model which where used for footers on my REL Britannica B3 subs! They are there purely for aesthetic reasons.

"One odd thing that also might be causing it is that the filament of one tube may start to die out."

If it is a tube thing, I haven’t eliminated it because I’ve switched all positions with known "good" tubes.

Are there other components susceptible to becoming microphonic? First time encountering microphonics in my home audio gear. Very familiar it with my tube guitar amp.

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You're absolutely right, I completely blanked on the SUT / design. I recalled the Fosgate being very noisy/microphonic on the MC setting, so I ended up getting an EAR MC3 SUT to pair it with.

So you're right, something else might be causing the issue electronically beyond the toggle switch, but looking into the switch might be a good first start.

Best of luck in addressing it!

Microphonics seem to go with certain tube (DHT) designs. Turns out that I generally love the sound of such tubes. They say isolation is the key, but I have found that to be quite a challenge when addressing the problem. In some cases, the answer turned out to be part of the problem! (I wonder if those copper tube sleeves might not be helping).

  I did not get a good look at the circuit board, but it likely has those snap in connections that I hate so much. They can seem to be solid with problems not seen. Maybe you can use De-oxit on those connections to see if that has an effect on the problem. Switching back and forth, can in rare cases, have the just vibration from the mechanical switch action to temporarily remake a connection. I doubt that it is the switch itself, but something out there hanging by a thread (electronically) that is making and breaking contact. 

 BTW, the switch itself is likely sealed and cannot be cleaned with De-oxit. That is also to say that oils and other contaminants won't be seeping in over time. 

As much as I loved the sound of the Fosgate, it was noisy so I moved on. I hope you can figure out your issue.