Weird Oscillation?

[acdc-angus]

Hi,

I did the maintenance on a mid 70's 2203.

- 6 filters caps
- 2 Bias Caps
- Add 1ohm resistor on each power tube socket for bias setting
- Changed all the pots
- Changed 3 preamp tube sockets
- Relocate the 68k directly to the socket tube
- Relocate the NFB on 8 Ohms instead of 4 Ohms
- Put a shielded wire on the V1
- Redo all the solder joint
- Set the bias on 36mv

I tried tubes that work really good on my Replica, same issue. Big fart uncontrollable noise randomly come.

(The pink and green wires are disconnected after the shielded wire install, same issue before the shielded wire)

Here the video of the sound, what it can be? Transformer? Broken wire? Faulty standby switch? (Same on the Torpedo Captor, than Speakers cabinet, both volume to 0 or 10)

https://youtube.com/shorts/raUXZClVUok

[acdc-angus]

Nobody ?

[Dana,s blues]

The forum is under repair. Read Surf's post. As for your amp, how did it sound before the maintenance? From the sound of your video it is hard to tell what it could be. Not familiar with some of your modifications. If it is not tube related then something is creating interference with the signal path. Anything from a bad transformer or faulty cap or transistor. If you plug in a guitar do you get any sound other than the hum? If not and you can't hear any notes over the oscillations then you have either a short or power bleed in your signal path. Unfortunately you will have to trace and test the signal path to find what it is. Very complicated. Test your transformers first. If it isn't a blown Tranny then you will have to check the path as I mentioned.

[acdc-angus]

The forum is under repair. Read Surf's post. As for your amp, how did it sound before the maintenance? From the sound of your video it is hard to tell what it could be. Not familiar with some of your modifications. If it is not tube related then something is creating interference with the signal path. Anything from a bad transformer or faulty cap or transistor. If you plug in a guitar do you get any sound other than the hum? If not and you can't hear any notes over the oscillations then you have either a short or power bleed in your signal path. Unfortunately you will have to trace and test the signal path to find what it is. Very complicated. Test your transformers first. If it isn't a blown Tranny then you will have to check the path as I mentioned.

The noise come randomly, sometime it doesn't came at all. The guitar sound perfectly when it doesn't oscillate.

I thought about the standby switch, I have a Superbass that works flawlessly, I'll try his switch. Then I think it will be the transformer.

How can I test a faulty coupling cap?

[Dana,s blues]

I thought about the standby switch

I think it would have to be something else. Steve D would have a better idea as to where to look next. Was the amp working before your mods? You can use a voltmeter to test along the signal path. But you have to know the resistance of each component and see if there is a discrepancy. Good luck! you may need an experienced technician that knows this amp.

Dana's blues

[acdc-angus]

I thought about the standby switch

I think it would have to be something else. Steve D would have a better idea as to where to look next. Was the amp working before your mods? You can use a voltmeter to test along the signal path. But you have to know the resistance of each component and see if there is a discrepancy. Good luck! you may need an experienced technician that knows this amp.

Dana's blues

Actually the amp work really good, until the noise came in, sometime it doesn't appear.
Before that, my friend told me the amp has a kind of "feedback" signal. Didn't notice when I tried it. But lot of crispy noise, bubbling filter caps a' D really bad bias. The pots was loose.

I maintened and converted the Super Bass he brought me too, this one works flawlessly