Hot Rod Deluxe noise

[Leevi]

Could the root cause be bad filter caps?It comes even if the preamp is disconnected (power amp in plugged)
The amp is about 25 years old and the caps are original manufactured by IC.
https://rikstone.fi/Recording 119.wav

/Leevi

[Platefire]

Please describe what kind of noise---buzz---static---hum. Yes I would say it's definitely time change the filter caps but that's not necessarily what's causing the noise. You could change them for the process of elimination to see if that helps. One question I would ask is, has it been sitting up or in constant use?
 

[Leevi]

Please describe what kind of noise---buzz---static---hum. Yes I would say it's definitely time change the filter caps but that's not necessarily what's causing the noise. You could change them for the process of elimination to see if that helps. One question I would ask is, has it been sitting up or in constant use?

Please listen the sample
/Leevi

[AlNewman]

Does it still make noise if you unplug your PI tube?
Have you tried changing your power tubes?

[Leevi]

Does it still make noise if you unplug your PI tube?
Have you tried changing your power tubes?

No


Tubes are new


/Leevi

[AlNewman]

So it stops making noise if you unplug your PI?

[Leevi]

So it stops making noise if you unplug your PI?

Yes


/Leevi

[AlNewman]

Try rolling in another tube to the pi and see if that's the culprit...

Otherwise, it looks like your reverb is still in circuit with the pre amp bypassed, look around your board for heat damage in the low voltage supply.

[Leevi]

Try rolling in another tube to the pi and see if that's the culprit...

Otherwise, it looks like your reverb is still in circuit with the pre amp bypassed, look around your board for heat damage in the low voltage supply.

It is a bit difficult to do troubleshooting since the issue comes up randomly.The SS reverb is there but I have turned the pot off.No visible heat damages.
/Leevi

[Leevi]

Try rolling in another tube to the pi and see if that's the culprit...

Otherwise, it looks like your reverb is still in circuit with the pre amp bypassed, look around your board for heat damage in the low voltage supply.

1st tube removed: noise
2nd tube removed: noise
3rd tube (PI) removed: no noise
Power amp in plugged: noise
/Leevi

[AlNewman]

Well, it isn't intermittent, so you can troubleshoot it.
So as long as the switching works in your power amp in jack, you've isolated the problem to between the jack and the PI plates.
If it isn't the pi tube, it's time to start tracing signal and see where the noise begins.
Could be a bad solder joint, bad resistor or cap, or perhaps one of the ribbon cables or other connections.
Either with a scope, a listening amp, or as tubeswell has explained in other threads, using a .1 cap and shorting signal to ground, you should be able to narrow down the source.

[Leevi]

One cause could be the filter cap of the PI.If I start change the caps I would start from that.

/Leevi

[AlNewman]

Yes, as mentioned earlier, it's not a bad idea to change the caps either way.  And if the caps are the problem, you shouldn't have to trace anything.
You may even be able to see it just by checking voltages.

[Leevi]

I changed the filter caps but the random noise was still there.

After I re-soldered some points around the phase inverter the situation improved a lot.
I think the basic problem is the the old PCB having bad contacts.
/Leevi

[stratomaster]

For some reason the PI plate resistors in this amp like to die.  If you had success in retouching those joints consider replacing both plate resistors.

[Leevi]

For some reason the PI plate resistors in this amp like to die.  If you had success in retouching those joints consider replacing both plate resistors.

This was an excellent tip, thank you.
The 100K:n plate resistor had changed its value to 250K.
Fortunately the resistor was visible and in a good position on the PCB so I was able to change it.
All the disturbances are gone now.
/Leevi