Vox AC15VR Oscillation

[telecaster]

Amp had a severely burnt R36 (Zobel network) and audible high frequency oscillation. I replaced R36 and C46 and also the LM1875 IC. No improvement, I can see oscillation of 100mV peak-to-peak high frequency (couldn't get scope to sync on it). Any ideas on what to look at or try next?
vox_ac15vr_sch.pdf

[g1]

You must not put your scope ground to the speaker neg. (or spkr jack sleeve), and the ext.spkr. jack must be isolated from chassis. Are those conditions being met?

[telecaster]

g1 - I hooked the scope directly to the external speaker jack tip and sleeve. The external jack ground (sleeve) is isolated from the main (chassis) ground, is that what you mean? How do I safely hook up the scope, do I need some type of isolation transformer?

[Helmholtz]

Scope probe ground goes to amp chassis.

[g1]

g1 - I hooked the scope directly to the external speaker jack tip and sleeve. The external jack ground (sleeve) is isolated from the main (chassis) ground, is that what you mean?

The speaker (-), and sleeve of spkr. jack do not connect to ground, that is why it is isolated from chassis. If you put your scope ground there you short it to chassis which defeats the NFB circuit. R35 is important and you are shorting it out.
So you may have repaired the amp and are introducing oscillation with your test set-up.
As Helmholtz stated above, connect scope ground to amp chassis. It's easier than putting the scope on an isolation transformer.

[Helmholtz]

It's easier than putting the scope on an isolation transformer.

A scope must never be put on an isolation transformer for safety reasons.

[g1]

A scope must never be put on an isolation transformer for safety reasons.

I won't argue with that here. I will also mention that any electrical appliance that has it's 3rd prong broken off the AC plug must never be operated.

[Helmholtz]

The question about floating a (safety grounded) scope arose every once in a while in our lab.
The answer of the company's safety engineer was a clear "Never!"

If unavoidable and observing safety precautions a trained expert may float the DUT but never the scope.
Another condition was that the person must not be alone in the lab when doing so.
All of our design and qualification engineers had to attend safety trainings on a regular basis (IIRC at a 2 years interval)
I did too even when I was a manager.

Safer solutions are described here: https://download.tek.com/document/3A..._MR_Letter.pdf

[g1]

If unavoidable and observing safety precautions a trained expert may float the DUT but never the scope.
Another condition was that the person must not be alone in the lab when doing so.

I don't know what situation you are referring to, but it must be quite different than ours. In audio and TV service, many shops connect all DUT via isolation transformer, working alone. Maybe not absolute best practice, but that is not always possible.
We're getting off topic and this would make a good discussion in the 'tools & test gear' section.

[telecaster]

Thanks for catching my error on the scope probe ground. I hooked the probe to chassis ground and External Speaker +, still oscillating as before (audible, can't sync scope to it). Any ideas or suggestions?

[g1]

Just for the record, the load also must not be connected to ground any other way.
Do you see the oscillation at the power IC pin1, or U5 pin7 ?

[The Dude]

Have you checked or changed C46? If R36 burnt, there's a good chance of enough excessive current through the cap that it is also defective. I don't see that we know the frequency of oscillation, or if you're hearing a harmonic or the fundamental. The zobel network should prevent any super HF from sitting on the amp's output.

[telecaster]

g1 - the load is not connected to ground in any way. There is oscillation at U4 pin1 (higher amplitude about 2V peak-to-peak) and at U5 pin 7 (about 6V peak-to-peak).

The Dude - I did change C46 when I changed R36. I'm still trying to get my scope to sync so I can measure the frequency.

[Helmholtz]

Can you post a scope pic even if not perfect?

[telecaster]

Scope picture attached. While trying to get a good picture, I noticed the amplitude of the oscillation increases with turning up the Master Volume.



Maybe a little better picture, 0.1 volts/cm and 1 microsecond sweep (limit of scope)