Peavey Heritage VTX white noise on phaser (1982)

[rotorcraft230]

Hello, I have a Peavey Heritage VTX that had several problems, HT power rectifier shorted, power transistor shorted and the phaser not working. I have worked through all those problems and now have the amp running, nice and clean and sounds good. The problem is in the phaser, it has a lot of white noise when I turn the depth pot up, no noise below 2 or so but above that it's there. I have a Tektronix Oscilloscope and only have very limited experience in using it, but can see the sign wave. I can see the signal and its a nice sign wave without the phaser and can see the sign wave get very fuzzy with the phaser depth knob turned up.

here is what I have done so far on the phaser

I installed new TL074 in U5,U6,U8 and a new RC4558p in U7, that got he phaser working.
checked all the e-caps for value and ESR in that circuit and was all good.

Traced out the circuit with the oscilloscope and can see the noise, when I got to the junction of R116, R117 and U6D pin 13 with the probe of the oscilloscope the noise totally goes away, I can see and hear it go away.
Why is that, I cant seem to figure that out.

https://schematicheaven.net/newamps/...ritage_vtx.pdf

I typically work on all tube amps and not very good at SS but really enjoy troubleshooting and like to fix broken stuff. I hate broken amps and would really like to get this one up and running completely.

thanks for your help.

Mark

[glebert]

I would guess (but have no way of knowing for sure) that there is some difference between the 4741 op amps and the TL074 and when you stack five of them in a row the differences are magnified.

[Helmholtz]

Could be HF oscillation.
Try to wire a 50p to 100p cap from U6D pin 13 to nearest ground.
Should have the same effect as connecting the scope probe.

[rotorcraft230]

I would guess (but have no way of knowing for sure) that there is some difference between the 4741 op amps and the TL074 and when you stack five of them in a row the differences are magnified.

I have read this is a common replacement and haven't herd of any issues when using the TL074 in place if the 4741

[rotorcraft230]

Could be HF oscillation.
Try to wire a 50p to 100p cap from U6D pin 13 to nearest ground.
Should have the same effect as connecting the scope probe.

I can try that, I'm at work so it will have to be later today. is that a 50pf-100pf like a silver mica or ceramic cap? would that be a fix or just a troubleshooting step to further the diagnosis.

thanks

[Helmholtz]

I can try that, I'm at work so it will have to be later today. is that a 50pf-100pf like a silver mica or ceramic cap? would that be a fix or just a troubleshooting step to further the diagnosis.

Both cap types should work.
Might be a fix.
I suspect the replacement opamps are faster than the originals, causing HF oscillation without additional frequency compensation.

Is C69 good?

Of course it would be best if you could scope the oscillation.
As it could be in the MHz range, you might need a µs/div or even ns/div time base setting.

[rotorcraft230]

Both cap types should work.
Might be a fix.
I suspect the replacement opamps are faster than the originals, causing HF oscillation without additional frequency compensation.

Is C69 good?

No, I'll check it tonight when I get home and report back.

thanks for the direction, I really appreciate it.

Mark

[x-pro]

I would guess (but have no way of knowing for sure) that there is some difference between the 4741 op amps and the TL074 and when you stack five of them in a row the differences are magnified.

They're a little different:

[rotorcraft230]

They're a little different:

I did look at the data sheets but with my knowledge of ICs I couldn't get to any conclusion and went with what I saw most people had put in. I did all of the 4741s that are in it to the TL074. Not really sure which one was bad so I did all 3

[tedmich]

no name silver micas have a bad rep, CDE are better but I always go with film for opamp bypass, good luck!

[rotorcraft230]

no name silver micas have a bad rep, CDE are better but I always go with film for opamp bypass, good luck!

thanks, I have some mica to test out but will probably go with film as you suggest.

[g1]

Sometimes when using a more modern IC, the poorer spec of the older chip will have prevented oscillation in the circuit. The higher bandwidth of the modern part can actually create problems.
Or there may be compensation pins and circuits with the old chips that the new chips don't have.

[rotorcraft230]

Sometimes when using a more modern IC, the poorer spec of the older chip will have prevented oscillation in the circuit. The higher bandwidth of the modern part can actually create problems.
Or there may be compensation pins and circuits with the old chips that the new chips don't have.

I can see what your saying. I'm going to try a 50pf-100pf cap across U6D pin 13 to ground and see if it stops as Helmholtz suggested. that is the location the noise stopped when I was looking at the signal with the oscilloscope.

everything past that had the noise

[rotorcraft230]

Could be HF oscillation.
Try to wire a 50p to 100p cap from U6D pin 13 to nearest ground.
Should have the same effect as connecting the scope probe.

Last night I tried the 50pf and it helped a lot, I could now rotate the depth knob to 7 vs 2.5 where it was before. I then tried a 100pf and that was the ticket, all the noise was gone. it was so quiet I had to put on the signal generator to make sure the Phaser was even working, yep it was.

Thank you Helmholtz for your replies and input, I really appreciate it.

thanks to the rest of you that had input as well

Cheers Mark

[Helmholtz]

Glad I could help.
Actually your result when connecting the scope probe suggested the solution.
Nevertheless I would check C69 and see it it's well connected.