Fender Pro 185 distortion

[Randall]

Got a Fender 185 in for a cleaning, loud hum and broken jacks. Cleaned it, replaced filter caps and jacks. Now I find a raspy, ripping distortion on louder low notes or big, full chords. Low volume sounds OK. I ran a 1K sine through it and find this distortion on the pre out jack:



And I also see this also on the pre out jack when the tone is fed into the power amp in jack:



Both of these waves are actually before the distortion is audible. I get the distortion thru the speaker if I turn the signal into the amp up a bit. I don't know what this tells me, should I look at the preamp or the power section, or maybe the power supply? I will say that power supply resistors R175 and R176 get hot at idle, over 200 degrees at the +/- 50v sides, and that it has NTE branded MJ15003 outputs in it.

185 Series (Pro,Stage, London).pdf

[Enzo]

It is oscillating. MAke sure your zobel network is intact.

I don't know what this tells me, should I look at the preamp or the power section, or maybe the power supply?

Why, all three of course. Plug something into the power amp in jack to break the preamp connection. is this still going on? Is the preamp out signal clean or like this? Does any of this ride on the power supply rails?

Isolating the problem is the very essence of troubleshooting.

[Randall]

Enzo, I have done this already, which is why I am confused. Plugging straight into the power amp in jack does it if the signal is high enough, and yet I still see it on the preamp out jack, so perhaps it is feeding back somehow? The first photo above is the pre out jack when plugged into the input jack. I hadn't thought of looking at the rails, but yes it is riding both +/- rails.

Is the Zobel network L1 and R174 going from the output rail to the speaker jack? If so, they look good.

[g1]

Are you grounding your scope to chassis or to speaker neg. side?
I recall funny issues with this model due to the .22 ohm between speaker minus and ground. Incidentally the same thing you were asking about with the Randall RG you are working on.
I could be completely off base, but you may be looking at a test equipment induced issue rather than what is causing the bad sound.
I also recall problems with these at high volumes due to bad solder, cracked pads or component leads.

[Randall]

This is with scope grounded to chassis. I just tried it grounded to speaker neg side and it was much worse, even causing the amp to howl.

[g1]

Ok. Also, just to be sure, your load must be floating and not grounded by any other test gear.

[Randall]

Right. What I am looking at presently is scope grounded to chassis, probe on pre out plug, and output connected to 4 ohm load.

Just found an interesting twist. With the above connections, I get the oscillations, but with a dummy plug in the amp in jack, the oscillation goes away at the pre out jack and on the rails. So there's a start. I also tried sending a guitar thru a different amp pre out into this amp's power in jack and the noise is still there. I've also noticed it doesn't seem loud enough for a supposedly 185 watt amp.

I also lifted R174 and it measures good, and L1 measures a short in circuit with one speaker connector lifted.

[Enzo]

It does it on the power amp alone, fine, but it is also on the preamp, OK. And you found it on the power supplies. Well the power supplies connect to both PA and preamp, so... that is how it can be both places.

R175,176 are dropping to 5w zeners, and they do get hot. I never took their temperature, but they are 7w size for a reason. Are the two 16v rails intact?

Look lower left from L1, measure R171 or 172, do you get about 24 ohms? (two 47 ohm in parallel) That is the zobel I refer to.

[Randall]

" Are the two 16v rails intact? Look lower left from L1, measure R171 or 172, do you get about 24 ohms? (two 47 ohm in parallel) That is the zobel I refer to."

Yes, the rails are intact, and I do measure 47 ohms on the zobel resistors. And please see the post above your last for new info.

[Enzo]

Unless you lifted them, if you measured 47 ohms on the zobel resistors, one of them is open, it should measure half that.

[Randall]

Sorry, that was a slip, it is actually 23.7 ohms.

[Enzo]

Cool, it is just the first thing to check when an amp is unstable.

[g1]

Just found an interesting twist. With the above connections, I get the oscillations, but with a dummy plug in the amp in jack, the oscillation goes away at the pre out jack and on the rails. So there's a start. I also tried sending a guitar thru a different amp pre out into this amp's power in jack and the noise is still there.

When you sent the pre out to another amp, did you have the dummy plug in the pwr.amp in jack?

[Randall]

g1. I think you got it backwards. I went out from the pre of another amp into the PA of this amp, still have the raspy distortion and low volume.

I just did it the way you asked, pre out to another amp, and it sounds good with or without the dummy plug in the problem amp pwr in jack.

[g1]

Ok, yes I did get it backwards.
But it is odd that you see the oscillation on the scope at pre-out but can't hear it from there going into another power amp.
This further implies to me there may be a test gear glitch that is confusing the issue. I'd try scoping the pre-out at the same time you run it into that other amp. If it introduces a different noise, or you still don't hear it, then you at least will know you are chasing a ghost.
Also recommend tapping the board quite hard to check for any physical issues.