Fender Pro 185 nasty distortion in boost mode

[Randall]

Everything seems to work fine with this amp except when switched to "boost" mode it makes a ratty dirstorted sputtering sound. This also switches on the dirt channel 2. I am trying to trace signal, but I don't seem to find an exact schematic. With the one I found, I get good clean signal at the front end through channel 1 input U1 pin 7, and boost amp U2 pin 7 with boost switched on. I see the two JFETs Q2 and Q3, which I think switch either the signal from the clean gain or the boost gain pots depending on if the boost switch is on or off, so I would expect to see signal from one or the other at C11, but I don't. I don't even see signal from either gain pot on R19 or R21, which is odd, because the print has it directly connected. But if I do a continuity check from the output of the ch 1 gain pot to R19, I get 280K ohms.

So what is going on here?

185_Series_(Pro,Stage,_London).pdf

[Chuck H]

No way I'm going to trace that schem to isolate probabilities. But I would say you're halfway there by tracing the clean signal. If YOU trace the schem you will probably find gain stages exclusive to "boost" and "channel 2". Check voltage conditions on those circuits to determine operating parameters. That should lead you to any potentially failed components or signal/ground leads/traces. Probably a good idea to plug signal into any "power amp in" or effects loop in jacks with a signal at line level just to be sure the problem can be isolated to the preamp. That's all I have on this one.

[g1]

That file you posted has revisions J, and G, on different pages. I also have one slightly older revision F.
Does the board have a revision letter, or a date on it anywhere?

[g1]

And instead of looking at C11, look further down the line at the output of U3A. (D5 area of schem.)

[Randall]

I find no revision or dates anywhere on the board. Just copyright 1988. FMI

"And instead of looking at C11, look further down the line at the output of U3A."

U3 has a rounded off wave at pin 1, yet nothing on pin 2, so where is the signal coming from? It has smaller more distorted signal on pins 5 and 6, and larger version of same on pin 7.

I also took voltage readings on U3. 1=0.8v, 2=-6mV, 3=0v, 4=-17v, 5=66mV, 6=66mV, 7=66mV, 8=16.7v

This was with signal applied, and boost switch on.

[g1]

There have been other discussions here about virtual ground in op amps. It can be almost impossible to see the signal at the inputs (when the op amp is set up for inverting mode with negative feedback). You'll notice that U3a is using inverting input, where U3b is not.
That's the very basic scenario.
I'd suggest you set the amp up as in note #4 on the schematic and input the 10mV 1khz signal shown at ch.1 input jack. Then you should get the approx. voltages shown in the ovals at the TP's.

[Randall]

My generator only goes down to 22mV, and the test points all give me a bit over 2X, so that seems reasonable.

[Enzo]

My generator only goes down to 22mV,

Slap a 1 meg or 500k or whatever pot on the output of your genny, and it goes down to any level you want below 22mv.

[Randall]

OK, well I guess I was chasing my shadow trying to solve the boost circuit. I traced a clean boost signal on the outputs of all op amps right out to the preamp out jack. I really should have looked there first. So then I fed the preamp out into another test amp, and I could push it as hard as I liked with no nasty distortion noises. I then reversed and fed the test amp preamp into the Fender power amp input, and same thing, I could really drive it, but no problems. The test amp is a GK 400RB, so of course it doesn't push the preamp nearly as hard as a high gain preamp.

So this leaves me scratching my head, because when I went back to the Fender alone, in boost mode with Boost control full down there are no issues until I turn up the control a bit, then it just loudly farts out. So, I wonder what if the power section isn't liking the extra gain the boost signal is feeding it, and how would one test for this?

It does have a quad of NTE MJ15003 in it, FWIW.

[g1]

Do some tapping around on the board with a signal running (doesn't have to be loud).
If memory serves, the main filter caps broke loose a lot on these, and I think there was a big power resistor somewhere in the output circuit that also tended toward bad solder. Seems to me for that power resistor, you could get at it without removing the board as it was right near the edge.

[Randall]

Tapping and flexing was the first thing I did, it changed nothing. I went after all the cement resistors and caps. If it was anything like that, wouldn't it have done it feeding an external preamp into it?

[g1]

I don't know. This one is not very logical. If it's not the preamp, then it shouldn't matter if it's clean or boost channel.
If it is the power amp, then you should be able to make it do it with some other preamp.
Have you tried with a patch cord from send to return?
And I've also run into cracked pads at solder joints where tapping doesn't do it, it only responds to real big vibration or agressive wiggling of the actual component. Seems to me that may have been on this model as well.

[Randall]

I know, not very logical. I apologize for giving wrong signals, but now I have more evidence. I am now thinking the preamp is not at fault, but that the power amp is faulting somehow when input reaches a certain threshold. I once believed it was only the boost signal that ratted out, but now I find if I push the dirt channel hard enough without the boost switched on, it makes the same nasty ripping tone. it's harder to tell because it's a dirt channel, but now I see it's there. The test preamp I fed it with to see if I could determine if it was the pre or power amp maybe doesn't drive the power amp hard enough to cause the issue.

I am certainly confused here.

[g1]

You weren't giving wrong signals, it's just sometimes logic doesn't seem to prevail.
Can your sig. gen. put out a hot enough signal straight into the 'power amp in' jack?
Another thing, when you can get it to exhibit the fault, try substituting a load instead of a speaker. If it does it with the speaker and not a load, it could be the speaker itself, try another. Or it is due to the vibration and the fault is mechanical (bad connection) rather than a faulty component.
Just make sure with the load you connect your scope ground to the chassis on this model. Speaker neg. does not want to be grounded.

[Randall]

It does it quite readily into my shop speaker with a guitar input. I do not believe it is a mechanical issue.