Fender Performer 1000 distortion channel

[52 Bill]

Looking at the schematics and the fact all the voltages at TP31 seem within range (including using all 3 fascia switches) im inclined towards the issue being U8.

Agree, the outputs of U8 are off.

I'd also suggest that you meter test all of the diodes in that part of the circuit, including the Zeners. Just make sure that there are no open or shorted ones in there.

[showmaster78]

Followed the schematic about 8 times now and im getting muddled up lol.
Any ideas which CRs im looking at in particular?
I know it'll probably be a page full...
Like i say, im a noob but I'm determined to crack this...

[52 Bill]

Any ideas which CRs im looking at in particular?
I know it'll probably be a page full...

Yes, CR 39, 43, 44, 46, 47, 48, 49, 50, 51 and 59.

[showmaster78]

Awesome, I'll get amongst that lot tomorrow

[showmaster78]

Right, measured all the diodes (the fact that CR49 is actually a transistor confused me for a bit lol).
I measured in both directions (red to anode, then to cathode) as to my understanding i should get a voltage rating forward potential and then a 1 (ive listed it as "i") to the left of the readout in reverse potential.
Ive listed the result as CR#, forward result, reverse result using the diode test on my DMM. I presume this is all in volts.

CR39, 518, 539
CR43, 742, i
CR44, 769, i
CR46, 625, i
CR47, 622, i
CR48, 765, i
CR49, 677, 1348
CR50, 628, i
CR51, 624, i
CR59, 629, i

I also checked CRs 56 and 57, purely because the board looked like it had got rreeeaaallly bloody hot at some point.
CR57 is fine, but CR56 is reading 013 in both directions!!
What does this one do and should it be that low? I know because I've tested it in circuit it'll be lower than if I'd removed it but still...

[Enzo]

CR56,57 are 5 watt zeners used as power supply regulators. They do generate considerable heat. They work on opposite polarities, but ought to test the same in circuit. If one tests as shorted, then remove it and test it out of circuit. If it is still shorted, then replace it. If it checks OK alone, then find out what is shorting the +16v rail to ground.

CR39 and 49 are in circuits whose parallel current paths cause the reverse readings.

CR49 is not a transistor, it is just in a three legged TO92 case. AS far as we are concerned it is a zener diode.


Have you taken the step of checking power supplies? If the op amps do not have +15v and -15v to run on, nothing will work right.

[showmaster78]

I'll have to take CR56 out and check it.
The step-downs at the top of the amp (the big ceramic bar things) are all reading ok.
Otherwise, from what i can gather without taking them out and re-testing, it looks to me like CR39 and 49 are shot.
This would make sense because (from what I'm gathering) CR39 is actually a high speed switch, and its part of U8s circuit.
I'll have to take them out and check.

I was checking a couple of noisy components as well, tried to check the voltage from R64 and it screamed!!!

C48 is causing some noise as well but i dunno if caps can be tested in-circuit?

[showmaster78]

C46, having traced it, is part of the reverb circuit. Seeing as the reverb pan in shot im not concerned about that. The rev. pan is shot anyway and ive no intention of playing about with that unless i have to.
R64 is on the same path as the drive treble control, the reverb and the fx loop, so again, unless told otherwise, not concerned about that.
Just thought id add this so you know im ignoring it until i get the switching issue sorted. Just wanted to check them in case they were linked.

So "if" the diodes have gone (pending tests) will that likely damage the ICs or are they likely to be ok?

[52 Bill]

Before you do anything else, test the low voltage power supplies! As Enzo pointed out, if one of the two supplies is off, then nothing will work or test correctly.

Read the dc voltage at CR56 and CR57. Do this by connecting the black lead to ground and then use the red lead to read the voltage at the test points TP24 and TP27. There should be approx. +&- 16 volts dc at the test points.

Don't get distracted by other things like noisy caps. Fix the big problem first and then look at the little ones.

[showmaster78]

Right, I've preempted a little bit here and if any if the information is irrelevant it was a "just in case". I followed the circuit from TPs 23 and 26 back to the diodes. Im gonna split it into + rail and - rail.

+Rail:
TP23/R172 is getting 42.6vdc in, 17.2vdc out.
That leads to R160, 17.2vdc in, 6.8vdc out
On to TP24, the CR56 is only receiving 6.8vdc

-Rail:
TP26/R173 is reading -43vdc in, -24.9vdc out
R161 reads -24.9vdc in, -17vdc out
Thats feeding TP27/CR57 -17vdc.

I dunno what to make of that. On the one hand R160 is losing WAY too much voltage making me think the problem is that, but on the other hand R173 should surely be dropping the voltage down to -16vdc? By the time it gets to CR57 its near enough right (would a -1vdc difference be harmful?) But is there still something wrong?

[Jazz P Bass]

Something is not quite right with the +16Vdc rail.

Either the zener diode (CR56) is bad/ in backwards or the supply itself is being loaded down beyond C73.

[52 Bill]

+Rail:
TP23/R172 is getting 42.6vdc in, 17.2vdc out.
That leads to R160, 17.2vdc in, 6.8vdc out
On to TP24, the CR56 is only receiving 6.8vdc

I dunno what to make of that. On the one hand R160 is losing WAY too much voltage making me think the problem is that, but on the other hand R173 should surely be dropping the voltage down to -16vdc? By the time it gets to CR57 its near enough right (would a -1vdc difference be harmful?) But is there still something wrong?

You are looking at this the wrong way. The resistors reduce the voltage based upon how much current is being drawn through them. So my guess would be that all of the resistors are okay, but something is drawing too much current on the positive bus, pulling down the voltage to 6.8vdc.

This is where testing CR56 comes in. If the Zener diode CR56 is shorted or leaky, it could be causing the excessive current draw and therefore the reduced voltage. But it could also be bad C73 cap that is doing this or for that matter something else in the circuit.

When you meter tested CR56, you got a reading of 013 in both directions, so the next step will be to test the diode removed from the circuit. To do this, all you need to do is to remove one side of the diode from the circuit.

To do this correctly you will need to remove the entire PC board and unsolder one lead from the board and then retest the diode with your meter. If it still reads 013 then you have found the problem and you will need to replace the diode. If the diode now reads normally with a high and a low reading, then the diode is okay and the problem is in the circuit.

The quick and dirty method would be to leave the PC board in place and to clip one of CR56's leads at one end. Retest the diode and if it is okay solder the two ends back together. I don't like working this way, but at your level of experience, this may be the safer approach for now.

A different test would be to set your meter to ohms and read the resistance across CR56. Then read the resistance to ground at different points in the circuit that connect to the +16vdc bus, like pin 8 of any of the ICs. If the lowest reading you get is across the diode CR56, then that would suggest that the diode of C73 was at fault.

A another test would be to see if any of the ICs are getting hot. Or if there is any dc voltage on an output pin of one of the op amps.

I'm suggesting all of these thing to try and keep you from having to remove the pc board from the chassis. While not really all that hard to do, it can cause multiple problems for you if it is not re-assembled correctly.

And by the way, all of the readings and testing of the switching circuit was probably not needed, as the loss of the +16vdc supply would probably cause the problems that you are having.

[showmaster78]

I didnt need to test it all? Hahahaha, now you tell me...

I don't like the "dirty" approach. While it would undoubtably be easier, it sounds like its asking for a disaster.

I've already had the board out checking for dry joints and put it back together again ok. I'm smart enough to label everything i need to disconnect before i do lol.

I've got rehearsal tomorrow night so I'll be checking this on thursday. I'll do it the proper way to be on the safe side.

Dc voltage on an IC output pin? Did you type that correctly? They all have a DC voltage as far as i know?

Other than that i will test C73 if CR56 is ok.

Would this (if its goosed) cause the switching issue with CRs 39 and 49? From what i can gather CR39 explains the footswitch issue and CR49 the loss of my Gain1. But does that all (potentially) hinge on CR56?

[showmaster78]

Well its not in backwards, I checked that. Im guessing its the zener thats goosed judging by the reading but I'll do a proper off-board test and try to eliminate C73. The less i have to test now the better. I've filled 2 pages with test notes already lol. Its more because I'm trying to get to grips with it all but I've definitely learned tons about how the amp works (i managed to work out CR39 being off explains the loss of my footswitch and CR49 the loss of Gain1).

[g1]

And by the way, all of the readings and testing of the switching circuit was probably not needed, as the loss of the +16vdc supply would probably cause the problems that you are having.

I didnt need to test it all? Hahahaha, now you tell me...

I'm surprised the amp was otherwise working fine (or was it?) aside from the switching issues. I think this is probably why we jumped to the switching circuit without checking supplies first.

As far as whether it can all hinge on CR56, yes it could.
And normally with a split supply like this, there should be no DC on opamp output pins, that's why finding DC there can help identify defective IC's.

Edit: As noted by 52Bill below, it's only audio opamps that shouldn't have DC on the outputs (with split supply), switching IC's may have DC on outputs.