So, I did some more testing and measuring on the power supply board since those voltages going to the preamp board were off.  To be honest guys, I really don't know what's going on with it because I don't know what's normal.  Besides the lines going to the preamp board there's no other voltage readings I am seeing in the service manual.  So here's what I'm observing at the moment:

There's two different windings coming off of the transformer- the red lines are reading around 54v, the brown lines are swinging between +20mv and -40mv.  Is that what it should be doing?  I was expecting a stable voltage.  If the transformer is blown then I'm giving up on this guy.  If that behavior sounds okay though, then I'll press on. 

It should be known that I was getting the proper voltages on those lines running to the preamp board before the big catastrophe that I discussed in my post from August 7th.  So, that event must have done something to something- I just don't know what.  The 4 amp fuse on the power board looks a little worn, but it's still passing those 54v fine. 

-I started writing this post, got curious and did some more testing and....

There's these two big weird metal things on the power board, I looked up their part numbers (S5151 and S5151R) and apparently they are diodes that form a rectifier bridge.  It looks like they are doubling the voltage because one of them is reading 110v when I touch the casing, the other one is completely dead.  So.... I look at what's feeding the dead one and there's 4 diodes, which I'm thinking is another rectifier, with two big ol' 35w caps right after it.  The little diode network is still carrying voltage and all together it looks like it's producing 24vdc.  There's a little jumper on the top side which enables me to read the voltage coming off of the first big blue cap without having to remove the entire board to take measurements on the solder side.  There is absolutely no DC on that jumper... so could that big blue cap be the problem? 

Here's a pic of the schematic, my pen is pointing to the big blue cap in question... and there's a photo of the board after it for reference.  *Sorry again for the huge pics.





Thanks y'all!

Just an update- the preamp output to the power amp is pin 1 on the 4th 4558.  Sure enough, when I apply a signal there it amplifies pretty well for about 10 seconds and then it slowly fades down.  A signal is still passing, but it's just my signal driving the speaker without the amp  My voltages to the preamp board are also off now, the -15 is fine, the -20 tap isn't working, the +20 isn't working, and the +15 is reading -8.9.
Gotta take a break for now, nap-time just ended. 

Tinkered around on it a little more last night.  New things and observations:

1. I probed all over the preamp board, alligator clipping from the input to different parts of the board so as to skip over certain sections so I could narrow down where the fault may be... I don't know what this is actually called so I'm just going to call it "bridging" from here on out.  I'm able to pass a signal, albeit a very quiet one when I bridge into pin 7 of the opamps, which is the output of the opamp.  I'm getting an even quieter signal from solder points in between the opamps, but the loudest signal comes from the opamps.  This is a big deal because before I was not getting any signal to pass through the speaker at all before changing the output transistors.     
    * When I was doing all this probing I thought I had narrowed down the problem to a 0.047uf cap and a 220pf cap right after the input to the preamp, so I replaced them.  No change.   :duh 
    *I don't know the dB level of the signal I'm passing through the input is, but it is REALLY loud.  I'm playing music from my computer, which is being amplified through a usb mixer, and the headphone out is connected to the input.  Through the headphones the signal is intolerably loud, just FYI.   


2.  Now, this is really weird... when I try to bridge from the input to certain places in the signal path BEFORE the 1st opamp I will get a very quiet signal after holding the probe on there for a few seconds... BUT THEN the signal will continue to be present for a few more seconds after I lift the probe off of the solder point.  Weird, right?  If there is no signal being applied then how in the heck is it still present?  Unless there's another short somewhere that I haven't found yet?

3.  The speaker will jump a little bit when I adjust the level on the para EQ section.  I kinda eluded to this in an earlier post.   

4.  Would any adjustments made on the "center voltage" and "output waveform" possibly affect this problem? I haven't touched those trimmers on the output board, but I wonder if the dude who had it before me played with them possibly knocking it out of wack.  I don't have a scope, is it possible to perform the right calibrations with a DMM or another method?

Again, I appreciate all of your suggestions.  Don't feel like you have to respond, it's good to just have a running log of what I'm doing on this board.  But... suggestions are always helpful. 

I only messed around with it for a little bit last night, and it still won't pass a signal.  I hooked it up to a different speaker with the same result.  I guess I'll go back in there and do some more probing around on the preamp board, I think that's where the problem is.  If I can hear the channel switching pop and the EQ level knob turn then the latter part of the preamp must be getting through.  Is there anything else that could be causing the signal to die at the first opamp besides a bad opamp?

Thanks DrGonz!  I took the limiter out and nothing bad happened!  I tested the DC offset and I can't get a stable reading, they just jump all over the place.  I also did the idling current adjustment- at first the drop off was 17mV, but I got it 5mV.  I'll do a little more testing this evening, but I wanted to let you know about those things first.  Thanks!

Alright folks- 1 month later and here's where I'm at...

-Sorry again for the gigantic pictures-

Light Bulb Limiter - Check.


New output transistors, new insulators - Check.


New resistor and replaced nearby charred caps (far right of pic) - Check.


And I replaced that first jrc4558 I mentioned earlier. 

I fired it all up with the light bulb limiter on and nothing bad happened!!!  I was ecstatic.  But I'm still kinda scared to fire it up full power, and I'd like input on what to check first before I try it on full power again. 

A few observations:
-The light bulb immediately fades out when I turn it on, and from what I've read that means its doing what it is supposed to do.
-I checked the 15v and 20v lines on the preamp board and they are measuring where they should.
-I don't think it was doing this before, but when I turn the "Level" knob on the EQ I actually here a change in the noise coming through.
-When I press the channel switch button there is a small pop that I can hear.
-When I audio probe around the opamp I replaced I'm not hearing anything different.  Everything that was/wasn't happening before the swap is still happening.

So yeah- I'm optimistic and feeling pretty all right about this.  If anyone has any "next steps" I'd love to hear them!  Thanks again everyone!

Thanks DrGonz-  I bought the parts I didn't already have lying around for the limiter today and will be putting it together before I try to turn it back on.  I have to order new transistors, mica insulators, and thermal compound too- all probably coming from the slow boat so I'll have a few days, haha.

As an emitter resistor in that position- it doesn't matter what voltage rating it is as long as its not lower than what is specified, correct?  And it being metal film over carbon film shouldn't matter either, right?

And yes- the speaker will be disconnected from now on.  :dbtu:

Alright... now I've really gone and screwed up.  So, in a moment of carelessness while I was probing the board, I decided to touch my probe (which was coming from the input) to a point on the DC board which fried a resistor on the MA board.  To give you a visual: if you look at my first post and see the board captioned "Power Board with Trannies", you'll see two green chiclet caps and a small 1/4w carbon film resistor on the bottom of the picture.  That carbon film resistor is completely fried.  It's a 150 ohm resistor, and I have several 150 ohm 1/2w metal film so I replaced it.  Turned on the amp and it fried the new resistor immediately.... and this time I wasn't probing and I made sure nothing was touching something it shouldn't have been.  That resistor is coming right off the pin 1 connector between the DC and MA board.   

Now, the spot I probed that initiated that fried resistor was a joint on the DC board where a white wire leads to the big cap right before the output jack (the DC board is the picture captioned "Power Board up top" on my first post).  It doesn't look like anything else is damaged.

Doing a little bit of searching around on this site and others, I think the fried resistor is the emitter resistor.  I've read that they fail sometimes because of output transistor failure.  Soooo... I should be testing my transistors to see if they are good, correct?  And if I do find a faulty resistor, that should be the problem, right?  I'm going to run in that direction for now.  If I'm wrong please stop me from blowing something else up.

Also, I replaced that first opamp- but I haven't been able to test it now that its acting crazy.  Interesting note-  when the amp started smoking a loud hum was coming from the speaker... more confirmation the speaker works, yippee! Thanks for any help anyone can provide. 

EDIT- Tested the transistors and sure enough, one of them isn't reading anything between the base and emitter.  So it looks like I'm off to get a new pair of power transistors... ugh.

Gotcha.  I see what you're saying.  I'll do some more testing and let you all know what I come up with.  Thanks again!

Thanks for the wisdom Enzo.  I definitely am a rookie when it comes to this stuff. 

A couple of questions:
-I've been attaching my probe (which is just an alligator clip jumper with an old soldering iron tip on one side) to the direct out, thus bypassing the vast majority of the circuit, just trying to see how far the signal is getting into the circuit before it dies.  Is there anything wrong with this method?  What do you mean by "injection"?  I'm thinking you mean doing something like alligator clipping the output of the opamp and seeing if signal passes through as I try to bypass parts of the section before it, just to see if the rest of the circuit works. 
-I have another 4558 on hand, so that won't be difficult to replace- I don't know the technical name for certain regions of the circuit, but could the network of diodes and resistors before the opamp affect the output of it?

I'm getting very faint signal from pin 5, a blast of noise from pin 6, and the same blast of noise on pin 7.  The "blast" silences very quickly, and if I move my probe around on the joint it might blast again.  The blast is much louder than the faint signal I get on pin 5.  So, to say I'm getting silence from pin 7 would be incorrect, but its not exactly a signal.   

Given the evidence of where the signal is going (or not going rather) and that there has been work done on the opamp (I'm thinking it easily could have been fried if too much heat was applied when that jumper was installed.  And with the legs looking all chewed up... its anybody's guess what the opamp has been through).  Thanks again for your help, its much appreciated! 

Alrighty-  So I finally got a little time with the amp again tonight.  Like I said in my previous post, the signal is dying after I get past the initial lines to the PA board.  I get a very very faint signal on pin 5 of the first opamp (jrc4558dv).

Here is the component side of the PA board.  The first op amp is down there on the lower left. (Sorry for the huge picture size)


To Enzo's questions:
1. The speaker works fine and I've been doing most of the testing using the direct out jack. 
2. The big cap is also connected. 
3. I checked the voltage of the lines to the PA board and they all measure as they should.  I checked the voltage to the first op-amp and it is getting power, although its only measuring 10.2v on my crappy DMM.  Pins 6 and 7 look kinda chewed up from the component side view, and then there is the issue of the messed up trace that was re-done (see picture in first post, it's pin 7 of the 1st op amp to a 0.0056 cap), but otherwise I can't SEE any other problems.  If I'm reading the schematic correctly I should be getting audio signal from pin 7, but instead all I get is a burst of noise when the probe first touches the pin.
4. I did what you said with the reverb footswitch and I got no signal. 

All the lamps and LEDs work, and they switch on and off properly according to which channel is engaged.  All the electrolytics look intact and I don't see any other traces that need attention. 

Before I start thinking I need to replace that first opamp, is there something else I should be looking at?  Thanks again everyone for your suggestions! 

Thanks for the schematics, and thanks for the feedback Enzo!  I'll work on it this week and see if I can't figure it out and report back.  Thanks a lot for all your help!

Hello all!  New to the forum.  So before I lay out the problem, know that I have some experience with electronics.  I've been building effects pedals for a few years now and I can debug a pedal alright.  If I had a schematic this may be a little easier, but I don't and I can't find one free online.  If anyone has one to share I would be very grateful! 

Okay-  so I got this amp in a trade.  I've heard awesome things about them for years.  It powers on, everything lights up, but there is no sound.  No signal, nothing.  I probed it and it sounds like the input jacks are working, but the signal dies once it gets past the input jack solder joints on the PCB.



The guy I got it from says he doesn't know anything about it, and that he never did anything to it.  In the chassis I found two old power transistors, so it looks like they were changed at some point.  There is a spot on the preamp pcb where it looks like someone tried to fix a busted trace and wired in a jumper to reconnect it. See picture.



I took out the power board the transistors are mounted to, and cleaned up the contacts leading to the second power board (at least thats what I think it is).  They seem to be making good contact.  I've checked the fuses and they're all good. 

Power Board with Trannies:


Power Board up top:


Besides the jumpered trace the only thing I can find that is suspicious is a blue solid core wire that looks like its supposed to be a ground-wire since it is bolted to the chassis (see 4th picture).  It's just hanging out and I'm not absolutely sure where it goes, but I do have a guess:  On the 2nd power pcb there is a blue stranded wire running from the preamp board.  On the underside of the pcb the solder joint where that stranded wire meets the power pcb has a little brown dot in the middle- almost looks to me like something had broken off at some point (it's right on the top edge of the PCB marked with a "-" above it).  I want to alligator clip that solid wire to that joint to see if its just a ground point- but since I don't have a schematic I'm not all that confident.   



Since the signal is dying at the preamp pcb, my guess is that the preamp pcb isn't getting power somehow, or the signal is grounding out somewhere.  My primary request is just for a schematic, but if you have any bright ideas I would love to hear those too.  Thanks everyone!