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**Mark Black** · 05-08-2009, 03:13 AM

R23 & R24 getting hot makes me think shorted IC, since those resistors are in the +/-15V supply circuits.

Any IC's getting hot (or warmer than others) as well?

**Enzo** · 05-08-2009, 03:26 AM

Instead of a sea of voltage numbers, isolate the problem. There are a lot of things I'd be checking before taking voltage readings.

Turn teh reverb up half way and rick the amp to crash the springs. Does that come out loud and strong, or is it weak?

Plug into the FX return and turn the return level all the way up. Does that come out strong or weak?

There are three 12AX7s in the preamp, plus the one in the power amp phase inverter. Take a good 12AX7, and substitute it for the others one at a time going down the row. If one of those tubes were bad, this will expose it.

Those two resistors do run hot naturally - they are the dropping resistors for the +/-15vDC rails. SO check at a couple IC - is there +/-15v on pins 8 and 4?

Apply a steady signal to the input, set the controls up for pretty loud normally, regardless of how weak it is now. Then trace the signal through the stages. I usually go to the plates of the 12AX7s. You may have to switch channels back and forth to find signal in all places, but do what you have to. I use a scope for this, but if you lack a scope, set your meter to AC volts and follow the signal with it.

**ahamay79** · 05-08-2009, 04:14 AM

Thanks for the input.

I posted the numbers to keep from being asked for them. If any numbers are blank I got no reading on that pin.

All the tubes are new and good as stated in the original post.

R23 and R24 are melting solder joints. They test well within spec.

I'll try your suggestions.

thanks

**Enzo** · 05-08-2009, 06:30 AM

Fair enough. May I suggest that a zero volts reading is not the same thing as no reading. In this case zero volts on the grid pins is a healthy sign.

If those resistors are running too hot, you could replace them. I would mount them up off the board a ways. That allows better air circuilation plus the extra lead wire length acts as a heat sink. Seriously. ANd on the solder side, instead of through the hole and clipped off, clean the trace off along the tracec going away from the hole, then bend the lead wire flat along the trace and solder the length of it to the trace. That extra solder and lead length soldered down also acts as additional heat sinking and also spreads the solder so that even if parts of it soften, it all won;t. Then a blob of silicone to glue them together or to somthing else to keep them from swaying.

And heed Marks suggestion as well. Are the 15v rails up to speed or are they dragged down?

**ahamay79** · 05-08-2009, 01:55 PM

R23 and R24 are already about an inch above the board and still melting solder.

I'll check the 15v rails today. Can you tell me how to test the IC's and voltages I should be seeing?

Thanks,

Dave

**Enzo** · 05-09-2009, 12:32 AM

Well, if they get that hot, and if we find no excess current draw through them, you could always mount heavier resistors. They are already 2W, is there room for a couple 5W? Or mount a couple of thos small aluminum ones on the chassis wall and run wires to the board.

SO check at a couple IC - is there +/-15v on pins 8 and 4?

Start there. If the chips don;t get good power supply, they won;t work right.

After the amp has been on a bit, touch each IC with your finger. If any are hot, they are probably bad.

Pins 1 and 7 are the output pins for the dual op amps. They should sit at about zero volts DC.

**ahamay79** · 05-09-2009, 03:46 AM

I replaced R23 and R24 and now the amp doesn't make a sound... at all....dead. I put it aside today because I had other amps that needed to get out.

Plus, I know nothing about how to check op amps and IC's.

What would be "excess current draw" through R23 and R24?

thanks,

Dave

**Enzo** · 05-09-2009, 07:18 AM

It used to work, and now that you have changed these two parts, it no longer works. One forever rule of thumb is: if you do some work on something, and the function of the thing changes, then it is MOST likely due to the work you did.

Those two resistors are dropping resistors to the zener diodes that set the +/-15v for the op amps. When you replaced them, you may have cracked or broken a copper trace on the board, or a solder joint didn;t take. And another possibility is a drop of solder fell and shorted across something and either blew a fuse or grounded a signal path. Check F3, it is the fuse they run off. Pull it from the clip and test it with a meter. Are the new ones staying stone cold?

But meanwhile I get the impression you spotted these hot resistors and have focused lazer-like on them as your problem for the low sound. I suspect it is not.

But let us get to them first anyway. Look at your schematic. Those two resistors come from a basic half wave supply. D1 and D2 rectify AC to DC, which then flows through R24,23 to zeners ZD1,ZD2. They are 5W zeners so they tend to run hotter. There should be 15v across each zener. That means there should be 15v on one end of each of those resistors. I don;t know what should be on the other end of each resistor - 18,20,25? It won;t likely be wrong, unless it is really low.

So find out what you did. The fuse F3 is open, or the voltages are not getting somewhere in that circuit. Ince you get +/-15v, then you can "check your ICs."

How? I already told you how.

Quote:
SO check at a couple ICs - is there +/-15v on pins 8 and 4?

Start there. If the chips don;t get good power supply, they won;t work right.

After the amp has been on a bit, touch each IC with your finger. If any are hot, they are probably bad.

Pins 1 and 7 are the output pins for the dual op amps. They should sit at about zero volts DC.

There is no secret, are they getting proper power supply voltages? Are they not getting hot? Are the outputs centered more or less on zero volts? That is about it. That is about all I ever need to do. Yes they can fail in other ways. If signal comes up to one and doesn;t exist on the output end of one, then it might be bad.

If you don;t find a bad one with those simple methods, chances are they are OK. You wil find 15v dropping resistors overheating on all kinds of amps. Fender amps are notorious for it. I don;t usually consider it a symptom of anything else, though it could be.

The op amps in this amplifier run the FX loop, the reverb drive, the preamp out or line out. None of those things are in the basic signal path of the amp. You could remove every op amp in there, and the amp should still play. No reverb or FX loop, but it would still play. You DO need the +24v for all the channel switching FETs and stuff to work. It runs off that same F3 fuse and D1. SO you do need those voltages.

So then go back to the signal tracing I suggested. APply a signal to the input and follow it through the amp. Find out where it disappears.

**Enzo** · 05-09-2009, 07:25 AM

Excess surrent through the resistors? That means enough to overheat them.

I am going to guess there is about 25v at D1,D2. And ther is or should be 15v at the zeners. That means about 10v sropping across each resistor. Power is volts squared over resistance, so (10v x 10v)/270. That is 100/270, or about .37 watt. That shouldn't be too much for a 2w resistor.

Are either of the zeners hanging lower than 15v? I mean by a few volts, not worried about a couple tenths. A failing zener could be acting like a partial short on the power rail. There are also filter caps on the two 15v rails, one of them could be leaky.

**ahamay79** · 05-19-2009, 03:19 AM

Thanks Enzo. I've been so busy getting other repairs and cap jobs out that I just set this Marshall aside. I'll look at it tomorrow and give you an update.

Thanks again,

Dave

**ahamay79** · 05-20-2009, 11:34 PM

Enzo,

This amp has some of strangest places to ground that I've seen. I'm getting the weak output again.

I'm geting 25 vdc across R23 and R24.

The large zeniers have hardly any voltage across them.

The IC's are cool to touch.

All fuses are good.

Can you call me please about this amp. My cellphone is always on: 225-718-6350.

thanks,

Dave

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