Fender SRM 6302 PA bad hum

[nosaj]

Fender srm 6302 PA loud hum. Initial guess bad filter cap(s). I would like to see if someone can help validate my findings or if I'm going in wrong direction.
Meter readings on PA1 output 2.76 vdc 3.87vac
PA2 output 7.98 vdc 7.60 vac


Test point data

Phantom on
All level Controls max
EQ an Tone set mid
10mv 1khz sine
no load on output

Should be is
tp1 .998v 1.5v
tp2 .961v 7.36
tp3 25.09 8.2vdc 7vac
tp4 25.08 7vdc 7.3vac
tp5 .998v 7.4vdc 7.2vac
tp6 25.05 7vdc
tp7 .96 7.39vdc .9vac
tp8 24.58 7.1vdc 7.2vac
tp9 24.58 7vdc 7.55vac
tp10 24.57 7.vac
tp11 48.26 47.2
tp12 15.58 15.4 vdc 0vac
tp13 -15.60 -15.50vdc 0vac
tp14 50v 46.1vdc .2vac
tp15 50v 31.2 vdc 14.9vac

Transormer with just 120vac
blue blue 39.4 vac
blue brown 19.9vac

red red 73vac
orange red 36.2

Scoping on d2 (TP 14) get a regular sawtooth(which I believe is correct for power filtering
scoping on r247(tp15) waveforms look a mess
Scoping D1 D3 also shows bad waveforms So I'd say c3 was bad cap, but replace them in pairs/

If anyone can corroborate or repoint me in a better direction.

Thanks,

Nosaj

[g1]

Can you upload the schematic/service manual or provide a link to it?

[Jazz P Bass]

A lot of work there.
Just put another cap in parallel.
If it helps in lowering the ripple, then that cap was bad.

[nosaj]

SRM 6302-8302 Schematic -11x17.pdfSRM 6302-8302 Schematic -11x17.pdfYes but I would like to understand if I am arriving at the right conclusion based on what the tools are telling me. Anyone can replace a cap and see if it helps. I just wanted to see if someone could tell me if measurements supported my spidey senses. I don't want to just fix I'd like to understand what tools are telling me.

Included are pics of waveforms and schematic. Waveforms are of each 6800uf/80v cap.

Thanks,
nosaj

[g1]

Well, if the scope settings are the same for both pictures, then the second one has massively more ripple than the first, and I would expect that cap. is bad.
It would help if you state what the volts per division setting is for each of the pics. Also, what is the DC reading on each of the caps when measured with a multimeter.

[J M Fahey]

You give us no data on each scope picture (Sweep/amplitude)

Anyway:
1) start testing with *no* signal.
Fender suggestions are to check a basically healthy amplifier , here we have gross DC problems which must be addressed first, so set all controls to 0 (or at least the Master volumes) and recheck DC.

2) that said, TP15 readings already tell us that C3 is dry or open.
Or, as Enzo says, its solder may be cracked open, so start by resoldering it.

3) you *also* have DC at TP5 which you shouldn't.
But remeasure without signal or load.

Post results.

[nosaj]

Re-Test all levels 0. no signal input

PA1 7.5 vdc 7.3 vac
pa2 7.5 vdc 7.3 vac

tp1 .002 vdc .003 vac
tp2 7.6 vdc .3 vac
tp3 8.3 vdc 7.2 vac
tp4 6.9 vdc 7.2 vac
tp5 7.7 vdc 7.5 vac
tp6 .002 vdc .003 vac
tp7 .001 vdc .261vac
tp8 7.3 vdc 7.1 vac
tp9 7 vdc 7.3 vac
tp10 7.5 vdc 7.4 vac
tp11 47.3 vdc 0 vac
tp12 15.4 vdc 0 vac
tp13 -15.5 vdc 0 vac
tp14 46.2 vdc .012 vac
tp15 -32.1 vdc 15 vac


In ref to the pics. .5 volts/Div 5 ms Time/Div probe on x10

In ref to C3 Flipped board over no solder on one leg of cap. Resoldered and ran a sine wave through it. Pics still same so cap is shorted.

In ref to g-One. Should I take a measurement at d1 and d2 to get vdc for caps?

In ref to JFahey looking at the schematic I see c115 c121 c122 would one of these be responsible for letting DC pass? They are film caps so I have to flip board over after disconnecting heatsinks so I don't see an easy way to test them except by removing them to test.
Thanks,
nosaj

[g1]

You don't need to check the diodes as TP14 and TP15 show you the voltage at the caps.
TP15 shows it is low on DC and high on AC (plus your scope shows excessive ripple compared to C2) so the C3 appears to be bad. If it were shorted it would overheat and pop or blow the fuse, so it is more likely open. Either way, if you resoldered it and that didn't help, replace it.

On second look, you say the scope is set to .5V/div with x10 probe. So 5V per division. C2 shows 4 divisions ripple which would be 20V. Not good for a 50VDC supply cap. Yet your meter reading for TP14 shows .012VAC, this does not agree with the scope.
Might as well replace both the filter caps while you're in there (I think that was your plan anyway).

[Enzo]

I am guessing you didn't run a sine wave through C3. C3 shorted? I bet it isn't, or you'd be blowing fuses hard.

Without good power supply, no circuit will work right. You have +50 and -50 supplies. TP14 is fine, and TP15 has obviously lost its filter cap. If resoldering BOTH terminals of that cap C3 doesn't smooth it out, then the cap is probably OPEN, not shorted. But check the copper traces too, it is possible a cap terminal has broken the copper trace. it may be soldered to the copper, but the copper may have broken away from the rest of the tracery.

Turn your scope vertical down. Lower the vertical gain so the good supply has just a little ripple visible, now look at the bad supply so the ripple is visible within the screen image.

If you don't get TP15 smoothed out, NOTHING else is going to be right in the power amp.

You learn by doing. Your -50v supply has massive ripple. A reasonable diagnosis is loss of filter cap. OK, but we don't stop there, what do you do next? You determine if the cap is bad or if it has become disconnected from the circuit. You resoldered it, or at least one terminal. OK, that should take care of disconnecting problems. Not sure? Scope the actual pins of the cap. Or turn it off and measure resistance. Is there continuity from the grounded pin to the other cap grounded pin? (One will be the positive and one the negative) Is there continuity from the - end of C3 to the output transistors?

But what if it seems the cap is bad itself? You think so, I think so. We parallel another cap there to see if it makes the difference. That isn't just randomly slapping a cap on there hoping it is the issue. It is the logical next step in the process. I am working on the amp, I get to where I think the cap is bad, so I clip in a new one to find out if I was right. Nothing cheap about that, it isn't a shortcut, it is the way we do it. The only thing short-cutty about it is we clipped it in parallel, rather than replacing it on the board. We verify it makes the difference first, and THEN install the new cap if so.

D2 and D4 are fine, because they serve the +50, which is OK. You have -30v on C3, so D1 and D3 are at least one of them working or you wouldn't have any voltage. Forget the diodes. You can check each one with a meter if you like.

C115,121,122 are highly unlikely to be bad. They are not in the path to the speaker anyway. YOur problem is not that something is letting DC pass, your problem is your main power supply is all cockeyed and so then is the output of the amp.

You want to know why it is what it is, well, the power amp expects to have +50 and -50, and the output centers between them. That center point would be zero volts DC. But you don't have +50 and -50, you have +50 and -30. And what is midway between those? +10. Well, I rounded, so using your numbers: +46 and -32, the average between them would be +7v. And lo and behold, that is exactly what you report having. The amp is working, it just has faulty power supply.

[J M Fahey]

***FULLY*** agree with g-one and Enzo, they nailed it.
Only thing I'd add is that those caps are not *that* old to be dry, but they certainly must have been shaken a lot.

Being heavy and fat all that twisting gets supported by the legs.
They crack themselves visibly, or they crack just inside the can or they crack solder and pads or all of the above.

So get a new pair (because the other is as beaten as this one, only this cracked *a little* earlier than the other), teplace both, before inserting them in the PCB make a "toothpaste line" all around the base, so when you sit them down they squeeze some all around.
When it dries it will form a "cushion" all around, between cap can and PCB .
And of course check tracks and pads as suggested above.

Probably, as Enzo suggests, the 7 something DC volts at the output will disappear, but even if they don't and you need to continue troubleshooting, this was an *indispensable* first step.

[Enzo]

Sure, they don't have to break at the solder. I can think of any number of times I unsoldered one leg of a cap, and it came off the board, the other leg still soldered into the board, but pulled right out of the cap. Vibration can do that.

[nosaj]

I will check the cap. The only reason I mentioned the diode is because it is one of the test points I believe. And thank you for the well written explanation, I appreciate that much more than just stick a cap in and see if it fixes itself.
Another thing I've noticed with Peavey amps that have 6800 uf in the power section. Peavey actually sells replacements of 4700 uf. What does this change really do for an amp other than make it work?
So after looking at mouser's stock my choices would probably be the 4700uf unless someone could tell me otherwise or show me where to get the 6800uf80v or higher.

What would be a good resource on power supplies that is written in layman terms?

Thanks for all the help

[olddawg]

MCM Electronics - 2011 Full Line Catalog - page 340

[Enzo]

Peavey no longer stocks 6800uf? What of theirs uses 6800?

Used to be all the amps had those blue cans 5000uf at 55v. Those were custom and when they were gone, they shipped 4700 in their place. Are they shipping 4700 when 6800 is ordered?

In any case, it is not critical. What does such a change do? It allows Peavey to stock one cap for many more applications, rather than many different values all of which do the same job.

Looking at Mouser for 6800uf 80v caps I find 9 choices in a snap-in package. Altogether they have about 2500 of them in stock. $5 or so each. So I see no reason to change to 4700uf.