Peavey Musician III (400GH power amp) buzzing & no output

[Tone Meister]

Peavey Musician III with the 400GH power amp buzzes when speaker is connected and no output.

Initially used light bulb limiter to power up, but it DOES NOT blow fuses, so now have it connected to full line voltage with a 6.25A fuse installed for testing purposes. Stock fuse is 8A.

• Line voltage = 122.6 VAC
• Measured 1.2 mVDC on MAIN speaker output jack
• Measured ~38.2 VAC on main power rails (schematic indicates +52)
• Both 1A fuses are good
• ~38.2 VAC at anodes of CR9 and CR11 - both diodes test good
• ~16.6 VAC at anodes of CR8 and CR10 - both diodes test bad
• R39 and R40 are dropping voltage
• Measure -26.6 VDC at C36, 0 VDC DC leakage across C36, tests fine for ESR
• Measure 26.5 VDC at C37, 0 VDC leakage across C37, tests fine for ESR

Anything else to check while I'm in here? Is it a good idea to install fresh 5000/55 and 100/35 electrolytics while I have the board out?

[Enzo]

You may need new caps, but fix the amp first before wholesale parts replacing.

If your +52 and -52 (BOTH??) measure a lot of AC, then the filters are not working. First thing is to resolder them, it might be that simple. If the caps have actually failed, you will not find the 5000uf/55v caps. Those were custom for Peavey and have not been available for many years. The current product to replace them would be a standard value 4700uf at 63v. That is what PV will send you if you order the old blue caps. You can of course use higher voltage caps if you have them or for that matter 6800uf or something, but if I am ordering something, I just get the 4700s. You will probably have to carefully drill a hole or two for the legs of the new caps, they won;t fit the old holes.

Please post or link the schematic you are using. I have 400G and 400BH, but I have no 400GH. Part numbers you are calling do not match the schematics I have to look at.


Whatever the circuit, you need the +/-52v rails, and there will be some +/-15v rails too. All those need to be up to voltage and clean of ripple.

[Tone Meister]

Wow, sorry I failed to attach the schem. Here is the GH module and the Mark III preamp as well.

[Enzo]

You know, I probably do have that in my paper files. I am so used to looking on my computer I forgot to walk over to the shelf. Sorry. But now everyone can see it.


Note the 16v supplies with C36,27,etc are only for the preamp board. The power amp op amp has its own 15v zener supplies off the 52v rails.

[Tone Meister]

... Note the 16v supplies with C36,27,etc are only for the preamp board. The power amp op amp has its own 15v zener supplies off the 52v rails.

Yeah, I had already picked that up from previous a thread you participated in some time back on this same amp. I based a good bit of my troubleshooting off that thread before starting this one.

[g1]

What is your VAC on power rails after replacing the bad diodes?

[Tone Meister]

I don't have any diodes that size on hand, so I'm at a standstill for the moment

[Tone Meister]

What is your VAC on power rails after replacing the bad diodes?

38.4 VAC after replacing the diodes and reflowing several solder joints on the power board. There are some other changes from what I measured before.

• Line voltage = 122.1 VAC
• Measured 0.0 VDC on MAIN speaker output jack
• Measured ~38.4 VAC on main power rails (schematic indicates +52)
• Both 1A fuses are good
• Measured 50.7 VDC at C32
• Measured -51.4 VDC at C31
• R39 and R40 are dropping voltage
• Measured -5.2 VDC at C36, 0 VDC DC leakage across C36
• Measured 48.2 VDC at C37, 0 VDC leakage across C37
• Measured 2.7 VDC at C38
• Measured -11.1 VDC at C39
• Measured 15.9 VDC at R48
• Measured -15.7 VDC at R50

I'm reading AC ripple in the 300 mV range at R48 and R50.

[Tone Meister]

OK, measured 2.2 VDC at CR18 and -45.5 VDC at CR19 zeners. Replaced CR18 with an IN4448 and I got a whiff of smoke when I put power back on it.

**OK, I know why I got smoke** I got the 3-pin molex connector between R39, R40 and R50 off by one pin.


I guess it's back to square one.

[Tone Meister]

Boy, you fellas sure let me swim in that one over the weekend, didn't you? Just kidding ... Actually I had shows all weekend and didn't get back on it until this morning. I now have the amp working and am getting output to the speaker, but I do have a low-level hum that sounds like 120 Hz that I'm trying to track down.

First, I have a question, or questions, about the 52V rails off the power transformer.

At TR2 (Red-Red/Yellow-Red): with the 3-pin Molex connected to the board, I get ~38 VAC on both the top and bottom pins at the connector; however, when I disconnect the connector from the board, I get ~52 VAC on the bottom pin and ~ 18VAC on the bottom pin at the connector. With the connector still disconnected, if I use the Red/Yellow center pin as a ground reference, I get ~ 38 VAC on each of the Red wires off the power transformer. All other connectors were connected to the board while taking these measurements.

Will someone shed some light on that for me, please?

[Enzo]

Here is some light. For a voltage reading to mean anything it has to be in a circuit. You know the transformer is OK because it makes the two 50v rails and the amp works, so this is academic. When you measure AC voltage from the center tap to either end red wire, you are measuring the voltage across a transformer winding. When you disconnect the thing and measure "to ground" then what circuit is there? Take a 9v battery and sit it in front of you. Connect your meter black probe to ground somewhere. Now measure either terminal of the battery for voltage. get anything? You haven't made a circuit. Unplug the transformer wires and they are sitting in the air. There is no circuit between them and ground. ANy voltage reading you get is simply the meter picking up radiated signal.

Let your meter leads dangle, now lick your fingers and pinch the red probe tip between the wet fingers. Sitting here today I only get about 100mv, but you will usually get a reading. You are applying the meter to one wire of a transformer, and the other end of the meter to something else. Might as well measure the voltage between my sandwich and my milk. (I prefer chocolate milk)

If you have hum, then classic troubleshooting says...

Isolate the problem.

Your power amp has a cable to the preamp in the corner. Unplug that from the power amp. Is the hum still there? If so, the power amp is hummy, if not, the preamp is hummy. And re-read post #4.

If the preamp is hummy, then do ANY of the controls have ANY effect on the hum in ANY way?

[g1]

OK, measured 2.2 VDC at CR18 and -45.5 VDC at CR19 zeners. Replaced CR18 with an IN4448 and I got a whiff of smoke when I put power back on it.

Where are these diodes? Are they zeners or not (1N4448 is not a zener). Not sure where you are measuring those voltages.

[Tone Meister]

Where are these diodes? Are they zeners or not (1N4448 is not a zener). Not sure where you are measuring those voltages.

Another misstatement: They're not zeners, and they're located between VR1 and VR2 on the 400GH power amp schematic. The stock part was an IN4148 and I subbed in an IN4448.

I have everything on the amp working, but I do have a low level hum in the preamp, with or without an instrument plugged into the input. I pulled the preamp connector and the power amp is dead quiet.

I can eliminate nearly all the hum by either: 1) turning CHANNEL B POST GAIN all the way down, or 2) turning CHANNEL A POST GAIN all the way UP.

If I engage the REVERB, the hum increases by about double. The amp is most quiet with both POST GAINS all the way down and the REVERB disengaged at the footswitch.

Digging in to the preamp now ...

[Enzo]

How much ripple is on both the PREAMP 15v rails?

Your report is confused. First you say it is least hum with B post all the way down or A post all the way up. Then you say the amp is quietest when both posts are down. We can't have least hum with post A both up and down. Pick one.

In the preamp, you have a lot of op amps, plus other chips like the TL604. Go through the amp and see if there is DC on any of the output pins. I point out the other types since they don;t have the same pinout as the op amps.

This amp is what 35 years old? In the preamp are a bunch of little 2uf 35v electrolytics, I'd wager they all need changing. And the few other lytics in the signal circuits.

[Tone Meister]

I tried to post this before the servers froze up three hours ago, but I lost everything I had typed.

I had ~350 mVAC ripple on the preamp 15V rails and around 90 mVAC on the 52V rails. Took a very close look underneath C31, and C32 and noticed some bulging on both caps. I went ahead and installed some fresh 5000/63's there and a pair of 1000/35's at C36 and C37.

She's quiet and purring like a kitten. After running it for awhile on the dummy load everything checks out fine. I've been playing some lonesome blues with my Frankencaster for the last hour or so and this thing sounds awful good.