Peavey Musician Mark III...No +15V

[earache]

Hi Folks

Looking at a Peavey Musician Mark III with the 400 B/C power amp.

MUSICIAN_MK_III.pdf

will keep the light bulb lit pretty bright on my light bulb limiter without blowing the fuse.

While troubleshooting I discovered that the +15 voltage is missing as measured at the positive end of C40

Would the problem be VR1 or the diodes before it?

I should note that I was able to measure -15 and also +52/-52 as per the schematic.

Thanks as always!

Earache N

[The Dude]

With the unit off, measure the resistance across C40 (+15V to Gnd). Do you see a short or low resistance? Is there voltage input to the regulator? My bet is that you have a short on the 15V line, but we'll need a bit more information before we can go further. If there is indeed a short, it could be anything connected to +15 including the regulator itself.

[Enzo]

yeah^^^

While troubleshooting I discovered that the +15 voltage is missing as measured at the positive end of C40

Would the problem be VR1 or the diodes before it?

You tell us. either that 15v rail is being loaded way down, or the voltage is just not getting there. If the +15 is loaded to zero - shorted to ground - then I'd expect the Vreg to be darn warm. Unplug the cable to the preamp board, does the 15v supply come back up? I doubt your rectifier diodes are all bad, but measure voltage at the input pin of VR1, is it there? I don't know what it is, 20-25v maybe, but your -15 works so it will be the same voltage as the other Vreg input, just opposite polarity.

Could it be a bad VR1? Or course. If I have no output voltage but i do have input voltage, and the circuit is not shorted to ground at its output, I'd have a new Vreg in there before i could finish typing this.

[earache]

yeah^^^



You tell us. either that 15v rail is being loaded way down, or the voltage is just not getting there. If the +15 is loaded to zero - shorted to ground - then I'd expect the Vreg to be darn warm. Unplug the cable to the preamp board, does the 15v supply come back up? I doubt your rectifier diodes are all bad, but measure voltage at the input pin of VR1, is it there? I don't know what it is, 20-25v maybe, but your -15 works so it will be the same voltage as the other Vreg input, just opposite polarity.

Could it be a bad VR1? Or course. If I have no output voltage but i do have input voltage, and the circuit is not shorted to ground at its output, I'd have a new Vreg in there before i could finish typing this.

The diodes tested out OK, and C40 measured like 2.5 ohms across it in circuit, so I pulled the voltage regulator and saw a browned spot on the board underneath. Soooo....I replaced the VR1 and also C40 and C41 and now have both +/- 15V and the light bulb is dim when I fire up the amp! However.....I have extremely low output - I have to crank the gains all the way to be able to hear it at all, very faintly. Same problem, both channels. Also when I flick either of the switches that turn the graphic EQ in/out I get a loud POP and with the reverb on at the same time but I can hear that the reverb works well.

Is there a common area to the two channels that I should be looking at? I think the problem may be on the preamp board but am not sure where to start.

Thanks In Advance!
Earache

[Audiotexan]

The diodes tested out OK, and C40 measured like 2.5 ohms across it in circuit, so I pulled the voltage regulator and saw a browned spot on the board underneath. Replaced the VR1 and also C40 and C41 and now have both +/- 15V and the light bulb is dim when I fire up the amp!

Positive progress!

However.....I have extremely low output - I have to crank the gains all the way to be able to hear it at all, very faintly. Same problem, both channels. Also when I flick either of the switches that turn the graphic EQ in/out I get a loud POP and with the reverb on at the same time but I can hear that the reverb works well.

Is there a common area to the two channels that I should be looking at? I think the problem may be on the preamp board but am not sure where to start.

Thanks In Advance!
Earache

Have you tried making sure that all the jacks are clean? I do notice some switching jacks in there...Particularly the "Automix" jack...but I'd also make sure that both Inserts ("Out/In B" and "Out/In A") are clean as well, since they're direct feeds to the graphic/main/reverb busses. Also same for "PA In" jack.

Also, check to make sure any switching jacks are making proper contact. Inserts in particular. If a switching jack gets bent just the least bit out of shape, you could have a signal break there.

Anytime you have a "loud pop" in switching with circumstances as you describe, generally says to me that it's "trying" or close to being working, but something minor is holding things up.
However, it could also mean that a cap's discharging somewhere along the line. So... 50/50. But given you say the reverb works well, then I'd bet on the former: you're close.

"Loud" means it's amplifying (if nothing more, at least in short duration) so hopefully you're close to having it wrapped up...

[Enzo]

A pop like that usually means to me some DC on the signal path.

But your low level:
You have a PA IN jack, so use it. Plug your signal into the PA IN jack. Does it come out clean and strong or diminished like the problem? I suspect you are right about the preamp, but we need to be sure about the power amp first.

There is an OUT/IN jack for each channel, is there DC on the tip contact of either jack? An east way is to plug a cord into the jack then measure for voltage at the other end of the cord - don't have to open the amp even. We want to see zero voltage there. Lokewise the PREAMP OUT, LINE OUT, and PA IN jacks could all have DC on them potentially, so check them too.

Play into INPUT B, have a cord plugged into the IN/OUT B to the PA IN. Does that come out OK? Now try connecting OUT/IN A to PA IN and play into INPUT A, same questions.

[earache]

A pop like that usually means to me some DC on the signal path.

But your low level:
You have a PA IN jack, so use it. Plug your signal into the PA IN jack. Does it come out clean and strong or diminished like the problem? I suspect you are right about the preamp, but we need to be sure about the power amp first.

Plugging a guitar into the PA IN jack returns a clear signal.

There is an OUT/IN jack for each channel, is there DC on the tip contact of either jack? An east way is to plug a cord into the jack then measure for voltage at the other end of the cord - don't have to open the amp even. We want to see zero voltage there. Lokewise the PREAMP OUT, LINE OUT, and PA IN jacks could all have DC on them potentially, so check them too.

Play into INPUT B, have a cord plugged into the IN/OUT B to the PA IN. Does that come out OK? Now try connecting OUT/IN A to PA IN and play into INPUT A, same questions.

I tested for DC on the jacks as described. Results:
Channel A Out/In has 14.9 V on it
Channel B Out/In has -15.1 V on it
Preamp Out measures -.004 V
Output Line measures .037 V
Power Amp In measures -.034

As far as sending a plug from Channel A Out/In to Power amp In while playing a guitar into the appropriate "A" or "B" input,, the results were the same as plugging into the regular channel inputs - had to turn the gain WAY up to hear anything and what I could hear was very distorted. Same with Channel B.

What devices should I look at next?

Thanks for your time!

Earache

[Jazz P Bass]

ChA & ChB In/Out should have zero volts on them.

For starters, check ChA CR22 & CR23 diodes for short conditions.
ChB check CR25 & CR26.
The diodes should be blocking the + & - 15Vdc voltages, not passing it.

[Enzo]

That^^^^

[earache]

ChA & ChB In/Out should have zero volts on them.

For starters, check ChA CR22 & CR23 diodes for short conditions.
ChB check CR25 & CR26.
The diodes should be blocking the + & - 15Vdc voltages, not passing it.

Excellent advice - I did just that and found that one out of each pair of the diodes that are associated with Channel A and Channel B were bad. Replaced them and now the amp works completely!
Thanks again everybody (Enzo, JazzP, Dude, AudioTexan), you are helping me gain more and more confidence fixing solid-state gear as I work thru each problem.
I am very grateful - thanks!