Peavey Session 500 Mark IV - Loud buzz when turned on

[YYZ]

Hello,

I am hoping that somebody can give me suggestion on how to fix
my Session 500 Mark IV.

When I turn it on it makes a loud buzzing sound. Like some kind
of grounding hum/buzz.

I can plug my guitar into the POWER AMP IN and the amp is good, loud
and clean.

If I disconnect the pre-amp board from the power amp board, the amp is
silent - no loud hum/buzz.

I have plugged the PREAMP OUT into another amp (my Peavey 260C) and I
all I hear is the same loud hum/buzz.

So the hum/buzz must be what the pre-amp board is outputting.

I have measured the power going to the pre-amp board, it is +15v and -15v
like it should be.

Now I need help/suggestions on how to troubleshoot that board. There
are many little op-amp ICs and stuff on that board. There is no
obvious evidence of anything burning up on the pre-amp board, smell-wise
or visually.

How does one go about troubleshooting this issue?

Thank you for your time,
brad

[ambiophonics]

I would make sure all the voltages from the power supply are correct and free of AC ripple first. I believe that amp runs +/- 50 volts for the outputs, +/- 15 for the opamps, there may be secondary supply with its own filter cap that runs something in the +20volt range. If the main filter caps failed I would expect you'd have hum in the poweramp section also (as peavey probably derived the 15 volt supply from the main 50 volt supplies) but it's always wise to confirm correct and clean power before moving on.

[Enzo]

Yeah, look for ripple on the 15v rails. But my first suspects are the input jacks themselves. Broken ground connections?

So:
Does it make this noise with nothing plugged into the amp?

Which controls have ANY effect on the noise? ANy that can turn it up and down, any that affect the tone of it?

Kill the reverb help?

Disconnect any footswitch help?

You measured 15v rails, but all those dual op amps have two output pins each. Look for unwanted DEC voltage on pins 1 and 7 of each.

[YYZ]

Yeah, it makes the buzzing sound whether or not there is a guitar plugged in. I can actually here the guitar below
the loud buzzing noise when it is plugged in tho. The guitar sounds weak, but I actually think it is as loud
as it should be, just that the buzz is MUCH louder.

The buzz sure seems to be some kind of 60Hz signal tho.

The RATE switch, when turned on, makes the buzzing sound louder, and turning that pot up makes it even louder.

The REVERB pot makes the buzzing louder when I turn it up.

No other controls affect the level of the buzzing.

I don't have any footswitch any more.

I can tell that the pre-gain pot, string effect and equalization sections still work on instrument
signal/sound.

One other thing. I plugged the PREAMP SEND into my other amp (260C) and it sounds fairly normal, although
there is some buzz. But I can't decide whether that is the SAME buzz tho, it is much weaker and sounds like your
more 'normal' ground buzz.

How can I test for any ripple on the +/- 15v rails? I don't have a scope or anything. You think its possible
to use a tiny speaker (with resistor) or something?

Thanks,
Brad

[YYZ]

Hello,

I got a tiny speaker and wired some leads from it to use as a probe. One lead I included a resistor.

Using this probe with one end connected to ground, I probed the +/- 15v rails.

On the +15v rail I couldn't hear anything since it was pure DC voltage.

But, on -15v rail I could faintly hear the hum/buzz.

I think you guys are correct. I think this is the bad noise I hear from amp, only then it is much amplified, etc.

Any suggestions would be appreciated again.

Thanks,
Brad

[YYZ]

Now I see my power board schematic that I got from Peavey is not correct. They sent schematic for 500BH.

On my power board (don't know model then) there are 7815 and 7915 voltage regulators, but the 500BH schematic
does not show these.

Does anybody have schematic showing these regulators on them? And also board layout diagram?


Thanks,
Brad

[Enzo]

Look on the solder side of your power amp board. See any model names?

You can also contact PV again with your exact model name and serial and tell them the schematic they sent has 500BH, but your amp doesn;t match that. Your first post mentions Mark IV, did you include that when you talked to PV before?

Since your power amp worked anyway, I don;t think that is an issue.

You hyave +15 and -15 at each IC, so measure it as DC voltage. Now switch your meter to AC volts and measure the exact same places. You SHOULD get close to zero AC volts.. If you get a few AC volts, that is ripple.


So, if you turn ALL the controls to zero, you still have the hum?

[Enzo]

Look on the solder side of your power amp board. See any model names?

You can also contact PV again with your exact model name and serial and tell them the schematic they sent has 500BH, but your amp doesn;t match that. Your first post mentions Mark IV, did you include that when you talked to PV before?

Since your power amp worked anyway, I don;t think that is an issue.

You hyave +15 and -15 at each IC, so measure it as DC voltage. Now switch your meter to AC volts and measure the exact same places. You SHOULD get close to zero AC volts.. If you get a few AC volts, that is ripple.


So, if you turn ALL the controls to zero, you still have the hum?

[YYZ]

Here is picture of my power board...

EDIT: Pin 1 on TL074 is on lower-right side of chip

Brad

[YYZ]

Here is picture of my pre-amp board...

[Jazz P Bass]

Looks like a 400BH to me.

[Amp Kat]

Try using the send out to another amp and see if it buzzes then try the return in and see if the problem is preamp or power amp related. At least it cuts your time in half. If it only hums in the return mode I would suspect output transistors or maybe a voltage problem dragging the supply down. What do the output transistors read ?

[YYZ]

Hello all,

I have measured for AC on the power pins of the TL074 on power board
at locations A & B (see previous image). I have indicated which multimeter leads went
where and have listed readings for all the permutations thereof (lol) so maybe you
can interpret the results from that. I don't think I can, I'm not sure if the readings
can even be trusted with this multimeter. But this is what I got:

Multimeter
Black Red AC Range
======================================

GND A ~30v 150v
GND B ~6v 15v
A GND ~6v 15v
B GND ~30v 150v

* A = TL074 +Vcc
* B = TL074 -Vcc
* GND was the ground post on upper-left of board.
* Readings are same if use +15v power post for A, -15v power post for B.

Later,
Brad

[Amp Kat]

Think your op-amp supply rails are ok if you get +/_ 15 volts DC. Enzo was trying to see if the ripple was bad and from the readings it seems like it could be but they are modulating so it's hard to tell. Did you do those checks I asked and if so what did you find ? The output transistors if it's a BH would have about 80 vdc on the collectors if they are 6530 and 6533 complementary.

Edit : One more thing is the 7815 and 7915 are very basic regulators for these type of designs and could easily see them there even if not on the schematic.

[YYZ]

Good morning,

If I patch PREAMP SEND into another amp, it sounds fairly normal. There IS a buzz, but I am not sure if this is same buzz or not because it is not nearly as loud. But it could be the same. (Maybe that buzz gets amplified somehow going through all the pre-amp circuits or something and so comes out much louder.)

If I plug directly into POWER AMP IN, the guitar is loud and clean sounding. There IS a buzz, but not very loud at all. The sound of the guitar is fine though, no distortion or anything. If buzz was gone it would sound great, especially now that I cleaned gunk from the black widow speaker as mentioned in another thread

Here I have rated the buzzing loudness in each scenario, 0 to 10, with 10 being VERY loud, screwed-up state its in now.

1) Guitar into POWER AMP IN --> 1 (hmm, i've heard worse, think i'll ignore it)

2) PREAMP SEND to another amp --> 3 (Somebody prolly thinking you got a problem)

3) Guitar into PREAMP RETURN --> 10 (turn it off, neighbors might call police or something)

4) Simply turn amp on with or without guitar plugged in --> 10

Please remember that with guitar plugged into INPUT, I can still hear guitar, just that buzz is drowning it out...

Thanks,
Brad