Peavey 400 B/G Module - Supply Voltages Low

[TomCarlos]

Friends...

I picked up a Peavey Mark III bass amp that has the 400 B/G power board. The amp has a problem, beginning back at the power supply.

I disconnected the front Preamp board and two feeds that connect to the output transistors. When I measured across the high side supply, I was seeing plus and minus 28 volts. This module is looking for around +-50volts. I also looked at the low side regulators. I was seeing 14.7 and 13.6 volts- so there is a problem with those too.

I pulled out the 5,000uf caps. Both seem to measure within spec but being original, I will replace those.
I checked the diodes CR17, CR18, CR19, CR20 and they seem to be ok.
I measured the AC (unloaded) coming out of the Transformer (705-16735) - Red leads 50vac, 25vac at CT and Orange leads 26.8vac, 13.4vac at CT.

So there must be a problem downstream on the power board somewhere.

Anyone work on these boards and know of common problems? This might be tricky to try and debug if I am not seeing the correct power supply voltages.

Thanks in advance... Tom

[nashvillebill]

This sounds exactly like the symptoms I had on a Peavey Classic; power supply voltage was half of nominal. I too immediately thought there was something downstream loading the power supply down, or some other power supply failure--and even posted here, asking what could cause the problem.

Fortunately, Enzo came through again, and had me change the power supply filter caps. Bingo, problem solved.

Did you measure how much AC ripple was on the 50 volt DC rails?

[Enzo]

Forget "common problems' and just fix it.

When I see two power supplies both sitting at half voltage, I can;t let myself think, they both must gave failed in the same manner at the same time. You schematic says 52vDC on the main rails, 50v is close enough. Ignoring the volt we lose in the rectifier diodes, what AC voltage do we need to make 50vDC? 50 x 0.707 = 35vAC. Your AC is 10v low, Likewise, the orange wires are for the low voltage rails, and that seems pretty low AC as well.

Any chance this amp has been set for 220v but is plugged into 120?

Before tearing out filter caps, see if they are working. You had 28v on the rails. Was that 28v clean DC? Or was that 28vDC and 20v or more of ripple too? If it is smooth, the caps are working.


5000uf 55v caps are not available. Those are not standard values and were made custom for PV many years ago. In a modern cap, 63v is the next standard value up, or even 80v or 100v. 5000uf is not standard, but 4700uf is.


You really should not run this amp with the output transistors disconnected. You run the risk of damaging the predriver transistors.

[TomCarlos]

Thanks Bill and Enzo...

I am going to run through the match and make sure the incoming AC is adequate for this amp. And I will change the filter caps. I have some 6800uf, 63v somewhere here in the house. I will check other caps and probably change those too.

I have other Transformers from Peavey amps... going to measure those and see if I have a suitable replacement just in case something happened to the stock Xfrm.

Will report back.

Tom

[52 Bill]

I measured the AC (unloaded) coming out of the Transformer (705-16735) - Red leads 50vac, 25vac at CT and Orange leads 26.8vac, 13.4vac at CT.[/B]

Please explain these readings, 50vac from red wire to red wire and 25vac from CT to each red wire? etc.

So there must be a problem downstream on the power board somewhere.

There are two or three burned resistors on the board in the driver stage, in your photo. Look for shorted outputs and driver transistors.

[J M Fahey]

Quote Originally Posted by TomCarlos View Post
I measured the AC (unloaded) coming out of the Transformer (705-16735) - Red leads 50vac, 25vac at CT and Orange leads 26.8vac, 13.4vac at CT.[/B]

I also have a problem with those voltages.
*How* can you have voltage on a floating transformer wire?
Where do you put the black probe and where the red one when you measure that?
How can you have 25VAC on one centertap and 13.4VAC on the other?
Relative to what?
Answers to these doubts are more important than anything else, without them you can't advance.

[TomCarlos]

Gents.....

Looks like the transformer has failed. The voltage readings I gave to you were from the outer pins of the molex connects.

For the high side, I should be seeing something like 70vrms across the outer pins of the molex connector, 35vrms from the center pin to the outer.

The low side is also a problem. So I connected my Kilowat meter and Current Limiter do-hicky and yes, the lamp is a tad too bright and with absolutely no load on the transformer, I am pulling 250ma. Bad....

And Bill, yes, I see the burned parts. When I got this amp, the molex connectors to the OT board were disconnected. So chances are, something happened to the predrivers and it took out surrounding parts. And whatever the chain reaction was, it looks like the Xformer went too.

I have another Xfrmr in the house. The low side would be ok. But the high side is a tad short- 60vrms/30vrms at the center tap. It would probably work, I just wouldn't see full power? That is my guess.

So perhaps I will go on a hunt to see if I can dig up a suitable replacement Xfrm from a salvage project.

Thanks for the replies.

Tom

[J M Fahey]

You are not answering what was asked.
And you add more meaningless data, such as:

The voltage readings I gave to you were from the outer pins of the molex connects.

Where is the black probe connected?
Is that Molex connector still plugged in the PCB or is it floating in the air?
plus:

I have another Xfrmr in the house. The low side would be ok. But the high side is a tad short- 60vrms/30vrms at the center tap.

How do you measure those 30V RMS at the center tap?
Again: "Where is the black probe connected?" .
"30V RMS" relative to what?

Edit: and "250mA", very probably reactive or magnetization current, is nothing unusual for a 300/350 VA power transformer meant for a 200W RMS amp.

[TomCarlos]

Juan...

See picture #1. The transformer secondary has two outputs. One set is Red the other set is Orange.

Let's focus on the Red set (and molex) connects to the main PC board. There are 3 wires. The outer two wires are solid red. The center wire has a dab of yellow on it. I call that the CT. I disconnected that cable from the PC board. Using a TRUE RMS meter (1 red lead, 1 black lead), I am measuring the voltage on the outer pins coming off the transformer- the 2 solid red wires. I would expect to read something like 70 Vrms across those wires. I am reading only 50Vrms. That does not seem right.

What else can I tell you...

[Enzo]

Before we go too far, are these readings taken with the full 120v across the primary? If you are on the bulb limiter, that will lower your voltages. The bulb is a powerful tool, but it is not transparent. This thing isn't blowing fuses, so get rid of the bulb and test the transformer alone. Your watt meter will be OK. You can also leave the transformer fuses just in case, but disconnect the secondary wires, plug it in and let it cook. Does the transformer get hot?

If the transformer is bad enough to drop the voltages 10v, it is going to be pretty darn hot.

And let's check another thing. PV used the same wiring scheme and color scheme on most of their transformers for a variety of products. SOmeone else may have put a different transformer in there. The schematic calls for a 705-16735 transformer. Look on yours, is that the type it is?

[TomCarlos]

Enzo... once again, you are the man!!!

I would never have expected that the bulb limiter would have such an effect on the transformer. I removed the limiter and put the transformer directly into the 120ac outlet. Voila... I am now reading 75.2vrms on the red leads and 41.2vrms on the orange. Yes, it is the original 705-16735 Transformer. THIS WAS A VALUABLE LESSON LEARNED!!!

Back in business... now onto replacing caps and checking the pre-drivers. As Bill noticed, there are burnt parts on the pc board. I will go through carefully and look at those.

Enzo, I can't thank you enough.

Tom

[J M Fahey]

Fully agree.
*Any* condition (say, a shorted turn) bad enough to pull down *unloaded* transformer voltages by 20% means a ton of power is being dissipated *inside* it, meaning it will smoke and melt in a relatively short time (say, 10 to 30 minutes).
Now a series bulb limiter drops voltage , even if the transformer is unloaded.
Why?
Besides the obvious point that its inductance is not infinite, so at 50/60 Hz it will pass *some* current, standard silicon steel (transformer core steel) is highly non linear and needs some "starting" current through it ... there's the 240mA you measured.
But those are reactive, meaning highly inductive, and don't actually mean that the transformer is dissipating 1/4Ax120V=30W which still would make it *quite* warm , but a fraction of that, those 240mA into the primary winding resistance, probably a couple Watts.
Enough for an idle transformer not being stone cold but not much more than that.

Now the bulb is not reactive but purely resistive, and will dissipate 240mA x its much higher internal resistance.
In fact, if you use a small bulb (25 W), it will become visibly orange ..... and knock a lot of wall power off ... yet the transformer is fine.

But glad you solved it, specially thanks to Enzo's eagle eye, who can see through the clutter

[Jazz P Bass]

Back in business... now onto replacing caps and checking the pre-drivers. As Bill noticed, there are burnt parts on the pc board. I will go through carefully and look at those.

I would recommend repairing the amp before gutting the ECaps.
If there is now a problem after replacing the caps, you will be hard pressed to identify the cause.

The 'burnt parts' are R44, L1, & R61.
That leads to Q12, which is probably shorted.
If that is the case, the output transistors that Q12 drives should be checked for shorts.

[TomCarlos]

Thank you JM and JP....

Yes, Enzo had a sharp eye to detect my original complaint. Once that was resolved, I was able to fix the other problems. Yes, Q12 was burned out too. So I replaced that, Q2, the resistors (and inductor) hanging off both emitters (R44, L1, & R61) and replaced R26 too. There were 2 bad OTs, replaced those too. I replaced the filter caps for good measure.

I had (and still have) some extra Peavey PC boards that I was able to salvage some parts. But cap? I always install new caps.

I hate to plug anything here but the Peak Atlas Component Tester is way cool... much better than trying to use an ohmeter to test a transistor.

Back up and running... thanks again everyone.

Tom

[altazepsta]

parts question

Thank you JM and JP....

Yes, Enzo had a sharp eye to detect my original complaint. Once that was resolved, I was able to fix the other problems. Yes, Q12 was burned out too. So I replaced that, Q2, the resistors (and inductor) hanging off both emitters (R44, L1, & R61) and replaced R26 too. There were 2 bad OTs, replaced those too. I replaced the filter caps for good measure.

I had (and still have) some extra Peavey PC boards that I was able to salvage some parts. But cap? I always install new caps.

I hate to plug anything here but the Peak Atlas Component Tester is way cool... much better than trying to use an ohmeter to test a transistor.

Back up and running... thanks again everyone.

Tom

Bro TomCarlos, I have the same board, same area burned. replaced all the resisitors in that area, knocked out pre's, replaced those. also q12 replaced. one prob so far. C26 is burned. little mustard yellow resistor looking fellow, what is it and where do i get it???? the C26 thing is throwing me. C= cap , but sure looks resistor to me. in either case, i cant find one anywhere. hope someone can help on this one. Even sent email to Peavey parts dept, they gave me a "Dunno"