Peavey special 212 Loud hum

[AutoTech]

Hi guys,
I'm brand new to this forum.
I've been looking through the other forums and found that I'm not the only one with this problem...
A friend gave me his Peavey Special 212 Trans Tube amp. When I turn it on (no guitar attached) there is a loud hum. The speaker cones push in and stay there. I've measured 7 VDC across the speaker wires. I also read about 17 VAC. I don't think the DC voltage is normal based on what I've been reading here. Is the AC voltage normal? I've tried resoldering the filter caps (which looked fine). I want to check the output transistors but there appear to be four of them? also not sure how to tell if they are bad. I sent an email to Peavey requesting a schematic so hopefully I'll get that soon. Another side note is it seems that this amp may have been stored in a damp environment for some time. Does this effect what I should be looking for? Any help would be appreciated. Thanks.

[Enzo]

Disconnect the speaker. DO not re-connect the speaker until we get rid of any DC on the output.

Now measure the speaker wire voltages. Is the DC still there? Or just the AC?

Broken solder on a filter cap can remove it from the circuit, but just because the solder is OK doesn't mean the cap is. We need to measure the voltage across each cap. SInce that may not be convenient, then find the main power supply voltages at the output transistors and measure them with respect to ground. COnveniently, ther eis a row of 6 output transistors on the heat sink. The three to the rioght should have negative on their cases, and the three on the left should have positive. what should it be? Beats me, I'd guess about 50 volts. What we want to find is both polarity supplies at about the same voltage (but at opposite polarity), and with very little AC on it. If one measures about 50vDC and maybe 1v AC, and the other measuresa like 35vDC and 20v AC or something, then the filter cap for that side is likely bad or disconnected.

ANother convenient spot to measure this would be across the two large filter caps in the corner by the fuses.

With a speaker load, DC on the output will severely load the power supplies, which will cause great increase in ripple. If the load is removed, then the DC won;t have to supply current and the ripple should disappear.

If there is a DC offset and power is OK, then we troubleshoot.

First, check the IC power for the power amp. All the ICs in the amp run off +/-15vdc, but the one in the power amp has its own 15v supplies. This IC is the one closest to the left-most output transistor. Got both 15v voltages at its corners? If you do, what is on pins 1 and 7, its output pins? If either has DC, I;'d probably change ot the IC, it is a common 4560. A 4580 would be fine too.

[AutoTech]

The DC voltage is still present with the speakers disconnected. I'll try working through the rest and let you know how I make out. Thanks

[AutoTech]

Okay I measured the DC voltage across each of the two large filter caps. They both read 51V. I checked the output transistors. Like you said three were positive and three were negative. The positive ones read 57vDC and 125vAC. The negative ones read 43vDC and 95vAC (I had to switch the meter leads to get the AC reading, I assume this is normal because it's AC?). Does this mean that both my filter caps are bad? I had another thought, could the rectifier feeding the filter caps be bad? I'm getting both AC and DC (the values found at the output transistors) on the output legs. I'm not sure if the rectifier is bleeding through the AC or if this is just reflecting bad filter caps?

[Enzo]

If switching your meter leads on AC volts changes the reading, then your meter is the problem. AC has no polarity.

What sort of meter are we using? I suspect your meter can't block DC, so it interprets it as some AC voltage.

It would be extremely difficult for the amp to put 125VAC on those transistors.

[AutoTech]

You're right about the meter. It's a cheap Cen-Tech (always worked for what I needed it for). Do I need to purchase a better meter ie. Fluke? What about the DC readings? Do you think I should replace the filter cap on the 43vDC side?

[Enzo]

Meters like that are usually older ones. Today, even the meters they give away at Harbor Freight work reasonably well. FLuke meters are top notch, mine has been working well now for decades. But you don;t need to spend FLuke money, even radio Shack has some fairly nice meters.

Or borrow someone's meter, or borrow them to go with it even.

DOn;t start changing parts just because... We need to get a good reading first.

[AutoTech]

Okay. I bought a new meter and ran through the tests you suggested...
voltages across filter caps are 57V and 44V. These also correspond to the voltages on the output transistors. (no AC voltages with the new meter!
Hooked up the speakers again and didn't see any significant AC bleed through.
Checked all three of the 4560 op amp ICs they all read the same with 15V on on corner and 4V on the other (outputs were around 2V).
Guessing from what you have said, I should replace the filter cap that is reading 44V across it?
Thanks

[Enzo]

WHOOOOAAA.

Op amps with 15v on one corner and 4 on the other? First off, we always want to include polarity. It means a lot whether there is -4 or +4 on that bad power rail. Pin 8 has +15, and pin 4 has -15.

Fix that first, then I bet your power amp will fall into place.

There are two 15v 1N4744 zener diodes in the power supply, CR36, CR39. WHichever one now has 4v across it is probably bad. Although we should also check the appropriate 150 ohm 5 watt R133 or R148. They feed the zeners. WHatever is wrong, those 15v supplies need to both be there, it shouldn;t be hard to fix that.

[AutoTech]

I got busy and just got back to working on this. the OP amp in question has -14vdc on pin 4 and +4vdc on pin 8. I realized that we are looking at two different units (my Peavey dose not have R133 and R148). I have the Peavey Special 212 Transtube II. having said that, I checked all nine 5W resistors. all of them were good. I checked Diodes CR36 and CR39 and both were good. I'm that doesn't mean a thing since we are looking at two different amps...Do you have any other ideas? I can email you the correct schematic if you would like. Thanks again for all the help so far.

[Jazz P Bass]

Can you post the schematic?

[AutoTech]

Attached is a copy of the schematic...

[nashvillebill]

That's not a schematic, that's the manual with only a block diagram...

Peavey will send you the schematic if you call them.

[AutoTech]

My bad. I grabbed the wrong file on my computer

[Jazz P Bass]

So the +15Vdc supply is loaded down.
Yee Haa.
The techs worst nightmare is when the supply itself is good but it is being loaded down beyond the initial supply.
The first thing you have to do is verify that the +15 volt supply is indeed good.
Check the voltage going into the regulator. +25Vdc.
If that is being loaded down you could try disconnecting the cable to the reverb board and/ or remove the +15 regulator.
If the +25 is good, somehow the +15 regulators v out must be isolated from the board & checked.
If that is good, then you have a "nightmare".
If it is bad, isolated from the circuit, then you need a new regulator.
(Didn't Peavey have a Tech Note on a certain brand of regulators being bad?)
(Something about "if you see xxx regulator, yank it!)