Peavey Mark III Series 260 C

[howarda]

I was just given this amp by my drummer. It was stored in an attic with no climate control for years. When I turn it on, it howls. None of the controls have any effect. The sound is not high pitched. I disconnected the front control panel section and the results are the same. Any idea what might be going on? (I'm not an electronics techies but have some minor skills.) Thanks in advance.

[Enzo]

I don't know what howls means. I assume it makes a loud noise all by itself. Is the noise a loud hum? Hum being the sort of noise you get if you touch the end of your guitar cord. Of is it more of a WOOOOO sound? Or is it a high pitched SCREEEEECH? It is capable of making noises, but the nature of the noise suggests different problems.

If hum is the problem, then we look for either DC on the output jack or we look for loss of filter cap function. And that latter would be either a cracked soler connection to a main filter cap, or the cap itself has failed.

[howarda]

Hum being the sort of noise you get if you touch the end of your guitar cord

Yes. That's it.

DC on the output jack or we look for loss of filter cap function. And that latter would be either a cracked soler connection to a main filter cap, or the cap itself has failed

How might I go about identifying these? I have no schematic. I know enough to possibly be guided through a repair though.

[Enzo]

You look at your speaker cones. When you turn on the amp, do they move one direction and stay there? If so, turn it off immediately, that is DC on the speaker. Or you can disconnect the speaker and measure for DC voltage across the output jack.

If there is not DC on the speaker output, then you probably have lost a main filter cap - one of the two large cylinders side by side in there. It is possible one has simplpy cracked fre of its solder, check for that. Otherwise we probably need to replace it.

You can get any PV schematic from customer service at Peavey. But here, does this match your power amp?

[howarda]

Thanks Enzo. This is a huge help. If there is DC on th speaker output - what kind of voltage range are we looking at and what might be the remedy?

[Enzo]

If the amp is blown, you would see something like 20-40 volts there. ANything over a quarter volt is a problem. But this won;t be subtle. You'll find 40v there or not, most likely.

Rather than go through a technical discussion over its remedy, let us find out if that is the situation first. if you have a bad filter cap, we needn't waste time on bad transistor talk.

[howarda]

Thanks again. I'm in the middle of selling a house andgetting divorced, so it may be a little while before I can check these.

[howarda]

39.7VDC reading taken on the speaker output pins......................

[Enzo]

OK< blown power amp.

First check all the large power transistors for shorted. Check the two small transistors on the indiovidual finned heat sinks, the 5331 and 5332. There is also a small 47 ohm resistor associated with each of those. Check for open resistor.

There are a number of large 10 watt rectuangular cement power resistors. Check all for opens.

Look at the schematic. There are DC voltages at points all over the powr amp. Check them. They should all be reasonably close. Let's say within 10%.

[howarda]

check all the large power transistors for shorted

Are these the 67376's

[Enzo]

According to the schematic, though various other types work there.

[howarda]

Enzo,
I removed the base screw on all four power transistors. One of them read a short from B-E and C-E. I guess the mystery is solved.
I found a previous post where you said you buy MJ15003 power transistors from Allied to replace failed 67376's. As you stated earlier, there are four 67376 power transistors in this amp. Two are 67376-7943-H's and two are 67376-8029-H's. The failed component is a 67376-7943-H.

Do I need to replace all four transistors or just the two identical units?

Is the MJ15003 a suitable replacement for both the 7943's and the 8029's?

Couldn't find the MJ15003 at Allied but it turned up on here on Mouser http://www.mouser.com/Search/Refine....eyword=MJ15003 with a slightly different part number. Do you think it will work as a replacement?

Thanks again for all your help. You're a lifesaver.

[52 Bill]

Is the MJ15003 a suitable replacement for both the 7943's and the 8029's?

Those number are date codes 43rd week of 1979 and 29th week of 1980.

[Enzo]

What Bill said. the part is a 67376, those other numbers are just when they were made.

They are all four the same part because this amp is quasi-complementary. But two of them run the positive side and two run the negative side. Two on the left and two on the right, I forget which is which. You want the two one a side to be the same type, not a mix. You don;t have to have the positive ones the same as the negative ones, but you do want the two positive ones to be identical.

SO if you have a couple good ones still, put them both on one side, and put the replacements on the other. Ignore those date codes.

MJ15003 and MJ15003G are the same part. The G stands for Green, and all it means is that the part was made in compliance with the new environmental regulations for lead content - it has no lead in it. YOu may mix and match MJ15003 with or without the G.

Yes, looks like Allied is out of stock. Please, never buy the NTE versions of transistors, wherever you shop.

[howarda]

Thanks Bill and especially Enzo. I'll give you a full report once the repairs are made.

never buy the NTE versions of transistors

What's that about?