Peavey CS400 power board, wrong resistor value?

[aminalaudio]

Hey everyone,

Figured I may as well join in, since I've gotten a lot of help from the minds here via internet searches. Thank you all, and hello from west MI.

I have a CS400 that has a blown resistor, R16 on the Channel B power board. It is split in half, and measures shorted. The schematic says it should be a 50 ohm, but it is a 5.6 ohm on the board, like the rest of the 5 watters. I checked channel A, and it is a 5.6 ohm as well. Is this a thing that happened? The board shows no sign of rework, so it was built this way I assume. Should I put a 5.6 or 50 ohm in? Should I replace the other channels R16?

I should add that Channel A seems to work ok. Initially, Channel B would stop outputting sound, and it's DDT LED would light up. I thought the DDT circuit was going bad. After toasting a speaker, I discovered it had the rail on the speaker terminals when the DDT was on. So I get to testing everything, and turns out someone stuck a 20A fuse in the holder... That's when I started digging deeper and found the bad part.

Any advice here would be great!

[Enzo]

You should replace it with what is in it. probably intended to be 5 ohms all along. The CS800 uses 5 ohms there.

It doesn't measure shorted, it is in parallel with a coil of wire, which has essentially zero resistance, thus it measures that zero resistance. I beet if you remove it from circuit it won't be shorted any longer.

However, for that resistor to be damaged, usually it needs that coil to be broken. So tug on the coil to make sure it hasn;t broken free.

Has it really split in half, or is the cement just cracked? I mean any sign of overheating?

That resistor should not be able to create DC on the output.

[aminalaudio]

Ok. I figured I should use what's there, but needed to make sure it wasn't some fluke error by Peavey years ago and it just happened to survive for a while.

And yes, the coil would make it show shorted. I typed before I thought. Thanks for the reminder!

The coil is in there pretty good. It doesn't wiggle or anything. The connections from the coil to various spots on the board are good, and don't change when I wiggle the coil.

Here's what it looks like:



It could be just the cement. I haven't taken it out yet. But it looks like the wire inside the resistor is a bit burned/broken. And it does look a bit black under the resistor.

Thanks for your help Enzo!

[Enzo]

Oh, I'd have to say that blew out for some reason, certainly DC to the speakers through ti wouldn't help anything.

[g1]

Are you positive you have the correct version of the schematic for your particular unit?
Maybe the 50ohm resistor is in a different revision.

[Enzo]

I vote it doesn't matter. If they made that change, it would not have required changes elsewhere.

The wind tunnel version used 22 ohms. The coil is doing the work, the resistor lowers its Q.

[g1]

Ok, your familiarity with these has me at a disadvantage.
I have no schematic to look at, so I have no idea what that resistor is doing. My point was only that there are probably more than 1 iteration of schematic for this model, and it can't hurt to check you have the right one.

[Enzo]

Let me see if this one will post:




The resistor in question is parallel the inductor in the output line. Sorry.

[Jazz P Bass]

If channel A has a 5.6 at R16 AND R17, it may be that a previous repairer got it wrong.

The zobel network resistor R17) IS a 5.6/5w resistor.

I would go with the schematic.
R16: 50 ohm
R17: 5.6 ohm

PEAVEY_CS400.zip

[Enzo]

I am sticking with the 50 ohms as a typo on the schematic. Unfortunately, that is the one resistor with no value next to it on the layout.

But again the A, B, and C versions of the CS800 all have both zobel AND the series resistor as 5.6 ohm 5w.
And I have never seen any model with a larger than 22 ohm resistor there.

[Jazz P Bass]

Interesting that the different CS units with the 3mH inductor all use a 5.6/5W R.

When they use a 0.8mH inductor, the resistor value rises to 22/1W R.

It would help if we knew the manufacture date of the amp.