Peavey cs-400 Blowing Fuse

[barnacle]

I just purchased a Peavey cs-400 Amp and when demoed it worked great. Then I brought it home, plugged it in , hooked up the speakers I purchased with it and then turned on the power switch----Sound was terrible for about 1 minute and then the 8 amp fuse blew. I replaced fuse and it blew again in 1 second. I removed all inputs and outputs and it instantly blows its fuse again. Am I missing something here? Advice would be appreciated.--Thanks

[olddawg]

I just purchased a Peavey cs-400 Amp and when demoed it worked great. Then I brought it home, plugged it in , hooked up the speakers I purchased with it and then turned on the power switch----Sound was terrible for about 1 minute and then the 8 amp fuse blew. I replaced fuse and it blew again in 1 second. I removed all inputs and outputs and it instantly blows its fuse again. Am I missing something here? Advice would be appreciated.--Thanks

You are drawing some serious current to blow an 8 amp fuse at idle. It sounds like you may have hooked the unit up to a dead shorted load (bad cable, shorted jack, speaker, etc) and blown one or more output transistors. Here is a link to a schematic but be aware that there are several versions of this amp:
http://fileshare.eshop.bg/download.php?fileid=33343

[Enzo]

Get your serial number off the bottom, and any extra stuff on the model name. For example CS400 is not the same amp as CS400 '89. COntact Peavey customer service with that info and ask for the schematic set.

I'd bet my lunch money one of your channels is blown. This puts a dead short on the power supply inside and will blow fuses right now.

I have not been in one in a while, but the power amp circuits are on a large heat sink assembly, it should unscrew from the chassis walls, then you can flip it over next to the chassis on your bench. If I recall there are two four-pin Molex connectors to it - one per channel. This is separate from the several Molex connectors on the small circuit boards screwed to the larger one. The two smaller boards are the driver circuits and have a row of pins all across one end where they conect to the larger board.

These two connectors are the power for the channel. Unplug them both and fire up the amp. Does the fuse hold now? I bet not. If fuses still blow you almost certainly have a shorted bridge rectifier or a piece of hardware rolling around stuck up under a circuit board. But if fuses now hold, power down. Go to the rear and measure resistance between the red and black output terminal posts. DO both sides. If one side measures shorted or low resistance, there is part of the problem. ANd the channel associated with that side is the blown one.

Leave power off. REconect the two four-pin Molex connectors. Set the amp heatsink assembly in place on the chassis. Measure resistance from the RED speaker terminal post on the rear to the top of each power transistor mounted on the heat sink. If any transistor measures low resistance to the red terminal, it is likely shorted and thus bad.

Run that test to each red post.

If you are lucky, it will be just a couple shorted power transistors.

[olddawg]

Enzo, don't these amps have a speaker protection relays? If that is so you will not be able to measure from the positive speaker terminal back to the collectors on the output transistors. It will be an open connection even if the transistor is internally shorted because the relay is open.

[Enzo]

THe CS400s I know are from the 80s and had no relays. They had the crowbar triacs across the output to protect speakers. If this is a newer version and has relays then of course that test doesn;t work.

[olddawg]

If you do have one or more shorted output transistors you should replace them in pairs and use as close to the originals as you can find. Do not use a generic TCG substitution. Also be aware that you should check the driver transistors and associated components as well. This is assuming that the bridge rectifier mentioned earlier isn't shorted of course. But I seriously doubt that is the problem. After the amp is fixed, meter out anything associated with the load...speaker cables, speaker cab, etc and check for shorts.

[Enzo]

And if the output terminal posts appear to be shorted together, replace the SAC187 triac.

Actually you don't need to worry so much about the transistors opposite, you need to have all the transistors on one side all the same in the row. The driver on the end can be different, but the rest should all be the same. Those are all in parallel, so if one comes on early it will hog current and try to do the whole job itself. And if it likes to turn on late, then it never comes on and might as well not be there. SO if all the outputs in a row are the same type - usually Motorola/On on a PV - then things are OK.

Yes, you want the opposite side same as themselves and similar in type to the first side, but to me it is far more important they share chores on THEIR side than it is to match the other side.

[olddawg]

As with most things I'm sure that there is more than one opinion on this. I was always taught to pay attention to semetry and that if one output transistor in a a push/pull circuit was damaged, its associated pair was probably stressed and should be replaced. I usually did it to avoid call backs. That's not to say that I have never replaced a single output transistor out of 8 with an obscure cross and had the amp last for years for whatever reason. A lot also depends on how hard the amp is being used, divine intervention, or whatever.

[Enzo]

Yes, usualy one on each side shorts, and at that point there is no longer any stress on the parts. But unless it blows, it is very hard to tell which transistor across the way took the stress. The one direct across may not be it, it could be over two. Or wherever.

Keeping the sides balanced across is good for fidelity, while making all of one side be the same is for reliability. By side I mean polarity, not amp channel.

[barnacle]

1 bad part

I removed the main Molex from each power board and then turned on Amp...No blown fuse. I plugged in 1 molex to power board A and still good. I plugged in Molex to B and the fuse blew. I removed power transistors and found one was shorted. New transistors on the way from Mouser. Any other part I should check before powering up to ensure that I do not blow another transistor?

I measured the speaker terminals from red to black and one channel measures open while the other channel measures 140 ohms. ?? All power boards disconnected.

where is the SAC187 triac? Do not have schematic yet.

Thanks for all the help
Bill

[olddawg]

It is very bad trouble shooting proceedure to test by blowing fuses. Everytime you blow a fuse with shorted output transistors you may be damaging other related components. You should check the outputs and drivers before powering up the unit. Most of us use a variac or at the worst case a light bulb current limiter and a suicide cord to run up a repaired unit. You should check your driver transistors for the shorted channel and related components. However, you may have been lucky and just blown output transistors though.

[Enzo]

The SAC187 triac is right next to the speaker terminals on the circuit board they are on. The tiny little transistor looking thing next to it is almost certain to be OK. The SAC187 is a TO220.

You are probably lucky. In my experience with those old PVs, usually there is a blown output or two and that's it.

[barnacle]

triac ?

The SAC187 triac is right next to the speaker terminals on the circuit board they are on. The tiny little transistor looking thing next to it is almost certain to be OK. The SAC187 is a TO220.

You are probably lucky. In my experience with those old PVs, usually there is a blown output or two and that's it.

I measured the Triacs and was surprised that the center lead is not used.

When the measurement is taken between the outer 2 pins one triac reads 190 ohms and the other reads 50 ohms. I get the same reading with the multimeter in the diode position and reads the same even when polarities are switched. Are these readings normal? The SAC187 triac cannot be found at Mouser, Digikey, Allied, in case I need to replace.
Thanks
Bill

[bnwitt]

Peavey will email you a schematic. You can also get the parts from them. Or, Antique electronic supply has alot of peavey parts too:

http://www.tubesandmore.com

search sac187

attached is a triac testing paper.

[Enzo]

In the TO220 package, the center leg and the mounting tab are one. They don;t use the center leg, but they do use the mounting tab. Electrically that is the same. Replace the one for the bad channel, the good channel one ought to be working.

SAC187 is equivalent to 2N6346A. it is a 200v 12A triac. It fires in all four quadrants. Not all triacs do that. SO the sub is not at all critical, but you would want one that fires in all four corners.

PV will sell you SAC187s.