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Old 27th January 2009, 03:37 PM   #1
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Default Peavey 400 blowing fuse on power up

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
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Old 27th January 2009, 04:41 PM   #2
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Default Re: Peavey 400 blowing fuse on power up

Quote:
Originally posted by 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

That amp has a crowbar board mounted on the speaker out jacks. It protects the speaker in the event of a failure, but almost always leaves the triac on the board shorted. This can damage outputs if not replaced before power up. This circuit also will not come up slow on a variac because the DC at slow power on cause the crowbar to trip and blow the fuse even in a working unit.

Also, while you are in there, check the bridge rectifier for short.
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Old 27th January 2009, 05:36 PM   #3
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Hi Barnacle

The likely cause is failure of one or more power transistors. When this happens, the crowbar circuit will be activated to protect the speakers. It does this by diverting energy to ground, thus "blowing" the fuse.

As mentioned by Steve, the triac in the crowbar is often damaged in the process. It is customary to replace the triac as well as the power transistors.

Mike
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Old 27th January 2009, 09:06 PM   #4
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Default peavey cs400 blowing main fuse

Keep replacing the main fuse will not fix this problem only keep wasting fuse's.

Remove outer casing and start checking the circuit out, start with the power supply via removing conections from it to the main pcb's.

Because you want to 'see' if the psu will power up with no loads. then remove the o/p stages and check them cold with no power applied.
Sometimes if the crow bar triac has gone it can take the o/p transistors with it or you could be lucky there. you dont have to have this device in there when repairing these amp's just hook the amp to a power mains trip breaker and save the fuse pop. I've serviced many of these old amps from cs400 to cs1200 amp's.

They all suffer from muck build up due to fan pulling in dust and crud in to the 'tunnel' were the to-3 transistors oh and peavey like to use there own brand of o/ps device's so replacements devices's I use are mje15024 and mje15025's alot better than there 180v 16amp one's.(npn/pnp).
oh here the bad new's if the ddt chip goes the amp is spare part's has this ic is no longer made:
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Old 27th January 2009, 09:06 PM   #5
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Default peavey cs400 blowing main fuse

If I rember on this amp the dc supply rail's is +/- 56 volts and you can use a mains 6ow light builb to discharge the filter cap's or a high watt resistor of 100 ohms via a fly lead and a rubber handle screw driver. you get the idear very easy to make discharge tool.
IF you get the unit to say on with out popping the fuse, don't connect a speaker just yet! check for large dc at the o/p because high dc will chrips your speaker's. use a speaker resistor load of '8' ohm or 10 ohm and if this load get's hot quick unplug and recheck your amp.

Do you have a scope/sine tone generater signal to check o/p wave.
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Old 27th January 2009, 10:51 PM   #6
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In these circumstances you need to inspect the cct logically.

Start with the power supply on its own.

I would then check the transistors for shorts.

I would then just power up the driver board and link the o/p back into the LTP with no output transistors connected.

Set the bias as low as possible before continuing.

Connect the output transistors again and fire it up.
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Old 28th January 2009, 12:59 AM   #7
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Barnacle,

I can email a schematic to you if you need it, let me know email and what vintage of CS400 it is. Peaveys are usually pretty dependable if nothing else. Older ones are much easier to deal with IMO. Most have Motorola/ON semis available anywhere.

Craig
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Old 1st February 2009, 06:20 AM   #8
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Default Ripped it apart

Thanks all for the advice. I disconnected everything form both power boards and unit stays on. Connected one power board and still good. Connected the 2nd power board and good by 8amp fuse. I will now check the output Motorola transistors on that board and replace if necessary and also check the Triac. Non of the transistors measure shorted so the Triac must be the source of the direct ground.

Lots of new things to fix here. I just got 2 Phase Linear 400s with all 8 meter lights gone in both of them. I think I will replace with blue leds. Does anyone know the voltage of the lamps?

Dynaco 120 Tube amp on the way to power my Dahlquists for that easy listening. Hope it works. Don't need more repair projects right now.

Thanks again for heading me in the right direction.
Barnacle
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Old 1st February 2009, 06:20 AM   #9
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Default Peavey blown

Sorry
Forgot to answer a couple questions. Yes, I have a scope and yes I would appreciate the schematics but I do not know which Peavey it is.

Barnacle
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Old 1st February 2009, 03:55 PM   #10
llwhtt is offline llwhtt  United States
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Barnacle,

Of the five different CS400s only the "C" model had the output triac, so that pretty much tell us that you have the "C" version of the original 400. If that's all you find wrong the amp will run without it if only for testing purposes. The part # is SAC187 or 2N6346A, 200V 12A Triac.

Craig
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