Peavey CS800C blows fuse only on start ups.

[Jack W.]

Another tech passed this repair on to me.
CH A blows the fuse but only when switched on.
Explanation: CH A has been repaired before, the tech couldn't get it to stop killing fuses.
Every time it blows a power component is blown, sometimes an emitter resistor & xsistor others just the xsistor.
CH B works fine.
* I tested all xsistors driver & power for leakage.
* Q10 was shorted. There was a mix of different brands of PWR xsistors so I Replaced all power xsistors w/ MJ15024 ON semis & all new silicone pads.
* Driver board all test good & cold spec measures match the CH B driver board. New IC's.
* All electrolytics on CH A driver board have been replaced by previous tech & test OK.
* The triac crowbar ckt components have been replaced correctly.
* ALL solder joints were very thoroughly gone over & re-sweated on both power & driver PCB.
* All molex pins clean & Deoxit & retension - nice & tight.
* All wiring rings out good.
* It has the original AC filter caps they read ok uf's and show ok ESR.

Here's what testing I did.
* Disconnected the rails supply off both power boards & check the rail power up voltages
- results - +/- 1 VDC from each other reaching 80 VDC no load.
* Removed the Traic protection on output & reconnect only the CH A boards.
* Power up using no load but a scope on output.

Now if I use a variac & slowly bring up voltage to 120VAC I measure .56 Vdc across R7.
The rails +/- 77 Vdc.
I then set variac @ 60 VAC & use the power switch on the front panel to turn it on. No faults.
I did this every 10 volts until I reach 120VAC. No fuse blew.
I could put a 1Khz thru no problem clean waves. Up to 154Vpk.on scope.
AC current to amp reads around 760 Ma 120 VAC
Put on a 4 ohm load, did the same power up as above. No fuse blew.
Put out full rated power @ 1 Khz & the bias returned to a stable .56Vdc across R7.

NOW with the Triac protection put back in everything works as long as I power it up slowly on the variac.
BUT if I set the variac 60 VAC or above - load or no load - & use the switch or the switch on the variac - the fuse blows & kills a power xsistor.
I also tried switching the triac cards from CH B. Same results..boom!
I put shrink wrap on those fastons at the output to maybe insulate it from the triac PCB.
So the board makes contact at the posts only..(?)

Put a new Xsistor in take the triac out amp works..!? The triacs test ok, new caps...both PCBs (CHA/CHB) cold read the same..

Seems to be with the input surge current or the rails under load aren't true, tripping the triacs (?) but I'm not sure of procedures for live testing those conditions ?

I know its hard with out actually experiencing the conditions, but please any ideas before it's time to give it back will help.

[The Dude]

The triac will turn on if the amp is putting out DC voltage. When it turns on, the output is shorted and there go/goes your output transistor(s). I would leave the triac board out of the unit until you are sure the amp is fixed and not putting DC at the speaker jacks. Next, fire up the amp and measure the DC voltage at the speaker terminals. Let us know what you find. You'll need the triac removed for a meaningful reading.

[Enzo]

Are you using a bulb limiter AND a variac? Don't.

SO you have a good channel and a bad channel? UNplug the bad channel from the heat sink - four pin molex on the heat sink board. Does the good channel come up good and consistently? No load, just flip power on or off. Now load the good channel. Does it come up good every time. And does it pass signal well?

If that works, pull the driver card off the bad channel and install it in the good channel. Bring it up on the variac at first. Does that driver card work flawlessly or does it have issues?

Unplug good channel and reconnect bad channel four-pin. Install good driver card.

We ALWAYS check all the resistors associated with bad transistors. SO we check all the 10 watt resistors on the power amp board, but also there are the small 100 ohm ones and 4.7 ohm and 47 ohm. Check them.

Work with NO LOAD until the amp is stable. And agree with Dude, disconnect the triac until we know it won't be triggered.

On the bad channel, see the large inductor at the output? Brown paper in it. L1 with parallel R19. Measure R19 at its leads. YOu should get zero ohms. If you get 5 ohms, then the coil is open. Fix it.

[Jack W.]

Thanks for the quick response.
I have worked on several of these amps before with great success. I haven't had one that acts like this.
All of the suggestions have been done & redone & will check ALL components again today.
Also I have cleaned & re-tensioned the xsistor sockets on the power board.
I will try swapping the driver boards one more time though.

I can power the amp up with the switch under these conditions.

* Both channels installed, NO triac boards installed NO LOAD
* 120VAC NO Variac, NO bulb limiter - Iac at plug measures 760mA
I'm not using a bulb limiter on this one, I need accurate readings at this point.
* DSO (scope) on output reads 0 mV DC at 20V and or 10V range.
I'll fire it up again later today and & remeasure using a Fluke 87 on the outputs for tighter accuracy.
* With unit on no triacs, no load I get a clean signal thru both channels reading 154 Vpk.
* With a 4 ohm load both channels produce full power signals.
Ran it on the bench into the load 1Khz for a half hour with no problem. BOTH channels!

* I have disconnected the bad CHA
* connected CH B WITH it's traic board installed. with or with out load.
* amp will power up without blowing fuse (using the switch & at 120VAC no variac) REPEATEDLY

* Removed CH B installed only CH A no TRIAC PCB.
* amp powers up same conditions as above.
* 0mV DC showing on scope @ 20v range

This is what I can't find - if I install either triac PCB on the output of CH A & use the switch the amp blows the fuse.
IF I slowly bring it up on the variac it wont blow & the scope shows no DC on the output.
Seems something throws the rails off putting DC on the triac firing it...?? Something can't handle the in rush current...? I'll look closer at L1 &R19. R19 has been replaced but maybe blew out again..
I'll post my findings.

[Enzo]

Did you replace the triacs? And if so with the same type? You want triacs that fire on all four quadrants.

Reread what I said about R19. I am concerned the COIL is open, not the resistor. Though we check the resistor too.

[Jack W.]

The Triacs are not original, seems the other tech replaced them with NTE65004. the spec sheet mentions triggering on four quadrants. I have switched the triac boards between channels & still have the same fuse blowing problem on CH A. The only triacs I have in stock at this time are these BTB24-600B from ST Micro if they may be a better fit.? The resistance test of the triacs seem ok, no shorts showing & trigger between gate & MT2 produce low ohms. The SBS14 seem to test ok using a DAC55 component analyzer & a resistance test. (the DAC won't work on power triacs).
I have a parts order going out soon to digikey...I can add a better replacement as needed.

Rechecked L1 & R19. Across L1 (R19 removed) reads .2 ohms on the Fluke.
R19 reads 5.9 ohms.

[Axtman]

I hate to ask, but....... are you are using fast blow fuses instead of slow blow fuses?

[Jack W.]

I hate to ask, but....... are you are using fast blow fuses instead of slow blow fuses?

Actually that is a good question. The amp did come in with a blown 15A normal blow.
I've ruled out fuses because:
* tried some from different boxes & makes (I am using normal blow during testing in an effort to limit damages)
* why the amp will power up with both channels in but NO Triac boards.
* when a fuse does blow there is a shorted power xsistor to be found in CH A. (it has been either Q10 or Q12 so far)

I'll have this back together with more detailed readings on Saturday. I want to do a little A/B comparisons again from the working CH B. See if I can find a difference. Will post results.

[Jack W.]

Well, I went back over the amp over this weekend.
Did some cold ohm checking on the power & driver & triac boards & found no significant difference between A & B channels.
Output posts on Ch A read 3.79K ohm (no triac PCBs) & CH B read 4.79K ohm
Powered up both boards under the following conditions to compare some voltage readings.

120 VAC (brought up slowly) fuse @ 6A normal blow.
NO LOAD /NO TRIAC BOARDS.
VDC across R7
CH A = .563 V CH B = .540 V
VDC across Output posts w Fluke 87, VDC / & with a 1Khz signal amp input jack Vol controls full up.
CH A = .001 Vdc / .165 Vdc CH B = .002 Vdc / .175 Vdc
Powered off. I'm just not seeing any DC on the outputs to track down once the amp is powered up.

Used power switch @ 120VAC to see if it would blow. It stayed on no problem.
I then put a 1 Khz signal thru both channels : see pic 1 below
Powered off & connected a 4 ohm load.
Switched back on, no problem. Put 1 khz signal in Vol. full both channels : See pic 2.
During these tests the voltmeter on the outputs ( once amp powered up) read .001/.002 Vdc
Powered off & disconnected Ch B.
Installed triac PCB into CH A. with NO LOAD.
Powered up slowly on variac to 120VAC. DC on output read .000 Vdc, across R7 read .561 Vdc
with 1 Khz signal in (still no load) output DC read .177 Vdc. See pic 3.
Powered off. Set variac 120VAC used front panel switch to power up = Fuse blew!
Found Q12 shorted. Replaced w/another MJ15024 from same box as others.

Swapped the CH B driver board on to CH A. Powered up slowly with triac PCB & no load.
Powered up no problem. Set variac to 120VAC used panel switch. Fuse blew, haven't found a short or blown resistor on power board.
I'm truly stumped. Had to walk away from testing. I'm gonna try swapping channels into the wiring harnesses tomorrow.
Any ideas will help guys.