Peavey 8.5c dead

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tjsilva
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Peavey 8.5c dead

Post by tjsilva » Sun Jan 26, 2014 6:49 pm

I picked up a peavey8.5c from someone who said it didn't work, come to find out they had it all apart already. I have found the schematics parts list and have repaired the driver board. Multiple cooked resistors and transistors, all back together now. My question is on the output board is there an easy way to test the power transistors before un soldering them from the board? I have the board out of the chassis already. If someone knows a quick and easy way I would appreciate it. Also the large ceramic resistors are reading 2.2 ohms and up does this mean they are bad? others show no resistance should I remove them from the board to test? Thanks for any help you can provide.

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Enzo
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Re: Peavey 8.5c dead

Post by Enzo » Mon Jan 27, 2014 7:26 pm

The resistors are mostly 0.33 ohm, which looks like a short on your meter - zero ohms, but careful, some of them are 0.68 ohm. Your meter has a hard time that low, so touch your meter probes together to see what the reading says for that. If might say 1 ohm or something just for probe resistance. In any case now you can determine better what the resistance is. 2.2 ohms means bad unless your meter just can;t handle the low readings. Unsolder one end of each resistor and lift it from the board, not there is no confusion as to anything else fudging the reading.


You have two rows of eight power transistors. One row is NPN for the V+ rail, and the other row is PNP for the V- rail, the two channels are the left and right sides. They are not all outputs, the four on the ends are wired as drivers, however they all have their collectors wired together. I flip the board over to expose the solder side, I stick one ohm meter probe on a screw from the collectors up top, then probe the emitter legs down each row. The left and right halves of each row are not wired together, please note. ANy emitter to collector shorts you find need to be explored.

One trick is to look closely at the resistance. If one transistor is shorted, it can make the rest on its group look that way, so if you go down the row and read 1.2 ohm, 1.2, 0.3, 1.2, pull the 0.3 ohm first, test it, and with it gone, check to see if the others were not really shorted after all.

Note the bases of your outputs are only 0.68 ohms from ground normally.

tjsilva
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Re: Peavey 8.5c dead

Post by tjsilva » Tue Jan 28, 2014 6:41 am

Thanks Enzo I was hoping you would comment, you helped me repair a cs800 before and it is still going strong. One other quick question, on the driver board the power transistors Q100, Q104, Q200, Q204, when I ordered my parts the parts list I had from the service manual saved on my pc called out 15030 amd 15031. After I received the parts and my laptop was not accessible to me I looked up the parts list on my pc. That parts list calls out 15033, and 15034 I am assuming either will work and that the 15033 and 15044 where from a newer rev BOM. Do you think it is ok to use the old numbers or should I order the new?

Thanks

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Enzo
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Re: Peavey 8.5c dead

Post by Enzo » Tue Jan 28, 2014 9:55 am

MJE1503x series, your new numbers are just higher voltage versions of the original types.


The schematic worked with original parts. I would generally replace parts with whatever they were on the unit already. The higher voltage ones are probably statistically more reliable. Make sure to put NPN and PNP in the proper places.

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Re: Peavey 8.5c dead

Post by tjsilva » Tue Jan 28, 2014 11:33 am

Thanks Enzo, I will update when I get a chance to do some work on it.

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Re: Peavey 8.5c dead

Post by tjsilva » Fri Feb 19, 2016 9:29 pm

I know it's been awhile but i finally got back to the amp. I rebuilt the supply driver board, replaced four blown output transistors put it together and it powered up. I inputted a signal from my phone with a meter at the output posts no DC. It then started to hum and one of the large capacitors burst open. was wondering if i hooked up one of the transformer wires wrong there are four, two red ones, one solid yellow and one yellow with red stripe. they all hookup to the left of those caps I had them red to j210, red to j212, yellow to j110 and yellow red stripe to j112 thats what looked like from the picture the previous owner had taken. wondering if anyone can tell me if that is how there's is hooked up. Also where can I get those large 7500uf 100v caps. Thanks for any help.

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Enzo
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Re: Peavey 8.5c dead

Post by Enzo » Fri Feb 19, 2016 11:50 pm

Wrong wires is conceivable. Are there not six secondary wires?

You have two sets of wires, one per channel, disconnect them, and get out your ohm meter. determine which wires are on the same windings. Now power the transformer, still disconnected secondaries. Use AC volt meter to find that they have (I think) about 120v across the pairs. Or 60-0-60 across the ends and the centertap.

7500uf is an odd value, you will find 8200uf more readily at Mouser or Digikey. Then again you can order the caps right from Peavey. They will either have those exact ones or theywill send you some 8200uf themselves.

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Re: Peavey 8.5c dead

Post by tjsilva » Sat Feb 20, 2016 7:18 pm

Thanks Enzo I found the pairs 120v across the red pair and 120 across the yellow pair. So I am assuming the yellow with red stripe and red with yellow stripe are the center taps each have half the voltage to either matching color. Unfortunately I believe I had one of the 120v lines where the half voltage tap should have been hence the blown cap. Does that make sense to you? Thanks for all your help.

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Enzo
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Re: Peavey 8.5c dead

Post by Enzo » Sun Feb 21, 2016 12:38 am

Yes, stripes are generally center taps.


Usually, the center tap goes to ground, or in this case, the filter cap common and speaker, then the resulting pair of 60v half-windings go to either side of the rectifier bridges. If you mistakenly connect one of the end wires to ground/common, then one side of the rectifier will see 120v, and the other still 60v. That would put double voltage on the one cap. And that certainly could blow it up.

tjsilva
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Re: Peavey 8.5c dead

Post by tjsilva » Sun Feb 21, 2016 9:15 am

I ordered my cap from digi key, I will let you know how it turns out after its all back together again.
Thanks again Enzo

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Re: Peavey 8.5c dead

Post by tjsilva » Wed Feb 24, 2016 9:15 pm

OK so I put the cap in and powered it up, inputted a signal to channel A and got DC voltage at output. channel B no DC so i tried a speaker got a weak scratchy signal through speaker. After I powered it down i checked ohms at the outputs channel A had 28 ohms and channel B had 15 ohms. wondering what next steps should be.

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Re: Peavey 8.5c dead

Post by tjsilva » Fri Mar 04, 2016 12:04 pm

Ok, continuing on my check I found 8 blown ceramic resistors on the output board and two smaller resistors R264 and R150. I will continue to post and as I find more problems.

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Enzo
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Re: Peavey 8.5c dead

Post by Enzo » Fri Mar 04, 2016 11:11 pm

OK, now you have discovered the original problem they were looking for when they pulled the wires off and got then back on wrong.

You probably have blown output transistors, the resistors won't burn up otherwise. So check every associated resistor and the transistors as well. Whenever outputs blow, we always check the drivers and predrivers. Somewhere near the output wires is likely a SAC187 triac, I bet it is shorted. In fact measure resistance between red and black speaker posts. SHorted? If so, that is a bad triac.

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Re: Peavey 8.5c dead

Post by tjsilva » Sun Mar 06, 2016 12:18 pm

I found 4 blown pnp output transistors. Enzo I have the output board out of the chassis can I check the triac on the board and if so how. Is it just on left leg to right since middle is ground? The blown transistors are all on the - rail. Q114 q117 q217 and q214.

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Enzo
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Re: Peavey 8.5c dead

Post by Enzo » Mon Mar 07, 2016 6:49 am

Look at the schematic. It is a triac from output to ground. In trouble, it shorts the output to ground - this prevents speakers from catching fire...seriously. SO what you don't want to find is either leg shorted to the grounded tab. That is why I suggested just measuring between the black and red speaker posts.

besides, grounded or not, the thing has three terminals, so three combinations of two. A good part won't have any of them shorted together.

If a row of outputs are blown, I'd replace the driver driving them too. Also check all resistors.

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