Peavey Mark III Bass 400 B/G? PA Problem - diyAudio
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Old 27th May 2009, 02:37 AM   #1
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Join Date: Jan 2009
Default Peavey Mark III Bass 400 B/G? PA Problem

I recently acquired this bass amplifier with a blown fuse. I removed the rear assembly and transformer from the cabinet and started to troubleshoot the circuits. The numbers 99009015 & 082878 are on the module board. First, I checked the power transformer and it was OK. Subsequent checks of the rectifier diodes on the 52v and 15v circuit showed them to be OK. I removed both C34 & C35 and tested them on my capacitor tester, both OK. Looking at the assembly from the back so as to see the transistor covers, I found two shorted SJ5367 output transistors (OTs) in the left bank, which I replaced with exact replacements taken from an old CS-400 amp. I tested them with my DMM on diode check and both biosed ok. I also checked CR11 and CR16 (MR752) and they both responded like normal diodes. A quick check of Q2 (5331) and Q12(5332) in circuit showed them biosed on normally. This board does not have an L1 (100uH) coil but has a 4.7 ohm resistor in parallel with R44 (4.7 ohms). I wanted to do a quick in-circuit power on check of Q2 & Q12 so I unplugged both OT boards. When I powered on the amp both 47 ohms R26 and R61 smoked. I replaced both resistors and didn't try that again. :-( Excessive current took out the resistors which indicated to me that both Q2 and Q12 were biosed on when I applied power. Is this normal? Another In-circuit test showed Q2 & Q12 to bios normally. I reconnected the OT board and applied power with no output load (speaker). The amplifier voltages held for about 2 seconds and blew another fuse. A check this time of the OTs showed another SJ6357 shorted but in the other bank of OTs. All the OTs I had put back in checked OK, so for whatever reason, this one blew. I'm tempted to put another transistor in and give it a try thinking that this transistor may have been marginal to begin with but I'm hesitant as that is not the way I usually troubleshoot. I wouldn't think an OT would just short without a load and it didn't check with any less biosed ohms reading (close to 500 ohms) compared to the others when tested.

For info, his is NOT a B/H model. On the solder side of the board it has B/G Module but now I'm not sure what this board is to be honest. At first glance, it has some components similar to the B/H such as the 47 ohm FP resistors off of the Q2 emitter and the 47 ohm off of the Q12 collector, the two parallel 4.7 ohm resistors off of the Q12 emitter instead of the B/G 4.7 and a 100 uH coil. My board has 78M15C as VR1 & 79M15C as VR2 instead of the B/G 340 & 320 shown on my B/G schematic. The board does not have the two fuses and the layout is different, especially in the Power Supply area.

Has anyone got some ideas about what I'm dealing with here and what to do next.
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Old 27th May 2009, 03:44 AM   #2
llwhtt is online now llwhtt  United States
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Join Date: Jun 2008
Location: SoCal
With the 78/79 series regulators it would seem that it is a BH model but you say it's not. Before pulling out any more of your hair call Peavey Tech dept. and ask for Gene. He's been there forever and should be able to tell you which schematic to use and will probably email it to you if you don't already have it. Also you can get whatever parts you need from them.

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Old 27th May 2009, 03:50 AM   #3
llwhtt is online now llwhtt  United States
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Join Date: Jun 2008
Location: SoCal
Forgot to tell you to change the 5500UF filter capacitors if they are still the old twist-lock style. That type isn't available anymore and you'll end up with some 4700UF snap-ins if you order them from Peavey. I realize that is NOT what's blowing your transistors but they need to be changed no matter what they measure.

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Old 27th May 2009, 05:55 AM   #4
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
I am discussing this with you over at Peavey and won't repeat myself here, other than to say the LM340 and LM320 are the same thing as 7815 and 7915. You have the BG not the BH. REally, you do.

If your 5000uf caps are working, even poorly, the amp will settle and at worst be hummy. I'd recommend sorting out what is wrong before updating everything. The less work you do to it while troubleshooting, the less likely you will mask some symptom or bad part.

4700uf/63v snap ins is the replacement. Usually you will have to drill an extra hole for the leads to fit into the circuit board.
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