Peavey XR600b blows fuses

[marty1]

I have a Peavey XR600B (400BH) Amp that I am trying to fix for my son. When I power on, the 8A fuse blows. What are some of the things I should test? - Thanks

[Dano]

First, disconnet both the high and low voltage plugs to the circuit board.
Use a variac to power up. At this point, we are testing the transformer.
If the transformer passes the test then turn off the amp

Next, plug in the low voltage and power up on a variac. If there is too much current, then there is a problem with the low voltage circuitry. Turn off the amp.

Next, unplug the low voltage and plug in the high voltage. Power up on a Variac. If there is too much current, then there is a problem with the high voltage circuitry. Turn off the amp.

Problems could be a shorted rectifier, but usually blown output transistor(s).
If any output transistors are shorted, replace them, and you will also have to replace the current limiting transistors and diodes.

[marty1]

How to I determine if the rectifier or the output transistors are shorted?

[Jazz P Bass]

Shorted component test

The rectifiers ( diodes) are in a bridged configuration.
A DVM on diode test will tell if they are shorted.
Key word. "Shorted". You don't want a short.
The output transistors are a little tricky.
Both sides, the transistors are in parallel & there is a ballast resistor in circuit.
The "best" way to test them is to unsolder them & then check as you would a diode.

[marty1]

Referring to Dano instruction:
I did not have a variac so I tried to measure the current draw with a DVM (with a 10A max load). I removed the fuse and connected the meter in series with the fuse holder. I removed the high voltage plug, inserted the low voltage plug, turned the amp on and took a reading. Did the same thing for the other side (removed the low voltage plug, inserted the high voltage plug and took a reading). In both cases the meter registered something initially and then went to 0a. I was afraid to take a reading with both high and low connectors plugged in. Any suggestions?

[Enzo]

Is your meter set to AC amps, not DC amps?

When the amp blows fuses, do not connect a speaker to it until we are certain it is not producing DC voltge on the output jacks.

An amp putting DC voltage on the output will usually blow a fuse when speakers are connected. Often the amp will not blow fuses when no speakers are connected.

Try your test again making sure the meter is set up for Ac current readings. If neither the low or high voltage wires alone make it draw excess current, then go ahead and connect both. If it no longer draws excess curent, then set the meter up for DC volts, and measure across the output jack. You will have to put the main fuse back in of course. If ther is like 40-50VDC on the output, we need to fix that.

[marty1]

Yes I am a goober. I had the meter set to DC. After switching to AC the current draw was 0.38a on one side and 0.36 on the other.
I do not have the speakers connected. I connected both high and low voltage lines and did not blow a fuse this time. I noticed some sparks coming from the SCR(?) 5332 (Q12 on the schematic). The middle lead was green. I will replace it and see what happens. Where can I get one (from Peavey)? Thanks

[Enzo]

5331 or 5332 is a transistor, and you can order one driect from Peavey parts, CALL them on the phone first thing in the AM.

If Q12 5332 is ify, then make sure to check R58 and R54, R55. In fact check everything in that whole vertical strip up through Q2.

[marty1]

I replaced Q12 5332. I also noticed that one of the posts for the High voltage plug was loose so I soldered it. I am still blowing fuses when I power on. I measured the current after connecting and disconnecting the low and high voltage plugs (as Dano suggested) again. I measured 0.388 A with only the low voltage connector attached and 0.365 A with only the high connector attached. Why do I draw more than 8 A when both are connected when individually they draw less than 0.4 A each? Should I test the power transistors next (remove them from the circuit and test for a short as Jazz suggests)?

[Enzo]

Because the amp does not function with only one source of power connected. What that test tells us is that the power supplies themselves are probably OK.

PLease look up "light bulb limiter" and make one.

Your problem is a blown output stage. Either you have shorted output devices and possible open resistors, but also possible open in the bias circuit.

[marty1]

Enzo thanks for your quick reply. I don’t understand what you mean when you say the amp does not function with only one source of power connected. How should it function?

[marty1]

I removed the power transistors (6357) from the circuit and found one of them to be bad (shorted). I measured (with a digital millimeter)the diodes CR11, CR16 and got .486v, .456v, Capacitors (5uf) C12, C45 and got 4.4uf and 4.7uf, Resistors (47 ohm) R27, R63 and got 46.6ohm and 46.8ohm. Aside from the power transistors should I replace any of the other components?

[Enzo]

Those other parts seem OK, but the shorted transistor has to be replaced. You can get a replacement from PV, though it will now have a different number on it. Since that amp made, the transistor number system has changed.

You have the two sets of transformer wires - the two sources of power - and the amp doesn;t blow fuses when you connect just one, but it does when both are connected. The amp needs both power sources to even function, so that explains why neither power source by itself blows fuses.

[marty1]

Wanted to say thanks for all the help. The amp is up and running. Ended up replacing one power transistor and one smaller transistor.

[RDY2RMBL]

Hello, since the original problem in this thread seems to have been resolved, does anyone know the exact part number or dimensions of the fuse holder cap for the Peavey XR-600b? I bought the amp without it because the people who sold it to me lost it while they were trying to replace a fuse to show me it worked. I ordered one from Uncle Ikes Music and it doesn't fit, and the one from my Peavey XR-400b does not fit either. Or would it be easier to simply just install a new fuse holder all together? and if so, which would you recommend? Cheers!