Peavey XRD 680 PA (dual power amp, one side bad) diagnostics

[misterc57]

I picked up a Peavey XRD 680. It is a dual power amp unit, with two speaker outputs per amp (total of four speaker outputs).

Power Amp 1 (PA1) speaker outputs work fine. Power Amp 2 (PA2) has no output. There are two 10 amp internal fuses inline to each Power Amp (PA). PA2 fuses are blown.

I replaced the fuses and hooked the amp up to a light bulb current limiter. The bulb lit and stayed very bright so I did not attempt to run the amp that way. When I disconnect PA2 fuses the light bulb is very dim.

I have no experience diagnosing transistor amp faults so I am hoping for some step by step guidance. I have the working PA1 side I can use as reference points.

At this time I have no schematic for this amp but will check with Peavey on Monday.

Thank you so much!

[Jazz P Bass]

Here is the power module schematic.
With luck, maybe it is simply the power transistors that are shorted.

Remove both of the connectors to the power amps. Go across the terminal header, P11 & measure the restance from pin 1 to pin 2 & from pin 7 to pin 8.
Do the same for the other amp, P21.

A very low reading will indicate a short.

I am not aware of Peavey putting fuses on each power amp.
There are two fuses on the power supply but they are for the AC side of the + & - power rails.

[misterc57]

Thanks for the info, will get back with those readings.

The 2 blown fuses were from the power supply feeding PA2. See attached image for fuse location.

Thank you.

[misterc57]

Right Schematic?

I am not sure the attached schematic is the right amp. More specifically my amp is a "XRD 680 PLUS". The output board is called "304S OUTPUT 8".

I do not see the 8 pin connectors (P11 and P21). Instead this board is connected by two sets of 3 wires coming from that fused section in the prev image. Each set has two red and one yellow wire.

Here is a poor quality pic of the board ( I will get a better pic tonight ).

Thank you.

[Jazz P Bass]

Alright, I contacted Peavey.
Here is the 304S power module schematic.

[52 Bill]

Instead this board is connected by two sets of 3 wires coming from that fused section in the prev image. Each set has two red and one yellow wire.

It looks like the fuses are in the ac wires that come from the transformer secondary and lead to each channel power supply, so you could have a shorted power supply diode or a shorted output transistor.

Follow the pc traces from where the two red leads connect on the power amp end and they will lead you to the power supply rectifier diodes. There will be four of them. Test them with the diode test function of your multimeter. If they all test okay, then test the output transistors for shorts.

[misterc57]

Awesome and thanks to all. Will get back after some analysis.

[misterc57]

Is there any way to test the diodes and output transistors in circuit, or do they all need to be removed? Thank you.

[Jazz P Bass]

Does your meter have the 'diode check' function?

That is how I test diodes.

A transistor is basically two diodes (in form, not in function).

Testing the power supply diodes on the board is easy.

The transistors may have external components that will 'confuse' the readings.
The schematic must be consulted to verify that.

[misterc57]

My friend and I looked it over, at first I suspected the bridge recetifier diodes because I was getting .5 volts in both directions (in circuit) with the DMM set for diode check on the bad side.

We then removed and checked all 6 output transistors and discovered two shorted, one 70473180 and one 70483180. Will see if Peavey has replacements and will update after the new ones are installed.

Thank you everyone!

[Jazz P Bass]

Make sure the 0.33ohm/10 watt ballast resistors are still good.

[52 Bill]

Most components can be tested in circuit, but sometimes there are other elements that are paralleled with with them causing false readings. In these cases, the part will need to be removed to get an accurate reading.

If you look at the service manual that Jazz posted, page one is a board layout showing where all of the parts for the amp are located. The lower drawing is of the power supply and output transistor board. This is the one that you will need to look at first. The photo that you posted shows this board on the top with the shiny solder side facing up.

If you look at the drawing and then the board, you should be able use the drawing as a road map to identify the real items on the board. The board contains the power supply and output transistors for both of the channels. The two sides are more or less mirror imaged from the center out. Because the drawing is shown from the component side, you will have to mentally flop the image or use your computer to flop the image for you. Some people find it helpful to print out the drawing like a real map.

The power supply diodes are located at either end of the board. They are mounted on end, in between the two large filter capacitors. If you look at the drawing, there are schematic diode symbols printed to show the polarity or how the diodes should be mounted in the board.

To test the diodes, turn off and unplug the amp. Let it sit for a few minutes to allow the filter caps to discharge. Set your meter to the diode test position or a low ohm range. Place one meter lead on each end of the diode and see what reading you get. Now reverse the two leads and see what reading you get. A good diode will conduct in one direction only, so with the leads connected in one way you should get a low reading and with the leads reversed you should get a high reading.

Because the diodes are connected to the filter caps, the low reading will be fairly immediate, let's say 0.7 on the meter. When you reverse the leads, if there is any charge on the filter cap that the diode is connected to, the meter may take a while to react. Hold the leads on the diode for a few seconds and see if the reading starts to rise.

If the diode reads low in both directions then it may be shorted and then you will need to remove it from the board for a final test. Or at least unsolder one lead from the board to isolate the diode from the circuit. Test all 4 diodes on the side that is blowing fuses.

Edit: I see you already figured it out, so never mind. I will leave the post here for future readers that may need the assistance.

[misterc57]

In regards to purchasing transistors. Any better sources?

Peavey wants $10.29 per transistor, makes me wonder if I should look for less expensive sources as there are many priced far less (online searches for MJ15024 and MJ15025).

I also wonder if it is ok to just replace the two or is there any need to replace all six of them for matching purposes.

[Jazz P Bass]

The 10.29 cost may be because Peavey tested them to insure that they will function properly.
Then again, maybe not.

In a perfect world, the other transistors may have taken a 'hit'.
(Under a microscope, it may be apparent. Which is a destructive test.)
If in doubt, remove them & diode check them.
There is not really any 'on the bench' test that will prove that they will work. (Short of testing them in circuit)
Most times the remaining good transistors will be fine.

There is no need to match them.
That is what the ballast resistors do. (kind of)

[J M Fahey]

Just personal opinion: if replacing just two, I'd buy the Peavey laballed ones; if replacing all, would get 15024/25 , but:
a) bought "äll together from the same supplier" to have a reasonable chance of getting all same batch
b) ONLY from a respected supplier such as Mouser or Digikey .
Forget EBay even if they cost 50 cents and they send Miss Alabama in a tight bikini to deliver them.