Peavey XR 696F - I made a booboo

[jondoe]

After a decent run of small repairs I came upon a Peavey XR 696F with intermittent signal problems, so I bought it as a cheap project to play with. On inspection someone had fudged in a non-oem power switch and fitted a different fuse holder, along with a 12v cooling fan with a bunch of resistors to replace the original 24v. I removed the power amp board/panel and inspected the work, it's rough but functional.

On reconnecting the transformer to the poweramp board I made a mistake, while referring to the pictures I'd taken of the wiring I didn't realise the phone had rotated the image 180 degrees, so I ended up plugging the L/N mains wires into the earth tabs and the earth into the L/N tabs. The magic smoke followed shortly after powering it back on, followed even closer by swearing and self flagellation.

I can only see one small burnt out resistor on the power amp board (R273), but I don't suppose I've been that lucky. I've tested the transformer on it's own, it's not suffered any damage, but I don't no where to start with the power board/preamp board. I guess I should concentrate on the power board for now and worry about the preamp board later?

Any advice for this idiot is welcome, I'm feeling pretty special today!

Pvy XR696F.zip

[glebert]

I've done worse. I would focus on voltage rails from the power supply, especially if it is a separate board. Unplug whatever you can, and if you have a variac and/or light bulb limiter use those to bring it up.

[jondoe]

I've done worse. I would focus on voltage rails from the power supply, especially if it is a separate board. Unplug whatever you can, and if you have a variac and/or light bulb limiter use those to bring it up.

This is the worst thing I've done for a little while, another free life lesson

So far, I've replaced R273 as I had one on a scrap board. I've tested connecting AC to HDR100 (1st page, top left) which is the power supply for the preamp board side. I plugged in preamp board and it lights ups, the effects board initialises and no magic smoke, it passes signal to the monitor output, so that's probably OK. That leaves the two high/low voltage sides for the power amp.

I have no variac and I should probably make limiter bulb, I've got away without one for a long time

[jondoe]

I have built a light bulb limiter as it blew another plug fuse (i tried the two high voltage feeds), it blew R273 again and let lots of smoke out, I think from Q103 from what I could tell. With a 60w bulb in the limiter it lights up bright if you plug in either of the two high voltage, I guess I've found a rough area of my problem

[The Dude]

I'm admittedly old and blind, but can't for the life of me find R273 on the schematic. Anybody else?

[glebert]

I'm admittedly old and blind, but can't for the life of me find R273 on the schematic. Anybody else?

It is in the power module pdf, left edge of the page. Top of vertical section C.

[nosaj]

I'm admittedly old and blind, but can't for the life of me find R273 on the schematic. Anybody else?

Far left side of schematic by edge between c and d markings on schematic side.

Q103 is for 48v phantom voltage.

nosaj

[The Dude]

Thanks fellas!

That resistor is between 2 different grounds. I'd be looking for a burnt trace somewhere and quite possibly more.

[jondoe]

Gah, I didn't say where R273 was on the schematic, sorry about that, it's well hidden over there.

I couldn't find a burnt trace/track anywhere on the board, but I'll go over it again with a magnifying glass and a bright light, just to be sure. I'll take the heat sinks off again and have a look around. Those big transistors look like a pain to remove and place back, I hope I haven't popped too many of them!

[jondoe]

So far we the dead transistors I've found are;

Q216 (A1387)
Q208 (C4793)
Q115 (A1837)
Q221 (A1837)
Q100 (C4793)

It's a little cold out in the shed tonight so I've left it for now and I've ordered up a bunch of the above. I will test some more parts tomorrow

[jondoe]

I have so far replaced

Q215
Q208
Q121
Q221

Q100 was not faulty when tested on a component tester, so I've put that back for now.

I've plugged her back into the mains with the bulb limiter and tried the AC feeds for high voltage side one by one. With JC107 plugged it I get a brief glow and then a tiny faint glow. When I plug in JC202 I get a strong constant bright light.e bulb limiter and tried the AC feeds for high voltage side one by one. With JC107 plugged it I get a brief glow and then a very faint glow. When I plug in JC202 I get a strong constant bright light.

I'm still learning but I assume that one half of the circuit (JC107) might be OK now, the other (JC202 side) still has a short/dead component?

[g1]

When you connect JC202 and the brighter light comes on, you have disconnected JC107?
Then yes, the JC107 circuit may be ok, and JC202 side still has a problem.

[jondoe]

Correct, I tried them both independently of each other the JC202 shines brightly. Thanks for confirming that I take it the light bulb will go some way to protect the circuit from further damage?

Looks like I've been 50% lucky so far

[glebert]

I take it the light bulb will go some way to protect the circuit from further damage?

Generally, yes. As I understand it the magic of the light bulb is that its resistance goes up when it gets hot, so when current is high most of the voltage drops across the bulb. Depending on the wattage of the bulb it may or may not protect it fully (high wattage bulbs will give less protection). I usually have a few different wattage bulbs available to choose my level of protection.

[jondoe]

Generally, yes. As I understand it the magic of the light bulb is that its resistance goes up when it gets hot, so when current is high most of the voltage drops across the bulb. Depending on the wattage of the bulb it may or may not protect it fully (high wattage bulbs will give less protection). I usually have a few different wattage bulbs available to choose my level of protection.

Good to know. I searched all over the house and only found a 60w incandescent, hopefully that will tide me over.

As JC202 supplies AC for the high and low voltage side of that circuit, do you think it would be safe to test each supply idividually? That is, supply AC power to each bridge rectifier within the circuit idividualy by removing the AC feed (temporarily) from the plug from the transformer. This might allow me to further isolate the location of the problem, or is that a stupid idea