AAARRRRGGGHHH! 5150 Question(s)!

[Danglin' Fury]

I'm working on a 5150 head. Has the Eddie Van Halen sig on it. It came to me with a blown R69 (4.7k) and blown on the power board, R200 and R201 both 100 Ohm, 1watt. Checked other R's and C's and they all seemed good. I changed the components afformentioned. I also changed C200 (100uF) just in case. Cleaned the areas of any flux VERY thoroughly, turned it on and SIZZLE SIZZLE. Had it on for less than a second and now R69 is reading 460 Ohms. I am going to disconnect the transformer wires and see what the voltages are. Has anybody had a similar problem or can anybody tell me what's goin' on? Possible bad transformer/rectifier? Thanks for looking and I appre3ciate all responses.

[Enzo]

yeah well, pull those power tubes out, and then fire it up - after replacing any burnt parts.

As you pull each power tube... in fact BEFORE you pull the power tubes, look at them. See the innards through the glass. Are all the innards facing the same direction? Forget the labels on the glass, are the insides all facing the same? And as you pull the tubes out, look at the plastic center post on each. Is it broken off of one or more? Any way one of the tubes could be in the socket the wrong way?

Either that or a simple bad tube internally shorted is about the only thing that will burn up those parts. The high voltage gets shorted to ground through those resistors.

I sincerely doubt your transformer voltages are anything other than what they should be.

[Danglin' Fury]

Checked the tubes and the center pins are not broken. Innards are cool. Replaced the parts and the tubes with a different set. Same thing. Also the ribbon cable from the power board to the main board is smoked bad. Pin 4 I think. From the dark stripe on the ribbon cable the 4th one in is black as hell.

[Enzo]

Then I suspect maybe a tube socket arcing between pin 3 and pin 2. That wouold likely still happen if the amp were turned on without tubes.

Look closely at the tops of the tube sockets for arc marks. Look underneath as well as you can. To get at the sockets if we must, we drill out the 8 rivets and drop the whole assembly out. When it is time to install it back, use a pop rivet gun and 8 pop rivets. Use a 1/8" drill.

if you must replace a socket, remove the dead one, then park the new one in the holes unsoldered. I usually raise it up to just below where it neds to be and tack solder one leg. That holds it in place while I rivet in the other three sockets. Then I untack the new socket, slide it into position, rivet it, and THEN solder its legs. That way I don;t have to guess the right depth, and there is no stress like there would be if it were off a little.

I don;t know which pins are what on your ribbon, look on the layout or follow the trace work. Whatever is burning o0ut the resistors has all its current flowing through some connector somewhere, so it could be involved. or it could just coincidently be a burning heater connector. Hardwire around it if so.

[Danglin' Fury]

Apparently this guy has been worked on before because it has no rivets, just standoffs with screws. I've already had all the boards off the chassis. I've also had to fix the trace between J6 and J15 on the power tube board as it was burned in half. I'll take another look and see If I can tell if it has arc marks or not . Thanks Enzo.

[Enzo]

The sockets are on standoffs? Hmmm, they usually mount flush.

[Danglin' Fury]

I meant the main board has standoffs. Somebody tapped the sockets and put screws in them. Anyway, I ohmed the tube sockets and none of them read under 15 Meg. I replaced R200 and 201 again. I also replaced R69, the resistor after F2 and between the 24V. I turned it on and sizzle, sizzle. Those exact same parts went tits up again. The ribbon cable from the power board to the main board is pretty cooked I did a continuity check on it, it was fine. I checked to see if the wires in the cable were touching. They weren't. I turned it off as soon as I heard the noise and the 2 amp fuse (F1) didnt blow like last time. Any suggestions? My schematic from Peavey doesn't show what voltages are supposed to be at the secondary of the transformer. Do you know what color is what voltage. I really appreciate your assistance.

[Danglin' Fury]

I just measured the rectifiers without the power board in and on one I got 24V DC. The other measured 523 V DC. Looks like something on that power board is bunk.