Wacky 5150 schematic

[Enzo]

And the shorted C44 doesn't leap out at you like a clue?

Get rid of C44 and see if things come closer to normal. Slap your extra 2200uf in its place. Yes, I realize it is half what the circuit expects, but it is close enough to see if the rest of the amp works.

[Valvehead]

well... i replaced C44 with a 2200uf temporarily, and put in a new 47 ohm for R77. guess what....

R77 started smoking within 10 seconds.

[Enzo]

Does the new 2200 cap measure zero ohms across it? In other words, the old cap might have been OK, and a trace or tube socket somewhere is shorted. Does the old C44 measure short now that it is out? Are any of the rectifier diodes right bext to it - on the drawing - shorted?

[Valvehead]

i have 2 new 2200 here, both measure about 3.9M ohms. I did test the 4700 i took out. It also measures 3.9 meg NOT shorted. I took the old 2200 out just to be sure, it measured up out of range in Meg ohms.

those 4 diodes all check good on the meter--out of circuit. in circuit-they test weird (i get voltage across them in both directions) .

It cant be a tube socket--i have all the tubes, pre and power , unhooked from the main board. The board looks clean, no burns, etc..

enzo, is there another cicuit that may be using that ground path?

[Enzo]

OK, so C44 IN CIRCUIT no longer reads short, right? The part we removed was in fact not shorted, so we have to think something else was sshorting that supply rail to ground. I believe the parts are good, but I want to make sure the circuit traces and stuff are OK too. Even if there is no C44 on the board, what does it measuer between the points where it was?

The main power supply ground is sort of wired to itself. Main filters, PT CT, power tube returns. Then that R77 connects it to all the rest.

Did you check CR21,22?

The four diodes would probably read more normal in circuit if you popped out the two fuses. As long as they are not shorted, I'm OK with them.

I really think you have a solder bridge, or something else creating a short circuit. Even a ribbon cable that is off one pin to the side or something.

[Valvehead]

i tried something today-- i clipped in the new caps (2200 + 4700) . and fired up the amp with ONLY the 24v tap hooked to the main board ---no other taps or tubes--nothing . R77 still smokes .

On the 5150, you can unplug each tap of the PT on the main board.

[Enzo]

I believe you that the caps aer OK, but I still haven't heard what the resistance across C44 measures now. Or i missed it if you said already.

[Valvehead]

It cant be a cable or ribbon cable--there is only ONE cable going to the board at this time---the 24v tap from the PT.

resistance with the 4700 out of the board . 204 ohms at the 2 points where the cap was.


HOLD THE PRESSES !

one of those diodes tests bad ! CR21 i think. I took it out of the board and it tests bad . looks like a 1N4148 .

i will get over to the local yokel electronics house tomorrow and MAYBE they will have one.

i will keep you posted of course !

[Enzo]

OK, so you have a low low low 2 tenths of an ohm right square across one of the 24v supplies. THAT is what you need to solve. WHAT is shorting that rail to ground?

[Valvehead]

im sorry enzo i didnt notice the period there. I meant 204 ohms .


i hope its the shorted diode i found. Going to get one today.

[Valvehead]

replaced cr21 diode and put in a fresh 47 ohm (r77). and that part of the amp is working great now !

However, i have horrible luck.

Now the amp has super low volume. this is on both channels. I plugged the guitar into the fx return and get full volume. so its in the preamp section. i jumpered all the ribbon cable--they are all ok . i guess ill pull the board out for the 75th time.

this amp is going to ruin my brain

[Enzo]

Then you get a bran new brane.

[Mark Black]

Are you sure that diode was a 1N4148 and not a Zener?

[Valvehead]

Are you sure that diode was a 1N4148 and not a Zener?

yes, 1N4148

thats what the peavey list says

[Valvehead]

scem attached







ok....maybe not--trying to figure out how to attach this