Peavey Ultra Plus Blowing Fuse

[chunkitup]

I bought this amp for $50 in non-working condition. Someone had done a bias mod which looked really sloppy. I have returned the bias to stock. The problem is the 2A fast blow on the secondary blew and the big round black cap bridging the power switch to the primary tap/ground switch lit up like a christmas tree. Now this is with all the tubes out and I also replaced all power caps. I checked all power supply diodes and they tested fine. The switching circuit and the heater circuit have no problems. When I disconnect the B+ from the board and take a reading it is 375v, perfect. What am I missing? What else should I look for?

[Gtr_tech]

Caps and diodes is pretty much all yer left with. Use yer meter and sleuth for dead shorts to gnd in the B+, bias, and heater sections.

[Gtr_tech]

And....if it has protect diodes on the output tube plates, check those for shorts. Bah....pretty much falls under the previously stated junk anyway...

[g1]

"the big round black cap bridging the power switch to the primary tap/ground switch lit up like a christmas tree". That's not a cap but a thermistor (CL-60). It's shown on the last page of the schematics here:
http://music-electronics-forum.com/a...-schematic.pdf
Not sure if it will still be ok or not, it basically limits the current surge when the power is turned on. Make sure the mains fuse is the correct value, if someone put in an oversize a fault could maybe fry the thermistor.
As far as the 2A secondary fuse blowing, there is also a cap across the secondary, C50 (pg.3 of schems.) which could cause the fuse to blow if faulty.

Edit: Also, check the protection diodes, CR13 & CR14 on the power tube board.

[Enzo]

Two issues.

First that black disc from the power switch us the inrush limiting thermistor, a CL60 type. You can get them from Peavey parts, PV part number 70240200. Or from suppliers like Mouser. replace it. But during testing, you can short across it, but do replace it before you are done.

BLowing B+ fuse. Tubes would be first suspects, but if it does it with them removed, then do what Gtr suggested, check the protection diodes CR13, 14 on the power tube board. Basically just check pin 3 of each power tube socket. SHorted to ground? Those diodes are type SR2873 2000v 250ma diodes. PV part number is 70402873.

And the red/blue/brown wires from the output transformer plug onto the end of the tube board. Does disconecting the red one stop the fuse blowing?

Bottom line, most likely problem is shorted protect diode, next on the list is output transformer primary shorted to frame or to secondary or arcing, and I don;t see it on PV amps much, but sometimes the power tube sockets themsemves can arc.

[chunkitup]

You guys are right on the money. I finished checking CR13,14 and they read .003 and .02 respectively with the diode check. I will also order a new thermistor. Thanks for the part numbers also.

[Enzo]

Remove the shorted diodes. They are like a seat belt or an air bag in your car - they are protection, but not part of the basic function of the amp. So the amp will work just fine without them, you just lose their protection against a lack of speaker. This allows you to run the amp and find out any other needs the amp might have before ordering stuff.

[chunkitup]

Thats what I will do. Thanks again

[chunkitup]

WTF!!! So the power tube board is only connected to the chassis by pop rivots for the tube sockets. So I have to drill out all 4 tube sockets. I need to think of a happy place.

[g1]

Yes, and it's a royal pain. Enzo has suggested elsewhere that repairs can sometimes be done without removing the board. Remove the solder from the bad components and push them out of the board. Mount the replacement parts on the foil side and resolder. Hope there is enough clearance that you can do it this way!

[Gtr_tech]

Yep....its either that or desolder all the output tube sockets and keep the rivets (what I had to do on my Ultra 120 head that sits at the after hours jam spot). Either way it ain't my idea of fun.

[Enzo]

Oh please...

Drill out eight whole rivets???!!!??? WHy the horror, it must take all of a minute and a half to do? And then having to actually install all EIGHT pop rivets later? Oh NO!

I can't imagine having the patience to unsolder all those socket legs, and THEN trying to get them all back into their holes at the same time.


But really, drilling out a few rivets is no big deal. WHen I leave the board in place and push out dead screen resistors or something, it isn't about not having to drill out the rivets as much as it is I need not take the time to remove the board AT ALL. Parts don;t really care of they are on the "wrong" side of the board, as long as they are facing the right way.

[chunkitup]

I didnt know that you can get a rivet gun so cheap. I was worried about reinstalling. I also worry about a stray piece of metal ending up left behind in the chassis. Just have to take my time.

[g1]

Oh please...

Drill out eight whole rivets???!!!??? WHy the horror, it must take all of a minute and a half to do? And then having to actually install all EIGHT pop rivets later? Oh NO!

Well I am shocked, sir, that you would deny us equal opportunity to also play the role of "crusty-old-stick in the mud-opposed to change-anti modern/progress/social networking-kind of guys".
But seriously, a bit of dust or dirt in high voltage areas can cause arcing, socket damage, or board damage. So we are going to drill there routinely and hope we remove all traces of metal filings or dust? Unless there is a rivet removal tool which does not involve drilling, I'm not prepared to accept rivets as anything but a permanent fastener.
I know you do a lot of Peavey stuff and have way more experience than me so it is probably old hat for you, but in over 25yrs of repairs I've never had to remove any rivets aside from this type of Peavey build. Maybe I've been sheltered (we have mostly Yorkville rather than PV up here).
Oh wait... if memory serves Fender used to supply replacement transducers for reverb tanks, rivet removal required. Sorry for the rant .

[Enzo]

Like anything else, I just try to remain cognizant of details like metal shavings and where they will be falling. And drill at a slow speed.

Since I am going to drill from the outside, if I am worried shavings will fall down th socket, a slab of masking tape over the holes solves that. Doesn;t work on aluminum scrap, but for anything ferrous, I have a magnet for cleaning up. One of those scribe tools with a magnet on the other end (and a pocket clip) is easy to get into tight spots for pickup.


ANd I have to do this so seldom, it just isn;t a concern for me. Hopefuly I won;t be teaqring that board out time after time. And remember, my first line of defense is not removing it in the first place. Pop out old parts and install new ones on the foil side.