Trace Elliot V8...need tech help

Wasnex · #1 12-25-2011, 10:55 AM

Trace Elliot V8 help...

Can anyone point me to an active link or provide directions on the best way to upgrade the poweramp heater circuit?

Just bought the head and the front panel power light started flickering. A few days later, no light and all KT88s are cold and dark.

Opened it up and thankfully the transformer is still working...When I touched the fuse holder with a probe I noticed sparks between the holder and the fuse. Pulled out the fuse to look at the previous attempt to repair the problem, which involved running a purple wire under the fuse and etching a line in one of the board traces to break the circuit...the part of the wire under the fuse was black and melted but had not shorted, but the fuse holder has come competely undersoldiered from the back of the board. Couldn't remove the second fuse as it has welded itself to the fuse holder. Apparently the resistance in the fuses cause a lot of heat to build up in the fuse holder.

So would it be better to put pigtail fuses in or, I have seen some reference to just jumper the fuse holders with heavy gauge wire. This scares me a bit regarding what would happen if one of the heaters shorts without fuses to protect the circuit...seems like pigtails are the way to go providing a suitably rated fuse exists.

I have searched for days and have found multiple links regarding a repair that might have been recommended by the designer of the amp...unfortunately none of the links are active.

BassmanPaul · #2 12-25-2011, 11:27 AM

It's not the fuse that has resistance. The fuse holder has become corroded adding resistance to the circuit which manifests itself as heat. The circuit is a low voltage 6.3V AC but carries a significant amount of current. You need to replace both fuse holders with NEW identical ones or use wire in, pigtail, fuses of the correct value.

Most tube amps are built without heater fuses, relying on the main fuse to pop if there is a short. IIRC it's a European rule that mandates a fuse on every cable coming out of the power transformer.

Wasnex · #3 12-25-2011, 12:12 PM

Sounds like pigtails is the way to go in order to prevent the oxidation which causes the problem. I'll try to track some down. Course the down side is the soldier trace is on the back side of the board...meaning the preamp and power amp boards will have to be removed everytime a fuse blows.

Any idea on whether these should be fast or slow blow? Littlefuse makes a 500V 20A fast blow ceremic fuse...exact same size with pigtails.

BassmanPaul · #4 12-25-2011, 02:07 PM

I have to admit I'm not a fan of heater supplies running on PC board traces. I like them running in tightly twisted pair cable lifted high above the other circuitry.

The fuse you mention should work fine. In your place I would see if I could get turret terminals into the fuse holder spots and wire the fuse to those above the board.

Wasnex · #5 12-26-2011, 05:22 AM

Gotta be some way to make it possible to service these fuses without removing the circuit boards. I'll look into the turret terminals and regardless of the mounting solution. I will make sure there is significant space between the fuse and board...

Thanks for enlightening me about the oxidation issue...I was talking with a couple of my audio techs a few days ago about why manufucturers use pigtails...I thought it was mainly to get the fuse away from the board to minimize damage if things go ballistic...Now I know that the upgraded continuity can also possibly prevent a meltdown from happening...and in this case I do mean meltdown as there was a pool of soldier lying in the case directly under the fuse holder.

Wasnex · #6 12-26-2011, 08:11 AM

Do you recommend a threaded or none threaded post.

Found these threaded posts for amp projects, including some in approximately Trace green.

Angela Instruments Online Catalog - Terminal Posts

Seems like an easy repair if I can push a screw through the board, soldier the screw head (or perhaps a washer) to the trace and then screw on the terminal post...but if the screw loosens up I could be back to the oxidation and heat....perhaps there is some sort of conducting gel that should be applied to the threads?

Don't think a push through terminal post would be a good choice as there would be no mechanical way of securing the joint independently of the soldier (which will likely melt if a fuse is replaced).

Also found some terminal posts that require some sort of tool to essentially press fit the terminal into the board...I imagine this is a bit like fitting a rivet. I am thinking this should probably be left to experienced pros.

Sorry if I am being a pest. Although I do have a military electronics background buried somewhere in my past, I don't have a lot of experience repairing circuit boards or instrument amps...but then how different could working on my V8 be from crawling around in a 3Kw (klystron) RF power amp.