Amp repair, weird faults

[VacuumVoodoo]

Have you had an amp with a weird fault that took hours to troubleshoot ?
Pray, tell us all about it...

[BJF]

Hi,

Oh yes several, the story of my life.
I'd start with this one though:

There was this Hagstrom amp that upon start up would make this wierd sound sort of IIIIIIIIIaaaaaaaaaaaaaaahumppf and then it'd work as normal.
This fault was something I got in third hand as repair and it was traced to a faulty bybass capacitor that de coupled the whole amp, but one of the sections was not working right and so isolation was insuficient.
This could only be corrected by disconnecting this section and replacing it with a seperate cap.

One can wonder though how bothering a peculiar sound during heat up could be?

Oh well. it was not working properly so- RIFA to the rescue

Have fun
BJ

[VacuumVoodoo]

A Tone Master misbehaved badly, working fine on low volume but couldn't take higher volume setting than 9-10 o'clock without going into heavy oscillation with OT making sound of a parrot in heat Loud. Scoping the output showed a squarewave going up and down in frequency at full power...

Opened the amp, removed the chassie....somebody has dumped a plate of ricenoodles inside, no, that's the wiring. So, on to tracing the circuit and untangling the noodles with a chopstick all the while the amp is powered up and volume set on threshold of oscillation. Found two wires that were extremely sensitive to being moved - Aha says I!

These two wires connected power tube control grids to the circuit board, bundled toghether with B+ wire, the one feeeding transformer's center tap so carrying a lot of rectifier ripple. Separated from the bundle and moved away from B+, the amp could now be cranked past 12 o'clock but later than that the parrot was back big time...

Another go with chopstick over the noodles, the two wires from control grids connected to two resistors on the board which were found to be the gridstoppers...checked again, yepp, they were gridstoppers. Now, any old textbook on tube circuit design will tell you that gridsoppers must be postioned as close as possible to the tube, preferably soldered directly to grid pin on the tube socket.

Heat up the iron, remove the gridstoppers from the board, solder them on the sockets and replace the noodle with nice shielded cable. Routed the cable away from any power carrying wiring. The parrot was gone, demised, dead, gone to it's maker, nit sleeping, just expired, had beutiful plumage though...

Next time: a 50 cent component gone halfway bad causes power transformer to overheat and blow internal thermal fuse - all because of erroneous safety thinking. Till then - remember: a hot soldering iron looks exactly like a cold one.