Problems with Trace Elliot Brat

[Canard]

I am not an electronics guy at all. But I can fix easy things - fuses, corroded contacts, corroded/seized pots, etc. And equipped with my paranoid rubber gloves and insulated tools and capacitor discharger, I can sometimes fix basic things on simple point-to-point wired tube amps when I have a schematic.

Anyway I bought a dead Trace Elliot Brat, a UK-made 15 Watt solid state amp, today because it was super cheap, it had its original 8" Celestion PG8A-15 speaker (which I needed for something else), and cosmetically it was in perfect condition. It was advertised as dead. I got it for much less than I would have to pay for the Celestion on its own.

All it would do when plugged in was hum loudly - hum volume is independent of how any controls are set. I could get no instrument sound out of it at all.

I dismantled it. I cleaned all the jumper wire contacts with Deoxit and re-seated them. I cleaned all the pots and (sound) switches with a deoxidizing lubricant cleaner - the pots where very, very, very stiff - some almost seized. I checked and re-seated fuses, not that this was necessary since the beast did power up. I still have the hum, but I get sound now. It is crackly, and it I turn the volume up, it wavers quickly back and forth between very loud and very quiet.

Any speculation here as to what the problem might be?

I will not try to attempt a fix here myself. I have done what I can with solid state circuit-board gear of this sort. If it can be repaired cheaply, I might get it done and then resell it.

Or if fixing is prohibitive, I might re-purpose the cabinet to house a little 1950s 5 Watt single-ended class A tube head I have - sort of like a Champ projector amp but with more tone controls and a different rectifier.

 

[Nuuska]

Bad capacitor somewhere ?

 

[Canard]

Bad capacitor somewhere ?

Thanks for the reply.

There are two large capacitors and number of smaller one on the single circuit board. Visually they look OK. No bulges. No puffy tops. No leakages.

I am waiting for a reply from a reputable repair guy who with the current plague has moved his business into his house/garage a few blocks away. He, quite reasonably, charges for estimates but does not state what his estimate rates are.

I found some posts online in a repair tech forum where a similar Brat problem was being discussed. There it looked like maybe somebody had plugged something into the input jack that was powered and shouldn't be plugged in and in doing so cooked an IC. The ICs are not in sockets but are soldered in.

Without a schematic, I do not want to do other basic testing myself, such as testing the power feeds from the transformer. And while it is only a single layer circuit board which is not densely populated, my soldering/de-soldering skills suck so I am not going down that rabbit hole. LOL.

 

[Nuuska]

You might check the op-amp supply rails - measure both DC and AC-voltage from pins 4 & 8 to see if they are even and wo AC on them.

If you find AC there - then it is in the PSU circuit. Without schematics no further guesses.


But happy for you that you got that desired speaker at good price.

 

[SFIV1967]

In that case the dead part was the 4558P Op Amp (probably a Texas Instruments one):

Trace Elliot Brat no reverb - Music Electronics Forum

Ralf

 

[Canard]

Thank you Nuuska and Ralph. And thank you, Ralph, especially for the link to the Forum. There is a downloadable service manual pasted there.

Whatever is wrong with the amp is beyond my skill set and my limited diagnostic tools. Repair in a shop that charges fair rates (where techs can actually eat and their pay rent) does not seem feasible - the value of the amp is not high enough.

I am trying to decide what to do. Sell it on, honestly. The cab is in excellent condition. Or re-purpose the cab for use with a different amp in it.