Crate BV120H guitar amp, probably 1991

On Tue, 12 Dec 2006 12:19:42 +0000, N Cook Has Frothed:

Take another visual, if you smelled it, it should show up.

And you can't find where the burning took place, either by visual inspection or smell? (I see your statement above.) You think maybe someone let the smoke out of the output transformer? Desolder the leads and test it.

burst

inspection

THe owner smelled burning, I have'nt so far. Resistance checks on the matching transformer seem ok. All anode voltages seem ok. Not the slightest sign of overheating on any resistor. It all looks perfect inside except the pcb polyester is covered in marginal scratches, like left by a dvm probe tip. But they are all over parts of the main board and go under the comps, never seen anything like it, presumably cosmetic damage at manufacture. I've not met optocouplers in a valve amp before - anyone know details of their construction used by Crate, 4 lead, not DIL packs ? Will check through the preamp tomorrow

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

instability narrows the options.

I cant think of anything that might be described by both those terms, so dont know what you mean.

after the preamp? If so that gets the power amp out of jail free.

I'd just start by using a probe to listen to the audio at each stage thru the chain, see where it goes wrong. Would need to remove the preamp's nfb path before doing this.

I once had something like this that IIRC ended up being to do with dirt on a spark gap... but memory is less than sure, and it doesnt sound like youve got a power amp problem so this presumably wouldnt apply.

NT

I decided to have another look. The distorted and lower level signal than I would expect was at the input grids side of the push-pull splitter/driver valve so problem is presumably further back to V1,2 or 3 or op-amps

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Opto's aren't uncommon in guitar amps, usually used in channel switching circuits or as a mute if there's a relay doing the switching.

It's a Crate, so the first suspect is solders and mechanicals. My usual first step is to run signal and hit it with a fist, listen for any changes.

You mention checking voltages, did you go through the whole amp or just the output section. Output section, did you check the screen supply and their resistors. Very common source of a burning smell.

Ron

high pitch squeal usually indicates dying preamp tube, but should have no odor...

ty

burst of

I've not seen the track side of the main board but i'd expect to see trace of overheating of a copper track through the polyester/glass

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

followup see new heading

That could cause some damage to your amp since your speakers have less resistance than what your amp is rated to handle.

Michael Kennedy schrieb:

Michael, pssstt, it´s a tube amp....

Jochen