Crate BV120H guitar amp, probably 1991

[N Cook]

History , there was a smell of burning followed by cutting out.
Next time powered up high pitched squeel.
Now on powering up very muted output like half-wave and sometimes a burst of
high frequency squeel but all at low level.
Nothing obviously burnt or over-heated inside.
Stable HT voltage. 4x 6L6
Same distorted sound on the line out.
Before testing the valves etc, anyone have any pointers ?

 

[Meat Plow]

History , there was a smell of burning followed by cutting out.
Next time powered up high pitched squeel.
Now on powering up very muted output like half-wave and sometimes a burst of
high frequency squeel but all at low level.
Nothing obviously burnt or over-heated inside.
Stable HT voltage. 4x 6L6
Same distorted sound on the line out.
Before testing the valves etc, anyone have any pointers ?

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

 

[Phil S.]

History , there was a smell of burning followed by cutting out.
Next time powered up high pitched squeel.
Now on powering up very muted output like half-wave and sometimes a burst
of
high frequency squeel but all at low level.
Nothing obviously burnt or over-heated inside.
Stable HT voltage. 4x 6L6
Same distorted sound on the line out.
Before testing the valves etc, anyone have any pointers ?

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.

 

[N Cook]

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.

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

 

[[email protected]]

History , there was a smell of burning followed by cutting out.
Next time powered up high pitched squeel.

instability narrows the options.

Now on powering up very muted output like half-wave

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

and sometimes a burst of
high frequency squeel but all at low level.
Nothing obviously burnt or over-heated inside.
Stable HT voltage. 4x 6L6
Same distorted sound on the line out.

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

Before testing the valves etc, anyone have any pointers ?

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

 

[N Cook]

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

 

[RonSonic]

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

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

 

[ty]

History , there was a smell of burning followed by cutting out.
Next time powered up high pitched squeel.
Now on powering up very muted output like half-wave and sometimes a burst of
high frequency squeel but all at low level.
Nothing obviously burnt or over-heated inside.
Stable HT voltage. 4x 6L6
Same distorted sound on the line out.
Before testing the valves etc, anyone have any pointers ?

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

ty

 

[N Cook]

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

ty

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

 

[N Cook]

followup see new heading
16 ohm amp --> 4 ohm speaker block (ex speakers)

 

[Michael Kennedy]

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

 

[jh]

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

Michael,
pssstt, it´s a tube amp....

Jochen