Solid State Amp Compression Issue

[Oolooloo]

Hi fellow FX-addicts,

I was hoping you could help me wrap my head around this. Maybe someone's encountered a similar problem before.
A friend of mine has handed me a solid state amp - an Orange CR120 - with an odd issue.

I am led to believe that the issue is only apparent at moderate to high volume, after a while of playing.
It sounds as if the amp is compressing the signal, just as an aggressive compressor would.
You strum a chord, the volume drops, you mute the chord, the volume quickly recovers.
I heard the symptoms only for a short while. This phenomenon is apparent; I am unsure if other distortion or artefacts occur.

Could this be a supply issue? Bad filter caps? There's no significant hum/ripple noise present.
Or is it more likely to be an output chip issue? I would assume it's less likely to be a pre-amp issue.
It's unfortunate that the issue only seems to occur at high volume and after some playing time, as it complicates troubleshooting the amp in action.
I'd like to monitor the supply voltages as the compression is occurring, but I'm not sure when I'll get the opportunity to.

Sorry for the wall of text, and I hope at least that it is clear.
Many thanks!

Edit: I've found no reliable full schematic online. Here's a shot of the power board I found.

[iainpunk]

could you post the type numbers of the chips?

it sounds like thermal protection kicking in, does the heatsink get really hot?

cheers

[Oolooloo]

Ah, could very well be!
Didn't occur to me, maybe because it doesn't actually mute the sound.

Of course. Pair of TDA7294 chips.

I haven't had the chance to check heatsink temperature, I only heard the problem happening.
I think the thermal paste looked alright, but I'm not sure what's it's supppsed to look like. I'll check when I can and I can post pictures.

Cheers!

[Rob Strand]

I'm presuming the sound is still clean when the compression occurs?

A lot of solid-state amps have JFET at the input of the power amp for muting.  However, the TDA7294 chips have a mute feature so it's highly likely they are using he built-in mute and not the JFET.

So what I think is happening is the mute circuit is kicking in for some reason.

It would be good to get hold of a schematic.   That would give us some idea how the mute circuit could fail (we don't know that is true it's only an initial theory).   I couldn't find a schematic for the Orange CR120.   The next bet would be look for other similar Orange amps.

In the bigger picture of faults the mute circuit might not be the cause it might just be a symptom.  Perhaps there is an issue with the power supply or a bad connection which is causing the mute circuit to trip.   The could be easy as cleaning the connector between the preamp amp power amp, or preamp and power supply,  as sometimes the mute signal is goes from one board to another via a connector.

[Oolooloo]

I'm presuming the sound is still clean when the compression occurs?

Can't say for sure, but I believe so.

It would be good to get hold of a schematic. That would give us some idea how the mute circuit could fail (we don't know that is true it's only an initial theory). I couldn't find a schematic for the Orange CR120. The next bet would be look for other similar Orange amps.

Yes, I'll look for a schematic of a similar amp. If this was a single sided board, it'd be less of a pain to trace...

Perhaps there is an issue with the power supply or a bad connection which is causing the mute circuit to trip. The could be easy as cleaning the connector between the preamp amp power amp, or preamp and power supply,  as sometimes the mute signal is goes from one board to another via a connector.

It doesn't look like the mute signal is going across boards, but I'm definitely cleaning the connectors. Easy fix for a lot of potential trouble.

Thanks a lot!

[Rob Strand]

It doesn't look like the mute signal is going across boards, but I'm definitely cleaning the connectors. Easy fix for a lot of potential trouble.

Cracked solder joint's is another one.   Pretty common on connectors.