Randall RH300 G3 cuts out at high volume

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[Anthrax Zygote]

I have a Randall rh300 G3 that works perfectly fine until I bring the volume up past about 7 on the level and 4-5 on the master. I hear that these amps tend to overheat but it doesn't feel very hot and as soon as I bring the volume down a bit it seems to return to normal ( although somewhat quiet ) operation.

Any suggestions? Should I just bring it to my tech? I'm pretty good with electronic repairs so if this can be cured somewhat reasonably I'd like to know how!

[Anthrax Zygote]

I forgot to mention that I'm quite familiar with running high wattage amps into low ohmage cabinets and the overheating problems that occur with randalls specifically. This seems to be a non-heat related issue...

[J M Fahey]

Set it up so that it cuts off consistently.
Turn it off, pull it from the cabinet without disturbing any setting, and play again just as before.
Does it still cut off the same way?
When it does, direct a home-type fan towards it.
Does it have any effect?

[Anthrax Zygote]

You really think it's overheating? I've tried touching the heatsink and Ive verified that the fan is working it really doesn't feel too hot but I'll try pulling it from the box anyway. I've modified a lot of Randall head boxes to more efficiently dissipate heat in my days as a SS amp enthusiast but this is my first G3 series amp so I'm somewhat feeling it out at this point but this particular amp seems to be well vented, has a functioning fan, and so far the heatsink has seemed cool but I'll put a thermometer in it and try the house fan idea. Funny thing about that, I use house fans to diagnose overheating variable frequency fan speed drives at work all the time but those are usually so hot it's very easy to tell...

[J M Fahey]

You really think it's overheating?

Well, not *reeeeally*  , but I'd like to take it away from my list of suspects.
Anyway having the chassis out is a pre-requisite for further testing, so .....
What *I* would really do is scope it, but not everybody has one.
Second best is inject a continuous tone (1KHz, 440Hz, your choice) and check what happens when it cuts out.
Is a relay clicking somewhere? Does it have some kind of mute, or a bimetallic thermal switch?
Google the schematic, I've seen it somewhere, maybe inside MEForums.
Note: heating does not only refer to power transistors and big heatsinks, maybe there is some lowly component in there which only heats, say, 10 to 20 degrees and yet doesn't handle it, that's what freeze spray was invented for.
You could also lose voltage somewhere, a connector pin might expand when hot by current, not ambient temperature and lose contact, etc.
That's why tracing is important, you have to know "what" happened, "where", before you start guessing "why".

[Anthrax Zygote]

Yeah I'm definitely going to bring my meter and a schematic next time I visit my practice space! I'm an electrician by trade but I do commercial work so I very rarely work with electronics but I understand the principles.