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I'm refurbing a Carlsbro Hornet amp from the early 70s. It has a weird problem where if you leave it for a few hours then power it up everything is fine and very loud, but after playing guitar through it for around 15 minutes the volume starts to drop to a very low level within a minute or so. I've put a voltmeter across the power supply (there's a +49V and a +24V rail but I can't find any -ve rail – maybe it's not aClass B amp?) and the +49V line at least doesn't appear to drop when the scope across the output speaker shows the amplitude dropping after the initial 'cold' period. You then need to wait at least an hour before the amp powers up as normal and goes through the whole cycle again!
I need to check the +24V supply but are there any suggestions for what could be the issue? It doesn't like an electrolytic failure though they're all original and none have any obvious leak/bulge.
Once the output has declined to a low level it also behave oddly after turning off for a minute. On powering up with a tone injected at input, there's no output for at least 30secs as if it was a valve amp warming up! The output then appears looking very clipped but the waveform becomes more normal as it increases over about 30secs, though only growing to the 'reduced' level. Another baffling thing is that I set the input to around 100mV, but the input signal goes up as I wind the amp volume up! At max volume when the amp deliver just over 100W RMS into an 8 ohm dummy load the input grows to 300mV!
Can't find a circuit diagram anywhere for this which doesn't help. The power amp drives a pair of 2N3055s which don't feel any more than warm when the problem arises, but they're heatsinked by the aluminium chassis. Could a 2N3055 behave like this?
Thanks for any suggestions.
I need to check the +24V supply but are there any suggestions for what could be the issue? It doesn't like an electrolytic failure though they're all original and none have any obvious leak/bulge.
Once the output has declined to a low level it also behave oddly after turning off for a minute. On powering up with a tone injected at input, there's no output for at least 30secs as if it was a valve amp warming up! The output then appears looking very clipped but the waveform becomes more normal as it increases over about 30secs, though only growing to the 'reduced' level. Another baffling thing is that I set the input to around 100mV, but the input signal goes up as I wind the amp volume up! At max volume when the amp deliver just over 100W RMS into an 8 ohm dummy load the input grows to 300mV!
Can't find a circuit diagram anywhere for this which doesn't help. The power amp drives a pair of 2N3055s which don't feel any more than warm when the problem arises, but they're heatsinked by the aluminium chassis. Could a 2N3055 behave like this?
Thanks for any suggestions.
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