Carlsbro Hornet going quiet

[stratfordade]

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.

[sortedradio]

I think the use of freezer spray might help locate the problem. I would also give all the solder joints a good inspection. The fault may be a dry joint somewhere.

[stratfordade]

Thanks Martin I’ll try that. The board is awkward to invert as so many flying leads but I’ll see what’s possible without fracturing any of them.

[Diabolical Artificer]

Check the AC ripple on those big Erie caps. Have you checked the other blue philips caps? Also check the opamp socket.

Andy.

[stratfordade]

There’s no ripple I can see on the big caps. The other large ones measure OK but I haven’t checked the small coupling electrolytics. There are two 741 op amps (alas not socketed!). What could I check on those? They both have the V+ ok but the V- pins are both at ground. This is why I’m struggling without diagram - there’s a ground but I can’t find the V- rail I’d expect. The two large electrolytics T transformer end are wired in parallel not the usual series arrangement with the centre connection grounded.

[stratfordade]

All fixed - a decoupling electrolytic at the main volume control was leaking! I don’t know why I didn’t spot in hours measuring and prodding around the PCB. Whipped it out, replaced and the amp is transformed.

[stratfordade]

This is the failing cap which was hiding in plain sight!

[Diabolical Artificer]

Well done. sorry, my eyesight, thought the opamps were socketed. I go over PCB's and my soldering on point to point with a magnifying glass. The big caps may be ok for now but the others I'd change if they are used for de-coupling especially.

Andy.

[stratfordade]

Good idea Andy. Was planning to but didn’t have the correct values in my collection. I’ll order the right ones while I have it on the bench. I do miss Maplin sometimes as could just drop by, and miss even more the wondrous electronics shops of my childhood where most of my pocket money went! Do any survive?

[Diabolical Artificer]

Doesn't matter how many caps you have stashed you hardly ever have the right ones : ) Axial caps are getting harder to find though Vishay/BC still or did last time I looked do a selection. you can always use radials but they don't look right.

AFAIK the component shops of our youth are mostly long gone, there are a few left but not many. That said if I wanted a diode back in 1980 I'd have to take a train to the nearest city - Derby, walk miles etc, now I can get it delivered the next day without moving out of my chair. Still, I do miss those shops.

When I moved to Loughborough in 83 there was a very small shop that had every stylus on the planet and Taurus electronics about a mile away. Taurus is long gone but they still trade - http://sinequanon.co.uk/

Andy.

[stratfordade]

Sinequanon/Taurus is an interesting site thanks.

[nicam49]

I remember the Taurus electronics shop on Nottingham Road Loughborough.. and his dad's shop on Arkwright Street, Nottingham before that. Wonder if his 'lad' is behind the website or another family member's running it. I see that the phone number is a Loughborough code too.
Anyway, I've bookmarked the site... Thanks!