Cambridge P40 Amplifier

[nigelr2000]

I have a strange one on the bench this morning. It came in with one of the fuses blown. I replaced it and powered on only for both speakers to shoot out as there was power on the speaker line. I quickly powered down and disconnected the speakers.
Powered up again and metered the speaker sockets to get a couple of mV showing so I reconnected the speakers and it works fine. Power it down then on and you get heavy DC on the speakers again.

I don't really know where to look on this one, it doesn't have a protection relay so making it difficult to work on with speakers connected.

I can't find a manual online as the one for the P40 is not a split rail and looks completely different.

Any ideas?

[Herald1360]

Power it up with around 100R load resistors with meters/LED+resistor across?

[PJL]

If it's both speakers, maybe one of the rails is failing intermittently.

That's a great example of the mostly empty box trick. Most of the manufacturing costs must be in empty PCB and unnecessary metalwork and paint.

[Ted Kendall]

The original P40 was introduced around 1970 and is completely different from what you have there.

[AC/HL]

Those red Roe capacitors are generally reckoned to be suspect I believe.

[nigelr2000]

I must admit I didn't like the look of those caps but they both tested fine out of circuit !

I might just change the rectifier diodes as I have had several of the same design become intermittent then give up recently so perhaps it's becoming common.

[nigelr2000]

Well sorted what it was. Changed the rectifiers, worked first time turn it on and off and it did the speaker pop out thing, changed the roe caps even though they read ok same problem but something new as well, it buzzed/crackled with no input. Pulled the main smoothers, just in case both read really well, once they were back in it started with a buzz for around a second speakers popped out then settled and worked. Connected a meter to the speaker connections and it read 18 volts dropping to a couple of millivolts, Turned it off and on again and nothing, no power checked feed yes got 230 volts, checked secondary, no volts, metered primary and got infinity aha so that's what it was the mains transformer was failing hence the buzzy crackly interference. Shame really but that makes it a write off as the customer wouldn't want to pay the price of a transformer. At least I found it in the end strange fault condition though.

[paulsherwin]

You might be able to save it with a cheap generic transformer or one from a scrap amp. It might affect the maximum output but the amp would still be usable.

[nigelr2000]

I will see if I win it as it were paul but I feel the customer might take it away as he is pretty tight and he wouldn't want anyone making any money out of it, you know the sort, lol

[robin coleman]

I used to make these at Papworth the transformers were made locally I believe at St Ives windings
Regards
Robin