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#1 Cambridge Audio Topaz AM1 repair

Posted: Mon Mar 13, 2023 9:26 pm
by Dave the bass
This was an amp a chum brought round for repair last year, there was a quick discussion on it in the 'Nothing in particular thread'.
Dave the bass wrote: Sun May 01, 2022 5:58 pm I'm looking at a chum's Cambridge Audio amp, reported fault was sparks when the speaker leads touched! :shock:

I've found about 12 to 17V DC across the lhd speaker terminals, gut feeling is one channel of the LM4766T output device has popped. Has anyone here had an LMxxxx device produce this scenario? I'm working on it using a resistive dummy load as I don't fancy frying my bench test speakers.
Our-Nick replied...
Nick wrote: Sun May 01, 2022 6:34 pm Probably the LM, but check the supply rails, I have seen that if only one half is powered.
Rails checked OK, I looked up price of genuine replacement chip (about £13 IIRC) then chum decided he didn't want the amp as it didn't have a Phono stage in it and just left it with me. It's sat for about a year on my workbench waiting for attention, I didn't want to scrap it because, erm, errrr I don't know, erm, I like repairing things OK :)

It finally got some attention, crikey, what a battle. I ordered the LM4766T and set about replacing it, on a multiple pin device I often cut the legs off it and remove it so it makes unsoldering the pins a lot easier. I cut a few and noticed the chip moved far more than you'd expect... I looked closer and noticed 3 or 4 legs had cracked at the 90 degree bend where the legs rest on the PCB! Odd. That could have caused the supply rail issue Nick hinted at but having got this far I continued to snip the rest of the legs then set about removing each pin through the PCB.

Erks, 7 (!) PCB tracks were broken too, kerrrikey! I wonder what caused that? The heatsink and chip are very well supported. The chip doesn't run hot at sensible room listening levels so I doubt its major thermal cycling.

New chip in and 7 tracks bridged later, it works, whahey!



Ta dah <jazz hands>

You'll just have to imagine Robin Trower blasting out the bench speakers OK.


#2 Re: Cambridge Audio Topaz AM1 repair

Posted: Mon Mar 13, 2023 10:04 pm
by andrew Ivimey
Well well, well done! I've been thinking about a chum's fender pro junior - why there is so much murky crackling rumbling in the background and a sort of crap sound.....

Measured voltages are 11% over what they should be. The schematic is very sweet.

Nothing else seemed to be wrong except fir crappy JJ valves. One of the output valves looked 'suspicious' - really.

So, as I have a stash of Soviet military grade el84s I plonked a pair into the output of the junior.

Wow! So much glory from something so simple! I'm thinking now about the 230 volt transformer...

But right now it sounds so good.

#3 Re: Cambridge Audio Topaz AM1 repair

Posted: Mon Mar 13, 2023 10:11 pm
by Nick
Nice work Dave.

Andrew: you could make a voltage reduction autoformer out of a toroid to reduce the mains voltage into the amp and hide it away in the body of the thing to avoid replacing the mains transformer.

#4 Re: Cambridge Audio Topaz AM1 repair

Posted: Tue Mar 14, 2023 7:34 am
by andrew Ivimey
Aha, I'd not thought of that Nick, thanks. I was thinking of high wattage wirewound resistors. The pro junior can't use many Watts nominally; (it's a little el84 based push pull circuit) but hadn't gotten further than thinking what a nice schematic diagram and very well laid out pcb.