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Dead A&R Cambridge A60 amplifier

Peter Macleod

New Member
Hi folks,

I'm new to this forum, so I hope the DIY section is the right one for help in fixing faulty equipment.

So....for sentimental reasons I took a chance on an old A60 amplifier, £49 in P&P.

It is the last, 1985 model, smaller transformer and with RCA phono sockets for turntable and a CD input.

The unit powers on but no output and speaker fuses are blowing. Also there is a distinct resonance to the case when it is on. No noise, just vibration.

The power supply fuses are fine. When I got it I noticed that the white block that fits in the M.C. option slot was back to front. Hopefully nothing else is wrongly wired! I have only done 6 measurements so far, across the four diodes D204-7 and capacitors C209/10. They all measure around 40v.

Where to start? Any suggestions other than the obvious landfill suggestions welcome.

cheers
 
power it on with a 100w light bulb in series on the 230v mains
power on if the bulb lights up then goes dim no short
if it stays lit you have a short somewhere possibly the output transistors or dry power supply caps
 
There was a recent thread - with a very similar problem, it was solved by the replacement of output and driver transistors.

I'm pretty certain the white MC jumper connector can be fitted either way with no problem as it just links the input and output pairs - and they are symmetrical sround the centre of the connector.
 
Hi,
sorry for delay, been busy.
Yes, I have a lamp tester and I had it wired when I was doing a couple of tests the other day. When I powered up it lit and went dim, so OK in that respect. I have now received some fast blow 1.6A fuses so I can try things again.
First thing, I noted that the power leads from the transformer were also the wrong way around! Not sure how bad that is, but I am sure the vibration has now stopped. That is the two white and one black cables, going into a white block by the main fuse. Hopefully do some more checks later today.
Thanks for the replies so far.
 
The Delta 290 and others in that series have Mosfet output transistors, originally IRF240 but replaced by plastic type IRFP240. No joy there.
The A60 is their first model, from the beginning in the 1970s, so it has bipolar transistors, both TIP3055 plastic case type in a quasi-complementary arrangement typical then. Thankfully, these are still very common and very cheap transistors. If you are DIYing, I assume you know a little about output transistors and what also happens when they blow.....anyway, they tend to take out their driver transistors too. These are long obsolete though so let's hope they test OK.

There are plenty of websites explaining how to test transistors but when power transistors fail, they most commonly short from collector to emitter. Even a cheap DMM can test that on the low resistance range. If unsure, google the procedure as there are plenty of sites giving advice on power transistor testing, in and out of circuit. Some are clear and concise, others confusing but that's the character of the net.
 
Well, I got some new F1.6A's, I corrected the orientation of the white and black power cables and hey presto, it is now working and the vibration has stopped (perhaps still a light hum). I have no idea of the consequences of this block being inverted (pity it wasn't made asymmetrical!) but the sound from my spare Kef Q100s sounded fine. Only issue now was a rather thin tinny sound via my AKG 550 headphones. I attach a couple of images to highlight the two problem areas (red arrows). ...I say that, but can't see how to upload an image from my PC! Only seems to allow image URL?
 
Re: Tinny headphone sound. From the schematics available in the service manual at Hifiengine.com, the headphone output has a high impedance (about 300 ohms) since this is an old style amplifier designed for the types of headphones available in the 1970s but I don't see any revision for the later issues.

Today's 'phones are designed for only low impedance outputs, much like the speaker impedance at a nominal 8 ohms. If you drive a low impedance with a 300R source, it will lack the lower frequencies most, as they require more current, so this seems to be what's wrong. A fix is possible, but it requires a re-design of the circuit which attenuates the output down to 'phones level. As designed, the output is taken from the centre tap of series connected 300R and 200R resistors, effectively strung across the output terminals. This will need to be changed but the best solution would still be a small, dedicated headphone amplifier.
 


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