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A&R Cambridge A60 - rebuild thread

Robert

Robert

Tapehead
Folks,

I know many here use and enjoy this old amp so I thought perhaps I'd pen a piece on a cost effective rebuild of this lovely old amp.

I picked one up recently with a blown channel and priced up a kit of parts from RS to restore the output stage, extensively recap the unit and make a few modest tweaks. The idea is not to produce some sort of turbo A60 but to keep the amp close to original while making use of better quality (but modestly priced) modern parts in key areas.

Cost is looking to be around £30 - which includes new smoothing caps, a handful of high performance supply decoupling caps, a handful of Elna signal decoupling caps, a couple of new chip regs, replacement emitter resistors, replacement output transistors/drivers and instructions on how to reset quiescent current.

Any interest in such an article with photos?
 
Yes!! (And while it's not keeping it original, I'd love to know how to reduce the gain slightly. A friend of mine is using an A60 with Tannoy Lancasters and doesn't use the top 80% of the volume control)
 
Right, we're on :)

Doorbell just rang and I now how a couple of large RS Jiffy bags containing the goodies required.

I plan to complete the project this evening so keep an eye on the thread.
If everything works and sounds ok I'll take up Martin's idea and make it a FAQ.

The articles posted above in the link are excellent but I'm aiming at a different group.
A60s are dead cheap at around £50 and I'm aiming to get the things up and running for a modest outlay - back to factory fresh performance with just a little extra thanks to better modern components.
 
Ah..... look at the little baby..... :)

P9160941.jpg


P9160944.jpg
 
Well, its 4.30 in the morning and job done - the time just whizzed past :)

She's been simmering gently on test for about an hour and all seems fine.
I'll shove some music through it tomorrow.... err.... later today and report in.
 
Had it playing for about an hour and all seems well apart from it sounding a little too tight and dry at the bottom and a teeny bit of residual PSU related hum.
Mid and top sound very clean and lively so I'm pointing a finger at the reservoir caps chosen which were just cheap multi-purpose caps. I wanted to keep costs down but I think we may have to use something like the BHC Aerovox.

Here are some pics to be going on with. I'll post a full description of parts required and procedures once I'm totally happy with the main supply caps and run a scope over the thing. That will be next week as I'm at Heathrow for the next 4 days.



Before any mods

P9160946.jpg


One channel had been repaired previously but was distorting badly. I don't like odd transistors and emitter resistors in each channel. Contact area and package type on the 3055 transistors are quite different as you can see....

Oh and there was bugger all quiescent current on the repaired channel and about half that required on the other. Fingers down the blackboard and bleeding ears alert.

P9160947.jpg


Stripped and showing naughty bits:

P9160949.jpg


New output stage components, beefed-up emitter resistors and Panasonic FC decoupling caps:

P9170958.jpg


New Vishay smoothing caps (up for review.....)

P9170962.jpg


Completely recapped pcb. Signal caps are Elna Starget and anything rail or psu related is Panasonic FC. First stage input coupling caps changed to poly film. New 317T and 337T regulators fitted. These are good quality, inexpensive parts which work well when targeted for their application:

P9170951.jpg


Finally, voltage drop across the 15 ohm PSU resistor set to 1.5v for correct quiescent current (target 1.4-1.6v):

P9170963.jpg


and this is what gets removed:

P9170969.jpg
 
One tip if I may. Re-do all the mains connections with new crimp terminals. They get quite bad after all this time and can even fail completely.

And I like FC's in my GC's but am not so keen on them in most other equipment I have tried them in. The FMs are better IMHO, and the ECAs are always worth a shot too.

Oh, nearly forgot. I know you don't want to tweak but I still suggest replacing those old trim pots with some nice multi-turns. Not so much a tweak as a component replacement.;)
 
Nuuk said:
One tip if I may. Re-do all the mains connections with new crimp terminals. They get quite bad after all this time and can even fail completely.
Click to expand...

Thanks for the tip.
I've soldered and sheathed the mains connections so no crimps.
 
. A friend of mine is using an A60 with Tannoy Lancasters and doesn't use the top 80% of the volume control

Easy. Connect the CDP to the TAPE input and adjust the preset pots to get the volume control up to around 12 o'clock at normal listening levels.
 
Robert,

Would you happen to have a parts list?

Do you know what the volume pot needs to be replaces with? Mine's a bit crackly.

Richard
 
Richard, my last RS parts order has gone AWOL but when it turns up I'll post a list of components. I've made couple of tweaks and will post chapter and verse to go with the pics once the last few parts arrive.
 
Almost there with this.
The cheap main caps were fine. The problem was a pair of Panasonic low ESR caps sitting on the end of ..... yes you guessed it..... regulators. Should have followed my own advice :)
Changed to Starget and all is sweetness again.
I've now got some 1uf Wima caps doing coupling duties in the pre amp to good effect.

Full instructions and part numbers in a few days.
 
Bit OT I know, but what is the headphone output like on these? Having just picked up a pair of HD600's on 'bay, I need a headphone amp for my 82.......
& happen to have a faulty A60 in the attic that this thread reminded me of...

Is it worth resurrecting for this role...& plugging in to the tape outs on my 82?
 
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