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Saturday, 20 February 2016

Kerr McCosh DS1 & CWA 10 amplifier restoration.

Oh, what a thing of beauty!


Plexiglass illuminated front panel. Switchable tone controls and EQ for phono pre-amp. Two CWA 10 amplifiers, each push-pull EL84's with ECC81 phase splitter, developing 10 watts.

Came in with a missing EZ81 rectifier, so one was duly ordered and fitted. The power amplifiers, despite missing a rectifier, has been overhauled not long ago, and were in reasonable shape, although the second amp was missing it's safety earth, relying on the earth from the pre-amp via the "Belling-Lee" interconnect. Easy to sort that though...

The pre-amp was in a shoddy state. A good few hunts capacitors in poor condition, as well as a few Wima caps, which had started to show signs of physical degradation.







Channels are laid out symmetrically, one above the other, easily accessible once the covers were off. All the valves (5x ECC83) bar the tone amplifier are mounted on a sub-chassis, isolated by rubber mounts.







 Some of the Wima coupling capacitors had little copper shields...

A nice touch to keep the signal path shielded.

When I extracted the capacitor from the shield, it had a distinct oily feel. Not good.









The individual phono EQ's are mounted on B9A bases, and are removable. Nice touch if you want to change filters. These filters are in-circuit regardless of input selection, there's a switch to bypass should you require.















Cathode by-pass capacitors have had it. You can see where the electrolyte is escaping from the one of the left, and corroded.








Recapping with quality Vishay parts, should ensure many years of trouble-free service to come.









An on initial tests there's a problem. There's a switch to eliminate the tone control. It's fine when set to Flat (bypass), but when the tone control is in circuit, one channel goes dead.

This has me scratching around for hours....





I sketched out a diagram, and went fault finding. On the working side, there's a nice 230V on the anode of the tone amp, 1.5V on the grid and 3.5V on the cathode. On the duff side there's 280V on the anode, and nothing on the grid or cathode. It's just like the thing isn't drawing any current. I try another ECC83. No change :(

I check the AC path. It checks out. So what else could cause this? I checked the valve base to see if the anode connection was damaged. It wasn't.
Now the ECC83 is a double triode. (AKA 12AX7 Click here for details). That's two triodes in one glass envelope. This is the only valve in the whole amplifier that shares left and right audio. One side works , one doesn't....

The ECC83 is suitable for 6.3V or 12.6 heater operation, as the heaters can be wired in series or parallel.... a quick look in the top of the valve when it's on shows only one heater lit!!! I check the valve base wiring, as the two are wired in parallel pins 4 & 5 should be shorted together, and they are. The valve base isn't very accessible. I check the wiring, and it shows pin 4 has never been soldered! Easy fix restores my sanity.

So, back to the power amps to sort out the lackadaisical 60's electrical safety issue of no earth, and restoring the mains switching operation to the pre-amp which has been previously by-passed. The amps had been disconnected for about 30 minutes at this point. I put my hand in to remove the poor mains wiring, and received a most unpleasant surprise! I uttered some words my mother would not approve of...  A cap was left charged up to about 350V, and I'd brushed against it. Someone, in times passed, had replaced one of the power supply caps, and not refitted it's bleeder resistor. The bleeder resistor is there to ensure the capacitor discharges in a few minutes to a safe level... to "bleed" the charge away. I usually work on about 1000 ohms per volt. I fitted two 390K 5W resistors across the capacitor. See the warning and disclaimer at the top of the website? Read it again. I had become complacent. Don't let yourself fall into this trap. Thankfully I've lived to tell the tale ....

On a brighter note, it's all reassembled, and sounding really good. It really does command a presence.

7 comments:

  1. I'm stumbled across your blog about the Kerrs I don't know if you have seen this thread http://www.lencoheaven.net/forum/index.php?topic=1434.0 but I'm now the owner of this set of Mono blocks...

    Do you think your mono blocks are the original colour and if so what colour would you describe them as?

    Your Pre amp looks amazing how does it sound...what's the phono stage like?

    Thanks for any info regarding the Kerrs

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  2. Hi, They're a sort of hammerite grey. The one has undoubtedly had a replacement output transformer, hence it's black. The gentleman that owns they used to know Messers McCosh & Kerr personally from what I gather. The pre-amp is lovely. It's quiet, well made and performs well, much better than the contemporary Radford pre-amp from the same time.

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  3. By the way, just read your thread. You shouldn't really mix and match output valves. They should be matched pairs. A matched quartet would be better, and the delightful people at http://www.hotroxuk.com/ should be able to supply you if you feel the need, and they charge no more per tube for matching. If there are any wax/paper caps in that amp, I'd be evicting them. They will be at best unreliable now, and failure will kill an output valve in minutes if you don't pull the power quickly.

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  4. Thanks Andy, any idea what the writing on the Partridges output transformers read..it would be useful for future reference...mine don't have any details on them?

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  5. Sadly, they've departed back to original owner now. They're not special like a Quad transformer though, just a normal, good quality push-pull output TX. I'd expect about 8K anode to anode.

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  6. Was the Pre amp powered from the monoblocks or did it have its own transformer?

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  7. Hello Unknown Lencoheaven person! It's powered from one of the monoblocks. Thanks for popping by! Andy

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