Service or Repair my amp (Unison Research Unico DM)

andreweast

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I've used the Unison Research Unico Pre and DM as my main amps for a long time. I've had mosfets blow a couple of times in the past due to my error, but otherwise they've been very reliable. However a while back I changed some fuses incorrectly and this caused some issues which it took me a while to track down. S-Man on PFM was very helpful in finding a fuse issue, but it recurred. Then I had Armstrong Audio look at it and they seemed to have sorted the problem.

I've now got a slightly different problem. The power amp sometimes emits a high pitched whine on the right channel (the tone seems to be around 1.2Khz). It's odd that it only seems to happen at certain times, and it seems to be related to the protection circuits of the amps.

Both pre and power have a soft start mechanism for the valve input stage, and they also go back into standby automatically after about 10 minutes.

The problem seems to occur when the power amp has done its automatic switch off and I then turn it on again. Usually when the preamp is still on.

I described this to Twaleb at Armstrong Audio and he has suggested bringing it in for tests. But I'm hesitant about the cost, and thinking, should the money be put towards a more general service?

This leads to the question of when and whether servicing is necessary for an amp. The capacitors all look like they're in good shape, but should I have the amp fully tested and measured to check? I've had it about 10 years and Duvet had it before me for a few years, so it's far from new but far from old. I've had much older amps that didn't need servicing.

I could send it to Henley (the distributor), but having spoken to them they clearly don't know the model well and would inevitably send it to Italy. I suppose that would be the best option, but no doubt expensive.

I'd appreciate any thoughts or suggestions, either about repairing or servicing.
 

DUVET

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Depends on what your long term view is . If they are keepers then a fettle and a service in Italy maybe just what they need . Glorious combo . I’d hate to think what new and like for like would cost now
 

ESK

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Have you tried a change of tubes? I used to get a similar wheee when I had an amp with microphonic tubes.
 

andreweast

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Depends on what your long term view is . If they are keepers then a fettle and a service in Italy maybe just what they need . Glorious combo . I’d hate to think what new and like for like would cost now

Good point Danny. Maybe I should just bite the bullet and get them to Italy. After flirting with lots of other amps I've come towards the conclusion that I can't really afford what it would take to conclusively upgrade, so sending them to Italy would be a mark of my commitment.
 
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andreweast

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Have you tried a change of tubes? I used to get a similar wheee when I had an amp with microphonic tubes.
Yes that is always the first thing I try. It may be the issue is located in the valve output stage, but it's not with the valve itself.

I could just continue living with this small issue, since it rarely occurs and when it doesn't occur the amps are fantastic. It's just perfectionism that is driving me to iron out the issue, but my worry is that it wll be very expensive perfectionism.
 

Dave Brown

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It's not a model we know, as it does pre-date us taking on Unison Research, but we do hold all schematics and are in daily contact with the factory. As you mention it might end up being a "send back to HQ in Dosson", but sometimes that actually works out as a surprisingly cost effective solution. We can always inspect, check, test and quote. 9 times out of 10 these things can be resolved in house.
 
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