Samson Servo 260 amp repair

[WaveyDipole]

I have recently rebuilt the output end of one of these as several output and driver transistors were fried and 3 of the large ceramic cased resistors were open circuit. The amp now works, but I am having problems with the right channel intermittently not coming on at power up. The protection light goes off after a few seconds as expected and the clipping no longer comes on (it did originally).

I initially thought it was the volume pot as the sound comes suddenly when the volume is raised. There was no scratchyness or noise though. Once it comes on, it seems to stay on OK. However I have replaced the volume pot but the amp is still suffering the same problem.

I understand that the protection relays in these can stick sometimes and am wondering whether that could be the cause, but why would they suddenly come on when the volume is raised? Ditto in the case of a dry joint. So could it be an issue with biasing? Due to the number of output power transistors that were blown, it was cheaper to replace all 8 of them with modern equivalents rather than finding original NOS parts at inflated prices. The consequence is that the bias pots did have to be tweaked slightly to bring the transistors on properly. I have looked at the output - when running - on an oscilloscope while injecting a 1kHz signal and it looks symmetrical and with no clipping or anything. When it is working it sound quite nice.

So can anyone suggest why an amp channel might not come on but suddenly spring into life when the volume control is rotated?

[rg58]

Familiar problem seen in other amps. It's most likely the output relay. The relay contacts become corroded over time and the higher volume 'burns' it free, the contact resistance decreases. Clean the contacts if you can open the relay, otherwise replace it. Or think of a way to measure the contact resistance in circuit while the problem manifests itself - but this is awkward and not safe maybe.

[WaveyDipole]

rg58, thanks for that reply. It does seem to make sense. The other day, the problem manifested itself by the affected channel having a lower volume rather than totally cutting out. It also seems more likely to happen if the amp is unused for a while, but not always. Sometimes I can turn the amp off and back on again and the problem happens immediately. Other times I cannot reproduce the issue - as was the case the other day when I had some time and took it to the workbench to have a look at! It is curious that when it happens, it doesn't require much volume to 'burn' it free, but then I guess it only takes a very fine layer of oxide. I tend to run it for background music so there is not much volume getting generated. It only seems to affect one channel so perhaps only one set of contacts is affected at this time. I will see if there is a way to remove the cover to have a look and clean it.

[dmills]

It is the output relay, it is always the output relay on those amps. Well known failure mode.

The contact surfaces oxidise and it takes some volts to punch thru the oxide layer, at which point it once again works until it doesn't.
While you can open up and clean the relay, the only real fix is to fit a better one, those things were overly heavily 'Value Engineered' and it shows.

Regards, Dan.

[WaveyDipole]

Well I finally replicated the problem on the workbench and found that it was indeed the replay contacts playing up. I had what I thought was a drop-in replacement salvaged from another amp some time ago, but after checking the specifications found the contacts were rated at only 2A, but the original seems to be rated at 5A, which would make sense given output power capability of this amp. Fortunately the top on the original came off easily and I was able to clean the contacts. Both sets looked dark and oxidised. I was able to clean them using a bit of Servisol on a piece of paper which I sandwiched between the contacts and moved backwards and forwards. Quite a bit of muck came off and after 3 attempts the contacts came up nice and shiny for most part. I have replaced the relay on the PCB and it seems OK for now, but only time will tell and from what I have now read a replacement is the only proper solution so I've been looking for one.

I cannot find much information on the original marked HR-CR323DC24 and the closest information I found was this which I was not sure is same:

https://www.hobbielektronika.hu/forum/getfile.php?id=236968

I then stumbled across this thread which discusses the issue:

http://music-electronics-forum.com/t32060/

It mentions that this relay is no longer available but does also mention some alternatives. The last one, Finder type 40.52s is available from Farnell and I will have to place and order with them at some point so I will add this one to the order.

[radicalrendell]

I repaired my Servo 150 today.  As mentioned below it was the output relay.  I activated it in circuit with about 15v and measured the output resistance I measured about 2-4 ohms across each.   I could not find a new HR-CR323DC024 relay or at least one that seemed suspiciously harvested from old hardware.  Anyways a bit of research found a drop-in matching Omron relay for $11 CDN.  It works perfectly now.  Anyways hope that helps.someone.