Re: Trace Elliot Commando 10 bass combo repair.

[dave]

When I was a youngster I was into hobby electronics. In other words

>I've had no formal electronic education but I've had a lot of fun. > >At the music school we've got a number of non-working Trace Elliot >Commando 10 combos and it is about time to get them back into action. >So I am going through them one by one to eliminate the obvious >problems -like components that have been vibrated so hard the wires >have fractured and the solder joints gone - and then we plan to pass >the ones with real problems to a professional amp repairer. > >I have one which has stopped humming ever since I soldered the >smoothing capacitors back onto the circuit board :)

As a matter of interest, How were they "off" or was it dry joints?

but has a very >scratchy-sounding pot on the input gain. I've blasted switch-cleaner >through it and it has made no difference: the other pots work fine.

Well, the other pots have no relevance to the sus one - but it is possible to remove the pot and sub a new one? The larger Trace amps have a separate Power amp stage/ pre-amp. The PA is ok to work on but be preamp - not really. Trace pots sometimes have very long spindles and takes some hunting to find replacements.

>Something deep in my memory is telling me that the scratchyness is a >symptom of something else like an open circuit resistor or a failed >capacitor.

Have a good visual for dry joints - (as the caps probably were.)

It doesn't feel as if it is the input gain pot itself >because all the other pots are working fine and it's sort of "loud and >noisy" rather than the crackle I associate with broken or cracked >tracks . So can somebody suggest what's going on? Should I be getting >some meter readings off some other components? Or is this one to go on >the pile for the pro?

Well check the PSU dc supply if you can - even an analogue meter can sometimes indicate a ify psu rail.

[Huge]

I don't suppose you know where I could get an input select switch for an Arcam Alpha 9, do you? The switch in mine has gone noisy and no amount of cleaner seems to achieve anything. The switches are not available from Arcam or anyone else, it seems, and it's a common fault.

[Arfa Daily]

Is that the switch that's made from several square wafers stacked behind one another in a metal 'frame' ? If so, the switch can be removed and carefully dismantled, noting exactly the positions and orientations of each wafer, as they are different. The wafers can then be opened, and the contact faces and wipers burnished to remove the oxidation that's causing the problem. A drop of cleaner / lubricant in the wafer as it's reassembled, prevents a recurrence. Of course, if that's not the switch type, I'm telling you absolute bollox !! :-)

Arfa

[Huge]

Sadly not. It's a closed metal box, and what's worse, the switch is powered by a tiny motor (for remote control). I've thought about removing it from the motherboard & dismantling it, but each time I decide against it.

[Nick Odell]

I guess so. Whether the joint was badly made in the first place or whether the constant vibration just wore it away I couldn't say.

These ones are PCB versions of ones I use in electric guitars only the resistance values are different. If this amp repairing becomes a habit I may have to get a little stock of components in but at the moment I'm trying to isolate the problems with a multimeter and advice from you all and (I hope) some common sense.

Yup. That was it. Remade the joints on the capacitors between the input volume control and the input stage and all was quiet again. Thanks!

Thanks,

Nick

[Arfa Daily]

No, I think we are talking about the same one. That 'box' is a sort of frame that holds the individual wafers. Once the switch is removed - and you need a decent desoldering station to do that - the shaft can be removed and then the box sides 'flexed' to release the individual wafers. The trick is to mark everything with a Sharpie pen as you dismantle it, as once the shaft is removed, the individual wipers can be rotated out of sync with one another. Also, as I recall, not all of the wafers are the same, so you need to make sure that you get them back in the right place, and facing the right way. It's been quite a while since I last did one, but it's actually nothing like as difficult to do, as it sounds.

Just to be sure that we are talking about the same thing, I think I remember there being perhaps about five sections to the switch, with each section having I guess around twelve pins in a straight line ? Plus the motor drive, as you say.

I might still be giving you a line in bull, but I don't think so. I'm sure it's the Alpha 9 switch that I'm remembering here ... d :-)

Arfa

[Huge]

I think this is the sort of thing I will put off for a rainy day. Like, for ever. Especially since providing you only have a single high-level source, so you plug it into the tape player input, the amp works just fine. Indeed, it's plugged into my PC to provide music in my study.