scratchy pots on old bass amp - diyable?

Open up. Identify pots. Values should be printed on the back, or on the PCB. Just check which are logarithmic[1] (probably the volumes) and which are linear (probably the tone controls).

Probably worth trying a few squirts of cleaner before plugging the iron in.

Owain

Indeed - had a problem with scratchy pots and input selector switch on a Kenwood hifi amp. Quick squirt with WD40 followed by working the knobs/ switch back and forth a few times restored normal operation.

If you can solder its perfectly doable, assuming its not an oddity with custom pots. I once had an amp with a 4 gang volume pot with centre taps - no chance of replacement!

NT

Now, what can WD 40 _not_ do? ;-)

Dave

WD40 is not really the recommended stuff for this, in the longterm. However, it does work ...

Ah, ye should have defeated (removed, exorcised, castrated, mullered) the loudness switch ...

There are better products for this particular job - but it was to hand, and did the trick...

On Sun, 30 Aug 2009 14:24:52 -0700 (PDT), NT had this to say:

I once had a custom pot made by Morganite Resistors. It was a dual gang semi-log and reverse semi-log with a centre click. It cost me nothing; they were so intrigued by the order :-)

I think I might still have it somewhere - I can't remember ever getting the correctly-shaped tuit for whatever it was I was going to build - hell, it was about 30 odd years ago.

Not too bad. A squirt of contact cleaner into them by whatever means is worth a try. If that doesn't work, they're typically held together by bent-over metal tabs on the bodies, and you can carefully take them apart, clean directly, then reassemble.

If they're standard shafts, values and mountings (if direct on a PCB) or there's room for some (screen) fly wires then you can always replace.

Even if the shafts are something oddball, you may be able to mate the shafts from the damaged ones to a modern part.

The only time it gets really tricky is when the clearances are too tight to get an off-the-shelf part in there (oscilloscopes are notorious for this!) or the part's really oddball.

cheers

Jules

I would never have expected it to do the job. I always use Super Servisol 10 switch cleaning lubricant.

You knew that, didn't you ;-)

Dave

I have a hefty Trace Elliot bass amp and the input pot went slightly scratchy on the first few degrees. TE went bust (have returned now) and the only supplier for these pots I could find was in Canada! There's nothing special about the pots except the shaft length - which is rather long. I didn't buy the thing from Canada of course and gave up looking when switch cleaner fixed the problem. I can recommend EML switch cleaner. afaik WD40 is a moisture displacer.

Water dispersant, to give it its right title. Hence the WD in its name.

Dave

I once used switchcleaner on the pots in a Unaohm spectrum analyser. They froze solid and had to be replaced.

Bill

It sdepends if they are on the board, or connected by wires. If the latter its a tricvial excecrise..mut do take pictures of which wires go where.

If the latter its a bit harder.

As clearing solder from the pins and breaking them out of the board is a bit more skilled. If they are being replaced, cut them off and remove the leftovers.

Replacements hafe to physically fit, be the same value, be log or linear taper, and have the same shaft to fit the knobs (round, round-with flat, or splined)

Apart from that, butyy moulded track if you can find them. Lats longher.

Try a spray of contact cleaner first tho. It may be just dust..

It is possible to buy shaft extenders. Or cannibalise a set of drain- rods from a Lego Utility Workers set.

Owain

Normally John, I would hang on every word that you would have to say on this group, but not this time. Please please don't recommend that DIYers bring WD40 anywhere near electronic equipment. It is not designed as a switch cleaner / lubricant chemical, and whilst in some very few instances it may be effective in providing an apparent cure to some electronic ill, in the vast majority of places that owners try it - for cleaning and lubricating CD drives, tape decks etc - it spells instant and irreversible death. I have lost count over the many years that I have been involved in electronic service, of how many owners' pride and joy items have been wrecked by use of WD40. My heart sinks every time one appears on my bench with that characteristic smell wafting up from it ...

There are many different 'proper' switch cleaning compounds available at very reasonable cost from the likes of Maplin.

I agree that before trying to remove the pots for replacement - and this is often a very long and involved job on this type of amplifier due to the construction techniques involved (I repair them for a living so I speak the truth ...) - a good squib of switch cleaner should be introduced to each pot to see if that cures the problem. Put in one squib through the hole (usually) formed in the pot case by the pressing out of the end stop, with the pot at the minimum end, and the cleaner nozzle directed to the full end. Then set the pot to full, and put in another squib with the nozzle directed to the minimum end. Now vigourously 'work' the pot from end to end. Using a decent proper cleaner / lubricator, this will cure at least 80% of scratch pots.

Arfa