DIY JCM800 pre-amp volume pot going bad

[jasonvilla]

HI all, I've had two pre-amp volume pots go bad in my DIY JCM800 build.  Its a pretty stock layout, with an addition to a reverb circuit this isn't near the pre-amp pot. It took forever to figure out the problem the first time but it looks like the replacement has gone bad as well. When it goes bad the amps output gets very low and theres no bass. Replace, or bypass the pot with a jumper, and it operates normally! My friend suggested that current is going across it and killing it. Any ideas on how to test this or other possibilities? Thanks so much!

[shooter]

Any ideas on how to test this

there should be a cap proceeding the pot to block big DC, swap it

[sluckey]

I've had two pre-amp volume pots go bad in my DIY JCM800 build.

JCM800 is a family of amps. I'm guessing you are talking about the popular 2204 or 2203? If so, I don't see much that would kill that pot. I don't think the coupling cap would do it because there is still a 470K resistor between the coupling cap and the pot, but check it for leakage or just replace it. You may just have bad luck. What kind of pot is this? Manufacturer? How old is this amp?

[jasonvilla]

Sorry, yes it's a 2204. Do I check for current or do I just check DC? Ive checked DC and it does really see any. It's a(2) new alpha 1M pot and its a new build. However these pots have and open underside, by the lugs so stuff can easily fall into them. I may have just gotten extremely unlucky, twice! Thanks for the suggestions guys!

[sluckey]

Did the pot make a scratchy sound as you turn it before it went bad? I'd probably just replace it with another Alpha. I've had good luck with them. If the new pot fixes the problem again, I'd say great, but probably keep an eye on it.

[jasonvilla]

Nope, it didn't make any scratchy noises and it doesn't now either - So i assumed that meant no DC. It does make crackling noises and volume drop off. Kind of like when a guitar cable is going bad. Ill try a third and see how it goes! Thanks!

[jasonvilla]

So I just thought of something that may have been a problem. The extra tube I added for the reverb was causing too large of a voltage drop on the preamp tubes. I was getting a static sound on the decay of notes. I realized that the voltage across the bias resistor on V2 was around .9vdc. I fixed the problem and the static noise went away. I know the pre-amp volume is only between the V1 triodes but could that have caused my volume pot deaths? Thanks!

[Willabe]

However these pots have and open underside, by the lugs so stuff can easily fall into them.

Not saying this is your problem but, we've had guys here who got flux splattered onto the pots resistive track when they soldered them up.

I put a piece of electrical tape across the pots opening in it's cover when I solder them up to prevent that from happening.

[sluckey]

no

[jasonvilla]

You might be right about the flux problem. This stuff I use has A LOT of it. I'll try the tape, thanks!

[Willabe]

That blue painters tape works great too. 

[sluckey]

I've used a LOT of Alpha pots in the last 10 years and never had any problems with stuff falling into the pot body.

[Ed_Chambley]

I use the blue tape too, but no Alpha pots.  Most boutique builders use them and the have a great feel, but I am snake bit with them.  I had a number of them go bad so I now use CTS, Bournes on the less expensive.  I like Honeywell and PEC as well, but from my observations Alpha pots are used more than any other.

Just like in pedals, you will see alpha.  You want to know the last problem I had with a pedal was.

[jasonvilla]

Just fyi this is the opening I was talking about on the alpha pots Im using. Im guessing that if the issue is flux then I would be able to spray it out and have it work again.

[Ed_Chambley]

Yes, you can.  Put a meter on lug 1 and 3 and see if your lugs are making contact.  Unless it is a LOT of flux, you should be able to get it to work some unless the wiper has gotten bent above the carbon track.


Anyway, if you do replace it, break it open so you will know what happened.  All the ones that I have had fail that I opened the wiper was not touching.  I aint rapping bad on alpha pots, it is probably me being heavy handed.

[sluckey]

If you think it's a problem then you can always use a sealed pot. Sealed pots are considerably more expensive.

[jasonvilla]

Does anyone use the WD40 electronics cleaner?

[sluckey]

WD40 is not an electronics cleaner. There are spray cleaners made specifically for electronics but you won't find them at WalMart. Best to shop at an electronics supplier store. Caig has a pretty complete line of various cleaners for general and specific purposes.

     https://www.tubesandmore.com/products/tech_supplies?filters=Brand%3DCaig

[PRR]

> WD40 is not an electronics cleaner.

"WD40" is now a Brand, not a product.
https://www.wd40specialist.com/products

They do have an electronics cleaner-
http://www.homedepot.com/p/WD-40-SPECIALIST-11-oz-Contact-Cleaner-300083/206356189
SDS says main ingredients are n-Hexane, Isopropyl Alcohol, and 1,1 Difluoroethane; but there could be minor ingredients.

I have never seen it, and would tend to use the stuffs I have known for decades. Caig is one, CRC(?) is another.

[jasonvilla]

Haha, yea not regular WD40... Thanks for looking up the SDS! I ended up with the CRC brand, as thats what they had at Homedepot. I used to use the contact cleaner at radio shack but they've all gone out of business around me. So I sprayed out the volume pot and it MIGHT of helped a little. Either way I had to replace the volume pot for the third time. Fixed the problem... though now I'm hearing weird noise on note decay... I tested the broken pot like you suggested and sure enough I could intermittently get a reading from lugs 1 to 3. If I get ambitious Ill open it up and see what happened. Thanks!

[sluckey]

"WD40" is now a Brand, not a product.

Ah, I'm still learning.