Roland JC120 Head: Jacks Intermittent and POTs Crackle

[Zulu6]

Hi Guys,
Just found your forum. I have a 1984 Roland JC120 Head that I bought brand new 20 years ago. It's been mothballed for about the last 10. I hooked it up recently to an old EV 2x12 Cabinet that I used back in the day. Turned the amp on and the input jack pops as I move the cord. It's a brand new cord so I believe the sleeve is out of round. Tuner cleaner didn't alleviate any of the popping from moving the plug in the jack. The other thing is the POTs crackle when you turn them. It's pretty severe if I get the amp volume up. Funny the volume POT is the worst... lol. Anyway, replacing the input jacks is pretty straight forward. Can anyone tell me if the potentiometers are all Logarithmic or may some be Linear. I got a schematic off of another post in this forum. However there is nothing that denotes Linear or Logarithmic POTs. I know that volume pots are Logarithmic. I suppose I could measure the resistance with a DMM and see how fast the resistance changes with respect to the knob position. I haven't even cracked the head yet to investigate these issues. But I'm fairly certain the POT contacts are corroded and they need replacing.

Any info you'd care to contribute is desired.

Thanks
Jack

[g1]

Try cleaning/deoxidizing the pots before replacement. Not sure what kind of tuner cleaner you have, for the controls you want something that leaves a lubricant behind.

[The Dude]

+1 for g-one. I'll add: If you don't have anything handy, Radio Shack sells Deoxit, which is a very good cleaner/lube. I'm not sure what you (the OP) used, but all cleaners/lubricants are not equal.

[Mick Bailey]

Many cleaners sold for general electrical work will kill a pot. They may give an instant improvement, but shortly afterwards the pot will be much worse than when you started. You need a product specifically designed to clean and lubricate a pot. Deoxit, and Kontakt PR work very well and unless the track is actually damaged you should be able to get them working good as new.

Also, use the absolute minimum product to achieve the desired result. Flooding a pot does more harm than good.

Your input jack could be a bad solder joint. I'd try to get the original working rather than replace it.

If you bought a brand new 1984 head 20 years ago, is this now 2004, or am I on glue? (possibly both).

[Mick Bailey]

Many cleaners sold for general electrical work will kill a pot. They may give an instant improvement, but shortly afterwards the pot will be much worse than when you started. You need a product specifically designed to clean and lubricate a pot. Deoxit, and Kontakt PR work very well and unless the track is actually damaged you should be able to get them working good as new.

Also, use the absolute minimum product to achieve the desired result. Flooding a pot does more harm than good.

Your input jack could be a bad solder joint. I'd try to get the original working rather than replace it. Is there any external corrosion on the amp?

If you bought a brand new 1984 head 20 years ago, is this now 2004, or am I on glue? (possibly both).

[soundguruman]

Take the board out, re-solder all the jacks and controls to the circuit board.
Clean the controls.

Don't usually need to replace the controls. Just resolder and clean them out.

There IS one jack that usually needs to be replaced.
It's the foot switch jack for the chorus and vibrato.
The switch contacts on that jack goes bad quite often, and cleaning it is usually not enough to keep it working.

[Zulu6]

If you bought a brand new 1984 head 20 years ago, is this now 2004, or am I on glue? (possibly both).

Sorry Mick, 30 years ago.... Now I feel old... thanks for that.

[teemuk]

However there is nothing that denotes Linear or Logarithmic POTs.

I'm looking at the schematics now. I'm seeing denotes like "10KB", "1MB", "10KC", "250KA", "250KB"... etc. How are these NOT denoting linear versus logarithmic?

I actually see indicators for use of linear, logarithmic and even reverse logarithmic tapers.

Though I have to add, it fails to denote what specific logarithmic taperings were used (e.g. 10% vs 30%).

I know that volume pots are Logarithmic.

Ideally they should be. In this amp they aren't.

[Zulu6]

Ah a little wikipedia on Potentiometers tells me the A, B and C indicate the type of taper on the POT. Thanks for the Heads-up Teemuk.

[gbono]

Before you replace the pot I would check to see if there is any DC voltage present due to a failed/failing capacitor.