Jet City 20 combo Troubleshooting

[Enzo]

I think Justin was making a joke. The EL84 is so erroneously drawn it looks more like a dual triode. So instead of ECC83 for a dual triode, he named it the ECC84.

[Justin Thomas]

As Enzo said - I was making a joke. That is one seriously messed up schematic. The heaters are connected to the HVDC supply, plates connected to nothing... grids to the OT plate leads... And yes, the EL84s are drawn as triodes.

Justin

[flatrockmobile]

Sorry. I guess I'm not up to speed with tube amp humor. lol
With all the mistakes being pointed out on the schems, I had to look and see if they were in fact mislabeled.

[DrGonz78]

How hard would it be to just draw up the correct diagram symbol for the EL84 pentode? I mean they spent all that time writing out the damn thing and then used dual triodes, yuck! I mean that is a very bad way to show no connections on pins 1 and 6. After all that they did get the pin numbers correct.

Speaking of Chinglish: Reminds me of a Chinglish operator's manual for a drone I recently came across.

"The holder of this product needs to use safety method to play it,If controll it with wrong method,It may occur serious physical injury or property losing. We declear that we will not bear any responsibilities, Because we can not control their installing,using and controlling process."

"Kee away from the high-speed rotating parts. While the blades are rolling with high speed,the operator and people arround should far away from the rolling objects to avoid danger and damage."

"Keep away frow the hot soune."

There are too many to post, but reading the thing was just a great laugh. Trying to understand how to operate the thing proved to be that much more difficult indeed.

[nickb]

IIRC the JC2112RC is basically the same as the Soldano Astroverb 16. There are a couple of component value changes - I have a note of them somewhere but you probably won't need that.

[flatrockmobile]

Its been 10 days and a lot of playing. I'm declaring this one officially fixed! A couple of days after making the repairs, I stumbled on this vid of a JC 20w head with a bad bias pot. He says his volume was all the way up while making the vid. This is what mine sounded like too. Thanks again for all the help in getting this up and running. Awesome forum!

https://www.youtube.com/watch?v=sHW2q15FDwE

[flatrockmobile]

Wow, that was short lived. Started having the exact same symptoms about 6 weeks ago. I've done all the same troubleshooting with no luck. This time though, I can whack the cabinet and it will be normal until I strum. I've taken the amp out of the combo cab multiple times to try different things and every time it will work beautifully on the bench, even when whacking with a rubber hammer all over the chassis. I put it back in the combo cab and acts up again. I've backed all the mounting screws out just in case they were too close to the circuit (which they weren't) but no difference. Tried different speakers, different taps on the OT, different tubes, cables, everything I can think of, but no luck.

[flatrockmobile]

Update on troubleshooting efforts:
When the volume cuts out to 1/4-1/3 volume and sounds tinny, the bass and middle pots have no effect on the sound.
The treble pot (which I replaced along with the MV) does but when turned all the way down, I get no sound at all.
Looking at the schematic, I'm thinking bad bass pot causing the signal through the treble pot to have a path to ground when turned to zero.
I'll be replacing it next week when I have a little time, but....What do the geniuses here think?

[flatrockmobile]

Fixed...Replaced bass pot and mid pot. All is well with no cutouts