My official JTM45 kit build thread. Pics & CLIPS ADDED

franc · Post by **franc** » Sat Sep 01, 2007 1:03 am

I doubt it is the jacks that are wrong unless BOTH are bad. One of them should be working. To be sure, plug in a guitar cord and measure continuity between tip and 'front' of 68k and so on to the grid.

franc · Post by **franc** » Sat Sep 01, 2007 1:28 am

If the bright channel is working, the problem cannot be too big.

It's one half of a 12AX7, one 0.022u cap, the volume pot and then a 270k resistor...

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 1:58 am

Should I swap preamp tubes around? Or try a new 12AX7 in place of the one in V1?

If it was the 270k resistor, it'd be the first of the two, correct?

If it's the sozo cap, I'm screwed for the weekend because I can't get that in town. Any way I can test that cap while it's still in the board? How would I test resistors while they're in the board?

If I'm hearing some kind of noise floor increase when I turn that volume pot up, nothing connected, that would indicate that the volume pot is prolly ok, right?

It's probably possible that one of those allen bradleys is bad, though I did get readings off all of them before installing... There's not even a peep of a sound trying to squeek through when connected to inputs 2, either of them. That would have to be a full on short somewhere, right? I'm beginning to suspect that half a tube theory sounds most plausible right now. Of course there's a good degree of wishful thinking in that declaration.

franc · Post by **franc** » Sat Sep 01, 2007 2:11 am

Yes, the 270k with 500p cap across is for the bright channel. The other one is for the normal channel. As you can see, after that there's no difference between both channels anymore so any problem should be before that.

What you could try is swap both channels. You could take the left ('input') lug of the bright channel volume pot and connect it to the left lug of the normal channel input lug and vice versa.

What you're essentially doing is swapping both halves of V1 and both inputs! Be aware of that if you decide to give this a try.
So with a guitar plugged in to the Normal inputs, your amp should respond to the Bright Volume.

See what happens. If your normal channel (channel 2) is still not working, then problem is with the pot, the wire, the 270k or any of the connections there.
If now the normal channel starts working and the bright channel isn't, the problem is the 0.022u Sozo or the first half of V1.

Of course you could try putting in a new tube in V1 first, just to check whether it's a bad tube

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 10:39 am

franc wrote: What you could try is swap both channels. You could take the left ('input') lug of the bright channel volume pot and connect it to the left lug of the normal channel input lug and vice versa.

What you're essentially doing is swapping both halves of V1 and both inputs! Be aware of that if you decide to give this a try.
So with a guitar plugged in to the Normal inputs, your amp should respond to the Bright Volume.

See what happens. If your normal channel (channel 2) is still not working, then problem is with the pot, the wire, the 270k or any of the connections there.
If now the normal channel starts working and the bright channel isn't, the problem is the 0.022u Sozo or the first half of V1.

I've swapped the first lug wires on the two volume pots. The result:

The second (inputs 2) jacks still aren't working. Now if I plug into an input 1 jack, the NORMAL knob gives me volume. If I plug into input 2, the high treble knob does not give me volume. (this also rules out bad or miswired input jacks)

SO, that means it's either a bad.022 sozo or the 12AX7 tube in V1? And if it turns out to be neither of those, a bad V1 tube socket maybe? Is that what you meant by the first half of V1? Or are you also referring to the 68k's and the one 270k as well?

I can't get a .022 sozo in town here obviously, but I'll go get a tube when they open and try it. If it's not that, I'll be done troubleshooting I guess until I can get a .022 in the mail which would be like a week. That would suck ass. Because I'm already addicted to having my hands inside this thing.

If it could also still be the 68k's and the 270 let me know and I'll try those too if I have to.

Post by **VelvetGeorge** » Sat Sep 01, 2007 11:13 am

If you suspect a 68k, just bypass them temporarily. Wire a short jumper over them.

I suspect you double check the jack wiring. And check under the board for a pinched wire.

If the tube is the culprit, usually the cathode voltage will indicate it.

George

franc · Post by **franc** » Sat Sep 01, 2007 11:39 am

So it's definately not the pots or whatever's after them, you have ruled that out.

Do what George suggests to make sure it's not the jacks, the wiring to the 68ks or the 68ks themselves.

Another thing to try is swap the 0.022u Sozos.

Undo the other changes to avoid it becoming a mess... you already know what's NOT the problem

don't create any there.

So when you have done that swap the Sozos. If channel 2 still isn't working, you know the Sozos are okay. Actually, check whether ch 1 still works in case you have more than one problem!

Slowly working our way back to the inputs

we'll find it.

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 12:31 pm

George, swapping the first wire on each volume pot confirmed the inputs jacks are working and wired fine. When I lifted the board, I checked to make sure the top wire of the jack went to the 2nd hole on the board, and the lower wire went to the 3rd hole, so that part is right too.

I've now swapped out the 12AX7 in V1 and it made no change, so it's not the tube. Unless it's the tube socket, but George, you're saying nope on that.

I decided to look in the bins at the guitar shop repair area. A drawer labeled .022 had this used old mustard cap in it. Is 22n the same measurement as .022 uf? Here's a couple of pics of it:

Is this the correct old stock version of my sozo .022, or is this the wrong value?

franc · Post by **franc** » Sat Sep 01, 2007 12:42 pm

0.022u is the same as 22n so that one is correct. 400V might be on the low side, but aren't the Sozo's rated 400V also?

Try it.

franc · Post by **franc** » Sat Sep 01, 2007 12:43 pm

BTW: you have NOT proven yet that the inputs of ch 2 are working!!
You said that after the swap inputs 1 worked with the normal volume, but that doesn't mean that inputs 2 are working!!

Flames1950 · Post by **Flames1950** » Sat Sep 01, 2007 12:48 pm

I'm with George, I still have to believe the signal is grounded out right at the jacks.........

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 1:03 pm

franc wrote:BTW: you have NOT proven yet that the inputs of ch 2 are working!!
You said that after the swap inputs 1 worked with the normal volume, but that doesn't mean that inputs 2 are working!!

Shit. That's right.

Ok... In order of easiest to test first:

1: I'm going to swap out that mustard for the first .022 on the board (btw, yes the sozo's are rated at 400v)

2: If it's not the .022, then I'm going to bypass the 68k's... one question though, george said to wire a jumper over the 68k's to bypass them. How do I do this? a wire from one or BOTH (if one, WHICH one) of the 68k's bottom turrets to the single turret they meet at above them?

3: If the above don't work, I'll need to desolder a couple wires from preamp sockets to get the board lifted off far enough to really see underneath.

4: disconnect and remove the inputs 2 jacks unhook them completely and rewire them from the beginning again to find out if it's either the wiring or a bad jack. This really seems unlikely to me, the wiring part, I've looked at it a 100 times, comparing it to the wiring of the input 1 jacks and it's exactly the same. Unless that resistor on that set is bad... I tested the ground on that set of jacks and get continuity to ground.

Does this sound like a reasonable game plan? Is the 270 for that channel definitely off the table for being a possible culprit at this point?

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 1:07 pm

Flames1950 wrote:I'm with George, I still have to believe the signal is grounded out right at the jacks.........

Well, if that's the case, wouldn't the most logical thing to try first be to switch the green wires on the jacks going to the board so the input jacks (input 1) function when routed to the normal channel circuitry? Because the input 1 jacks for sure work fine.

That'd tell me if the problem was at the input 2 jacks themselves or after them...

franc · Post by **franc** » Sat Sep 01, 2007 1:11 pm

mushmouth wrote:
Flames1950 wrote:I'm with George, I still have to believe the signal is grounded out right at the jacks.........
Well, if that's the case, wouldn't the most logical thing to try first be to switch the green wires on the jacks going to the board so the input jacks in the input 1 position lead to the normal channel circuitry?

That'd tell me if it was at the jacks or after them...

Yes, diconnect both green wires from bothe channel inputs and connect ch 2 inputs to channel 1 green wires. Check ch 1, if it works, your ch 2 inputs are not the problem.

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 1:38 pm

Ok, I swapped the green wires from jacks 2 to jacks 1 and vice versa.

NOW, with the wires swapped. I plug into jacks 1 and get nothing when I turn up the normal channel. If I plug into the input 2 jacks, the high treble volume now works fine with those jacks.

So NOW we've eliminated the jacks and the jack wiring.