My official JTM45 kit build thread. Pics & CLIPS ADDED

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 6:32 pm

Is that "820" strapping the 330uf, the first thing on the board, is that supposed to be 820 ohm (grey-red-brown) OR 820Mohms (grey-red-violet)? Can't tell in the pic, but that's for the normal channel correct? The normal channel 1st stage Rk/Ck, whatever that means, wondering if that's the wrong allen bradley I have on there or if it's bad.

Which is it supposed to be 820 ohms or 820Mohms? If it's relevant, I'll swap it.

Post by **VelvetGeorge** » Sat Sep 01, 2007 6:38 pm

820.

What about the pot for the normal channel? Try swapping the wire from the .022 on the normal channel, to the pin 1 of the channel one pot.
That will route it through the bright channel pot, and the 270k/500pf mixer.

VG

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 6:39 pm

VelvetGeorge wrote:Touch the common terminal for the 68k's. And pin 2. You should have the same "pop" both places.

VG

black meter lead ground to chassis, amp on standby ON, volumes up a bit. No "pop" touching the red meter lead to pin 2 of V1 OR the point where the first two 68k's come together...

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 6:43 pm

I actually did this back on page 6, one of the first things I'd tried. Here's what I wrote:

mushmouth wrote: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.

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 6:44 pm

VelvetGeorge wrote:820.

(grey-red-brown) OR (grey-red-violet)?

OnTheFritz · Post by **OnTheFritz** » Sat Sep 01, 2007 6:54 pm

820 = grey-red-brown

There's a color code chart for resistors on the cd that George included in the kit, I printed it out for quick reference.

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 6:56 pm

OnTheFritz wrote:820 = grey-red-brown

There's a color code chard for resistors on the cd that George included in the kit, I printed it out for quick reference.

I know. I wanted to double check it. Any chance that one could be bad and killing the normal channel?

Never mind, just checked it and, connected, it's reading 842...

Edit to add: Where'd everyone go? Did I fart?

What, is it a holiday weekend or something?

New summary:

It's not:
the jacks
the first 68k's
the first 270k
the first .022 Sozo
the preamp tube in V1
or the v1a plate 100k (the left one of the v-formation)

With the amp on, if I tap the meter lead against the lead of the first .022 on the jack side I get a quiet buzz. If I tap it on the tube side of that .022 lead I get a loud "pop".

Tapping on the first 68k's common point, I get no "pop." Tapping on pin 2 of V1 socket, I get no "pop."

toner · Post by **toner** » Sat Sep 01, 2007 9:22 pm

It looks like almost everything has been checked.

If you haven't checked yet, you could try removing V1 and checking the socket pin holders on pins 1 through 4. Make sure the two halves of each are close together so they hold the pins securely.

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 9:35 pm

toner wrote:It looks like almost everything has been checked.

If you haven't checked yet, you could try removing V1 and checking the socket pin holders on pins 1 through 4. Make sure the two halves of each are close together so they hold the pins securely.

If that were the case, wouldn't I NOT get readings on those pins?

Edit: I pulled the preamp tubes and compared. The pin holders look about the same on all of the sockets.

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 9:57 pm

Would someone be willing to take this board overview and draw over the top of it the full signal path the normal channel takes from input to output jack?

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 11:43 pm

I created this graphic of overlays (wiring, components, and backside wiring together) from the various pics in the instructions as a learning device to try to get a grasp on the total signal flow of the normal channel, but it's of little help to me since I don't understand what happens in the tube sockets with signal...

Edit to add: I'm reading in the glossary of terms here, and looking at the marshall 101 page to try to put it together. My goal is to find every wire, every component, every path that is part of the normal channel that is NOT shared by the path of the treble channel. There's got to be a wire (under or at edges) or pin or SOMETHING we've missed. Otherwise it defies logic and is acting like the normal circuit itself is an incorrect circuit. And of course it isn't...

franc · Post by **franc** » Sun Sep 02, 2007 1:20 am

Sorry man, it got dark over here and I just had to get some sleep

I'm trying to draw you that picture.

To be honest, I almost suspected that 330u/820ohm pair too, but than again, it's shared cathode, so it is in use or both halves of V1, so if something was wrong there, you wouldn't get any sound at all.

mushmouth · Post by **mushmouth** » Sun Sep 02, 2007 1:31 am

franc wrote:To be honest, I almost suspected that 330u/820ohm pair too, but than again, it's shared cathode, so it is in use or both halves of V1, so if something was wrong there, you wouldn't get any sound at all.

Yup, I dropped that too as soon as I read that it's shared at the marshall101 page...

I'm seriously considering unsoldering the board from one whole side, flipping it over, and redoing ALL wiring that involves the path in question. Unless there's something with that v1 socket, and unless there's something we've missed, I fear I'll have little choice. I feel like I've totally worn out my welcome with questions on this problem here.

franc · Post by **franc** » Sun Sep 02, 2007 1:53 am

Okay, here's my go at the signal flow of the normal channel (channel 2):

I'm not much of a photoshop artist, so excuse the bad drawing (my 3-year-old is much better at this to be honest

)

I stopped at the tonestack-input because it gets to hard to draw and it doesn't really matter for this problem.

A little tube-theory might be of help. Let's see how I can explain this, because there's more than just the signal path to make it al flow, you'll understand.
Look at it this way: you have this big flow of elektrons going from cathode to plate. You have a very small signal coming from the inputs to the grid.
The signal on the grid 'irritates' the big flow of elektrons. If it's negative it slows down the elektrons and if it's positive, it speeds them up.
So what happens? The big flow of elektrons is reacting to the small signal on the grid -> you have amplified the signal, the big flow of elektrons is now your signal.
The elektrons have to come from somewhere, that's why your cathode needs to be connected. They have to be encouraged to go to the plate, that's why you have the filament 6.3V on pin 4/5 and 9 (to free the elektons) and high voltage on the plate.
The high voltage comes from the 100k resistors. One for each halve (there are actually 2 sets of cathode/plate/grid inside every 12AX7)
The high voltage is DC. You have 'added' AC (your signal) and in order to separate the two, you have these Sozo caps also connected to the plate. It stops DC but lets AC pass through. So at the tube side of the Sozo, you have high voltage DC plus AC (your signal) added, at the other side you have just your AC signal. That's also why you get different noise when you tapped both sides.

The yellow wire goes to the cathode of the first half, but also to that of the second half (shared cathode) and goes to ground through the 330u/820ohm combo.

Hope this makes any sense. Tried to tell it in simple words, but I am not a good teacher

mushmouth · Post by **mushmouth** » Sun Sep 02, 2007 2:33 am

I'm just about to read all of what you've just written, but I wanted to mention this. I just made a jumper cable with aligator clips. I used it to bypass the 68k, the 100k, and the 270k for normal channel. No difference.

Then I used it to create a new wire for each point at the normal jacks to board, and normal pot to board (to eliminate a front wire). Then I used the jumper to create a new wire from each normal channel turret that leads to a pin on the V1 socket (the 68k common to pin 2, the 100k V1A plate turret to pin 1. STILL NOTHING. So I've jumpered resistors, ruling them out. I've checked the wires going to V1 socket. I've checked the pot wires and jack wires to the board. I've had the .022 out and measured it... this is insane. Going to read your thoughts now.

Edit to add: I just read that. A lot of that is really over my head this early in the game. I know too little about electricity, the science of it. What I do know, though, looking at everything I've tried so far, it just seems like it has to be something going on at the v1 socket. If once it leaves the mixer, and I've eliminated the A side 270 being the culprit by jumpering over it, everything is the same from there on (and treble channel works fine) there is nothing else to check besides what's happening with the tube socket. I've already tried the tube.

At this point, it might be helpful for me to rerun a check of ALL my readings again just to see if anything is drastically different and jumps out. That might help.