Mojotone 18w TMB build - no sound (rookie builder)

[Returnsnull]

Hey folks. I just recently completed a Mojotone 18W TMB kit and there's zeroaudible output from the regular channel and the TMB channel only outputs an extremely faint hum if all controls are maxed out.

I've attached two voltage charts both with and without the tubes installed.

I don't know really what I'm looking at, but my issue appears to be in the preamp area, mostly around V1 based on other voltage charts I've seen. I'd appreciate any thoughts or tips and can provide anything you need - photos, videos, audio clips, voltage / current readings - just let me know.

I'll work to get some photos added soon.

[JMPGuitars]

Hi, welcome to the site!

Your description, and your voltages tell me you need to play the highlighter game. That means you need to print a copy of the SCHEMATIC and the layout, and verify every single connection and component (including values). Highlight each piece as you go.

Your rectifier (V6) voltages look fine, everything else doesn't. Your power tubes are poorly biased, but you need to fix the rest before you mess with that as the other voltages being off may be affecting it. V1, I assume is the input valve, looks WAY off. I assume V3 is your PI, and that is also way off except for pins 1 and 6.

Compare your voltages here, and ignore V3 on this chart, it's for the tremolo. V1 is input, V2 is PI. I may eventually add TMB voltages to another chart. Here's the classic voltages: storage/Voltage-Chart.htm

Thanks,
Josh

[Returnsnull]

Thanks for the advice, Josh. I've updated my original post with pictures and I'm off to start playing the highlighter game now.

[Returnsnull]

1 issue found so far, I had a couple of resistors swapped between the can caps. Swapped those out and voltages changed pretty significantly, but still pretty far off and the problem still persists.

Onward!

[Returnsnull]

Okay! Found two more missing turret jumpers and I now get sound out of the normal channel. Also, there was a pot lug that wasn't quite making its connection to ground.

Lesson learned: if you have an isolated turret out there not connected to anything, that has a 99.9% chance of being a problem.

I'll get an updated voltage chart after work tonight.

An interesting note is that my TMB channel has no output at all and the TMB controls (mostly Treble) seem to affect the normal channel. Not sure what I've gotten screwed up there but I'll keep up this systematic approach until I get it figured out. (Sure would be grateful if anyone recognizes this issue though )

Also, if I'm following Mojotone's input wiring scheme for the normal channel, I can only use one of the inputs (the one with the 1M resistor). If I break the ground connection (the red X in the picture) the other input becomes active (but, of course, breaks the one that was working before).

InputWiringScheme.jpg

Seems like the solution here is to separate the ground connections but I'm wondering if anyone else sees an issue with their proposal? Or maybe I'm reading it incorrectly?

[JMPGuitars]

Are your input jacks wired correctly with good connections? It sounds like probably not? Highlighter time!

Besides that, the diagram you posted, and consequently your amp build, has a fairly lame ground scheme (not your fault). Make sure the path to ground from the jacks, to the pots, through the bus wire, to ground is all good. We can discuss improving your ground scheme later (or you can search for my posts on the subject).

Thanks,
Josh

[Returnsnull]

Attached is the updated voltage chart as promised.

[JMPGuitars]

Attached is the updated voltage chart as promised.

You should ask mojotone if they have a chart of target voltages. Here's another thread with voltages to compare your voltages to: viewtopic.php?f=4&t=25324&start=15#p242334

Your B+ is better than his, but I don't know if they're final voltages or not. You'll also see that his V1 and V2 (Phase Inverter) voltages are closer to the voltage chart I linked earlier: storage/Voltage-Chart.htm

A little variance is okay as you can see on our voltage chart. But your stuff is still way off. Keep going with the highlighter. Start fresh if you have to. V1 is crazy, so you can try swapping with a different tube and see if it's a faulty tube or not. If the voltages are still crazy, check all the connections and component values.

EDIT: Some users here are listing their Mojotone tube voltages inconsistently. You need to determine if V3 is a preamp tube, or your Phase Inverter. V2 in our voltage chart is the Phase Inverter. From your build photos it looks like V3 should be your PI.

[Returnsnull]

I've requested the reference voltages (which are referenced in the documentation but not included, so hopefully they have those).

I'll try and confirm which tube V3 is. I know V1 is input, the other two I'll need to educate myself a bit more to determine.

[Returnsnull]

Well, as soon as I hit submit I saw the email from Mojotone come in. Okay, one day in the future someone else is going to come across this thread troubleshooting their own build that didn't come with the reference chart, so let me keyword the heck out of this:

Mojotone 18-watt 18w TMB voltage test points and reference chart for voltages - PDF

Mojotone_18wTMB_Voltage_References.pdf

I'll try to verify these today at lunch!

[Returnsnull]

See attached for the most recent readings for the Mojotone reference points. There's weirdness everywhere, but especially around 5/6 and 8/9/10. I've attached a 18watt.com voltage chart as well.

I've validated the connections and components (even continuity tested all the connections). I know it has to be something simple, especially the trouble I'm having with the inputs but I'm at a complete loss.

I'm going to highlight... again.

Mojotone_TestPoints_RefVoltages.pdf

Mojotone 18W TMB – 04102019 1945.doc

[JMPGuitars]

Rewire your inputs to match this: storage/18_Watt_TMB.pdf

Then check all the solder connections, or rework them if you have to around V1. Then the next tube, testing voltage at each stage.

Be careful testing continuity, it's not always useful, and sometimes dangerous in amps. You're often better off testing resistance.

Thanks,
Josh

[chunter]

I see that your V3 pin 3 voltages are similar to the readings I was initially getting using the new revision (3.0) of the Mojotone wiring diagram. I changed the wiring on pins 2 and 3 to follow an older revision and that corrected my low output volume and the PI voltage readings improved (now match reference voltages posted by others).

Slide1.JPG

[Returnsnull]

I changed the wiring on pins 2 and 3 to follow an older revision and that corrected my low output volume and the PI voltage readings improved (now match reference voltages posted by others).

I was going absolutely mad trying to figure out what I screwed up. Thank you for catching that issue, Chunter. I've made that change and I have *much* more sane voltage readings now. Only thing left to do now (I think) is sort the wiring on the TMB input jack. I had to change the wiring on the regular inputs to match the Trinity wiring instead of Mojotone's.

[JMPGuitars]

I had to change the wiring on the regular inputs to match the Trinity wiring instead of Mojotone's.

Trinity? Don't use Trinity, use Richie/Gabriel's layout I linked previously: storage/18_Watt_TMB.pdf

[geoff 1965]

hello,
the trinity TMB does'nt have a master volume it has hi/lo inputs so you can't use that as a reference,also the richie layout is different it does'nt have the 68K resistor on the TMB input.
i think i've found the problem here,the mojotone build guide says to wire the green wire from pin 2 to the rear left lug of the TMB input jack but it should go to the front left lug. have a look at this 2015 mojo layout.

18-Watt-TMB-WD.gif

[Returnsnull]

Thanks, Geoff. That layout matches what I ended up going with.

[geoff 1965]

good! i can see why you struggled with the jack wiring,apart from the error in the build guide the illustrated inputs on the latest layout don't show a clear wiring scheme like the older layout.

[JMPGuitars]

the richie layout is different it does'nt have the 68K resistor on the TMB input.

That's true, the layout doesn't have the 68K grid stopper resistor on the TMB input. Do you know what that resistor does?

[Returnsnull]

I initially wired mine up without the 68k resistor and had a lot of radio noise being picked up by the amp. Reinstalled it to see if it helped and the radio noise disappeared. Found this article (http://www.regiscoyne.com/tech/68kinput/) that seems to indicate that is its purpose as well.