Mojotone 18 TMB + No Sound = Sadness

[sethb85]

First time poster, but I sure do appreciate all of the knowledge that gets shared on here. I've been lurking for a while and I've learned a lot, and only wish that I had read more before starting this build.

I'm working on a Mojotone 18 watt TMB kit; had I known the collective wisdom before I started, I might have gone a different route. I had an easy time with a Weber Princeton kit, but this one is giving me some fits. I'm not getting any sound out of the speaker at all -- not a hum, hiss, or scratch. The cables are good and the speaker is good, so I don't think those are causing any issues.

I went through Rob Robinette's start-up guide and my light bulb limiter dims until I'm at the power tube step -- I'd like to believe that the bulb dims, but I'm not convinced that it actually does. At this point, I've traced the schematic a few times, confirmed connections and components, but I'm not seeing anything that jumps out at me. I've reflowed all of the solder connections and I can absolutely confirm that the jumpers noted on the diagram are there, but on the back of the board. I know that older tubes don't have the 1, 6, and 8 IC pins that the layout would use, but the EHX tube sheet notes that they are "free".

You'll notice that I've got that yellow/white wire from the PT floating towards the ground terminal. The layout had it connected to pin 6 of EZ81, which is how I did it at first; however, that is not what the schematic shows, so I have it disconnected and will insulate it once I've got things working. It looks like it is an IC, though, so I'm not sure how much it matters.

I've attached the schematic and the newest version of the Mojotone layout, in addition to some images.

Here are my measured voltages, with Mojotone's specified voltages in parantheses (given the nature of the amp, they are higher than the voltages listed on the document from the Downloads page).

V1
1: 163 (151)
3: 1.1 (1)
6: 225 (190)
8: 2.0 (1.5)

V2
1: 165 (231)
3: 1.1 (2.5)
6: 169 (148)
8: 1.1 (1)

V3
1: 246 (213)
3: 86 (68)
6: 260 (213)
8: 86 (68)

V4
3: 31 (12)
9: 404 (330)

V5
3: 31 (12)
9: 404 (330)

I've obviously got problems somewhere!

The following are things that I know that I need to do:
- use shielded wire for the inputs
- move those 68K RFI resistors closer to V1
- mount the cap can to a better spot

Once I get an actual signal, I'll work on making it better.

Any help is most appreciated!

[Daviedawg]

Easy stuff first.
Good job you are using a limiter. Dimming should be very noticeable and should be almost off. So if you are in doubt at all then you have an issue. This is confirmed by your voltages being so far from the targets (but see big question below)
The white/yellow should not be connected to any part of the circuit according to the schematic. The layout shows it connected to non operational pin 6 on the rectifier valve as somewhere to keep it safely out of the circuit. Your intention to terminate it with insulation is just as good a method.
The voltages on v1, 2, 3 are workable but the voltages on the power valve cathodes (pin 3) are very high and the HV is way high. Pin 7s are presumably even higher. Big question - Was that measured with valves in place or empty sockets? Voltages on empty sockets read high and inconsistent.
You should also measure and record the voltages at the OT secondaries and after the rectifier valve to see a full picture. Do not run it without the limiter or for more than needed.
I am not sure that the total silence is related to the voltage issue as normally you would get noise and/or poor audio. There may be two issues.
If you post what you find you will get help here if you need it.
Dd

[sethb85]

Thanks for giving me a few places to look, Dd! I'll have to check those other voltages when I get back to the amp this afternoon.

The posted voltages are with all tubes installed. I know that I've measured pin 7 on V4 and V5, and it was most definitely higher than what it should have been.

Again, I really appreciate the help.

[sethb85]

Just for a bit more information, I've listed the voltages for the EZ81 rectifier:

1: 310
3: 416
7: 311

For both V4 and V5, the voltage is 416 on pin 7.

I've got to admit to a bit of ignorance here, but I'm not sure what you mean by the voltages at the OT secondaries. Don't the secondaries go to the impedance selector switch?

Sorry for being slow on this one.

[Bieworm]

Just for a bit more information, I've listed the voltages for the EZ81 rectifier:

1: 310
3: 416
7: 311

For both V4 and V5, the voltage is 416 on pin 7.

I've got to admit to a bit of ignorance here, but I'm not sure what you mean by the voltages at the OT secondaries. Don't the secondaries go to the impedance selector switch?

Sorry for being slow on this one.

I think Dd meant primaries ( G2, pins 9 on the EL84 tubes)

[Daviedawg]

I think Dd meant primaries ( G2, pins 9 on the EL84 tubes)

Yup. Sorry for confusion.

Dd

Edit: You have given us the power valve plate voltages (pin 7) above which should be the same value.

[sethb85]

Sweet. I thought that I was really far off in what those secondaries were!

Pins 7 and 9 aren't quite the same on V4 and V5.

[sethb85]

As I'm working through Rob Robinette's Troubleshooting guide, I'm noticing that I'm not getting any grid pop when probing. What I am getting, though, is a very quiet squeal through the speaker when probing pin 7 on the power tubes.

Count me as stumped!

[Bieworm]

I bet it’s a wiring error or something touching and/or grounding the signal somewhere. All you need is a DMM to find that and probe the signal chain in regard to continuity to ground, starting at the input

[JMPGuitars]

Do NOT probe continuity with the amp powered on!

[sethb85]

No joke there, JMP!

Thanks, Bieworm, for the reminder to do the simple things! Testing continuity turned into a 30 second task that immediately pointed out a ground issue. Just 2 minutes later, it was fixed and now I have a very quiet amp that sounds great!

Since I've got it up and running, I can turn my attention to tidying some things up. I'm curious about JMPGuitars' post about using Zener diodes to drop B+ voltages. Mine are still higher than 345VDC, so I'm wondering if it is something that I need to do.

Thanks again for the help!

[JMPGuitars]

Since I've got it up and running, I can turn my attention to tidying some things up. I'm curious about JMPGuitars' post about using Zener diodes to drop B+ voltages. Mine are still higher than 345VDC, so I'm wondering if it is something that I need to do.

Grab a voltage chart for the TMB from our downloads section and give us some updated voltages.

What is the voltage at the 1st filter capacitor (aka B+ node A)?

Thanks,
Josh

[Daviedawg]

Good job identifying the issue. It is all a learning experience despite the frustration at the time.

Dd