TMB Fades after 5-10 minutes (and MojoTone TMB doc errors)

[rideorsail]

Hello all,

I'm working on a MojoTone TMB build and I'm very close to done. Unfortunately, the latest MojoTone wiring diagram has a series of overt errors. I'll start by sharing those since it may add value to the community:

1) V1 - Pin 4&5 should be jumpered
2) V3 - the circuits to Pins 2&3 are reversed
3) Normal input wiring is incorrect. Also, the new diagram is really hard to interpret because of the angle of the part.
4) Test voltages should be included in MojoTone TMB documentation packet and the web site. (docs exist but are unpublished)
5) The web site includes both the old and new documentation, which conflict. It would be a good idea to get the new docs corrected and archive the old documentation to prevent confusion.

The docs from 4/21/2019 contain these errors
https://www.mojotone.com/site/TT_SB2_Fi ... B%20WD.pdf

I noticed that they have put the old diagram up recently, so I suspect they are aware of the issue. Also, everything else about the kit has been spot on, so please don't interpret this as a knock on MojoTone. I love their stuff.
---------

My final problem is that when I'm playing my TMB the volume rapidly fades to nearly nothing after 5-10 minutes. Before this happens, it's loud and operating normally (or at least normally to my limited experience with this configuration (I've always played/built Fender circuits).

If you shut the amp down for a few minutes, it's volume returns to normal until it heats up again and fades out.

Any thoughts on approaches to diagnosing this issue?

Thanks,

Rich

(BTW: This site is awesome and has made my troubleshooting so much more productive. This is a seriously motivated and knowledgeable group of people.)

[geoff 1965]

hello,
yes you can see these problems on the other mojotone posts, check your wiring to the TMB jack using the 2015 layout not the build guide.it helps if you can add some pics and your voltages.
good luck

[rideorsail]

Thanks! I'll let you know how this works out.

Rich

[Daviedawg]

Hi. Welcome to 18 watt. Your fading issue is not something I have come across before. Does the whole amp fade out or just one channel?

Assuming the whole amp is affected, can you measure your hv ac and rectified voltages before and after fading and maybe also while on standby? Depending what you find, trace the voltages at the power nodes until you see where the change happens.

Based on no particular evidence my first point of attack would be the rectifier. What is it on your model?

Dd

[rideorsail]

Hey all,

I corrected the input wiring on the Normal input and ran a long (and very loud and raucous) set of tests on my TMB. Whatever was causing the fading has been corrected and the amp appears to be operating normally! Thanks for nudges in the right direction. Jesus this thing is loud...

After everything appeared to be running nominally, I went through and re-tested all of the voltages on the board to see how it measured up to the MojoTone test voltage chart. Generally speaking it most voltages are fairly close, though my V3-P2 and V3-P3 voltages are both 36% below the target value. Generally speaking, my voltages are below the targets that MojoTone provided. Here's the reference and my actual voltages:

Voltage Test - TMB 18 RichM.png

I have included MojoTone's Voltage Test Chart for reference. (Note that the MojoTone voltage test chart includes the documentation errors on V1 and V3, so don't let that throw you off.)

My question is, are these voltage readings within a reasonable range, or should I be looking at making corrections? From the research I've done, it looks like the Heyboer transformer included in this kit tends to run lower voltages than the Mercury equivalent. Thoughts?

Thanks again for all the help. I have learned a lot in the last few days. My next build will be much more informed.

Rich

[geoff 1965]

if you could fill this voltage chart in it includes the ez81 voltages and el84 plate voltages etc,it will give us a better idea of your overall readings.storage/18_watt_voltage_chart_template.doc

[JMPGuitars]

You should do what Geoff suggested and complete the voltages.

You'll also want to look at their schematic: https://www.mojotone.com/18-Watt-TMB-SCH.gif

Are your resistor values the same?

[geoff 1965]

Rich,
one thing i've noticed that will cause problems if you have used the mojotone layout is the el84's cathode resistor & capacitor.the resistor is subject to a lot of heat and should'nt lay next to the capacitor.you want a good 7mm gap between them.
good luck
https://www.mojotone.com/site/TT_SB2_Fi ... B%20WD.pdf

[Returnsnull]

That's interesting. The revision date on that PDF is tomorrow.

[rideorsail]

Excellent advice all around. I'll be at the New Orleans Jazz Fest (among other things) next week, but when I return I will definitely pull things apart and get a full set of readings.

Thanks again for all of the information. This amp platform is great, but the depth of this community really offers huge opportunities for tweaking and geeking.

Thanks!

Rich