Another Mojotone 18w TMB Help Request

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braddock
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Another Mojotone 18w TMB Help Request

Post by braddock »

Hi all,

I've recently finished a Monotone 18w TMB build and cannot get it to run right. I've been troubleshooting it for days straight now and cannot seem to find the issue, so I'm turning to this forum for help please. I've searched and come up with a few threads on similar issues, I've fixed up my jacks and double checked V2 wiring off the back of these threads, but to be honest I think this is going to come down to something I've done wrong so the other threads weren't a lot of help.

The issue I'm having is extremely low volume from the speaker. This occurs with either a guitar plugged in or with a 440Hz tone generated from my phone. I've chopsticked everything, highlighted the entire circuit for completeness, checked every resistor value, and checked caps for short circuits with a multimeter. Can't find an issue no matter what I look for.

Images of the build are below. I've disconnected the TMB channel for now, the controls on it were affecting the normal channel (again, no idea why this is), I just want to get the normal channel up and running first.

Whole amp:
Full.jpg

Left:
Left.jpg

Centre:
Centre.jpg

Right:
Right.jpg

Preamp tubes:
Preamp Tubes.jpg

Power tubes:
Power Tubes.jpg

If I could please get some help on checking my work and pointing out any errors/problems that could be causing the issue I'm having, that would be greatly appreciated. Apologies in advance for the flux everywhere and the mojo ground scheme, these will be fixed one I get the thing up and running.

Voltages and schematic are attached.

I've got some scope shots as well, I'll put those in the next post.

Grateful for the help.

Cheers
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braddock
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Re: Another Mojotone 18w TMB Help Request

Post by braddock »

Input and V1:
Input and V1a.png

Phase inverter:
Phase Inverter.png

V4:
V4.png

V5:
V5.png

Output:
Output.png

Checking these on the scope I found that I had no idea what kind of voltages I was looking for when carrying out a signal trace, aside from looking for amplification from one stage to the next. So if anyone knows enough to comment on the signals I'm seeing through the amp, that would be really helpful too please.
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dgrainger
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Re: Another Mojotone 18w TMB Help Request

Post by dgrainger »

I'm just driving by and no expert, but it looks like your impedance selector needs more solder on the wire going to the jack.
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Re: Another Mojotone 18w TMB Help Request

Post by JMPGuitars »

Step 1: read the threads in my signature, especially the soldering thread and watch the videos. Bad solder joints cause 90% of problems with amplifiers. Your amp has a lot of questionable soldering joints (don't feel bad, we see this all the time, it just needs to be improved).

Step 2: Use a 1K sine wave, amplitude below 300mV for your input into the guitar if you're going to use a scope. Even 120 to 150mV is fine.

What's your wall voltage? Is your transformer 275-0-275 or 290-0-290?

I also suggest you look at the schematics in our Downloads section and compare your build to that. Look at a SCHEMATIC, not a layout. Highlighter test to that, and see how it goes. Make sure you don't have any extra connections, or any missing connections.

Other than being relatively low in general, only your V2 voltages appear odd, but I don't know what the Mojo target voltages are supposed to be there. You need to reverify your wiring after improving your soldering.

Thanks,
Josh
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Re: Another Mojotone 18w TMB Help Request

Post by colossal »

dgrainger wrote:
Sat 10/09/21 6:32 am
I'm just driving by and no expert, but it looks like your impedance selector needs more solder on the wire going to the jack.
Yes, that joint looks very dry.
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Re: Another Mojotone 18w TMB Help Request

Post by JMPGuitars »

Right.jpg
That's what they're referring to, and that could very well improve the output, but it won't correct all the issues. Start with fixing the soldering and ground scheme. Your entire preamp ground is a big ground loop.
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Re: Another Mojotone 18w TMB Help Request

Post by BlueBoozer »

I think there's a wire on the loose
Error.png
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Re: Another Mojotone 18w TMB Help Request

Post by Bieworm »

BlueBoozer wrote:
Sat 10/09/21 11:06 pm
I think there's a wire on the loose

Error.png
OP mentioned the TMB channel affecting the normal channel , so he disconnected it

The cathode connection of the PI is not solid on pin 8 of V3B. That's bount to get loose some day...

And take Josh 's advice on the grounding schemes. That Mojotone layout is terrible!!!!
There should be 3 distinct grounding points:
1 for AC mains ONLY
1 for the power supply and power amp
1 for the preamp. All those connections should end up at the input ground and that one should be connected with a single wire to a dedicated ground lug, very near the input.
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braddock
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Re: Another Mojotone 18w TMB Help Request

Post by braddock »

Thanks all. Will answer specific points first:

dgrainger wrote:
Sat 10/09/21 6:32 am
I'm just driving by and no expert, but it looks like your impedance selector needs more solder on the wire going to the jack.
Good spot, doesn't seem to have helped though I'm afraid.

JMPGuitars wrote:
Sat 10/09/21 9:16 am
Step 1: read the threads in my signature, especially the soldering thread and watch the videos. Bad solder joints cause 90% of problems with amplifiers. Your amp has a lot of questionable soldering joints (don't feel bad, we see this all the time, it just needs to be improved).
Can you please help to point out a couple of joints that are questionable in your view? I've reflowed all joints at least once, many have been taken back to bare and started from scratch. I have very low resistance across all of them. Not saying they're fine, but some examples of what not to do would help.

JMPGuitars wrote:
Sat 10/09/21 9:16 am
Step 2: Use a 1K sine wave, amplitude below 300mV for your input into the guitar if you're going to use a scope. Even 120 to 150mV is fine.
Done, is this just to replicate what the amp will see in service or for another reason?

JMPGuitars wrote:
Sat 10/09/21 9:16 am
What's your wall voltage? Is your transformer 275-0-275 or 290-0-290?
Wall fluctuates around 225V. Transformer is 290-0-290 off 240, I put my slightly lower AC voltages at the rectifier down to the difference between c.225 and 240V.

JMPGuitars wrote:
Sat 10/09/21 9:16 am
I also suggest you look at the schematics in our Downloads section and compare your build to that. Look at a SCHEMATIC, not a layout. Highlighter test to that, and see how it goes. Make sure you don't have any extra connections, or any missing connections.
Have done this more times than I can count. My feeling is that, if I've wired something up wrong, every time I do this I'm making the same mistake. I've checked against the schematic included in my first post and also against files/18wTMB_Richie.pdf

If there is an error, I'm so deep into this amp that I just don't seem to be able to spot it.

JMPGuitars wrote:
Sat 10/09/21 9:26 am
Your entire preamp ground is a big ground loop.
I de-soldered the bus bar from the back of the pots and de-soldered the pot terminals from the pot bodies. I left the TMB channel pots without ground, I'm not plugging into this channel until I get the normal channel working anyway. The normal input jack, volume/tone pot, existing preamp grounds (black 'traces' on turret board), and PI 47K resistor are now all grounded to one star ground on the chassis.

This should have removed the ground loop but has resulted in no change in volume at the speaker.

Following this, I removed all components related to the TMB channel, no change. I then reflowed all preamp components again on the normal channel, again no change.

I've still got to re-flow the PI and power section again, I'll probably have a go at that next weekend. Other options that I can't really isolate to at this point are tubes (I don't have any spares, will order some as I'll use them eventually) and the OT (though I can't find a short in it).

Is there anything in my symptoms that points to this or anything else? I've always been suspicious about the output from my phase inverter, shown in my second post above. Is it supposed to be this asymmetrical, both side to side (V3A vs V3B) and in the +ve and -ve direction?
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braddock
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Re: Another Mojotone 18w TMB Help Request

Post by braddock »

Bieworm wrote:
Sun 10/10/21 4:18 am
The cathode connection of the PI is not solid on pin 8 of V3B. That's bount to get loose some day...

And take Josh 's advice on the grounding schemes. That Mojotone layout is terrible!!!!
There should be 3 distinct grounding points:
1 for AC mains ONLY
1 for the power supply and power amp
1 for the preamp. All those connections should end up at the input ground and that one should be connected with a single wire to a dedicated ground lug, very near the input.
Another good spot, thanks. I'll fix that.

Yes, I actually always intended to upgrade the ground scheme to what Josh has posted, but wanted to see how the amp ran per Mojo's scheme first/wanted to get it running before I messed with it. Seems my hand has been forced, the Mojo layout should still work though, right?
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Re: Another Mojotone 18w TMB Help Request

Post by JMPGuitars »

For a 290-0-290 all your voltages are too low. If you have something in the amp dropping the voltage on purpose, remove it. If you connected the wrong mains lead, correct it. It could also be that you have something shorting out causing the issue. Pin 3 of V6 is your B+. In an 18W that should be 345VDC (+/- 5V). Typically that's too high on modern wall voltage. If it's too low in your amp, something is wrong.

You can also try swapping tube positions around. See if a different tube in the PI changes things at all. If they're JJ tubes, get rid of all of them.

If you don't know what I mean about the solder joints, then you need to watch the soldering videos linked in my signature. If after watching those a couple times you still can't tell, then I can take a few minutes and highlight one of your photos. Right now I have to run so I don't have time. Maybe Bieworm or somebody else can highlight a few spots for you in the meantime.

Thanks,
Josh
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Re: Another Mojotone 18w TMB Help Request

Post by Bieworm »

No time either right now. I'm doing Chris' favourit job at the moment.
16338794983148969787039488290689.jpg
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Re: Another Mojotone 18w TMB Help Request

Post by geoff 1965 »

You have the potential for a good amp there,the components in the mojo kits are very good just need to vet the amp in area’s already mentioned.
Your 1st issue is mains voltage,220 is too low obviously for the transformer which only has 120/240 primary options.if you get a variac with a slightly higher secondary winding you will be in the ballpark i.e. here in uk you can get variacs with 240V mains and 270V output for around £60.
220V where are you in Spain or Portugal?
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braddock
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Re: Another Mojotone 18w TMB Help Request

Post by braddock »

Thanks again all.

Josh, the tubes are JJ. What's your issue with them, tone or reliability? I'll see if I can pick up some others to test out.

Geoff, I'm in Hong Kong.

Didn't get a chance to touch the amp this weekend, I'll be back on it next weekend.
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Re: Another Mojotone 18w TMB Help Request

Post by JMPGuitars »

braddock wrote:
Sun 10/17/21 5:56 am
Thanks again all.

Josh, the tubes are JJ. What's your issue with them, tone or reliability? I'll see if I can pick up some others to test out.

Geoff, I'm in Hong Kong.

Didn't get a chance to touch the amp this weekend, I'll be back on it next weekend.
Reliability. I think others had issues with tone. I used to like the tone many years ago, but then I stopped using them after reliability became an issue.

Thanks,
Josh
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Re: Another Mojotone 18w TMB Help Request

Post by Bieworm »

Lately I tend to only trust new production Sovteks. From other brands I had new tubes that were crap. Just recently a new Mullard 12ax7 that was microphonic out of the box. Tung sols, EHX,.. that crapped out within a week. The Sovteks are the only ones that were all ok.
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Re: Another Mojotone 18w TMB Help Request

Post by JMPGuitars »

Bieworm wrote:
Sun 10/17/21 3:18 pm
Lately I tend to only trust new production Sovteks. From other brands I had new tubes that were crap. Just recently a new Mullard 12ax7 that was microphonic out of the box. Tung sols, EHX,.. that crapped out within a week. The Sovteks are the only ones that were all ok.
Weird! It could be supply chain issues too (QC and/or damage in transit, etc...). Though JJ has been a mess for years.
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Re: Another Mojotone 18w TMB Help Request

Post by cstenger5 »

Bieworm wrote:
Sun 10/10/21 10:25 am
No time either right now. I'm doing Chris' favourit job at the moment. 16338794983148969787039488290689.jpg
lolololololol. that thing looks amazing, btw.
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