Mojotone 18W TMB Problems

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chunter
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Mojotone 18W TMB Problems

Post by chunter »

Hi All,
I am in need of some assistance with my Mojotone TMB build. This is my second amp build so I am still a beginner (first was a 5E3 that went well). I completed this build over the weekend but have been having issues. I get output on both channels (TMB and top input on normal, the lower input on the normal chanel has no output) however the volume level is relatively low and it doesnt sound very good.

I have ran through the circut several times highlighting the layout when checking values and wire traces and can not find any problems. I have checked all resistor and capacitor values on the board with a DMM and all match the Mojotone layout. I swapped all tubes except the rectifier (dont have a spare on hand) but nothing seems to fix the problem. I have also checked continuity on all grounds and wire traces.

Below are some photos of the build and the measured voltages. Here is a link to the Mojotone layout that came with the kit https://www.mojotone.com/site/TT_SB2_Fi ... B%20WD.pdf. At this point I am at a loss as to what to check next, any advice/feedback would be greatly appreciated.
IMG_3508 2.jpg
IMG_3509.jpg
IMG_3510.jpg
IMG_3511.jpg
Voltages.JPG
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Daviedawg
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Re: Mojotone 18W TMB Problems

Post by Daviedawg »

Welcome to 18 Watt. I hope we can help. Your work is very neat which helps a lot with troubleshooting.

The first thing I see is that if one of the normal pair of inputs is working and the other is not then you should look closely at the way you have wired the inputs. It can be confusing. Solving that may affect your output volumes. But then again I do not think so. But do that first and review the effects.

You have been through the circuit looking for errors so the next step is to chopstick looking for noisy joints or components. You need to look closely for "whiskers" shorting connections at jacks and valve bases. Loss of signal is often caused by a short to ground. Moving leads often reveals noises which lead you to the problem.

If you need advice on chopsticking come back to us. But safety is key.

It would be useful to know what the target voltages on the preamp and pi valves are. Do you have any info? The power valves are in the right realms (they are much the same for all 18 watt versions) but should both show identical readings within a few millivolts. This suggests an issue somewhere there. But again review after you find the input issue.

Dd

Edit. Just been thinking about your inputs. There is not much to do wrong. I wonder if it is just that one signal is much less audible. Or is there really no signal at all?
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JMPGuitars
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Re: Mojotone 18W TMB Problems

Post by JMPGuitars »

I would suggest disconnecting the input jacks from the front panel, and posting a photo of the wiring here.

For mojotone voltages, you can still use our standard chart: storage/Voltage-Chart.htm

Except: V2 in our chart is the Phase Inverter (usually V3 on TMB builds). For the "other" preamp tube, the voltages would be nice to establish a standard, but we need more people to report their voltages once their amps are fixed. Another mojotone user reported these voltages (look at them for the voltage variance, don't try to copy exactly):
Pin 1: 173
Pin 3: 1.6
Pin 6: 218
Pin 8: 1.6

Side note: your B+ is a little too high, which will give you extra headroom and less distortion. Ignore this until you sorted everything else out and then test again.

Thanks,
Josh
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chunter
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Re: Mojotone 18W TMB Problems

Post by chunter »

Thanks for the guidance and advice, I solved the input problem last night. There seems to be an error in Mojotones layout I was following. I moved the green wire in the screen shot of the diagram to the upper contact and now I have signal on all three inputs. Still have low output volume but all controls and inputs seem to work. Unfortunetly I dont know the desired voltages as Mojotone did not provide those, perhaps I will reach out to them and see if they can send them along.

I will try to chopstick the amp this evening after work.

Thanks again for your advice
Screen Shot 2019-04-09 at 6.40.29 AM.jpg
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Re: Mojotone 18W TMB Problems

Post by JMPGuitars »

The diagram you posted is accurate(ish), what you modified is not correct.

Look at the input jacks. One side has an indent in it. That indent connects to the input cable, and separates the connection between the two sides of the jack. When no cable is connected, the input is connected to ground. When you insert the cable, it disconnects the input from ground.

So from what I can see, it was correct(ish), and you made it incorrect.

Look at the input on this TMB: storage/18_Watt_TMB.pdf

Try that instead.Your input resistors are next to the jacks, instead of near the tube. That's not ideal, but it's fine. Your ground wire also looks fine assuming all 4 ground tabs are connected.

Thanks,
Josh
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Returnsnull
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Re: Mojotone 18W TMB Problems

Post by Returnsnull »

I'm glad I'm not the only one completely flabbergasted as to how these Cliff jacks work. Understanding a bit better now that I know they disconnect from ground when not in use.

Kudos on the clean work in this build, I'm jealous.There's an art to lead dressing that I really need to learn. 8O
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geoff 1965
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Re: Mojotone 18W TMB Problems

Post by geoff 1965 »

hello,
have a look at this trinity build guide,there's a good explanation and diagrams showing the hi & lo inputs signal path.
18wattBuildersGuide31.pdf
your amp look's good,quality components and nice wiring/soldering.
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chunter
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Re: Mojotone 18W TMB Problems

Post by chunter »

Thanks for all the support, I have re-wired the normal channel input jack as per Josh's layout and all three inputs are working.

I also sorted the output volume, so now I have a working amp! I compared an older revision (rev 1.3) of Mojotones wiring diagram to the one that came with the kit (rev 3.0) and found a discrepancy on the PI wiring (connections to pins 2 and 3 are reversed). So I wired it up according to the old rev and everything worked. I will post details and my current voltages tomorrow as it is late and I need to be at work in the morning.

I appreciate all the positive feedback and assistance with this build.
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Daviedawg
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Re: Mojotone 18W TMB Problems

Post by Daviedawg »

That is great. Well done sorting the issues. Enjoy your amp.

But maybe there is a lesson here. If there is an inexplicable problem with a third party schematic or layout you can refer to the material in the downloads section here for comparison. Most of the circuits in the kits (maybe not components) are almost identical to the designs here, for the TMB for example. The material here is well tried and tested. I may add that to advice in future.

Dd
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chunter
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Re: Mojotone 18W TMB Problems

Post by chunter »

Thanks Daviedawg, I think your advice to follow a known wiring diagram is valid.

Here is what I ended up changing from the wiring diagram that came with the kit. It seems that Mojotone created a new diagram (revision 3.0 dated Apr 21 2019....?) that has some differences from an older rev they have posted on their website. I dont know if these are errors in the new diagram or if the changes are intentional, either way the changes dont seem to work.
Slide2.JPG
V1 does not have a jumper between pins 4 and 5, I added a jumper as all other diagrams show a jumper.

Slide1.JPG
The new diagram on the right has the phase inverter (V3) pins 2 and 3 reversed when compared to the older rev (1.3). I had low voltage readings initially, but after swapping the wiring on pins 2 and 3 to match the older diagram my voltages improved. This solved my low output volume problem.


Finally here are my voltages now that the amp is working.
Working Voltages.JPG
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