60 cycle hum in tremolo channel after shorting pin 7 to ground

[JMPGuitars]

Mojotone will only sell the new revised back plate in sets and they want 50 bucks for a front and back face plate.

I'll custom make you a back panel for only $100.

[bobtait]

So, I completely rewired this amp, exactly duplicating the mojotone wiring diagram. Pretty sure all connections are correct. First time I turned it on, heard a small pop and a little bit of smoke. Something blew but there is no apparent sign of anything burned. The tubes are getting power and lighting up, the filter capacitor is putting out 409 volts, pilot light on. There is no signal coming out of the amp at all. The pop sounded like it was coming from the new on off switch, but I couldn't tell for sure. Any ideas what I smoked when I turned it on?

[bobtait]

So after checking everything again and fixing one wire I left out and changing a few tubes ... everything is working except the volume is very low. But, both channels are working, the tremolo pedal works, everything. Just no volume. I don't have an extra rectifier tube yet to check that. But twice there was just a puff of smoke like a component was overheating, so something might have burned. But, it is passing signal now. Any ideas are welcome.

[JMPGuitars]

Back to the highlighter test with the amp unplugged. Check all component values, and for any shorts.

[bobtait]

Checked every component, everything looks good. However, even though the amp is playing program material at a very low volume there is continuity between the positive and negative on the speaker output jacks. When I unhook the OT there is no continuity. So, should I assume the OT is shorted?

[bobtait]

So, I checked every item and did tweak a few things and get 1 cap to the right value. After working on it it got louder but was still crippled and distorted. The first time I fired it up when I set the impedance on the back of the amp it felt like the knob was slipping, when in fact I was just turning the switch around backwards cause it was loose. This caused the switch to impact the second output tube and bend a resistor causing a quick short. So when I hit the switch pop and smoke. I think the OT was damaged, not enough to quit passing signal or to show up on multi-meter tests but enough to maybe make the amp do what it is doing. The thing I said previously about continuity at the speaker jack was just the amp showing a completed circuit with the speaker. So anyway, I ordered a new OT and we will see what happens. I have also learned that solid wire is great if you have the harness all laid out and don't have to move it too much LOL. It is terrible working in tight spaces cause it breaks. It is all part of learning this stuff I guess. I will keep posting updates for anyone looking in.

[JMPGuitars]

Your OT might be fine. Since you already ordered another, I guess you'll find out one way or the other. However, go thoroughly through the amp again and make sure there are no issues left so you don't damage the new OT. Make sure the mechanical connections are all solid too.

That's probably the main reason I don't use solid wire. The stranded PTFE I use holds its form well too, and it's much harder to break. Pre-bonded stranded wire also holds its form pretty well.

Thanks,
Josh

[bobtait]

The only thing I have not done yet is pull every component and measure it. The connections are good. It never hurts to have an extra transformer around. I think I am going to keep the solid on the heater wiring and change the rest to stranded. I am getting solid 6.5V on each tube heater. However the power transformer is putting out close to 370-400V to the filter cap and rectifier. Is that a little hot? I noticed that the pins on the tubes that came in it are kinda discolored. Oh, also when the amp first turned on the output tubes stopped working. I am probably in the end going to change this amp to your TMB design anyway, this is going to be a long time project. Gonna go real slow and check every component. I think its possible I have had my iron set too high as well, Duh. When I gig I always play through an 18 watt Marshall stacked on top of a 5e3. So I am just going to mount one of my 5E3s in the mini blues breaker cabinet that came with this amp and have another good stack since I have that TMB from your design in a identical cabinet. Thanks again for all the input.

[bobtait]

This will be my last post on this topic as I have solved my problems and learned a lot in the process. To review: I had purchased a mojotone british 18 watt with tremolo kit amp that was running "of sorts" and it was in a 1-12 combo cabinet. The tremolo channel was weak and when you turned it off the amp hummed. After consulting with JMPGuitars I decided to completely rewire and confirm all components. This was the first time I had ever done this so, after a few newby errors and things I eventually got sorted out, the amp still sounded like it had a huge drop in output. I thought it might be the output transformer but it wasn't it was a short in the wiring to the phase inverter tube and a ruined tube, probably because of that. So I fixed the wiring put a new tube in for the phase inverter circuit and now the amp works perfectly. No hum, no tremolo switch pop. It is loud and sounds like a mini bluesbreaker. I am looking forward to interacting with this site and future builds and repairs. I encourage everyone to try to donate as much as they can to the site. This site is a great resource. See you guys again soon! bobtait

[JMPGuitars]

Nice job! I'm glad you got it sorted. I'm sure the work you did besides finding the short was a huge benefit to the quality of the amp.

Thanks,
Josh