Some trouble with Mojotone FX loop

[Yoda]

Hello everyone,

I’m trying to install one of the Mojotone Ver. 4 effects loops in a Wangs 1987HW amp.

I installed it per Mojo’s instructions but I’m not getting satisfactory results. The amp passes audio but if I bypass the loop the amp gets much much louder and with the loop in I cannot hear any effects pedals. I am also only getting about 40v on the side of the 10k 2w resistor closest to the 10uF capacitor where the instruction say to check for <400v. If I try getting the B+ voltage from a power supply node farther upstream in the amp it starts to burn the first power supply resistor after the choke. What gives?

I did buy this effects loop secondhand off Reverb to save some money but don’t see any reason why it shouldn’t work but I’m open two ideas on checking the board itself for defects. I’ve attached a photo of the install so far.

I’ve reached out to Mojotone technical support and I’m ready to swap out with a new board if that happens to be the case but until I hear back from them maybe you guys can spot something I did wrong.

Thanks in advance for any help.

IMG_9325.jpeg

[R.G.]

If you could post a schematic we could make much better guesses.

Right now, best guess is "something after that resistor is pulling too much current."

[Yoda]

This is the only schematic that comes with the installation instructions from Mojotone. As for the amp itself there is no schematic for it from Wangs but it’s not much different from a typical Marshall 1987 so I’m just working off a generic 1987 schematic for the amp at the moment.

IMG_9326.png

[sluckey]

I am also only getting about 40v on the side of the 10k 2w resistor closest to the 10uF capacitor

This indicates a problem on the FX loop board.

[Yoda]

I am also only getting about 40v on the side of the 10k 2w resistor closest to the 10uF capacitor

This indicates a problem on the FX loop board.

Well that would actually be a little bit of a relief in a way lol.

The instructions are somewhat vague, they say as long as there’s less than 400v but more than 200v you’re good, but if voltage is too low then to move the B+ node upstream. As I said I tried that and that’s when it seemed to draw too much current so a problem with the board is starting to make sense.

[Ten Over]

I am also only getting about 40v on the side of the 10k 2w resistor closest to the 10uF capacitor where the instruction say to check for <400v.

What is the voltage drop across that 10k resistor when this happens?

FX Loop Mojotone Ver 4 stats.jpg

[Yoda]

I am also only getting about 40v on the side of the 10k 2w resistor closest to the 10uF capacitor where the instruction say to check for <400v.

What is the voltage drop across that 10k resistor when this happens?

FX Loop Mojotone Ver 4 stats.jpg

I have 119v across that 10k resistor.

[Ten Over]

I have 119v across that 10k resistor.

That's way too much. As R.G. and sluckey deduced, there is something wrong with this board.

[Yoda]

Well that’s unfortunate, but as I said earlier still better news then something being wrong with the way I installed it, I know I triple-checked everything.

I’ve never installed one of these Metro Zero-Loss type loops before so I don’t know what to expect as far as voltage drop or anything like that but I definitely wanted the B+ in the 300+v neighborhood if it’s supposed to be a high-voltage loop.

[Stevem]

Was this amp working properly to begin with?

With your input B+ to the board being dragged down so much that 2 watt resistor should be getting really hot, is it?

Do you have a thermal gun to check with ?

[Yoda]

I do not have a thermal gun/IR thermometer, I need to get one. That 10k/2w watt resistor was getting hot to the touch.

Yes this amp was working great before installing the loop.

[Yoda]

Double post.

[Yoda]

Double post.

[Yoda]

Double post.

[Stevem]

On Q1 and 2 what dc voltages to you read when you prob the side of the transistor with one tab on it?