NAD - 50w JMP clone

Ok, I think I changed the in/out as shown.

Loop2.jpg

Not doing what I expected...
MV seems to be acting as a kinda effects level control.
 
Of course there’s a possibility of the loop board arriving faulty. Maybe Mojotone can send you a replacement? Or maybe their support or tech department can help you diagnose the issue.

A short in the loop’s circuit, caused by a sneaky loose wire strand, could have caused a surge big enough for damage to occur within the loop board.

Have you checked that 10K 2W power resistor yet?

Reexamined your soldered connections for integrity?
 
syscokid said:
Of course there’s a possibility of the loop board arriving faulty. Maybe Mojotone can send you a replacement? Or maybe their support or tech department can help you diagnose the issue.

A short in the loop’s circuit, caused by a sneaky loose wire strand, could have caused a surge big enough for damage to occur within the loop board.

Have you checked that 10K 2W power resistor yet?

Reexamined your soldered connections for integrity?
Click to expand...
Planning to email Mojotone today.
Just checked the resistor and it's measuring in spec at 9.97k.
 
fitz said:
Planning to email Mojotone today.
Just checked the resistor and it's measuring in spec at 9.97k.
Click to expand...
Hoping for good news from Mojotone!

Resistor: But is it still doing what it’s supposed to do when it’s under a load? Even if you replace the questionable resistor with a new one, the damage might already be done further down the board’s circuit. The damage could be in the circuit’s pair of LND150 MOSFET’s!

If Mojotone denies you a free replacement, I would look into another brand of high voltage buffered fx loop boards. Mojotone’s loop boards are not built with protection diodes for the MOSFETs like Metro, Granger Amps, or Evolve. Also, all these brands make their loop boards with 1.5” spacing between the jacks like the Mojotone.

sites.google.com

SDM - Zero Loss FX Loop

Zero Loss FX Loop
sites.google.com sites.google.com

grangeramp.com

Ultimate FX Loop - Granger Amplification

Ultimate FX Loop The Ultimate FX Loop is our best selling modification kit. Our high voltage, zero-loss, true-bypass effects loop, available in kit form. Our Ultimate Effects Loop works perfectly for most vintage Marshall, and Marshall-style amps, as well as many other tube amps, that have extra...
grangeramp.com grangeramp.com

evolvediy.com

LND150 FX Loop - populated and soldered - Evolve DIY

LND150 based transparent FX loop PCB for your DIY builds. 1.5″ jack spacing. Using our PCB, we fully populate the board ready for you (or your tech) to install. This is the regular loop suitable for most master volume amps, 2203 & 2204 style. For the Marshall reissue amps (2203x) use our...
evolvediy.com evolvediy.com
 
fitz said:
Making headway.

Started with the heater wiring.

View attachment 110741

I think I got everything hooked up except the connections to the board.

View attachment 110742

DPST toggle switch with blue & yellow leads are for the EL34 / 6550 switching to the parallel resistors I added.

View attachment 110743

Stole the ground buss bar idea from a build by an MF'er instead of running a ground across the pot backs.
Long piece of Romex ground wire connected to the chassis at each end.
Seemed like a good idea if a pot needs changed.

View attachment 110744

Spliced in the pre-standby power wire to the bias supply.

View attachment 110745
Click to expand...
Nice work, I do like the buss bar idea.
I might try that on my old thrifty plexi build.
 
Ok, pulled the loop and returned the amp wiring to the MJT diagram.

I think my board connections look ok.

loop (1)s.jpg

Nothing in top or in the traces looks odd - other than the discolored 10k resistor.

loop (2)s.jpg

And said resistor is still measuring fairly close to spec...

loop (3)s.jpg

Ran the amp for a few minutes and banged out some power chords.
It sounds fine - for just one amp and no effects.

Low input volume is surprisingly lower than the high input.
Not sure if that's normal, or indicative of something.

Measured the voltages with 6550's biased around 40mA.
440 plate voltage on the Eurotubes probe, same reading at the rectifier diodes.
365v at the 10k from the cap at the power side, and 320v at the 10k from the cap on the preamp side.
Everything was within a volt or two from the MJT diagram.
 
In my previous post I forgot to add that yes, your connections to the fx loop board are nice and clean! But I’m noticing that the solder pads for the Send and Return wires are very close to those ground pads… too close, IMO. Since the pic is not sharp enough, can you verify that any of the solder did not flow onto either of those ground pads?
IMG_2605.jpeg

fitz said:
I do not.
Only other high/low input amps I have are the JTM45 & Lead12.
Click to expand...
Yeah, the JTM, aka Fender Bassman, high/low setup is very different from the 2204/2203’s. But using Rob Robinette’s annotated schem will help with why there is a big discrepancy in volume, gain, and tone between the high/low inputs of this type of amp. There are three gain stages in this amp: Hot, cold, and warm. High input will see all three stages. Low input will skip the hot stage and begin with the cold stage and then the warm stage.
IMG_2604.jpeg
 
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