Anyone ever seen a mod like this?

[bizzlebot]

This is my 3rd or 4th MP-1 and I was getting ready to mod it with the 3TM mod. When I opened it I found that it had already been modified but in a way I've never seen before and couldn't find in any of the lists of mods.

1) the daughter board has 4 TL072CP Op Amps
2) it runs from the rear and front inputs

Images: https://imgur.com/a/uiXaEdR

[MarshallJMP]

Can you post some more detailed and higher res pics? I can see he disconnected the headphone jack and connected it to this board, I also see you tapes off the signal from front vol pot. It looks like 2 identical channels. But I can't see what going on with the back input, is it also disconnected? A long shot and in the dark but maybe a stereo effect loop? Just guessing here.

[bizzlebot]

Adding Attachments below thanks!

[bizzlebot]

attachments 1 of 2

[bizzlebot]

Part 2

[MarshallJMP]

I see 2 green wires coming from what looks like the transformer to that small pcb with the bridge rectifier and cap and then I see a green and red wires leaving to..? Can you take some pics of these too?

[bizzlebot]

As I was recapping it, here are the things I noticed (although I might've missed something).

None primary board was significantly altered except for the following:
1) the front input was not connected to the rear input like my other MP-1s
2) The A/B of the pre-amp board (hopefully thats the right term) was disconnected from the main board A/B, and wired to the bridge rectifier (see images). A to red and B to Green
3) the transformers on my other ADAs do not have the 2nd pair of Green taps

[rnolan]

Wow this gets more and more intriguing, does the unit work ?

[MarshallJMP]

It looks like they used the rear in and headphone jack as in and outputs.

A and B of the tube board as we like to call it is the heater voltage of the tubes. It looks like they added an extra winding on the transformer for the heaters?
Maybe a higher voltage?

[bizzlebot]

Yes it definitely is a wild one and it does work. Sounded pretty good but not exactly what I was looking for.

I'll turn it on when I get home and get a voltage reading off the windings. Anything else I could test/probe? At this point I'm mostly curious as I'm replacing the board with the 3.666 board (which by the way is incredibly well put together!).

The traces all look good for a/b on the main board so I'll probably probe that too. Can you share what voltage the heaters are looking for?

[bizzlebot]

The transformers other green pair is about 12vAC and the new a/b heater is 15.6vdc

The A/B voltage on the main board is 9.8vdc so slightly lower.

[MarshallJMP]

15.6V is quite high for the heater voltage.

[bizzlebot]

Can you share if 10v is typical? If so I'm going to just pull that mod out, return it to stock and keep going. Thanks!

[bizzlebot]

For what it's worth, I ripped out all that wacky stuff and put in the 3.666 mod. Sounds great!

Rear input own't work but otherwise headphones, outputs etc all are working.

[rnolan]

  :thumb-up: