Alctron MP100 Preamp / UA710

[cageytiger]

Hi guys.

I have a MP100 (it's a direct copy of the UA710, looking online at the guts of UA710 it might even have better components?) I picked up for next to nothing a while back which has never behaved correctly, most of the gain seemed to be in the last 5% of the Gain pot and the tone blend knob seems to not do what it's supposed to, it changes the sound very slightly but feels most 'different' at 50% and seems to go back to same sound or there abouts at 0 and 100%. I've just built a load of those DIYRE units, so I'm feeling a bit inspired to sort this out next. I have very little electronic knowledge but can solder and have a multimeter. I've ran into a few issues and have a load of questions...

I've identified 2 expected issues; the gain pot is mental. The multimeter gives me wild jumpy readings from 0 to 80k on the wiper. It has '12CA20k' on the pot, it has 6-pins and 2 arms that I'm assuming are just there for support. I cannot find these anywhere, not even on google image search, god knows where Alctron got them from. Can I swap this out for any 6 pin A20k pot and just solder wires to the board to the corresponding pins? There's plenty of room to work with so I should be fine.

Secondly, this is where it gets hairy for me, there's some weird crap going on with the tone pot/board. The pot itself reads great. But as you can see in the picture, the track to the connector has been scraped/damaged and a wire soldered from the bottom left pin around to the track from the top right pin. I'm not sure if this wire came off when I was dismantling it, but it wasn't on there very sturdily either way and I assume is/was my problem. The first pin on the connector the track leads to gives no reading at all, the second pin looks good and corresponds to the pot readings. There is not really any room at all to work with in that space between the pot and the connector. Why has all this been done? Was it an error with the pcb? I can't really get my head around the purpose of breaking the connection and then soldering a wire from a different pin. Is there a way I can fix this?

Also, (sorry for all the questions) from all three pots, Gain, Blend and Output, I get ~10k full readings but they're printed as 20k. Is that right? It's very strange that they're all consistently 10k less than printed, is that to do with it being 6 pin?

Thanks!

 

[abbey road d enfer]

Can I swap this out for any 6 pin A20k pot

You need a Reverse-Log pot (usually marked "C"); however, for this application, a special type is preferred, with a sharper taper.

Also, (sorry for all the questions) from all three pots, Gain, Blend and Output, I get ~10k full readings but they're printed as 20k. Is that right?

If you're measuring the pots in-circuit, other components interfere; that may explain the lower value.

 

[cageytiger]

You need a Reverse-Log pot (usually marked "C"); however, for this application, a special type is preferred, with a sharper taper.
If you're measuring the pots in-circuit, other components interfere; that may explain the lower value.

Brilliant - thanks for that. They are in circuit so that would explain it, thought there would be something logical! I'll start hunting for a C version.

Would you have any ideas about the blend pot/board? I just had a thought, could I bypass the board and solder a wire from the pins directly to the top of the connector, or is that foolish without the schematics to see exactly what's supposed to be going on?