Boss DM2 Analog Delay Repair - Low supply voltage

[clarisso11]

Hi everyone,

I've been working on repairing a version 2 Boss DM2 analog delay, service manual attached. After sorting out a short at the potentiometers, the signal is coming through, but definitely not right.

Here are the symptoms:
- No LED
- Repeats are audible, but there is a constant echo feedback in background, at a lower volume
- Supply voltage is only 6V, measured directly at the DC jack, with a brand new 9V battery attached to the battery clip
- I just noticed burn marks where D1 would be touching the isolating plastic sheet

What I have tried so far:
- Checked all electrolytic capacitors with ESR meter - all good, and no sign of leakage
- Replaced D1 with a 1S2473 from a different Boss PCB I had lying around, no improvement
- Checked voltages of JRC4558DD. They are low, but the IC seems to be doing its thing
- Trim pots seem to be in factory positions, judging from the sealing glue
- Tried a new LED, but no change

I have a feeling that I'm missing something obvious here, with the low supply voltage and the D1 burn marks.

Any pointers in the right direction will be appreciated!


Boss-DM-2-Service-Manual.pdf

[g1]

I would guess someone connected it to the wrong adapter burning D1 and damaging other components. R2 may also be damaged but that should not affect battery operation.
Check D4 zener.and C37.

[Enzo]

I Agree with g1, D4 is more suspect than D1. Especially if someone put the wrong adaptor to the unit.

D4 is a protective zener, and I think I have replaced more of those in Boss pedals than any other repair. Always from wrong adaptor use.

[clarisso11]

Thank you both for your help, I will check D4 as soon as I get a chance.

I Agree with g1, D4 is more suspect than D1. Especially if someone put the wrong adaptor to the unit.

D4 is a protective zener, and I think I have replaced more of those in Boss pedals than any other repair. Always from wrong adaptor use.

Enzo, since you have replaced these a lot, what part should I use? The schematic shows D4 as 'RD11FB3', I don't think I have heard of those before. 11V Zener?

[clarisso11]

D4 was the problem! I replaced it with a zener from an old Boss NF1 board, all workings again now.

Thanks again for the help, this forum never disappoints!

[g1]

D4 was the problem! I replaced it with a zener from an old Boss NF1 board, all workings again now.

Good stuff. The DM2 service manual mentions the change in the D4 part. It is 11V zener, the early ones used 1/2W (RD11EB3), then they changed to 1W (RD11FB3) in the later versions.
Both will work, but the 1W will take a little longer to go bad if you connect the wrong adapter.

[Jazz P Bass]

Here is the zener datasheet.