Suggestions to Repair Boss HM-2

[Henry89789]

This is my first post on this forum. I know this section is about building and I am requesting assistance in regard to a repair. I hope it will be allowed for consideration. I would think that the same expertise  that applies to building a pedal would apply to repair.  If not I apologize. My problem is as follows:

I found an old MIJ Boss HM-2 that doesn't work for almost nothing and I want to try to fix it.

The Facts:

1. A sticker on the back says to use a Boss "ACA" adaptor only. I read online that if you daisy chain it with another Boss pedal the  "PSA" adaptor will work.

2. I daisy chained it and it doesn't work.

3. When you daisy chain it, the PSA adapter light starts flashing.

4. The red LED on the HM-2 does not light up.

5. When you put a 9 volt battery in it, the battery gets hot.

6. When you connect it to the guitar and amp, the guitar signal does not go through the pedal.   

7.  I opened it up and there is nothing obvious, i.e., no broken wires;  no caps,  resistors or other components that look defective or broken solely by visual inspection.

8. VOM test shows there is power where the red and black wires connect to the PCB. The power was obtained from a Duofone AC adapter with output DC 9v 300ma.

Does anyone have any suggestions on how to fix?  Suggestions on caps, resistors that may be replaced to make it work?  Why does the battery get hot?

I am going to try to post a schematic I found online. I hope that is allowed. Thanks in advance for any help with this problem (Couldn't figure out how to attach the image)

[smallbearelec]

Hi--

Battery getting hot is a sure sign of a short circuit. If I had to guess, I would say that someone reverse-connected a power supply to the pedal and blew out the diode (usually a 1N400x) that protects the board from just this mistake. I don't have the layout of the HM-2 board in my files, but someone else might. Find that diode, unsolder one end from the board and use your meter to see if I am correct. May be other problems as well, but that one is common.

Regards
SD

[Mark Hammer]

There are several "clusters" of diodes on that board.  The one you probably want is the all-black diode closest to where the wires join the board.  Do an image-search for the HM-2 and you'll see several gut shots that will easily identify it for you.

[Henry89789]

There was in fact an  all black diode close to where the wires join the board. I removed that and an electrolytic cap right above it and the red LED lit  up when I connected the guitar cables and  a 9 volt battery.  Progress!

[Mark Hammer]

That would indicate that replacing it with a new 1N4001 and sticking a new electrolytic cap in will yield you a good as new HM-2!

[runmikeyrun]

I was going to suggest the power filter cap as well.  I had one go bad in a boss pedal and it created a direct short.

[Henry89789]

All right! Thank you  Mark and SD.  

Runmikeyrun:  Any idea on how to identify that power filter cap?  

[DiscoVlad]

The power filter cap is 100uF, and is in the corner of the board underneath the wires marked 3 and 4.



It may pay to check that D1 and R2 (if present) aren't damaged, they were used with the old ACA wallwarts.

[Henry89789]

Discovlad:  Yep! That is in fact the electrolytic cap that I removed along with the diode that got the LED to light up. The diode is right below it; you can see the left leg of it beside the cap. Thanks for posting the pic.

I replaced both the cap and the diode and it works fine now. Sounds better than the MT-2,  I think. Less shrill and tinny.  Nice sounding pedal. A vintage MIJ Boss pedal for $5.00!  I think that makes it sound even better. 

Thanks everybody for the help in troubleshooting this repair problem.

[Mark Hammer]

I have one, that a nephew gave me, and I find it a tad harsh.  Part of that is the EQ-ing, but another part is the use of crossover distortion in addition to multiple clipping. 

http://www.hobby-hour.com/electronics/s/hm2-heavy-metal.php

D6 and D7 introduce crossover distortion.  They are germanium diodes such that the leading and trailing edges of any signal will not pass unless 250mv in amplitude, or thereabouts.  The good aspect of this is that whatever noise is created by all the gain applied before that point will be gated out unless the signal crosses that threshold.  The bad part is that it adds another kind of distortion that you may not want sometimes.  A simple and easy mod to do is to wire in a toggle to shunt those diodes, effectively taking them out of circuit.

Alternatively, D8/D9 are 1N914-type diodes, used for more traditional clipping.  A person could decide to make their ground connection selectable.

Or, even MORE alternatively, a person could use a 3-position on-off-on SPDT toggle, and select between crossover distortion+clipping (D6/D7 not bypassed, and D8/D9 connected), crossover distortion only (middle switch position), or moderate asymmetrical clipping (D3-5) only (D6-7 bypassed and D8-9 disconnected).

Finally, you will note that the HIGH EQ control actual adjusts two gyrator circuits, where the LOW control adjusts only one.  If one has space, the end of C29 that connects to C28 can be lifted and fed to its own 10k pot to provide more flexible EQ adjustment.

[Paul Marossy]

I replaced both the cap and the diode and it works fine now. Sounds better than the MT-2,  I think. Less shrill and tinny.  Nice sounding pedal. A vintage MIJ Boss pedal for $5.00!  I think that makes it sound even better. 

Knowledge is power, eh?

[Henry89789]

Mark:

Thanks for that link. I was looking for a schematic with the numbers for each component.

Yes it is a tad harsh but with the title "Heavy Metal" I thought it would be a lot more harsh and "nasally" than the MT-2 which it isn't. When I said nice sounding pedal I meant in comparison to the Metal Zone MT-2.

I found a mod for the pedal that involves changes to C12, C15, C30, and C6 (different value caps);     D3 (to IN4001);   (D5, D8, and D9 (all to LED));  D6 and D7 (jumper);  these changes lower the bass and add more bottom as well as remove fuzz and lower background noise.  I  bought it to try to fix it and do the mod but after getting it to work I am inclined to leave well enough alone; Also I don't know if I want to mod a vintage MIJ pedal. 

I haven't tried the types of mods you discuss that involve adding toggle switches  and pots. I am still intimidated by those types of  changes. Mostly because there is such a small area to work in. How do you fit a pot or a switch down inside one of those little boxes and solder tiny  wires to them without burning or breaking other wires or components? (The answer has to be: very carefully and with much patience).  I have just been doing mods that involve changing caps, resistors, and diodes with pretty good results. When I find a cheap DS-1, I am going to attempt the mod which involves adding a switch and another  LED and other component changes. 

[DiscoVlad]

Also I don't know if I want to mod a vintage MIJ pedal. 

^^^ This!

Also, what kind of crazy person would want to modify THE swedish chainsaw pedal? All knobs to 11 is the best sound.

[soares71]

I know this is an old thread but I got a very similar problem on my HM-2 and I am not been able to fix it and I am wondering if anyone could give a help on this.
I bought a not working Boss HM-2. This model was ACA adaptors only from 1983. When attempt to use my PSA off course no light on LED. On battery would brings LED up but no sound engaged or not.
I opened pedal and found few things:
The R2 and R3 resistor looks burned. The Blue cable near the ACA power jack had signs of short circuit.
The Circuit board had a broken connection near the electrolytic capacitor C2 which I have fixed.

I have now done the modification for the pedal to accept conventional PSA jumping D1 and R2. I have replaced D2 whit a new 1N4001S and C2 with another electrolytic of 100uf  50V (as I didn't find like original 100uf 16V).
Now when connected to the PSA I get a lot of noise if I engage the pedal the LED lights up but I get even more noise. On battery pedal also powers up but no noise at all.
In both situations on PSA/ Battery I get not sound when in by pass or engaged. Any one any ideas please I love to see this pedal working again!!

[soares71]

R3 has also been replaced for another 100K resistor as the original looks burned.
I am just getting a very hight pitch sound when enagaging it on PSA but LED is on. It seems as it still in short circuit.

[soares71]

The original D2 diode was clearlly faulty as whe I meassured for continuity it would give peefect continuity in both ways(both directions)