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Help me fix Boss BF-2 and DM-3

11/12/1999 7:14 PM
Help me fix Boss BF-2 and DM-3
Hi, I've decided to try and fix a couple of Boss pedals I've got lying around, and am seeking some help.  
BF-2: The resonance knob causes the unit to feedback rather severely when cranked, even with no input signal. Is this a result of a trimpot inside being incorrectly set? Someone here sent me a schematic a while back which I lost when changing computers, sorry. I believe there are three or four trimpots on the PCB - what does each do? Are they suitable for tweakign the sound, or are they best left alone?  
DM-3: There is an electronic-sounding overtone present with each delayed note, but not the original. When I asked about this a while back, someone suggested it might be the clock signal bleeding through, and this might be due to a bad cap not filtering it out. Is this possible? Could it be fixed by identifying the bad cap and replacing it, or is there more going on here? Does anyone have a schematic? I can handle switching components, but any serious troubleshooting of complex circuits like this are out of my league.  
I'd like to get these units working properly, cause I think I may want to sell them and am not comfortable unloading a pedal that I know not to be in 100% working order.  
And on a related note, both these pedals are of the ACA-adapter type. I haven't been using one. Can they be damaged by using the lower-voltage PSA adapter, or any other regulated 9V power supply? Or would they just not sound their absolute best?  
Thanks greatly for any help here.  
11/14/1999 8:38 PM
Mark Hammer
BF-2: Your suspicions about the trimpot's relationship to resonance and wierd oscillations are likely correct. If you trace the signal path from the centre lug of the resonance control, you will encounter a .047uf cap, and a 22k trimpot (in that order). The trimpot sets the maximum resonance for the device. It is one of those things that can conceivably drift over time. To adjust it, set the chassis resonance control up full, and tweak the trimpot until the oscillation just disappears. You may also want to consider replacing the .047 cap with a slightly smaller value, such as .039 or .033. This will take out some of the bottom end from the regeneration circuit and keep it from sounding too box-ey as you turn up the resonance.  
DM-3: No idea of the design of this thing, but if it is a bucket-brigade chip, then these sometimes incorporate a "balance" trimpot which is used to cancel out the clock signal (the likely source of the whining sound, especially if you notice its frequency changing with the setting of the delay time). Knowing something about the major chips involved, and the physical location of the trimpots, relative to the chips, would help out here.  
Power supply: There are 4 things to keep in mind about external power supplies...  
1) Not all wall warts use the same polarity on their jacks. BOSS and many other pedals use a convention in which the outside or shank of the plug from the power supply is positive, and the inside (pin connection) is ground. This is usually illustrated on the pack of the pedal beside the jack, since it is an important thng to know. Any wall wart you use should adopt the same convention, or alternatively, you can simply cut the adapter wire at a convenient spot, flip the leads around, and splice them securely.  
2) Current. The supply should provide enough current to do the job. Few pedals which have battery connectors can be expected to exceed 20-25ma current consumption (otherwise a battery would go in the middle of a gig, and that's kind of a bad marketing strategy). So, two pedals shouldn't expect to consume more than 50ma, which is generally much less than what most $5 adapters are capable of supplying (most will exceed 100ma).  
3) Voltage. Here you have some flexibility. Most stompboxes can actually behave very well with 12-15v going into them, and they can certainly tolerate more (up to 18v in many cases). The exceptions might be some distortion pedals, which will behave with 15vdc, but behave most desirably with 9vdc or less (since distortion is all about running out of headroom, and higher supply voltage generally = greater headroom).  
4) Regulation. A regulated supply is DC, and NOT AC (read that fine print on the back of the adapter, folks). You CAN use an adapter that puts out AC, but you would need to provide the regulation to turn it into DC, and it seems silly to spring the money for somethng you have to fix anyways. Having said that, not all "DC" is equivalent, since it really only has to be positive to count as DC. You could have an adapter with tons of ripple (heard as hum), because it isn't filtered properly.  
There are several alternatives to consider. One is to stick a couple of caps across the power lines (e.g., a 1000uf/16v cap, with + going to +, and - going to ground, in tandem with a 1uf cap of the same orientation). These will act like severe "treble cut" filters, and eliminate a bug chunk of any existing ripple. A second fix is to consider making an adapter box (you can, and probably SHOULD, use one of those little blue or black plastic project boxes from Radio Shack or wherever), and use a higher adapter voltage, which is then regulated down to 9-12v. The classic 3-pin regulator chips tend to like an input about 3v or so higher than what they put out, so a minimally regulated 12vdc going into a 9-volt regulator chip should work out nicely. This should deliver what you need. A third fix is to bring a meter with you when you go to buy an adapter, and use it to measure AC ripple. You can then have a sense of what might possibly be (or NOT be) in the little black sealer box that gets plugged into the wall.  
There is really nothing special about BOSS adaptors, other than that all these things are already taken care of. As a matter of fact, I'd be curious to find out from anyone if BOSS or other "musical" power adapter boxes have anything in them other than a rectified bridge and a couple of appropriately selected caps.  
11/15/1999 2:02 PM

Hi Mark  
Thanks for the help. The flanger is now working normally again. The trimpot needed quite a bit of adjustment to make the oscillations go away - odd, since I enver messed with it before. Oh well.  
I quickly opened up the delay again last night, and man can Boss cram a lot of components onto a little board! There are three trimpots - one is clock, one is bias and the third one's tag eludes me. It may be cancel, I'm not sure. I'll play with them a little tonight and see if I can affect the overtone if not make it go away. I'll report back here after.  
Finally, I didn't expect there to be anything really special about Boss AC adapters, I was just a little concerned that both by ACA-special-voltage-needed effects had developed some sort of problem after being powered by a non-ACA adapter for a period of time. It doesn't make sense to me that an effect should operate fine off a 9V battery but need different wall wart power. Is there a simple sort of mod to turn an ACA effect into a PSA effect? Damn Boss...  
11/15/1999 3:16 PM
Mark Hammer
I'm losing you here. What's the difference between an ACA and PSA adapter?
11/15/1999 4:02 PM

Sorry. That's Boss' nomenclature - apparently the ACA adapters were used on older modulation and delay effects. They supposedly supplied more than 9V to the circuit, and these pedals sounded better with the extra juice. PSA-model adapters supply a steady 9V to the circuit, and all new Boss pedals are specced with it. So an older CS-3 compressor may use an ACA adapter, while a brand new one will use a PSA. It's all a big mess, really.  
One aspect of Voodoo Labs' marketing of their PEdal Power unit is the ability to juice both ACA and PSA Boss effects. I use a Signalflex power unit that only puts out 9V, but my older Boss effects seem to work fine, except for the problems which developed as I described above.  
I don't think that using a normal 9V supply should damage the effects - maybe not run them at their full sonic potential, but damage? I've just been a little wary since both my older Boss pedals have developed problems that I wouldn't relate to the power supply used, but still...  
11/15/1999 4:24 PM
Jay Doyle

This may be a bit simplistic but why don't you pop a 9v battery in your delay and see if your problem persists. If it does then it  
prob. is a bad filter cap otherwise you know its a power supply problem. Just an idea,  
Jay Doyle
11/15/1999 5:26 PM

Hi Jay  
Tried that, and it the problem persists. Do you have any idea how to track down which filter cap has gone south?  
MY concern with the power supply was not that it was directly causing the problem, but that the lower voltage supplied had caused a component to fail.  
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