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previous: frank Hi, I've decided to try and fix a c... -- 11/12/1999 7:14 PM View Thread

Re: Help me fix Boss BF-2 and DM-3

11/14/1999 8:38 PM
Mark HammerRe: Help me fix Boss BF-2 and DM-3
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.  

 
Replies:
frank Hi MarkThanks for t... -- 11/15/1999 2:02 PM