Want to true bypass an old dm-2 I have laying around...

Started by nightendday, September 18, 2012, 06:48:44 AM

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nightendday

So I have a dm2 that I'm currently rewiring, and putting new pots and jacks into, It's been beat half to death and the enclosure is done for. I'm putting it into a hammond box. I've heard I can pull the switching fet and bridge them so it is always on, then wire true bypass accordingly. Are the switching FET the two 2sc945p ? and what would the bridge need to connect?


R O Tiree

The 2 x 2SC945-P transistors are NPN, and they form the flip-flop that changes state to switch the FET and put the LED on and off when you press the footswitch.

The switching FET is the 2SK30-ATM, which will be marked on the board as Q5.  You would put a jumper between pins 1 and 3.

The pedal is designed so that the input signal is buffered and treble-boosted (Q1 and pins 5, 6 & 7 of IC1) and then divided into 2 paths.  One path goes direct to the output mixer (pins 1, 2 & 3 of IC1, which also cuts the treble by the same amount as it was boosted by the input buffer - a kind of poor-man's Dolby, really) and the other goes through the delay path and then meets the other path at the mixer.  When Q5's Gate is held low by the flip-flop, it presents an insanely high resistance to the delayed signal, effectively cutting it off.  Press the footswitch, the flip-flop changes state, Q5 Gate goes high and now presents a very low resistance to the delay path, so that signal gets fed through to the mixer.  Simples!  The snag with this is that there may or may not be a small but discernable volume jump if you TB this circuit.  It is designed to be pretty much unity gain, but component tolerances might say otherwise.  If you try it, let me know which way the volume goes and I'll try to suggest which component to tweak to sort it out.
...you fritter and waste the hours in an off-hand way...

nightendday

I'm correct in assuming that 11 and 12 should be tied together, correct?

R O Tiree

Doesn't matter at all, since we have no need to change the state of the flip-flop.  You can get rid of that entire bit of the circuit if you want so, for completeness' sake, remove the following:

C24, C32, C33, C34, C35, C36
D2
R29, R38, R39, R41, R42, R43, R44, R45, R46
Q6, Q7

Re-route D3, R40 and either the existing LED or another one of your choice from 9V via one pole of your 3PDT TB switch to GND (as you would normally in a home-build). You probably don't even need D3... it's just there so, when the LED dims, you know you need to change the battery.

While you've got it in pieces and doing major surgery, it might be worth replacing all the electrolytic caps.  If it's as old and beat up as you suggest, some of them might not be performing quite as well as they did when it was all new and shiny.  C26 in particular, is only rated at 6.3V for a 4.5V line.  C38 has only 1V of headroom.  Modern electros have more headroom in the same package size (generally).  All the ones rated at 50V should be fine but, you know what?  If it was mine, I'd replace them all, so I'd have peace of mind that they wouldn't fritz out on me for years to come.
...you fritter and waste the hours in an off-hand way...

R.G.

R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.