Relay bypass mod to Ibanez and Boss pedals

[rtcook]

I know this is an old thread, but I'm looking for some help relating to the Latching Relay Driver circuit.  What are the 0.1uf caps across the tranasistors for?  I am using this schematic to make a MIDI switching board and am wondering if I need those caps.  I think I understand they are for relay popping noise getting into the audio.  For my use, MIDI switching, I don't think I need them as it should not have a way into the audio.  Also, are 2N3904's good enough or should I be using the 2N7000 or other transistor?  I have this breadboarded with the 2N3904's and no caps and seems to work just fine.  I want to be sure of the design before I make the PCB.  Thank you for any help or suggestions.

Roger

[JRM]

This is a quite useful topic! My only problem now trying to convert my Boss GE-7, BD-2 and DM-3 to TBP is sourcing the non-latching relay. I've only able to source NEC EA2-5NJ which are small but expensive. I know that I'll have to add a resistance to avoid frying the coils but I'm not sure how to calculate it as I never worked with relais before. Can someone give me a hint?

[R.G.]

This is a quite useful topic! My only problem now trying to convert my Boss GE-7, BD-2 and DM-3 to TBP is sourcing the non-latching relay. I've only able to source NEC EA2-5NJ which are small but expensive. I know that I'll have to add a resistance to avoid frying the coils but I'm not sure how to calculate it as I never worked with relais before. Can someone give me a hint?

(1) Read the relay datasheet: http://www.nec-tokin.com/english/product/pdf_dl/mini_data/relay_ea2_eb2_e.pdf This tells you that the 5V version has a 178 ohm coil.
(2) It therefore pulls a coil current of 5V/178 = 28.1ma when run from 5V. 
(3) To run it from another voltage, you need to make it pull nearly the same current. For running from 9V, you need a resistance of R = 9V/0.0281A = 320 ohms to get the same current.
(4) The relay coil will be part of this 320 ohms. So you need to put another resistor of value 320 - 178 = 142 ohms in series with the relay coil to get the same current at 9V. 140 ohms is a standard value. You could also use 2*270R in parallel, 2x68 in series, etc.

[Transmogrifox]

The right-hand part of the below schematic gives a clue for using the internal flip-flop in these pedals to drive a latching relay.

[JRM]

Thanks RG! I had read that datasheet but haven't found the coil current now I understand it clearly. And it seems quite easy to make this mod.
On BD-2 SW1 is Q6, SW2 is Q4 (PCB assy 70567845 http://www.freeinfosociety.com/electronics/schemview.php?id=116).
On GE-7 I'm having more trouble as I find only two FETs (Q4 and Q5 on PCB Assy 75215510 http://i890.photobucket.com/albums/ac102/mortadlanuit/GE-72schem.jpg); Q5 is on the led controling side and bypasses the main circuit so it's SW3 and Q4 is SW1 (no SW2 on this circuit).
On DM-3 the circuit is less clear to me but, Q1 is for sure SW1 and Q8 and Q11, the other 2SK30ATM-Y look like SW2 and SW3 on this circuit (http://www.freeinfosociety.com/electronics/schemview.php?id=2291).

[JRM]

I had the daughter boards lying on my work in progress box, and finally decided to take some time to install it. I've started with thelectronic DM-3 and it's driving me crazy. I've rechecked everything and can't make it work. I have the bypass sound but can't engage the effect. Just to be sure, the 2n7000 must be wired with the source to ground, right?
I've also learned that Q11 is the switch and Q8 the Sw1, removed the Q11 and add a jumper on Q8 from source to drain.