Help ID these parts in FD 2 circuit

[Dave Z]

Not trying to cause any trouble for any companies, just trying to learn some tricks for my own messing about. If this is verboten (or if the co in question too much trouble) moderators let me know....

I have a FM version FD2 and most of the signal path I can ID, and it's close to Steve Cerutti's, but there have been some changes:

1) Input coupling cap is .1, so big change there, lots more bass coming in

2) Input buffer has no biasing resistor:input looks to go straight into the transistor after the input coupling cap. So the 1K inline & 510K to Vr are missing. Self-biasing transistor? What's the advantage? Input buffer transistor has the part number sanded off, so no help there...

3) Output has two resistors to ground, a 1 M (as in Steve's schem) and a 82K (steve showed a 100K)...what is the point? Isn't this the same as having a 75K to grn (resistors in parallel..)?

4) The OD feedback loop/boost circuit is the most mysterious (I can hear MF guffawing in the background here) .. seems the design goes to great lengths to keep all Vr connections physically close together...for fewer RF problems?

Note, the filter that hangs off the feedback loop appears to be a .1 uF and 4K7 (does not have the two resistors as in Steve C's schem).

5) I really have no interest in figuring out his boost circuit as I don't use it...but there are a few components hanging around out there which probably are involved in the 'FM' mod.


- God knows what he has for clipping diodes but that's OK I intend to experiment

- We've got a couple of 22K resistors, and a 10 uF electrolytic that appear to be involved in the boost circuit. Also 120K, 18K resistors and .047 film cap.

The .047 is interesting as it was the original value used on the '808s feedback loop filter.  Since this is what Steve's "preFM" shows, was the FM mod bypassing the .047 to use a .1? If so, why leave the .047 on the board?

You can presumably cut a lead by that extra 10uF cp to get into "nonFM" mode, but it seems strange to have parts on the board no one but a hacker would use as FM seems to hate hackers...


other changes...


Resistor inline before output coupling cap is 100, not 470 ohm

Tone cap is .22 film. No tants in there at all. Probably why is sounds so smooth!

[Dave Z]

One correction - I have *not* found the "min drive" resistor...the classic 808 & Steve C's schem show a 51K..there is not one on my board. I'm wondering if he changed it to 18K..there is a 18K resistor inline with the drive pot..this would make sense, as when turned down the regular OD channel is very subtle.

[Dutchie]

Hi Dave,

I'm very interested in de new fd2 schematic, can u maybe take hi-res pics of the board , so 1 can trace it? i'm especially interested in te different tone settings ( vintage , comp , fm ) and also boost

Greetz

[Dave Z]

Sure I can take a photo of the board later, but how useful that is I do not know...all the traces are on the underside, it is epoxied down, and you won't be able to tell the values of the parts (except for resistors) as they are on the sides, in dimly printed ink.

All I could tell from close pysical inspection is noted above.

Comp-cut is lifting the clipping diodes out of the circuit

Do not know the FM/Vintage mod though I suspect the 'FM' is lifting the cap in the feedback filter (cap & resistor tied to pin2 of the JRC4558).