Farfisa OS42SR spring reverb modification

[salicet]

Hello there,

this is my first post, I really hope that my english isn't confusing since I'm from Germany.
I would describe my experiences as beginner with progression but so far it is hard to tell. My projects so far include building a few kits (Fuzz, Tremolo), building a Treble-Booster from a web-layout and a few modifications/repairs.

My next project is not really a stompbox, so please excuse but I didn't know where to put it here.
I recently acquired an old Farfisa preamp with a phaser and spring reverb. My plan is to make the spring reverb more versatile by installing a control for the reverb driver, the often so called "Dwell". In fact, I know this is mostly built into tube reverb, preferably Fenders.
The Farfisa is a solid state unit. Is there any chance to do the "Dwell" mod on a transistorized model?

Luckily I've got the schematic and so far, I've experimented on the transistor labeld Tr.2 and exchanged the resistor (R18) which connects base to ground and put in a pot. With no luck, this worked more like a reverb mix.

Has anyone got an idea? It won't make any sense for me to guess...

Another thing that I would like to do is a attenuation for the dry signal after it goes to the reverb. Sorta like a control to fade out the dry signal for a complete wet signal.
How and where in the schematic could I do this?  

I would appreciate any hint!
Thanks for reading, hoping for answers,

Cheers, Alex

[PRR]

> hope that my english isn't confusing since I'm from Germany

Many Germans use better English than many people who grew up in America.

> do the "Dwell" mod

Dwell is useful on the HIGH-power reverb drivers that used a powerful tube.

Dwell is much less useful on reverbs that use a small tube driver. As you say, it just turns down the amount of "wet" signal, does not really change the tonal quality.

Changing R18 will change the DC bias (and power) more than the signal. Not good.

This isn't just transistor. This is CHEAP transistor design. Every stage is matched to get enough performance from a minimum of parts. Adding a "feature" really means a major re-design.

Put a switch to break one end of R21. That will be like "Full Dwell" and "Half Dwell". I suspect it will sound the same only smaller.

There's full bass going into the reverb. That is usually "muddy". Try changing C700 0.22uFd to 0.01uFd.

[salicet]

Thanks for answering!
It worked. I excanged the suggested resistor und wired a pot in place. In fact, this really drives the springs a lot more and you can hear a more "thundering" effect. Side effects are that it sounds a bit more overdriven but that's something to live with.
The sound actually isn't too muddy (when Dwell is set moderate), maybe I'll try to make the cap switchable with different values. First thing I actually did was to exchange the spring tank because the the original one was a bit too weak.

Any suggestions for the "dry signal attenuation"? I still didn't really figure it out where to do it. When I follow the M. output (in the 2nd part of the schematic), the signal is connected with 47k resistor which has another 47k in series. Would that be "the spot"?
The schematic is a quite large scan, so I'm only able to post slices in 4 equal parts.
I would appreciate any help!

Regards, Alex

http://farm8.staticflickr.com/7102/6999158830_235e5d8e07_b.jpg
http://farm8.staticflickr.com/7105/6999159044_d7f89e53b0_b.jpg
http://farm6.staticflickr.com/5239/6999159160_620d5aa602_b.jpg
http://farm8.staticflickr.com/7276/7145245513_0411d07474_b.jpg

[PRR]

[salicet]

Thank you very much!

[mrhenry76]

Hello,
I'm restoring an OS42SR too, the preamps work, the reverb and the phaser seem to work but they  don't recieve any signal, the real problem is that I can't find the schematics. Alex you said you own the schems, please can you help me?
thanks
Enrico

[salicet]

Sorry for the delay,

sometimes, I just check in occasionally. Here's a link to the schematic. It's a large scan but it's missing the power supply. Guessing that it's just a problem with inputs the effect lines, it shouldn't matter much.

https://flic.kr/p/UTeG1r

Have you checked for any torn wires?

Good luck!
Alex

[mrhenry76]

Hello Alex,
sorry for MY delay (2 years ...) thank you very very  very much for the schem, I hope it will help me to save the unit!!!
last time I powered it up I had a black out .. so I guess the wires are so old and degraded that I have to rewire it completely and your schems will be super helpful!
Thanks
Enrico