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Author Topic: Revox G38 line in amplifier bypass mod?  (Read 2058 times)

Offline robgil

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Revox G38 line in amplifier bypass mod?
« on: April 29, 2015, 07:48:37 AM »
Hi, I have a Revox g36. When I record from my HiFi through any input (radio/diode) it would seem the input is too loud and I need to turn the levels way down, something about the Revox needing 50 Mv and line out from my amp being closer to one volt? I don't know if I've got that right but you get the idea?

Anyhow I keep reading about a bypass mod on the input amplifier so something like that but I have no idea how to go about it. I did find a step by step but I think it was for the older f36, cant remember now but I cant find the web page again anyway.

Has anyone ever done this modification?

Offline stellavox

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Re: Revox G38 line in amplifier bypass mod?
« Reply #1 on: May 02, 2015, 06:12:22 AM »
Yeah Robgil,

As you've experienced, the record amps on the 36 have a LOT of gain.  They (and other decks, like the tubed Tandbergs) could accept a microphone input directly; they then resistively "padded down" the other, high level inputs.

On some 36's I owned, I bypassed the high-gain stages (V1 and V2) by simply rewiring the "top" of potentiometers P3 and P4 (the record input level controls) originally connected to C4 and C8, directly to the "tuner" input jacks.   As I remember, the "stock" shielded wires coming from those jacks and originally connected to the input selector switches S3/S4, were just long enough to reach P2/P3.  This mod allowed line level input signals to drive the record amp "just right", with plenty of gain to spare.

BTW - I wrote about this and other mod / G36 info in the long defunct "Glass Audio" magazine 25+ years ago (holy ...., I'm old).  Email me: [email protected] if you'd like file copies.

Charles   

Offline robgil

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Re: Revox G38 line in amplifier bypass mod?
« Reply #2 on: June 01, 2015, 07:38:33 AM »
hI Charles, THANKS FOR GETTING BACK TO ME, oops sorry, caps lock!
I think I sent you an EMail a while back regarding your article, I would love to read it. I shall PM you my EMail address.

At present I am using the diode input which seems to have tamed things down a bit, the Revox needs a lot of work, one track (1) seems to have some low down mechanical noise, I really gave the heads a good clean which seems to have helped but not totally eliminated it.
I also have a lot of tape hiss.
When I got it the switches and pots were really dirty which gave me all sorts of problems, I also think the motors could be doing with a rebuild, the capstan motor is noisy, its not squealing out fore help but its got a quiet rumble, I don't think they left the factory like this.
And the tape counter doesn't work.
But apart from all this its a lovely machine and worth putting a little effort into to get it back up to spec.

Offline stellavox

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Re: Revox G38 line in amplifier bypass mod?
« Reply #3 on: June 01, 2015, 08:51:03 AM »
Don't forget to cramolin (or equivalent) the tube pins.  Don't know about the mechanical noise in one channel.  The motors can be rebuilt with new bearings but it's a lot of work.  On a lot of the machines I've seen,  the tape counter is broken - I believe it was the that they wouldn't reset to zero because the plastic "pusher flap" broke.

I'll send you the articles I have.

Charles