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Author Topic: More Revox mods  (Read 34227 times)

Offline sound signal

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Re: More Revox mods
« Reply #15 on: June 19, 2010, 04:17:33 AM »
Hi Seth,

Re. converting the usual 3-4/4 + 7-1/2ips version of the Revox G36 to higher speed operation.

First off I don't think it's realistic, or even desirable, to aim for 15 + 30ips operation with a G36.  7-1/2 + 15ips is the target.

Next, let's look at how the factory did it.

The factory only made 7-1/2 + 15 ips available on the series 3 machines. Series 3 G36's can be spotted from the photo-electric end stop which made a different shape of head cover necessary.  There were two batches of these factory high speed machines.

For the first series, the factory substituted a motor with 8 + 4 pole windings (to get the two speeds) for the normal 12 + 6 pole motor.  That gave them 50% higher motor speed:

12 + 6 pole motor, 50Hz: 500 + 1000rpm, 60Hz: 600 + 1200rpm.
8 + 4 pole motor, 50Hz: 750 + 1500rpm, 60Hz: 900 + 1800rpm.

To get the remaining 33% speed increase (50% x 33% = 100%), they fitted a capstan shaft 33% thicker.  Obviously there were two versions of the capstan shaft for the 4 + 8 pole motor, one for 50Hz and one for 60Hz.

Since the capstan shaft was only 33% thicker, Revox made no modifications to the tape path.  The same top chassis and base casting are used and the motors have the same fixings, so the 4 + 8 pole motor and the appropriate shaft can actually be retrofitted to the normal 3-3/4 + 7-1/2ips G36.  And this can be retrofitted to the Series 2 G36 as well, if the drive couplings from the existing Series 2 motor are swapped onto the 4 + 8 pole Series 3 motor.

However, since about the early 1990's the 4+8 motor and shaft have been "unobtainium".  I actually got an invoice for these parts from F.W.O Bauch in the UK when I was a student there, total cost 180 pounds sterling which I couldn't afford at the time... life is full of regrets.

For the second batch of 7-1/2 + 15ips G36's, Revox switched back to using the same 12 + 6 pole motor as for the 3-3/4 + 7 1/2ips version.  To get the speed increase they fitted a double diameter (+ 100%) capstan shaft.  Obviously there were again two versions of this shaft, one for 50Hz and one for 60Hz.  But the 100% increase in capstan shaft diameter meant that they had to modify the chassis and the base casting to shift the motor a bit towards the VU meters to compensate; this chassis and casting had different part numbers of course.

And this is where the misunderstandings began, because people thought they could just swap the 7-1/2 + 15ips capstan shaft for the 6 + 12 pole motor into a 3-3/4 + 7-1/2ips G36 and convert it to high speed.  Naturally that wrecked the tape path geometry and with it the scrape flutter and frequency response.

If anyone is interested, I will dig out my G36 manual and post all the parts numbers for the different motors, shafts, chassis and castings.

This brings us to the question of how to make the modification today.

Any modification involving only a +100% capstan shaft is, in my opinion, not advisable, since it is impractical to correctly modify the chassis, base casting or head block to re-set the tape path and factory modified parts are of course no longer available.

We will have to proceed as Revox did with the first batch of high-speed G36's and fit a capstan shaft that is only slightly larger, and spin the motor faster.

To get a firm of precision engineers to make up a +33% capstan shaft for the G36 should not be difficult, but it will cost money.

To spin the motor 50% faster, we only need to supply it with a frequency 50% higher, since it is a synchronous motor.  We can use an electronic power supply as used for upgrading synchronous motors for turntables.  There are a number of boards or kits available to do this.  And since the frequency will be electronically controlled, the mains frequency of either 50 or 60Hz will not affect the speed.  We will only need one version of the capstan shaft, and our modified G36 will be "internationalised", needing only a switch to set the capacitors for the reel motors to move between 50Hz and 60Hz regions.

I have two G36's and you, Seth also have two.  Any firm we can get to make the +33% shafts for us will probably charge less per shaft for a batch of four shafts than for a one-off but it is still not likely to be cheap.

This is not likely to be a beer-money project, but there are several things in its favour:

- Secondhand prices for professional tape machines with all-valve (all-tube for you transatlantic fellows) record and playback circuits are much higher than for later solid-state machines and parts for such machines tend to be expensive and hard to find.
- The G36 is a simple, rugged, reliable, long lasting and easy to maintain design of good quality.  There is reasonable availability of spares and a plentiful supply of used machines.
- And, of course, a 7-1/2 + 15ips G36 is by definition a Tape Project playback machine.  They came with IEC eq as standard; NAB was offered on the 60Hz machines only.

I would be very interested to hear from other G36 owners / users who could be interested in a 7-1/2 + 15ips conversion done as above.
« Last Edit: June 19, 2010, 04:20:06 AM by sound signal »
George Karaolides
Nicosia, Cyprus

Offline vladtm

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Re: More Revox mods
« Reply #16 on: March 25, 2016, 11:48:56 AM »
Revox B77 Reproduce Amplifier PCB, modification from 3 3/4 - 7 1/2ips NAB to 7 1/2 - 15ips IEC (original values in brackets):

R1, R27: 56k (220k)
R9, R30: 2k7 (3k9)
R17, R43: 3k9 (4k7)
R22, R42: 470k (330k)


First of all hello to everyone and thank you for all the valuable info on this forum, this is my first post here !! wohoo

I Successfully converted one of my b77 NS-NAB to HS-IEC, motor, tension board resistors, electronics.

I followed the diagrams from the B77 manual for HS IEC to do the conversion.
This is one of the first links that comes up when searching for B77 conversion to HS, so for a long time i wanted to ask this question:
Why did you put here R22, R42: 470k (330k) ? the original value for NS is 330k, but the HS repro board has 680k as a value in the manual, not 470k.

Thank you,

Best regards.