Tape Project Forum

Tape Machines => Reel to Reel Tape Machines => Topic started by: sound signal on November 08, 2008, 05:41:03 AM

Title: More Revox mods
Post by: sound signal on November 08, 2008, 05:41:03 AM

I also just received today a high speed Revox B77 with half track heads, which appears to be in decent shape, although the heads could use a relapping.  Other than it being an NAB deck, it's set up correctly for TP tapes.  I also found some spare B77 audio boards on fleaBay which should be readily modifiable for IEC 15ips (the PCBs are the same per the service manual, so all I have to do is swap out a few resistors and caps).  Is the reason you haven't been recommending Revox just that the high speed half track configuration is hard to find, or is that only part of it?


Hi bobschneider,

As far as Revox B77 PCB's go, the playback amps are all the same and you can change components for IEC 15ips.  But of all the record amp PCB's, the only one which isn't the same is the NAB 7 1/2 - 15ips board.  You can't make the mod to IEC without cutting up tracks.  This is a pity - the NAB high-speed record amp board is rare, I wouldn't cut it up.  I would rather find a secondhand 3 3/4 - 7 1/2ips NAB record amp board and modify that to IEC.

Most B77's sold were NAB 3 3/4 - 7 1/2 ips 1/4 track machines as this had become the de facto consumer standard in the 1970's.  It's easy enough to modify a B77 to 1/2 track by changing heads - but nowadays that's expensive.  It's also easy to modify the electronics to 7 1/2 - 15ips IEC.

But it's much less easy to modify the mechanicals from 3 3/4 to 7 1/2 ips.  To do it the Revox way, you need a different capstan shaft and a different support casting that shifts the motor so the tape path geometry stays the same with the thicker capstan shaft.  That casting is the heart of the machine.  The capstan motor, pinch roller arm and head block all bolt to it.  Replacing it would be a big job, even if you could get the part from Revox, which I doubt - it's not a wearing item so it's unlikely they still stock it.

There is also Arian Jansen's (http://www.tapeproject.com/smf/index.php?action=profile;u=170) published method for modifying a B77 (http://laocaudiosociety.net/tech/ReVox2.pdf), which spins the motor at double speed by modifying the motor control electronics.  I haven't tried that myself.  Both the B77 Mk.II's we have here are original high-speed machines, they only needed the mod from NAB to IEC.  I am curious to try it, though, and I do have a wow and flutter meter so I could easily check its performance against a factory high-speed machine.  All I need is a standard-speed B77 to try it on.  If I do try it and test it I will of course let the forum know.

With best regards,
Title: More Revox mods
Post by: bobschneider on November 08, 2008, 10:56:30 AM

As far as Revox B77 PCB's go, the playback amps are all the same and you can change components for IEC 15ips.  But of all the record amp PCB's, the only one which isn't the same is the NAB 7 1/2 - 15ips board.  You can't make the mod to IEC without cutting up tracks.  This is a pity - the NAB high-speed record amp board is rare, I wouldn't cut it up.  I would rather find a secondhand 3 3/4 - 7 1/2ips NAB record amp board and modify that to IEC.


Thanks, George.  While the service manual I have is clear that the NAB 7 1/2 - 15ips record amp board is a different PCB than the others, the spare audio boards I have appear to be the common 3 3/4 - 7 1/2ips NAB versions.  So converting the record amp to 7 1/2 - 15ips IEC should just be matter of swapping a few parts.  What I'm hoping to end up with are two sets of 7 1/2 - 15ips record and reproduce amps: one NAB and one IEC.  So switching back and forth will be just a matter of swapping the audio boards.  Not as easy as the switch on a MX-5050, but still reasonable, and basically factory stock in both cases.

     -Bob

Edited to fix typo (see Sound Signal's post below)
Title: More Revox mods
Post by: sound signal on November 08, 2008, 12:03:47 PM
Hi Bob,


Thanks, George.  While the service manual I have is clear that the NAB 7 1/2 - 15ips reproduce amp board is a different PCB than the others


It's the NAB 7 1/2 - 15ips record board that's different to all the others, not the reproduce board.  I have a feeling that's what you really meant to type...


The spare audio boards I have appear to be the common 3 3/4 - 7 1/2ips NAB versions.  So converting the record amp to 7 1/2 - 15ips IEC should just be matter of swapping a few parts.


Indeed, it is a case of swapping parts - and often of simply removing a part or replacing it with a shorting link.  Here is the list of swaps you need to make to change a set of B77 3 3/4 - 7 1/2ips NAB boards to 7 1/2 - 15ips IEC.

Be careful with that soldering iron, and make sure you have a good solder sucker to remove the parts.  It's easy to lift those Revox PCB tracks.  Don't ask how I know...


Revox B77 Record Amplifier PCB, modification from 3 3/4 - 7 1/2ips NAB to 7 1/2 - 15ips IEC (original values in brackets):

R6, R26: 1k5 (3k3)
C7, C20: Short (0u1)
R8, R23: Open (56k)
R14, R30: Open (4k7)
C13, C24: Open (33n)


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)


And, just in case anyone needs Nagramaster...

Revox B77 Reproduce Amplifier PCB, modification from 3 3/4 - 7 1/2ips NAB to 7 1/2ips NAB - 15ips Nagramaster (original values in brackets):

R1, R27: 68k (220k)
R9, R30: 3k9 (3k9 no change)
R17, R43: 1k2 (4k7)
R22, R42: 270k (330k)
Title: More Revox mods
Post by: ironbut on November 08, 2008, 03:01:28 PM
Wow George, what an asset to this community you've become. I'd like to personally thank you for all the excellent posts you've contributed so far. Do you have a web site or blog for information regarding past and ongoing projects you're involved in?
Title: More Revox mods
Post by: Tubes n tapes on November 08, 2008, 04:30:39 PM
Quote
There is also Arian Jansen's published method for modifying a B77, which spins the motor at double speed by modifying the motor control electronics.  I haven't tried that myself.  Both the B77 Mk.II's we have here are original high-speed machines, they only needed the mod from NAB to IEC.  I am curious to try it, though, and I do have a wow and flutter meter so I could easily check its performance against a factory high-speed machine.  All I need is a standard-speed B77 to try it on.  If I do try it and test it I will of course let the forum know.

George,

Thanks for your wonderful contributions to this forum !

One remark concerning the speed change of a standard B77. Keep in mind that it only works if your mains frequency is 60Hz. I guess that on Cyprus it will most likely be 50Hz.
Title: More Revox mods
Post by: sound signal on November 08, 2008, 07:30:22 PM
Thanks Steve and Arian for the kind word, it's great to be appreciated.  I spent most of my life in love with the analogue tape medium and I can't express how happy I am that the Tape Project, and the community that is growing around it, have come to exist.

Arian, indeed the mains here in Cyprus is 240V 50z.  Now you've got me curious about the high speed B77 mod.  Maybe I can think of something.

Anyone interested in my audio activities is welcome to peruse The Sound Signal Company's website. (http://the.sound-signal.com)  Expect to be underwhelmed by the (lack of) web design, but I hope the content has some appeal.  There isn't any reel to reel stuff (yet).  There's some vinyl LP playback stuff, and some loudspeaker stuff.

Just so you know what it's all about, The Sound Signal Company is a framework within which I try to do two things I have wanted to do for a long time:  design and build high quality audio electronics, and make high quality recordings on analogue tape.  Of course, the two go nicely together, since a high quality recording is a great source for testing audio equipment.  The chief objective is to have a few laughs while possibly getting something right, eventually, and also sharing what we find out from our research with like-minded individuals.

Best regards to all,
Title: More Revox mods
Post by: Ben on November 08, 2008, 07:47:55 PM
Since you are closer to Europe, if they made B77's for 50 Hz, 15IPS
would not option of finding one be easier over there.
Title: More Revox mods
Post by: sound signal on November 10, 2008, 06:56:48 AM
Hey Ben,

There are no special B77's for 50Hz or 60Hz.  All machines have the same power supply which is international and switchable for the local mains.

What Arian is saying is that his modification works when the local mains is 60Hz but not when it's 50Hz.  This makes me curious, both as to why it is so and as to whether there's a workaround.

As I've written above, factory high-speed B77's are much rarer than normal-speed machines, and modifying a standard-speed machine to factory-spec high speed configuration is a no go for several reasons which I've mentioned before.  The mod has to work differently than the factory mod.

By the way, the same goes for A77's and G36's.  Many people thought that it was possible to modify them to high speed simply by changing the capstan shaft.  The machine will run but the tape path will be wrong, you will not get the correct wrap around the heads.  I wrote a letter to Glass Audio in 1991 correcting a statement that the G36 could be thus modified - they printed it on the Letters page under "Don't give your G36 the shaft"...  When buying any high-speed Revox G36, A77 or B77, be careful you're not getting one that's "been given the shaft".  On the other hand, the A700, which is three-speed, features 15ips in its standard configuration and has a much nicer transport with rolling tension rollers rollers and servo tension control.  But those slider controls for input and recording level become noisy, and the audiophiles among us scorn the early audio chips it uses.  There's no such thing as perfection, even among Revoxes...

That's why it's interesting to see if 15ips could be achieved purely electronically with a mod like Arian's for all users irrespective of where they are.  There are a lot of standard-speed Revoxes out there.
Title: More Revox mods
Post by: Ben on November 10, 2008, 11:23:28 AM
Can't the title be edited. News from the bench. :)
That makes me wonder, what is the factory mods for 15IPS
B77? I think the paper says the reel motors will not be at
the correct speed or something like that, and new mechanical
Assembly was needed.
One other option if you have a 12V DC source, is to try
a STATPOWER inverter to get 120V / 60 HZ. Check the fine
print on the mods, when you get the reference signal for
the capstan. If is from the AC line we are out of luck.
Title: Re: More Revox mods
Post by: Tubes n tapes on November 10, 2008, 02:36:40 PM
Thanks Dan, for putting this under an appropriately titled thread.

George, Ben,

In my article I'm explaining why the electronic speed change doesn't work for 50Hz.

Ben's proposal of using an external 12Vdc to 120V/60Hz inverter would work, because the 60Hz itself is not used as a reference, but only provides the speed headroom you need to run it on 15ips. Keep in mind, however, that most (cheap) 12Vdc to 120V/60Hz inverters have a modified squarewave at the output instead of a sine wave. I don't how your average B77 reacts to that.

Some words about the A700. As George mentioned, the transport is very nice. In fact the A700 shares it's transport with the Studer A67/B67. A very beneficial trait of the A700 transport is that it regulates the tape tension during record/playback, during FF and Rew, and during braking. That last feature assures the gentlest tape handling under all conditions. A feature it only shares with large recording studio machines like the Studer A820.
On the flip side, the audio electronics of the A700 are an abomination and completely beyond salvation. So if you want to use an A700 because of it's excellent transport, run the head straight into a Bottlehead repro amp or something like that.
Title: Re: More Revox mods
Post by: Ben on November 10, 2008, 03:30:23 PM
I state STATPOWER as it more of a AC sine wave than the other guys if I remember
right. They may have clones of it out by now.
Ben.

Footnote:
Then again all the real audio people have surplus 1.2 kv DC generators, running the
845's for 100's of watts of power.
Title: Re: More Revox mods
Post by: sound signal on November 11, 2008, 09:26:45 AM
Greetings all,


what is the factory mods for 15IPS B77? I think the paper says the reel motors will not be at the correct speed or something like that, and new mechanical Assembly was needed.


Ben, the factory 7 1/2 - 15ips B77 had a thicker capstan shaft - it was double the thickness so the motor ran at the same speed.  This would change the amount of wrap around on the playback head and adversely affect the tape path geometry.  So Revox also redesigned the main casting to shift the motor a little upwards and keep the tape path constant.  Undestandably this means that the main chassis also has the holes in a different place.  The same goes for G36's (except for a rare version which used a higher speed motor, which was quickly changed to a standard motor + casting mods) and A77's.


In my article I'm explaining why the electronic speed change doesn't work for 50Hz.  [...]  the 60Hz itself is not used as a reference, but only provides the speed headroom you need to run it on 15ips.


Thanks Arian for the clarification.  Perhaps a modest change of a capstan shaft - to, say, 33% thicker - would allow the deck to run at 15ips with only a 50% change in motor speed (1.33 x 1.5 = 2.0).  Perhaps 50% higher motor speed is achievable with 50Hz as well as 60Hz mains.

Of course that would mean having a custom machined capstan shaft.  That would of course cost money but with that type of thing the more you make, the less the unit cost.  Perhaps if there is enough demand we could go to a firm of precision engineers and have them make us up a dozen or so.

This is actually what Revox did with the first series of high-speed G36.  They fitted a 4/8 pole capstan motor which ran at 50% higher speed, and a high-speed shaft that was only 33% thicker than the standard speed shaft, to preserve the tape path geometry.  Later high-speed G36's had the standard 6/12 pole motor, a double-thickness shaft and modified castings and chassis to restore the tape path geometry. 


Some words about the A700.  As George mentioned, the transport is very nice. [...]  On the flip side, the audio electronics of the A700 are an abomination and completely beyond salvation.


Having made some nice-sounding recordings with my A700, I wouldn't dismiss it totally out of hand.  As far as I'm concerned, the silly faders that go noisy and the equally silly fuse holders that go intermittent are much more of a problem than the actual audio electronics.  I want to get mine to work reliably and cleanly, it deserves it.  Time and other projects allowing...

Best regards,
Title: Re: More Revox mods
Post by: microstrip on December 02, 2008, 06:59:00 PM
Hi,
You can always use a PSAudio PS300 mains regenerator - it accepts 50hz but can output 60 Hz.
Even USA models can be switched to accept 230V input.
Title: Re: More Revox mods
Post by: microstrip on June 05, 2009, 04:32:04 PM
I would like to add some good news to 50Hz users thinking about joining the Tape Project    - the offset compensation for the larger diameter shaft of Revox 15 ips B77 and PR99 is made at the motor fixation holes, not at the chassis. It is very easy to shift the holes to the new correct position in the motor - around 2.3mm 140? looking from the shaft side with the wires on the left side.
The same people who redo capstan worn out shafts with great precision can sleeve an old 7.5 ips shaft to the 9.08 mm diamater, as Revox  15ips capstan shafts are very difficult to find - Revox manuals suggest you should replace the whole motor assembly if you need a new shaft! All you have to do it readjusting the pinch roller tension.

I have detailed photos of the motor fixing that I can share with anyone interested in such a modification.
Francisco
Title: More Revox mods, Specifically the G36
Post by: slorinczi on June 17, 2010, 02:57:58 PM
Hi there,

I'm dipping my toe in the dark and treacherous "moderizing a tube deck" lagoon. I recently bought a couple of ReVox G36s. Unfortunately they're both the low-speed version, and I'd like to convert one of them to high-speed (15/30 ips).

What I'm trying to figure out is the best way to achieve this. There is plenty of evidence that merely substituting a thicker (high-speed) capstan will cause an incorrect tape path. Is there any (practical) way to reposition the existing tape guides to approximate a better path?

Alternately, I'm told that the "correct" way to move up to high-speed is to use a high-speed shaft AND high-speed motor, as it has a slightly different footprint to compensate for the thicker shaft.

Anyone know if this is all there is to it (I say "all" with a smirk, knowing how difficult it will be to find one)? In the G36 service manual, I see a number of parts that are designated for high-speed operation, including the capstan, capstan motor, and head support block. So I'm wondering if there's more to this than meets the eye.

Finally, what's the "definitive" story regarding 50 hZ vs. 60 hZ.? Is it impossible to usea 50 hZ motor in my (North American) deck? I'm told there may be a conversion possible, but I'm none the wiser.

Many many thanks, in advance!

Seth
Title: Re: More Revox mods
Post by: sound signal 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.
Title: Re: More Revox mods
Post by: vladtm 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)

Hello,

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.