Revox A77 head mounting

[monaro0162]

Good morning all,

I am considering converting an A77 MK3 from 2-track to 4-track. I know this could be considered as sacrilege by some on here but I have my reasons:

I have bought and sold a number of A77s over the last year, I sold a few recently, and currently have three machines that are 'keepers' for the foreseeable future: I have a 2 track MK2 High Speed, a standard speed MK3, and a standard speed MK4, all of them in very good condition. To cover all tape track formats I really want to convert one of them to 4 track, so as to be able to play a lot of 4 track 3.75ips tapes that I have. I did have an AKAI 4000DS to do this, but since I've had the A77s, its has become very obvious how superior these machines are compared to the AKAI. Once you've had a 3 motor direct drive machine you will never want to use a single motor belt driven machine again! So I've now sold the AKAI, and I want to convert one machine, the MK3 (which has the most head wear) to 4-track.

There is a seller on e-bay selling new A77 2 and 4 track heads. According to the manual, there were two types of mounting on the heads - 1 hole fastening and 2 hole fastening. The seller says his heads are 1 hole fastening. I have has a look at the heads on the MK3 and I think that they are the 1 hole fastening. It is a bit difficult to see, photo attached, but below the bottom left hand wire, there appears to be just one screw holding the head in place.

So I am asking for advice on here on this potential conversion. Is there anything I should be aware of when doing this conversion? I know I could try and buy an A77 4-track, but I really don't want to only to find that the heads are worn and it needs lots of electronic work as they all do. I'd rather convert the MK3 that I have, with new heads, as its already a known quantity, and should guarantee perfect performance.

Any advice on the conversion appreciated.

[Radio Wrangler]

You can't see the real mounting screw(s) in that photo, they point upwards through the tilt plate into the head and they are countersunk.

I re-headed my machine in 1983 so memories may be a bit faded.

David

[Ted Kendall]

A Mk 3 is late enough to be single-hole mounting. You will also need to change the erase head and the oscillator transformer. That apart, it's fairly straightforward. The usual care in setting up height, face tilt and wrap will pay dividends in extended head life.

[monaro0162]

Thanks Ted. I wondered which way around the 1 or 2 mounting point fixing was applied relative to each MK. Intending to buy the erase head as well. Whats the reason for the oscillator transformer change?

[monaro0162]

Originally Posted by Radio Wrangler

You can't see the real mounting screw(s) in that photo, they point upwards through the tilt plate into the head and they are countersunk.David

The manual doesn't show countersunk screws.. shows a screw fitted with 2 washers for the single hole variant. 2 hole variant just shows 2 screws without washers. I've been caught out with problems like this before .. don't want to get caught this time!

[Radio Wrangler]

Pretty certain mine is 2-hole, I still have the old heads kicking around. I'm less certain they were countersunk.

David

[Ted Kendall]

The quarter track erase head is of different inductance - easiest way to deal with it may be to find a complete quarter track oscillator card.

Two hole heads had gone by 1974, when the A700 was launched - that machine was single hole from day one. I think the changeover was during Mk II - new, harder heads came in during Mk III, but all replacements available now will probably be of this material (Revodour) by default.

[monaro0162]

I found some photos of the underside of the base plate (which isn't shown in the manual). Flat surface for the hole (not countersunk) which confirms the view of the single screw with washers that I think I see in the first photo I posted. It's very difficult to view it properly, but it does look like just one screw and not two. So those new 4-track single hole heads should fit.

Regarding the different oscillator card, I will source one from one of the e-bay sellers in Germany .. would anybody know the correct part number for one to suit a MK3 4-track?

[Ted Kendall]

2 track oscillator is 1.077.712 (red dot on oscillator coil), 4 track is 1.077.731 (yellow dot on oscillator coil), according to the manual.

[monaro0162]

Christmas has arrived early for me: A four track oscillator board from Germany, 10 new reels of Recording Masters SM911 from France, and a set of brand new 4-track heads from Romania. All the parts needed to carry out the 4-track conversion. That's me sorted for the Christmas holidays!

On the conversion I was wondering about setting the head alignment: I only have a 2-track calibration tape. I'm assuming, or hoping that (in theory) the 2-track tape would be ok to use to set up the 4-track heads? (at least to do the initial setting?) I've done the head azimuth adjustment on all the 2-track machines that I have many times before without problems.

Once the heads are in, I have a pile of 4-track tapes that I haven't heard for years to review. I will also need to re-calibrate and re-bias the other 2 A77s that I have to suit the new SM911 tape.

I've fitted amps to all my machines and I'm listening to extremely funky live James Brown, and live Fleetwood Mac with Bob Welch, from the 70s at the moment. Ideal source material for checking out the Revox. Happy days ahead

[Ted Kendall]

Calibration tapes are generally full-track, as it's difficult enough to get one track sufficiently accurate, never mind two. Azimuth remains the same regardless of track width, but a quarter track head will appear down on the lower track if playing a tape made with 2mm track width half track heads, as it will be reading the edge of the recorded track.

[TIMTAPE]

Also if replacing heads it's important to confirm or adjust head height, zenith and rotation. Test tapes dont help for these adjustments. The heads need to be checked visually. An ordinary 1/4" tape can be used to check head height.

[mitajohn]

Originally Posted by TIMTAPE

An ordinary 1/4" tape can be used to check head height.

Even better to use a transparent 1/4" tape leader.

[Welsh Anorak]

Since most conversions are the other way round I don't think you'll have any trouble selling the old kit should you so wish.
Glyn.

[monaro0162]

Originally Posted by Ted Kendall

Calibration tapes are generally full-track, as it's difficult enough to get one track sufficiently accurate, never mind two. Azimuth remains the same regardless of track width, but a quarter track head will appear down on the lower track if playing a tape made with 2mm track width half track heads, as it will be reading the edge of the recorded track.

For some reason I assumed that the calibration tape that I have (from an e-bay seller nielsfup2010) was two track because it was made on a B77, but yes thinking about it full track would be the optimum layout to do the azimuth/phase adjustment. May be I'll try playing the tape backwards and see what I find.

[monaro0162]

Originally Posted by Welsh Anorak

Since most conversions are the other way round I don't think you'll have any trouble selling the old kit should you so wish.

I also bought a head carrier with the intention of fitting the new heads to it, thus enabling the 2-track carrier and heads in the MK3 to come out in one piece (so as to maintain original head height/azimuth settings etc) ... but I forgot about the seating platforms with the adjustment screws on them (that I didn't buy) so I'll have to remove the heads from the MK3 and use those seating platforms instead.

I'll shall probably retain the MK3 heads and the other parts .. to enable later conversion back to 2 track if required. This MK3 has the vari-speed so its the best option to use to play the 4-track tapes that I have, made on different machines, that may have been recorded with the tape running a bit too fast or slow.

[jamesperrett]

Originally Posted by monaro0162

For some reason I assumed that the calibration tape that I have (from an e-bay seller nielsfup2010) was two track because it was made on a B77

I've seen other people offer these too but essentially they're secondary calibration tapes which have been recorded on (hopefully) a well set up machine. The MRL full track tapes have been recorded on machines which have been modified for the job, are very carefully set up and are constantly checked against a known reference. There's a bit of background at

http://www.mrltapes.com/hodges_mrl.pdf

[monaro0162]

Yes I agree that they are secondary tapes .. although made using decent equipment by the look of it. On all 3 machines that I have set up, I found that only the very slightest azimuth adjustment was needed on each machine from the position set by the factory (no more than an 1/16th rotation). Which is an encouraging sign .. I suppose. All 3 machines have been set up using the nielsfup2010 tape so as to maintain azimuth compatibility when swapping tapes around between machines. I would guess that those MRL tapes are quite expensive?

eta - so where would a 'first generation' calibration tape come from? What machine would it be made on?

[Ted Kendall]

MRL's machinery is based around Studer A80 transports; BASF used Telefunken; the BBC made their tapes for some time on a Telefunken M10A which now sits in my studio.

Proper test tapes are individually made with an operator or control system correcting levels in real time, on selected tape stock, on full track machinery which is calibrated as far as possible from first principles, with the use of flux loops and the like. It is possible to establish absolute perpendicularity of the head gap without the use of a test tape by a rather involved process involving playing the back of the tape.

Given the time and hassle involved in making even a passable secondary test tape, MRL's prices are fully justified, in my view.

[monaro0162]

Successful conversion to 4-track completed. As said a straightforward job, although a couple of gremlins were waiting in the wings:

After I'd completed the head swap I found that it was recording and playing fine, but wasn't erasing properly. Turns out the 4-track erase head is slightly shorter, and requires a shim underneath it to raise it to the correct height to match up with the 4 tracks on the tape. The manual doesn't seem to show this part (part number 1.077.131-01) although the brass 0.1mm shim used on all variants (part number 1.010.006-23) is shown (index 33 in the diagram). This part didn't come with the head carrier that I bought (although the loose erase head was there) so I was unaware that it was needed. I solved the problem by cutting a piece of base plate off of an old Akai head which turned out to be exactly the thickness required and already had a hole for the mounting screw to go through.

The second gremlin caused complete havoc and a lot of head scratching: Suddenly during testing I seemed to loose signal routing through the back plane accompanied with intermittent full-scale deflection of the VU meters. After a lot of fiddling about with the wiring and swapping cards around I found that the input amplifier had a blown input transistor. Turns out that the locating pin in the back plane for the input amplifier had fallen out causing the card to be inserted with the pcb tabs pins offset against the back plate sockets ... you know what that can cause ... inappropriate voltages at inappropriate points in the amplifier circuit! Diagnosed to Q401 blown after replacing it normal service was resumed.

So after calibration the machine is now running fine in 4-track mode. Can't say I really notice much difference between 2 and 4 track at the moment, it sounds great. Before I removed the 2-track heads I took an Audacity trace of the frequency response and I will be re-running the trace under the exact same conditions to determine if there is any difference.

Happy days!