Revox B77 HS 4 track?

[Linnovice]

Hi all. I'm in the process of refurbishing a Revox B77 HS half track and I'm curious as to whether it is viable to convert it to a quarter track machine. Is it just a matter of putting in a set of qtr track heads or are there other factors to be considered?

[Ted Kendall]

The oscillator transformer is different - a 2 track board will overdrive a quarter track head and a 4 track board hasn't enough urge to run a 2 track head properly.

[monaro0162]

I've wondered this myself, considering this on a A77mk4 that I have. So, cost of the heads aside, it's a fairly simple conversion?

[Ted Kendall]

In essence, yes. However, the wrap, height and tilt of the new heads needs to be set accurately, and part-worn heads should be re-lapped to ensure good tape-to-head contact in the new installation. That apart, it's a doddle...

[Amraduk]

Hello Ted,

Originally Posted by Ted Kendall

The oscillator transformer is different - a 2 track board will overdrive a quarter track head and a 4 track board hasn't enough urge to run a 2 track head properly.

Does that also apply to the Revox G36?

Regards,

Dave.

[TIMTAPE]

I remember converting a 2 track B77 to 4 track in the '80's. It's not just transformer T1. There are also changes to C8, Q1 and R5 on the oscillator PCB. Each of the component changes is noted on the schematic with a star symbol.

[SteveCG]

Consider also that the record and replay heads will have different characteristics. Whether this is show-stopper issue I don't know the answer to.

[Ted Kendall]

No, there is enough adjustment to align either type of head as far as rec/rep is concerned.

[IvorBlister]

I am just wondering what the reason would be for going quarter track. I have owned high speed half and quarter track versions of this machine and was constantly plagued by cross track 'sound bleed' on the quarter track version. It was set up several times by Revox engineers but never fully cured and I ended up just using two of the tracks so any gains in tape consumption was lost.
Nick

[Ted Kendall]

This is an inherent problem, caused by the same fringing which makes it necessary to apply correction to LF readings from a full track test tape on narrow track heads. Since the use of high speed is either for high quality or ease of editing, I'd say the use of quarter track heads is counter-productive.

[monaro0162]

No doubt the two track layout offers better performance in terms of tape noise and signal dropout. This was the first thing I noticed when comparing my 2 track A77 with my 4 track Akai 4000DS, although some of the improvement can be attributed to the better overall design of the Revox. The only reason I considered doing the conversion, was to make it compatible with 4 track tapes and machines that I have, for comparison purposes. I'd only do it on one of the A77s that I have, and only if I could find some cheap 4-track heads suitable for lapping.

[TIMTAPE]

As Ted mentioned it's a low frequency issue. Often if there was little low bass in the recording you didnt hear any crosstalk between tracks.

Pre digital we recorded audio book masters to 1/4 track on long play tapes to match the US Library of Congress 1/4 track cassette format. I dont recall crosstalk ever being an issue on the reel masters but print through often was and with time it only got worse. Even back then people used to get the terms confused, calling print through "crosstalk".

[Michael Maurice]

I've owned a 1/4 track A77 from new and have always had problems with crosstalk, something you never got with the Akai or Sony or even the old Korting.

[ben]

I'm currently working on a Mk I A77 (half track) and even on that, I have noticed (minor) cross talk. My suspicion is on the crude track selector switch.

Back to the point. Unless you're after as much hi fi as possible or aim to edit, for general domestic listening such as making long mix tapes/ compilations, quarter track is probably best to get maximum use out of the tape.

In the interests of compatibility, I would also suggest going with the track format most your tape collection has - if what you have is nearly all in quarter track, it makes little sense having a half track deck.
If it seems like too much of a hassle to modify, you could always sell it and look for a quarter track model, Revoxes hold their value well.

[Restoration73]

Why not get a Teac X-2000M - it can replay 2 or 4 track tapes (switch) with a quality equal to the Revox without all the modifications that have been proposed.

[Michael Maurice]

Originally Posted by ben

I'm currently working on a Mk I A77 (half track) and even on that, I have noticed (minor) cross talk. My suspicion is on the crude track selector switch.

Its not the track selector switch, it is purely down to the head arrangements.

On 1/4 track machines you hear the low frequencies of the tracks NOT being played, ie if listening to tracks 1 & 3 you will hear the low frequencies of tracks 2 & 4.

[TIMTAPE]

Re crosstalk on quarter track, the B77 Service Manual quotes figures of 60db (mono) and 45db (stereo) at 1000Hz (pages 6-14 and 6-15). But interestingly these specifications are the same whether it's a quarter track or half track machine. No distinction is made. Whereas for signal to noise ratio, the stated figures are worse for quarter track, as we'd expect.

In stereo the tracks (1 and 3) are spaced further apart. If the crosstalk was because of the close proximity of the physical tracks we'd expect expect it to be worse between adjacent tracks (eg: tracks 1 and 2), not better.

That the stereo crosstalk is worse than mono suggests the crosstalk is related to amplifier and/or head coupling rather than track proximity.

[Linnovice]

The consensus of opinion seems to think my original idea to be not a good one. So no point in asking for advice and then not taking it. We'll forget about a HS quarter track B77.
Sooo, how about converting a standard half track to quarter track. I have the machine and a set of new 4 track heads. My problem is although I don't often record on a quarter track (A77) I do have a small collection of pre recorded tapes. Plus I do like having a project �� . . .

[Michael Maurice]

All the advice given above will apply equally to a standard speed machine

[Ted Kendall]

The frequency at which adjacent channel crosstalk becomes troublesome falls with tape speed, and high levels are more easily recorded at higher speeds, as HF crushing is reduced. In practice, therefore, adjacent channel crosstalk falls to more acceptable levels at domestic tape speeds. If you have a use for a standard speed quarter track machine, and you do, by all means go ahead.