Revox B77 guides

[jpmorris]

I've just got one of those Italian service kits which turn up on ebay fairly regularly (probably only in the UK and Europe though). I really wanted it for the replacement pinchroller, but the other stuff was very handy.

Anyway, the B77 has two large, round guides at either end of the headblock, which also form the pillars which the headcover it attached to. The left one rotates, the right one does not - apparently by design. Does anyone know why this is? What would happen if I replaced the stationary guide with the spare rotary one from the service kit?

 

[N7SC]

Although I don't know the answer to your question, I have pondered replacing the stationary right guide on my A77 with the rotating guide also. Actually the rotating guide is just a bearing.

The only thing that I can think of that would argue against doing it is that the drag of the tape across the fixed guide would be gone, and that might have an effect on the tension. It might cause the tape to wind more loosely on the take up reel. However, different tape formulations that were available in the days when these machines were made would all have differing amounts of drag due to differing lubricant formulas and surface characteristics. So the tension setting of the machine would only be exactly right for only one type of tape. There would have been a variation in the drag using other tape formulations. That variation is probably much less than the drastic reduction in drag that would result from installing a rotating guide.

Whether or not it is enough to make any difference to how well the machine handles and packs tape, I don't know. However, it would be much kinder to the tape, by reducing the number of fixed surfaces that the tape had to drag across. And, if you use a heavy tape (996, GP9, RMGI/BASF/Emtec SM900, etc) that was developed long after these machines went out of production, it may actually help the tape handling by making it easier for the machine to handle a tape that is much heavier and "draggier" than it was originally designed for.

I also recall seeing, in one of those eBay ads, the suggestion that you can replace the fixed guides with the rotating ones.

 

[N7SC]

As always, thought of this after posting above:

The PR99 has a rotating right guide. It is actually the input tachometer for the autolocator system in the Mark II and III units. If you have access to a PR99 service manual and a B77 service manual, you can see if the reel tension settings are any different from those in the B77 (which is actually a PR99 for the consumer market). If they are the same, then apprently Studer/Revox's engineers did not think the rotating guide made any appreciable difference.

I do have a PR99 manual on a hard drive around here somewhere, but am up to my ass in alligators, so it might be a week or two before I get the time to find it and compare it with my A77 manual. But I vaguely think that the reel tensions in the A77, B77, PR99 units were all the same. Would need to verify though.

 

[Beck]

Stationary guides near the heads on many open-reel machines function as mechanical “Scrape-flutter” dampers, so they are part of the overall transport design. Removing them can increase flutter. The most important thing is that it is round and smooth where the tape makes contact. If it is wearing flat, the best course is to turn it.

 

[Tim Gillett]

Eddie Ciletti used to recommend playing SSS tapes on a machine with all rotating guides for obvious reasons. I think he mentioned Ampex and Studer models.
I once changed an A77 to both rotating guides and it seemed to work fine. The standard stationary guide is made of aluminium and wears a bit. OTOH the rotating guides can also start squealing especially on FF and rewind, if they wear out or lose their lubricant.

Either way I doubt it will make a big difference except maybe to reduce the sticky shed that might accumulate on the fixed guide.

I believe the A77 /B77 were designed for long play tape (eg 457).

There should be no change in the "wind" on the take up spool as the tension is controlled not by the guide but the combination of capstan speed and the take up motor torque.

Tim