Thank you for that, Robert!
In the meantime I was given exact measurements by a friend in Germany and had 4 capstan shafts made to order (including heat treatment, hardening, etc) by a local Mercedes supplier of machined components.
Also, as I am planning to experiment with upgrading the G36 to HS, I also ordered 3 capstans to ensure the conversion without major modifications to the equipment. Meaning, the driving section being double in diameter.
The 3.75/7.5 capstans are spot on, they all work flawlessly. The HS conversion ones I still need to test, but I am concerned about the potential drop in contact surface between the tape and the playback head, owing to the fact that the capstan is twice the normal diameter. Will let you know.
G36 works very well, except for one aspect I noticed: towards the end of a 10" tape, the donor reel adds too much resistance and the tape slows down slightly. Any ideas where I should look to solve this issue? I do not own a dynamometer to measure break tension, should I get one?
Thanks for all the good words, my best regards,
Aega
First of all, may I ask why you are converting this machine to high speed?
Also, I am assuming you have a service manual and some test equipment available.
I made some observations on my G36 regarding the possible lifting of the tape from the playback head. In photo D00085a, notice the wear on the playback head. That wear is wider on the right side of the gap which can be seen. This, I am certain is partially due to the wear on the tape guide to the right of the playback head. Another possibility is a slightly incorrect centering of the playback head with respect to the tape but I'm inclined to doubt that.
Now, assuming the tape guide on your machine is similarly worn. This would cause the point of contact to shift to the right of the gap on the playback head. It is my feeling that by your adding the larger diameter capstan shaft, the tape will restore close to it's original position. The tape may lose contact with the guide's worn surface but will still maintain its vertical position. You just might get lucky. This wouldn't work on an A77 as the small low speed capstan is 0.177", larger than the G36's 0.118". The high speed capstan shaft diameter is 0.356" which will considerably affect tape to playback head contact. The correct procedure to make such a conversion on an A77 is to drill new holes in the aluminum casting so the motor can be repositioned to maintain the correct contact on the playback head.
I've had this G36 for a few years. I bought it essentially to add to a collection of A77's of which I have 6; two are two track high speed. All have new heads which cost me more than the respective machines, not to mention the cost of added electrical and mechanical parts needed to restore them and 2 calibration tapes at $135 each. But, the G36 has yet to be put on the bench. It does work but the switch for the tape tension isn't working properly; it's a mechanical problem. I have a full compliment of new tubes also. I haven't fiddled with it for a couple of years, until now so we're both going to learn something from this experience.
Now, to your end of large reel slowing speed problem. I doubt it's the brakes rubbing as that would very likely cause an uneven speed. What I'm inclined to think is the capstan pinch roll and /or related solenoid. The pinch roll may be hardened and glazed. As the supply reel diameter decreases, the load imposed on the capstan to pinch roll friction is increased. A hardened and glazed roller will lose it's friction. As for the solenoid, ifn the pinch roll to capstan shaft pressure is low, it may cause a uniform slippage which can easily be interpreted as speed slow down. Have you monitored the voltage to that motor prior to and when the problem manifests itself?
That the problem only exists with a 10.5" reel is puzzling. The added weight of the larger reel should have no effect but the smaller diameter of the 7" reel hub just might at the end of the 7" reel, if the pinch roll to capstan shaft friction is compromised. When the problem happens, add a little pressure to the capstan pinch roll arm. That may indicate a pinch roll problem. Another thing to try is manually rotate the supply reel so as to remove the tension on the tape. When doing the latter, it may be necessary to apply a very mild pressure to the tape near or between the left guides to keep it in position, else the tape may skew past the heads and into the capstan/pinch roll contact. Use an expendable tape if you have one.
The service manual gives these tips in section 2.1.3. regarding tape tension. (A) Faulty adjustment of the brake band. (B) Electrical fault in the feed motor (which I assume to be the left reel motor) (C) Electrical fault in the phase shift condenser, C72.
And, I might add that it takes 1.7 kg (3.75 lbs) to pull the capstan pinch roll from the capstan shaft enough to create an audible change in pitch (tone).
Robert