I have a REVOX A77 Mk I (bought new in 1968) that I have worked on over the years to correct electronic problems in the audio section but have recently cured a tape speed control problem. The machine had started to play at a tape speed of about 5.7 i.p.s when set to 3.75 i.p.s and 10.5 i.p.s when set to 7.5 i.p.s.
The Capstan Speed Control Board is Diag 10 in the REVOX Service Manual.
I checked the power transistor Q209 (RCA40318) and found it appeared to be OK. I was then advised to replace electrolytic capacitors C204, C209, C210, C211 & C212, which means board removal. That board has never been removed before and I had to devise a method which I thought others might like to see, in particular for replacement of the board.
Removal:
Remove the chassis from the box, then the front plastic cover, pull out the control knobs and remove the lower control cover. Remove the bottom aluminium plate (held by large X-point self-tap screws) that holds the seven push-in p.c. boards in place.
The Tape Drive Control Board (Diag 2) needs to be unscrewed and swung out of the way, it is held in by two small X-point screws (M3x10) at either end of the push buttons at the front. Before removal, label all of the wires to the rear spade connectors on this board and tie them in groups with pieces of cotton so they don’t get confused with the wires from the Speed Control Board when that is removed, then swing the board out towards the rear.
Remove the two central push-in boards (Oscillator & Record Channel II). You now have access to the Capstan Speed Control Board which is fixed to the main transformer body by three very small (M3 x 6) cheese-head slotted screws, each with a small black washer (I think they are insulator washers) which are very easy to lose! (eventually found the lost one after 10 minute search). Before removal, label all of the wires to all of the spade connectors on this board, pull them off and tie them in groups with pieces of cotton so they don’t get confused with the wires from the Tape Drive Control Board when the boards are replaced.
With the other boards out of the way, the three screws and washers can be removed using a long screwdriver, and the Speed Control board removed.
Replacement:
After replacement of the capacitors, the next problem was to get the three small screws and washers back in to fix the Capstan Speed Control Board to the transformer. I could not get the screw and washer into the holes, hold the board, place the screwdriver and turn it without dropping the screw and/or washer. In the end I used the following method which I thought others might like to hear.
Turn the frame upside-down and with the Tape Drive Control Board swung out and down, place the Capstan Speed Control Board aligned over the three holes in the transformer body. Take a short piece of 4mm heat-shrink sleeve and shrink it onto the end of the screwdriver with the screw and washer held in place so that the sleeve just holds the head of the screw and the washer to the end of the screw driver. Engage the screw into the hole and tighten a few turns then remove the screwdriver and pull the short length of sleeve off of the screw. Repeat for the other two screws. Tighten the three screws and check that all of the wires are replaced on the correct spade terminals.
Replace the Tape Drive Control Board after connecting all of its wires to spade terminals. Replace the two push-in boards and the lower panel, checking that the panel is engaged in the fixing cut-out in each board.
After re-assembly, I tested the machine using a timed method I have devised that takes into account thickness of the tape and diameter over the tape after a number of turns. With the machine set to 3.75 i.p.s, the measured speed was 3.755 i.p.s so I am happy that the fault is gone. The 7.5 i.p.s setting was within 0.4% fast so I did not adjust the trimpot P201.
Hope that hint about the heat-shrink sleeve is of use to someone out there. A question: does anyone out here know a modern replacement for the power transistor Q209 which is listed as TIP-27or RCA40318 in my Service Manual? Alternatively does anyone know the spec of those two original transistors?
In case it helps, the capacitors I got from RS Components are listed below. I could not get all the radial types as direct replacements so used axials mounted vertically; there was just enough room.
C204 & C210 25µF 25V [RS 850-6004]
C209 250µF 16V [RS 850-5982]
C211 10µF 35V [RS 228-6717]
C212 250µF 25V [RS 850-6133]
Cheers to all.
