Revox A77 - Sudden increase in speed (2) - UK Vintage Radio Repair and Restoration Discussion Forum

mabelis2 · 8th Nov 2006, 5:13 pm

I recently purchased this nice old A77, and it took me a while to get everything running smoothly and cracle-free. Finally, yesterday, I was very hapily listening to the old tapes I had not been able to play in years.

Then today: big disappointment.

The machine is running fast when set to 3 3/4 speed, and I don't mean just a bit. It seems like 7 1/2 inch to me (but I can't test that). It seems not to make any difference at all whether I set the knob on 7 1/2 or 3 3/4. No difference in motor-sound as far as i can tell.
Any ideas would be much appreciated.
Hans

caldirun · 8th Nov 2006, 7:17 pm

Hi Hans
I have just this weekend fixed a similar problem on my A77 (although mine was running fast on both 3 3/4 and 7 1/2 about 30%).
I have replaced the electrolytic capacitors on the speed control board (the actual values are obsolete so use nearest you can get, always go over the voltage, not below though. The circuit is very well controlled so a small change in capacitance has no effect, but do not change the polystyrene 1% caps C205,206,207, I did not change the paper/metal film cap C213 either.
Change C204,209,210,211,212, I found no adjustment was needed on T201 for 7 1/2 but a slight tweek was needed on P201 to set 3 3/4.
To gain access to the board take the mech out of the case, turn upside down, remove the tagged wires from along the front (make a drawing first), take a couple of the slot-in boards out from above and with the correct flat bladed screwdrivers (you will have to try a few) take out the 3 screws securing the board to the transformer.
Remember to fit the insulator washers back over the screws before you fit the board back in.
Hope this helps, it sorted mine.
john

mabelis2 · 11th Nov 2006, 1:00 pm

Thanks very much, John!
I will follow our advise - or at least give it a try.
Hans

MiMo · 26th Nov 2006, 5:03 pm

You should note that I had the same problem. Michael maurice here in the UK replaced the capacitors and it now runs fine. See the posts at https://www.vintage-radio.net/forum/...ad.php?t=11763.

Hope you get it sorted.

Michael

8th Nov 2006, 5:13 pm	#1
mabelis2 Retired Dormant Member Join Date: Nov 2006 Location: Amsterdam Posts: 2	Revox A77 - Sudden increase in speed (2) I recently purchased this nice old A77, and it took me a while to get everything running smoothly and cracle-free. Finally, yesterday, I was very hapily listening to the old tapes I had not been able to play in years. Then today: big disappointment. The machine is running fast when set to 3 3/4 speed, and I don't mean just a bit. It seems like 7 1/2 inch to me (but I can't test that). It seems not to make any difference at all whether I set the knob on 7 1/2 or 3 3/4. No difference in motor-sound as far as i can tell. Any ideas would be much appreciated. Hans

8th Nov 2006, 7:17 pm	#2
caldirun Retired Dormant Member Join Date: May 2006 Location: Sheffield Posts: 4	Re: Revox A77 - Sudden increase in speed (2) Hi Hans I have just this weekend fixed a similar problem on my A77 (although mine was running fast on both 3 3/4 and 7 1/2 about 30%). I have replaced the electrolytic capacitors on the speed control board (the actual values are obsolete so use nearest you can get, always go over the voltage, not below though. The circuit is very well controlled so a small change in capacitance has no effect, but do not change the polystyrene 1% caps C205,206,207, I did not change the paper/metal film cap C213 either. Change C204,209,210,211,212, I found no adjustment was needed on T201 for 7 1/2 but a slight tweek was needed on P201 to set 3 3/4. To gain access to the board take the mech out of the case, turn upside down, remove the tagged wires from along the front (make a drawing first), take a couple of the slot-in boards out from above and with the correct flat bladed screwdrivers (you will have to try a few) take out the 3 screws securing the board to the transformer. Remember to fit the insulator washers back over the screws before you fit the board back in. Hope this helps, it sorted mine. john

11th Nov 2006, 1:00 pm	#3
mabelis2 Retired Dormant Member Join Date: Nov 2006 Location: Amsterdam Posts: 2	Re: Revox A77 - Sudden increase in speed (2) Thanks very much, John! I will follow our advise - or at least give it a try. Hans

26th Nov 2006, 5:03 pm	#4
MiMo Retired Dormant Member Join Date: Oct 2006 Location: Radcliffe on Trent Posts: 7	Re: Revox A77 - Sudden increase in speed (2) You should note that I had the same problem. Michael maurice here in the UK replaced the capacitors and it now runs fine. See the posts at https://www.vintage-radio.net/forum/...ad.php?t=11763. Hope you get it sorted. Michael