The Revox A77 MKIII
This blog/article is a work on the progress of servicing my 1971 Revox A77. It will be periodically updated, amended, and corrected until it is finished. When finished, this paragraph will be erased. (02/07/2021)
A factory stamped 'November 1971' Revox A77 that's been in the family since 1972.
 |
| The same date was also originally stamped on the heads. |
This blog/article is intended to be a servicing account, let's see how far I can go?
Last time I switched this on was around 2003-2005. Before switching the machine on though, I checked that all three motors (Reels, and the capstan) were turning smoothly, other internal mechanisms ran/moved smoothly, contact and switched cleaned here and there, and finally changed some potentially dangerous capacitors that are known for catching fire.
14/06/2021: Report so far - it's working!
 |
| Playing original AGFA PE36 Tape. |
 | |||
| Record/Replay Head wear: the flat spots are quite wide. Tape Lifter: small amount of wear, but may need replacing. Erase Head: No signs of wear Left Tape Guide: Runs smoothly Pinch Roller: Free running and cleaned with Rubber Renue * Capstan Shaft: Cleaned * (*after this picture was taken) | |
Tape Drive Board: I initially replaced the said board's capacitors with metalised polyester 0.47uF 250v dc units, but later changed again to 0.47Uf 275 vac 'safety capacitors' in yellow. I'm not sure if there's any benefit to this, but it seems to be the general consensus?
 |
| Actual units in the Revox A77 now in yellow. |
Originally -
Initial replacements -
 |
The old original 2500uF canister type electrolytic capacitor has been replaced by a 3300uF 50v rated device - shown at the rear. A blue radial JB type is a 1000uF direct replacement for the old 1000uF axial type.
There are two dc power supplies on the A77 -
(2) A 21v dc regulated circuit where the 1000uF capacitor resides and serves its purpose for - audio electronics, capstan motor speed control, and the end-of-tape lamp.
So far the Revox A77 is stable, and generally working well.
- Complete the re-capping of all electrolytic capacitors, including tantalum types. (Most completed 2022)
- Main potentiometers to be replaced. (All but one replaced, 2022)
- New Motor Run Capacitors. (Done: 01/07/2021)
- End of tape switch off issue
- Tape guide bearings may be improved by replacing? (Replaced 2022)
- Reel brakes cleaned, better than before - re-clean?
- Tape lifter may need replacing or modifying?
- Reel motors may need some lubrication?
- Capstan motor may need lubrication?
- Bias trap band-stop filter refinement? (Done: June 2021)
- Bias amplitude trimming to obtain a frequency response 30Hz .. 16Khz ±3dB at 9.5cm/sec.
- New Record and Replay 1/2 track heads? - will need to create an azimuth tape, and calibrate playback levels from a known reference tape.
Bias Traps
A 120Khz high voltage 'bias' frequency generator on board the A77 is a neccessary building block to successful analogue recording on tape. A bias 'carrier' voltage is applied to the record heads, and the recorded signal (music. speech etc) 'sits' on top of this 120Khz bias signal. This modulating scheme is similar to Amplitude Modulation 'AM", although it is not mathematically the same. The result of applying a bias carrier is a linear, low distortion magnetic field image of the audio transfered to tape.
However, some parts of the overall A77 circuits need to be isolated from this 120Khz bias carrier, and so Bias Traps are employed - in particular, the recording, and playback amplifiers.
 |
| Underside of Revox A77 The Bias Traps for Record, and Playback circuit access points have been opened up ready for fine tuning. |
The recording circuit schematic is shown below, and a test point is established for adjustment of the bias trap. The bias trap is a standard bandstop filter where at approximately 120Khz, the inductive and capacitive reactances are (theoretically) opposite and equal in phase. It is at this resonant frequency where the impedance of the LC circuit is theoretically infinite, however, in practice owing to small internal resistances and leakages this is never attained, but is sufficient enough to work well.
 | |
| The Record Amplifier Schematic I later traced and then marked on the underside of the board a point where I could use an oscilloscope probe. |
Trapping the Bias Signal: Record Circuit
Pressing Record and Play initiates the record circuits and the bias carrier is generated. Turning the small trimmer choke L501 (for CHI on card CHI, and CHII on card CHII) with an insulated screwdriver I observed the 120Khz signal, and turned for minimum amplitude - it never diminishes to zero. At this test point, the minimum amplitudes for both CHI and CHII were down to approximately 50-75mV peak.
Trapping the Bias Signal: Playback Circuit
After engaging Record and Play again the trimmer chokes were adjusted for the playback circuit. The test point this time is at line out, ie at the phono sockets. And again, we aim for minimum 120Khz bias leakage voltage. The bias signal does not completely diminish.
With Ampex 499 tape wound on to the Revox A77, setting record and play set to ON, and with a 1Khz test tone applied at the input set to 0VU, the recorded 'NAB' playback 1khz output at line out was approximately 6v peak to peak.
Below shows the oscilloscope trace showing the best 120Khz bias bandstop attenuation results (without 1Khz modulation) -
 |
| Note the crossover distortion - probably due to the non-linear B vs I charactersitics of the choke at low levels of excitation current. |
Both original 390pF bias-trap circuit capacitors were replacd by newer 1% tolerances versions.
Revox A77 Motor Run Capacitors
Employed to produce neccessary current phase shifts so that all 3 motors experience the correct rotating magnetic field phasing.
Old motor run capacitors in the A77 are known to be suspect, especially after 20 years and more, so I need to change these. So far I've had no problem, but it's best to be safe than sorry.
 |
| Old A77 Motor Run Metalised Paper Capacitors 2 x 4.3uF, 1 x 3.5uF |
Just ordered new Ducati caps ...
New Motor Run Capacitors Fitted
 |
| New A77 Motor Run Ducati Capacitors: 2 x 4.5uF, 1 x 3.5uF (Note the small difference: 4.5uF compared to 4.3uF) |
All but one of the onboard potentiometers were replaced earlier in 2022.
On replacement, first measure the resistances either side of the extracted potentiometer, set the new one to the same value, then replace.
05/12/2022: As the blog/article and work on the A77 is unfinished, there will be alterations and corrections made to the above.
cassettedeckman@gmail.com
