Revox A77 Service Manual

9.2. Faults in Circuitry
Pre- Check:
Playback Amplifier Hums :
Playback Amplifier Noisy ( Drive Amplifier )
Playback Amplifier Noisy :
Recording Noisy :
See Pre- Check 9.
Break in playback head windings.
Flap shield doesn't close adequately.
Supply voltage 21 Vdc, hum voltage too high.
Capstan motor leakage field too high, check supply voltage,
100 V Of max.
Possibly strong interference from UHF- TV transmitters,
see. 8.3.
Line voltage too low, see 9 ( pre- check ).
with closed volume control :
Defective transistor Q 803 or Q 804.
Amplification too high from defective balance potentio-
meter.
Noisy zener-diode in power supply, replace with planar type.
Inadequate matching to following amplifier. Open VO-
LUME control completely, adjust volume in amplifier.
with open volume control :
Condition: Function PLAY; without tape in place ( optical
end- of- tape switch: Cover light ); before- and- after- tape
switch position NAB / IEC.
Defective transistor Q 801 or Q 802.
Check tantalum capacitors.
with closed record volume control :
Check playback amplifier.
Clean and demagnetize magnetic heads.
Procedure with pulled record amplifier, once fault is correc-
ted, record amplifier found defective :
Replace transistor Q 501 or Q 502.
Check C 515 for leakage current, check tantalum capacitors.
If fault persists with puffed record amplifier :
Defective oscillator or erase head - replace.
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Revox A77 Specifications

General

Wow and Flutter	0.08% (7 1⁄2 ips)
Frequency Response	30Hz to 20kHz (7 1⁄2 ips)
Signal to Noise Ratio	60dB
Type	Reel-to-Reel Tape Deck
Track System	4-track, 2-channel stereo/monaural
Reel Size	Up to 10.5 inch reel
Motor	2 x reel, 1 x capstan
Inputs	Mic, Line
Outputs	Line
Tape Speed	3 3⁄4 and 7 1⁄2 ips