Hello Michael,
according to your findings, the problem lies in missing ground of the CHI in STOP position: when the monitor is set to NAB/IEC and recorder is in STOP, the pinchroller in rest position actuates the switch S5 which mutes the playback amplifier by shorting the first stage (pin 13) to the ground (pin 8). This takes place on the motherboard, via selector S2 (NAB/INPUT/IEC) and the switch S5. Obviously the hum comes from pin 13 not being effectively shorted. A simple test will prove this: with a piece of isolated wire, carefully connect pin 13 to the chassis and the hum should be gone.
In order to further investigate the ground path on the motherboard, you will have to remove the front panel.
according to your findings, the problem lies in missing ground of the CHI in STOP position: when the monitor is set to NAB/IEC and recorder is in STOP, the pinchroller in rest position actuates the switch S5 which mutes the playback amplifier by shorting the first stage (pin 13) to the ground (pin 8). This takes place on the motherboard, via selector S2 (NAB/INPUT/IEC) and the switch S5. Obviously the hum comes from pin 13 not being effectively shorted. A simple test will prove this: with a piece of isolated wire, carefully connect pin 13 to the chassis and the hum should be gone.
In order to further investigate the ground path on the motherboard, you will have to remove the front panel.
I have removed the knobs and lower front panel and found a bifurcated copper contact which is operated by the Pinch Roller arm. I couldn't find pin 13 so I touched the copper contacts with a pencil and the Hum stopped. I am sure you have found the root cause of the problem Wiili and I thank you most sincerely for your in-depth help. I looked at the Cct. Diags. and couldn't work out the routings to give this result. I have now cleaned those contacts with a cotton bud charged with Contact Cleaner for I noticed that one of the bifurcations made contact with a much carbonized contact on the MoBo.That was the CHII, though. CHI contact is not as positive as the contact for CHII which was the root cause of the problem. So I made a 'contact bender' from a small length of thick coat hanger wire, which I sawed down its length for about 3/16" with a thin hack-saw at one end, and bent 90deg. This allowed me to adjust those contacts accurately from the back. In Strowger days, when there were relays, I used to work in Telecoms! I am so glad you were there and able to give such expert advice. Thank you, Willi.
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Michael
I have removed the knobs and lower front panel and found a bifurcated copper contact which is operated by the Pinch Roller arm. I couldn't find pin 13 so I touched the copper contacts with a pencil and the Hum stopped. I am sure you have found the root cause of the problem Wiili and I thank you most sincerely for your in-depth help. I looked at the Cct. Diags. and couldn't work out the routings to give this result. I have now cleaned those contacts with a cotton bud charged with Contact Cleaner for I noticed that one of the bifurcations made contact with a much carbonized contact on the MoBo.That was the CHII, though. CHI contact is not as positive as the contact for CHII which was the root cause of the problem. So I made a 'contact bender' from a small length of thick coat hanger wire, which I sawed down its length for about 3/16" with a thin hack-saw at one end, and bent 90deg. This allowed me to adjust those contacts accurately from the back. In Strowger days, when there were relays, I used to work in Telecoms! I am so glad you were there and able to give such expert advice. Thank you, Willi.
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Michael
Now I have put the A77 back together (almost) and tried to play a tape - to test it. Unfortunately I have a New Problem. The tape sensor Cct. with its PEC and light bulb, doesn't work. The tape starts and plays - so long as I hold the PLAY button down. But immediately I release the Play Button the tape transport stops.
Any ideas??
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Michael
I have measured the voltages at R155 LDR (PEC) to GND and I get:17.748VDC on one Green lead contact and: 1.693VDC on the other Green lead contact. I can't find any reference in the Manual for the Voltage at this point.
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Michael
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Michael
Did you plug back the dummy connector on the back connector panel after assembling the recorder back in the wooden case?
ReVox A77 - Dummy
You are right, Willi. I hadn't got as far as inserting the dummy remote plug 'cos I hadn't yet put the case back on. Now I've plugged in that dummy and all's well!! Many thanks again! I wonder how much more I don't know about this splendid machine!
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Michael
You are right, Willi. I hadn't got as far as inserting the dummy remote plug 'cos I hadn't yet put the case back on. Now I've plugged in that dummy and all's well!! Many thanks again! I wonder how much more I don't know about this splendid machine!
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Michael
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