Revox A77 MK IV on my Repair Bench.

TLS Guy

TLS Guy

Seriously, I have no life.
My Revox A77 four track tape recorder, is back on my repair bench.

I played a tape last week and everything was fine. A few days later I went to play another tape, and on switching on all the reel motors spun, and then the main fuse blew.

I bought this machine as a restoration project about twenty years or so ago. I had to be patient as I had to find the NLA oscillator IC for the capstan drive. This took many months to locate.

The A77 was produced from 1967 to 1977. It had four iterations, the MK I, MK II, MK III and MK IV. I have restored MK I, MK III and this MK IV. I have a MK II, but confess I have never got round to doing the restoration. The MK IV was produced from 1973 to 1977. The MK I was the world's first solenoid operated tape transport, and used at Abbey Road to record the Beatles' Sergeant Pepper album. This was the first major recording made with a tape deck that did not have an entirely mechanical transport.

The A77 MK IV is the deck on the right of the Cassette deck and the Brenell MK 6 id on the right of it.



Here is the machine on the bench. The layout is organized and modular, and not in any way a "bird's nest" of wires and cables.



The problem seems to be on the solenoid transport control board. One of the motors starting caps, seems to have shorted and burnt out its accompanying 4.7 ohm resistor.



Hopefully parts will be here this week. I will replace all three motor starting circuits. Hopefully that will do the trick.

This older caps, were aluminum/paper. I have ordered more modern aluminum/polypropylene X2 rated safety caps.

So, I hope that does the trick.
 
TLS Guy

TLS Guy

Seriously, I have no life.
My Revox A77 four track tape recorder, is back on my repair bench.

I played a tape last week and everything was fine. A few days later I went to play another tape, and on switching on all the reel motors spun, and then the main fuse blew.

I bought this machine as a restoration project about twenty years or so ago. I had to be patient as I had to find the NLA oscillator IC for the capstan drive. This took many months to locate.

The A77 was produced from 1967 to 1977. It had four iterations, the MK I, MK II, MK III and MK IV. I have restored MK I, MK III and this MK IV. I have a MK II, but confess I have never got round to doing the restoration. The MK IV was produced from 1973 to 1977. The MK I was the world's first solenoid operated tape transport, and used at Abbey Road to record the Beatles' Sergeant Pepper album. This was the first major recording made with a tape deck that did not have an entirely mechanical transport.

The A77 MK IV is the deck on the right of the Cassette deck and the Brenell MK 6 id on the right of it.



Here is the machine on the bench. The layout is organized and modular, and not in any way a "bird's nest" of wires and cables.



The problem seems to be on the solenoid transport control board. One of the motors starting caps, seems to have shorted and burnt out its accompanying 4.7 ohm resistor.



Hopefully parts will be here this week. I will replace all three motor starting circuits. Hopefully that will do the trick.

This older caps, were aluminum/paper. I have ordered more modern aluminum/polypropylene X2 rated safety caps.

So, I hope that does the trick.
The last of the parts I needed arrived yesterday. So today, I have replaced the components on the three motor starting circuits, as advised in the service manual. I brought it up gently on the Variac on the test bench. No smoke and fuse stayed intact!

The tape deck is up and running again and functioning normally. So far so good.

 
Eppie

Eppie

Audioholic Samurai
Thanks for sharing. Nicely laid out inside. I was going to sell my Univox EC80A Echochamber recently but it needs work. This is a tape cartridge based echo unit for guitar or microphone. Needed a bit of light lubrication on the capstan bushing but otherwise runs well mechanically. Can't get any audio though but I need to expand my equipment collection and get an audio signal tracer. Might be easy enough to build for that matter.

Hoping to build a capacitor tester as well. There is a guy in Alberta that has Mr Carlson's Lab on YouTube. He has recorded many repairs on vintage gear (radios and amps mostly). He engineered his own capacitor tester and it is quite the accomplishment. His device can test any cap for leakage without having to apply full voltage, just internal battery power. For audio purposes, it can test which lead is connected to the outside foil on a cap. You would be surprised how often manufacturers label the outside foil wrong. Great piece of kit. Have to join his Patreon channel to get the schematics though but worth it for this device. He demonstrates in another video how to test of the outside lead.
 
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