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monaro0162 · 29th Dec 2017, 7:59 pm

Successful conversion to 4-track completed. As said a straightforward job, although a couple of gremlins were waiting in the wings:

After I'd completed the head swap I found that it was recording and playing fine, but wasn't erasing properly. Turns out the 4-track erase head is slightly shorter, and requires a shim underneath it to raise it to the correct height to match up with the 4 tracks on the tape. The manual doesn't seem to show this part (part number 1.077.131-01) although the brass 0.1mm shim used on all variants (part number 1.010.006-23) is shown (index 33 in the diagram). This part didn't come with the head carrier that I bought (although the loose erase head was there) so I was unaware that it was needed. I solved the problem by cutting a piece of base plate off of an old Akai head which turned out to be exactly the thickness required and already had a hole for the mounting screw to go through.

The second gremlin caused complete havoc and a lot of head scratching: Suddenly during testing I seemed to loose signal routing through the back plane accompanied with intermittent full-scale deflection of the VU meters. After a lot of fiddling about with the wiring and swapping cards around I found that the input amplifier had a blown input transistor. Turns out that the locating pin in the back plane for the input amplifier had fallen out causing the card to be inserted with the pcb tabs pins offset against the back plate sockets ... you know what that can cause ... inappropriate voltages at inappropriate points in the amplifier circuit! Diagnosed to Q401 blown after replacing it normal service was resumed.

So after calibration the machine is now running fine in 4-track mode. Can't say I really notice much difference between 2 and 4 track at the moment, it sounds great. Before I removed the 2-track heads I took an Audacity trace of the frequency response and I will be re-running the trace under the exact same conditions to determine if there is any difference.

Happy days!

29th Dec 2017, 7:59 pm	#20
monaro0162 Pentode Join Date: Sep 2016 Location: Stansted, Essex, UK. Posts: 185	Re: Revox A77 head mounting Successful conversion to 4-track completed. As said a straightforward job, although a couple of gremlins were waiting in the wings: After I'd completed the head swap I found that it was recording and playing fine, but wasn't erasing properly. Turns out the 4-track erase head is slightly shorter, and requires a shim underneath it to raise it to the correct height to match up with the 4 tracks on the tape. The manual doesn't seem to show this part (part number 1.077.131-01) although the brass 0.1mm shim used on all variants (part number 1.010.006-23) is shown (index 33 in the diagram). This part didn't come with the head carrier that I bought (although the loose erase head was there) so I was unaware that it was needed. I solved the problem by cutting a piece of base plate off of an old Akai head which turned out to be exactly the thickness required and already had a hole for the mounting screw to go through. The second gremlin caused complete havoc and a lot of head scratching: Suddenly during testing I seemed to loose signal routing through the back plane accompanied with intermittent full-scale deflection of the VU meters. After a lot of fiddling about with the wiring and swapping cards around I found that the input amplifier had a blown input transistor. Turns out that the locating pin in the back plane for the input amplifier had fallen out causing the card to be inserted with the pcb tabs pins offset against the back plate sockets ... you know what that can cause ... inappropriate voltages at inappropriate points in the amplifier circuit! Diagnosed to Q401 blown after replacing it normal service was resumed. So after calibration the machine is now running fine in 4-track mode. Can't say I really notice much difference between 2 and 4 track at the moment, it sounds great. Before I removed the 2-track heads I took an Audacity trace of the frequency response and I will be re-running the trace under the exact same conditions to determine if there is any difference. Happy days!