I'm overhauling an 8-track M79 for a client, and one particular problem is troublesome.
Before I go further, I should say that the machine is regularly demagged, bias is set 3dB over peak @ 10kHz and operating level is exactly 250 nW/m. we're running at 15 ips.
Here's the problem - if the machine is in CUE mode (sync), as soon as you put track 1 into record, the meter on track 2 momentarily pins as the bias current builds and a heavy pulse is audible at the output. The same thing happens on 2 if I put track 3 into record, but to a lesser degree. As soon as the bias is steady there is no problem. The pulse only lasts a few milliseconds.
The same problem occurs between tracks 7 & 8 and 6 & 7 to some degree, but there's no problem on the centre tracks. Swapping amp cards around mitigates it somewhat but not in any particularly logical way.
I am figuring some kind of inductive coupling between adjacent heads, but what would cause this, and why is it only on the outside tracks or is this perhaps a red herring?
All comments greatly appreciated. Any thoughts?