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[MagnetoSound]

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?

[Matt Allen]

Really interesting!

Let me think on this and check some notes.

[MagnetoSound]

Cheers Matt, was hoping you would check in!

[Matt Allen]

Can you tell me if the brightness of the lamps change when you do this?
For example, the sync light gets dim on the channel, or the record lights go dim?

One thought. Is there any tape screwing happening on the record head?

[tinntonne]

Dear Matt,

It can be a bad coupling in the head or even the wires leaving the head.
Try to verify the origin of the trouble.

As:
switch off the master bias oscillator: problem gone?

short the input of the sync amp. problem gone?

How does the signal look at the first stages of the sync amp.
I assume there is a lot of bias riding on the signal and the sync amp starts
distorting and has to find a new operating point. Connect a scope to the sync amp TP or emitter of Q33/C44.  How does is look there.


Tinntonne

[tinntonne]

Besides, my scan of the M79 audiocard is bad, something is missing in the middle.

-.-.-.-.-

Meanwhile I found a better scan at:

http://www.3m79.com/SCHEMATICS.htm

Tinntonne

[MagnetoSound]

Matt, no tape screwing (the problem is there even with no tape in the path) and, as far as I recall, no dimming lights.


Tito, the problem is mine not Matt's, but thanks for your pointers, I will look into them.


I will be at the studio tomorrow, I'll check all the points mentioned and report back.


Dan

[drknob]

If you're in a jam, Dale Manquen was Lab Manager at 3M for the Model 56 16 track. you can track him down here: http://www.manquen.net/audio/index.php?page=9

[MagnetoSound]

Scuppered by the snow. I'll have to pick this up again next week lads.

[MagnetoSound]

A quick update ...


No head connections appear to be adrift. All 8 tracks perform perfectly well in terms of level and frequency response and all track functions are good. There is no excessive bias leakage, and there is no spurious erasure or loss of HF on any of the recorded tracks.

Here is a clip of what's happening, as each track is placed in and out of record. This is with no tape in the path and all tracks in CUE mode.

It still seems odd that tracks 5 & 6 are almost completely unaffected, and that track 4 is the only one that does it to itself!

Grounding any sync amp input (at point E4, between secondary and coupling cap C26) reduces the thump at output significantly, though the meter still twitches.

I am considering the possibility that these may be just unfiltered switching transients, perhaps from the sync relays. There may be a grounding or decoupling issue and I will inspect the backplane further and disable the bias oscillator to see what happens then. I shall replace the PSU output caps too for good measure.



Dan

[Larrchild]

Cue traps tuned?

[Matt Allen]

Any progress reports?

[MagnetoSound]

Matt, PM'd you.