More tape machine woes: crosstalk in sync only

[EmilFrid]

Since I fixed my teac 90-16 with the generous help from the heroes of this forum, I've stumbled upon another issue. After getting it up and running I swapped the sync head for a NOS head and adjusted the bias traps. I also did the routine alignment as per the manual as I've done a thousand times. Now, the odd thing I've run into is as sudden as it is confusing; in sync mode, the active channel (the one recording) leaks into the adjacent channel if and only if that channel is inactive (no recording ie no input monitoring but monitoring off tape). The audio leaking into the sync head amp rises in volume with frequency, so I suspect some sort of capacitative coupling somewhere, but I fail sto see how I could have done anything wrong, as I've only adjusted the bias traps and the regular alignment, something I've done countless times before. I should mention that the crosstalk doesn't stick to tape, only leaks into the sync amplifier while recording.


Does anyone in here have a similar experience that they managed to fix? I feel like I'm fumbling in the dark after making sure grounds are fine, which they are.

Edit: if I plug in the old head, the crosstalk is still there but if I have no head mounted there is silence

Edit 2: to be clear, the crosstalk is excessive and not the normal sync crosstalk that all machines suffer from

 

[JohnRoberts]

I am not a tape recorder expert but back last century I was forced to support a 4T cassette machine.

IIRC the special 4T cassette heads had magnetic shielding embedded between each winding to allow punch ins without crosstalk.

Stereo (2T+2T) heads lacked the shielding.

Since you mentioned replacing a head is it identical to the old one.

JR

 

[Brian Roth]

That is a CLASSIC problem that has always existed with probably every multitrack. I learned about that decades ago when I was youngster.

The first time it bit me, I was "bouncing down" some string tracks. It was something like bouncing tracks 11, 12, and 13 to 14. I set up the mix from sync on 11/12/13 while monitoring input on 14. Sounded good until I punched into record and it went into a feedback loop as track 14 bled into 13 through the record head.

It was an Ampex MM-1100 2" 16 track. I learned to plan ahead so the destination bounce track was was several tracks away from the bounce source tracks.

Bri

 

[EmilFrid]

@JohnRoberts the head is not identical to the one that was fitted before, but actually identical to the original head that the machine came with. But as I wrote, the problem persists if I plug in the head that I swapped out, and this never happened with that head before

@Brian Roth yes, this has always been an issue, but not at this extreme level. If I send a 10khz sine wave to channel one at 0dB, channel two will register an even higher level, like +3dB and beyond. Very strange.

 

[EmilFrid]

Realized I posted this in the wrong forum section. Sorry. Mods could move it to "The Lab" if needed

 

[Brian Roth]

@EmilFrid Perhaps a head grounding conductor has become disconnected/compromised. Also, is the problem universal across all tape tracks or are some pairs better/worse than the others?

Bri

 

[EmilFrid]

@Brian Roth yes, this was my suspicion as well, but this far I haven't found any discrepancies in the head grounding. But I will troubleshoot further. Yes, it's a universal issue and no channels are better or worse than others

 

[EmilFrid]

This is what it looks like when I send a 10khz sine wave to channel two, some serious crosstalk as even peak lights turn on:

 

[Brian Roth]

Wow! I vaguely recall seeing something a bit similar on a crappy Fostex 1/4" 8-track machine..the one that maxed out with 7" plastic reels. But I don't think it was THAT bad....

Hmmmmm.....

Bri

 

[EmilFrid]

@Brian Roth Yes, as this isn't a crappy fostex machine, this shouldn't happen. It didn't use to be like this, but just all of a sudden happened. Driving me crazy

 

[Brian Roth]

If the crosstalk increases at 6 dB/octave as the test frequency increases, that seems to indicate some sort of capacitive coupling as you mentioned.

Just to eliminate something (likely a red herring...) in the testing setup....what tone generator are you using? Is it connected directly into one input at a time? How does the channel in record "sound" when monitored in repro mode? Can you connect an oscilloscope to that channel? Also...what do you "see" on the playback on the adjacent track(s) in Sync mode??

Bri....Ye Olde Straw Grasper.....

 

[EmilFrid]

@Brian Roth thank you for the insight. The crosstalk increases a lot with frequency, but I haven't measured that closely, so I don't know about the specific db levels.

I eliminated the tape head and then the issue disappeared, and I've tried other heads but they all do the same thing.

The tone generator is the generator on my desk and I patch one channel at a time, but also tried sending the same signal to several channels. Still the same. The record armed channel sounds good in repro mode, no artifacts whatsoever. On the adjacent tracks there is no recorded crosstalk, it all happens only in sync monitor mode. I will connect the oscilloscope tomorrow.

I wonder where the capacitative coupling (if that's what it is) could be happening, as all channels are affected. I even tried pushing on the backplane board to see if there's a crack somewhere, but with no luck. Also tried wiring the sync head ground of an affected channel directly to circuit ground. Strange.

 

[Brian Roth]

The scope would be my next step, both to look at repro from the track being recorded as well as the sync playback of the neighbors. Glad I still have some straws in my kitchen that I can go grasp....lol!

Bri

 

[EmilFrid]

Lol I'm happy for any straws possible!

Yes, I will do this tomorrow if I find time.

 

[radardoug]

Your bias traps are mistuned. Also, its possible your bias/erase oscillator is off frequency. This will affect how the bias traps can be tuned.

 

[Brian Roth]

I didn't notice in his first post that he had adjusted the bias traps. Going way back in my memory....wouldn't mistuned traps cause a constant meter reading on adjacent tracks even without audio?

Bri

 

[gyraf]

.. don't most multitracks have reverse-phase-cancelling trims that subtract from adjacent channels in sync?

 

[juanito2008]

Do you have a DIP switch or something that narrow the bandwith from the repro sync ?
Like on studer machine (wide/narrow).

 

[EmilFrid]

Your bias traps are mistuned. Also, its possible your bias/erase oscillator is off frequency. This will affect how the bias traps can be tuned.

Wouldn't this lead to a constant reading though, like Brian says, or am I missing something crucial?

@gyraf no this machine doesn't have that, and never needed it before. My other 16 track one inch doesn't have it either. I've never encountered this issue before on any one inch machine. But I once had a 1/2 inch 16 track; crosstalk nightmare.

@juanito2008 no DIP switches here

 

[RoadrunnerOZ]

Do you have a schematic of the sync repro section and a cabling/wiring path diagram?
Seems to be that this problem is not related to the head as when you swap the old head back in the problem persists which was not there before the head change - so therefore something occurred while you were swapping heads - maybe a mental retrace of all the steps you took to change the head would shed some light. Perhaps a bent pin on a connector socket or a wire not making connection with a pin. If I have to test loom integrity I use a meter probe with an insulation penetrating needle tip to test connection paths in circuit.
Question: is the crosstalk audible on the adjacent channels or is it only showing on the meters adjacent without being audible?
(I always check off the steps I take using photos from my phone when doing anything just to jog the memory on reassembly.)