More tape machine woes: crosstalk in sync only

[radardoug]

Having the adjacent meters reading zero as you have shown, indicates the traps are not working at all. You should be getting a null when you tune the trap. Think about it, you have no trap action happening at all, when you should be getting 50 dB odd.

 

[EmilFrid]

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.)

Hello and thanks! Yes, the crosstalk is audible but only in sync mode, it never gets printed to tape. Photographing every step is a great idea. Here are the schematics. Disregard the scribbles on one of the drawings, that was just bored me trying to figure the unpopulated area of the PCB out.


@radardoug I understand, but wouldn't that be a reading of the bias signal and not the actual programme material? If I record silence, there is no reading on the adjacent tracks.

 

[RoadrunnerOZ]

It may possibly be to do with the board and the cabling of same that the heads plug into - IIRC they all plug into a common distribution board

 

[radardoug]

Have you put a millivoltmeter on the adjacent track and adjusted the bias trap for that track? Record silence, and you should be able to meter the bias crosstalk. You should be able to null by at least 50 dB.

 

[EmilFrid]

@RoadrunnerOZ I will have a look at that. My guess was that it had something to do with the grounding, but I fail to see how that would only affect the adjacent tracks rather than all tracks at once.

@radardoug yes, I have done that and have no bias leakage on any channel.

 

[RoadrunnerOZ]

I would be checking any points where you unplugged/replugged cables

 

[EmilFrid]

@RoadrunnerOZ there aren't many of those points though. Just the head connector and the edge connectors for the boards, and I've checked those thoroughly at this point. As I mentioned before, I suspected a crack in the backplane board but couldn't find any. Perhaps I should remove the whole amplifier/connector board unit from the machine and have an even closer look. A little uncomfortable to stand on my knees all the time

 

[RoadrunnerOZ]

Worth looking for any broken print traces around the board connectors, also possibly worth resoldering the solder joints on them all

 

[EmilFrid]

@RoadrunnerOZ that is very true. I will have an even closer look

 

[RoadrunnerOZ]

Also worth looking carefully for any bent pins in connector sockets

 

[EmilFrid]

@RoadrunnerOZ sounds like a good idea. This is a very perplexing issue. Seems like all the issues I get with my machines are of the most perplexing kind. My luck!

 

[RoadrunnerOZ]

 

[RoadrunnerOZ]

Is this machine the same architecture as the 85-16? - I have the service manual for that and looking at the way it’s structured the connector board for the heads is a prime suspect - I don’t have the manual for the 90-16 and I’m assuming they’d be similar in design.
If you set all tracks to armed and insert signal to say Ch8 do more than the adjacent tracks 7 & 9 get any sort of meter reading (like tracks 6 and 10)?

 

[Brian Roth]

A bit ago, I found the 90-16 manual in the Tech Docs section:

https://groupdiy.com/threads/tascam.65910/post-1161795

There is a strange message on the screen, but the file downloaded just fine.

There are a total of three bias traps.

1. Series LC across the primary of the input transformer into the sync amp. PDF page 203.

2. Series LC across the line output. PDF page 204

3. Parallel LC in series with audio drive to record head to prevent bias from "back feeding" into record head driver output. PDF page page 212.

Only traps 1 and 2 should have any bearing on bias leakage into output in sync mode.

As @EmilFrid mentioned in post #22, there is no reading on adjacent channels when recording silence. IMHO, bias leakage is not the issue.

Bri

 

[RoadrunnerOZ]

Yeah I agree - seems like there’s audio leakage occurring here

 

[EmilFrid]

@RoadrunnerOZ I guess they'd be of similar architecture at least. I have a 85-16 as well, but this specific part of the machine I've never had to deal with. To your question: no, there is no crosstalk if all channels are armed. The crosstalk takes place in sync mode. Say I record on channel two, channel one and three will pick up the crosstalk from channel two, but only as long as they're in sync mode, ie reading from tape on the sync head as opposed to input monitoring, which takes place once they're armed.

Edit: the affected channels do not read the crosstalk from tape, rather, it seems to be purely electrical crosstalk

 

[guy_4]

@RoadrunnerOZ I guess they'd be of similar architecture at least. I have a 85-16 as well, but this specific part of the machine I've never had to deal with. To your question: no, there is no crosstalk if all channels are armed. The crosstalk takes place in sync mode. Say I record on channel two, channel one and three will pick up the crosstalk from channel two, but only as long as they're in sync mode, ie reading from tape on the sync head as opposed to input monitoring, which takes place once they're armed.

Hello EmilFrid,

One suggestion : have you tried with a brand new tape ?
Not an old one that has been erased, but one virgin tape that you have unpacked from the manufacturer box ?

Best,
Guy

 

[EmilFrid]

@guy_4 hi! This is completely unrelated to tape – the issue is there even with no tape threaded. It must be something in the electronics

 

[Brian Roth]

Straw #568978657.221

I'm curious....when recording a "wideband" music source (symphony crescendo, maybe Chicago "25 or 6 to 4" or similar) does it sound "normal" except for being seriously high-passed?

Bri

 

[EmilFrid]

@Brian Roth yes, normal but seriously high passed.