More tape machine woes: crosstalk in sync only

[RoadrunnerOZ]

Normally across a large board I’d expect 1 - 1 1/2 Ω maximum total plus of course meter lead resistance but 15 Ω is big. Maybe try 1 - 8, 9 - 16 see if they’re half and identical

 

[EmilFrid]

Will do!

 

[EmilFrid]

Update: measuring directly on the back plane connectors I get only 1.5 ohms between GNDs of channel one and sixteen, and the same goes for digital GND, so we can at least rulle out any weirdness on that front. Tonight, I will hopefully have time to go through the machine with ny scope. It's an old tube scope with very few features but maybe it could tell me something.

 

[RoadrunnerOZ]

Set your ground for the scope at the same place as the tape machine. I assume the wall ground is ok and that the tape ground is not being achieved via audio connections but is fully achieved from the wall socket - ie the mains power cable ground is intact?

 

[EmilFrid]

Yes, wall ground is intact. Thanks for the instructions. So am I looking for DC offsets in GND? Haven't done this particular work before, so excuse my stupid questions.

 

[RoadrunnerOZ]

Yes, wall ground is intact. Thanks for the instructions. So am I looking for DC offsets in GND? Haven't done this particular work before, so excuse my stupid questions.

DC offsets and also audio on grounds at the various ground points in the machine like the power supply individual DC supply grounds, cable harness frameworks, head cable shields, headstock and the card grounds - there are different ground runs for the different voltage supplies - three I think on the connector board. Just look for audio leakage where it shouldn’t be.

 

[EmilFrid]

Alright, thank you!

 

[EmilFrid]

So I checked grounds and couldn't find much but a tiny tiny bit of bias leakage – no traces of audio though. I did a weird thing disconnecting the cable pair for channel two and wired it directly to the amp unit via a separate cable, and then voila! the crosstalk disappeared. What this means I have to think about, but something tells me the wiring isn't right. How this just all of a sudden happened is beyond my understanding at this point

 

[RoadrunnerOZ]

Check your loom - is this from a console?

 

[EmilFrid]

The loom between console and machine is fine, I just tried with my other 16 track. But I'd guess the loom between tape head and amplifier is acting up. Maybe I wasn't clear, but what I meant was that I removed the cable pair of channel two at the tape head, and soldered it directly to the amp card, where it is supposed to connect via the internal loom. I tried it a few times just to be sure, and that seems to cure the illness.

 

[RoadrunnerOZ]

So the loom from the connector board has issues

 

[EmilFrid]

Yes, this must be the case. I will investigate further. This starts to read as a murder mystery, like my thread about the blown motor transistors

 

[RoadrunnerOZ]

Yeah - I remember that one but we got to the bottom of that one eventually

 

[EmilFrid]

And I feel like we're going to the bottom with this one too eventually. I'll have to dismantle the whole amp unit again and see what I can find

 

[RoadrunnerOZ]

There is a loom to the head cable connector running from the connector board to a mounting plate where the cables meet is there not?

 

[EmilFrid]

Yes, it's this one that I bypassed in my test.

 

[EmilFrid]

My hypothesis is that for some reason the loom isn't shielding correctly. But when I measure the resistance to GND from the cable bypassing the loom I get the exact same figure as when I measure the shield of the loom to GND. But, as stated earlier, I should get the connector board out again and look even closer at it. Hopefully tomorrow