644 Midistudio - channel not working

[BillBrooks]

First off, thanks for the info on this group. Using it (and a bit of advice from Sam at Skywave) I managed to dismantle my 644 last night, clean and lube various bits of the mechanism and replace the belt. I'm fairly handy with these things, but was a tape machine teardown virgin so it was still a slightly squeaky bum moment!

The tape is now running smoothly, but I've realised that channel 3 basically does nothing. Doesn't pass any audio to the group, or produce a signal at the mic/line input. Not the end of the world, as I can obviously use channels 5 - 8 as inputs to the groups / tape and then direct tape outs to my DAW for the crunching I want to do. It would be nice to have a 100% functional unit though, as I've got time to spare, being on furlough at the moment.

Can anyone suggest where I should start looking to identify the issue in ch.3 please?!

Thanks in advance.

Bill

 

[BillBrooks]

Something I think worth mentioning, after a bit more time playing this evening, it seems the insert point on channel 3 works... I patched the direct tape out to the return leg of the insert jack and voila, I'm getting audio passing through the channel and out to the group output. This is almost a satisfactory work around, except I don't seem to be able to use the channel EQ.

 

[-mjk-]

Sounds like the insert jack isn't passing audio with a plug not inserted. Some consoles are known for that. Try cleaning the jack with a good contact cleaner.

 

[BillBrooks]

Thanks MJK. Gave it a good clean but has not fixed the problem. Any other ideas?

 

[-mjk-]

You could take an actual insert cable and connect the jacks together and see if it passes audio. The unbalanced insert jack is a "switch jack" and if the switching part is broken, the circuit is broken. The Soundcraft Ghost for example has something of a reputation for that. Often the jacks need to be replaced because they corrode badly or break. Broken/malfunctioning insert jacks are not all that uncommon.

 

[BillBrooks]

Thanks MJK, I should have mentioned that I did try that when I was testing the sockets on the back... Only by connecting the tape 3 direct out to the insert return do I get anything down that channel, suggesting there's a big issue before the insert point.

Could be an error with the patching matrix? If it's that I expect I've got no chance of fixing it... I tried factory resetting it... I'm clutching at straws here but could it be the internal 'battery'? It's never been replaced in 30 years.

 

[BillBrooks]

Any other ideas about this anyone?! Common faults in 644 channel inputs?

 

[wkrbee]

With the insert return working, it sounds like your problem is in the circuitry between the input and the insert send.