3M M79 problem...help

[plateson]

Hello,
I've just recently aquired a 3m M79. Everything seems to be working except for one thing. When I have already recorded a track and want to lay down a second track while monitoring the first track, the first track I recorded gets erased. It's like the record head for that channel isn't switching to the monitor only mode. Could this be a logic disorder? I have troubleshooted the remote transport and it seems to be arming the tracks fine and functioning. I borrowed another remote and patch cords from a spare machine I have and it does the same thing. I have also tried recording to different tracks but to no avail. It records the track down fine but when I go to put another track down while monitoring the first track gets recorded over and blanked.
Any suggestions? I appreciate any ideas from any level of experience you may have with this infamously troublesome but beautiful sounding machine.

Thanks!

Brett

 

[hitchhiker]

If you have a number of tracks (recorded together say) and then try your overdub, does it then erase all the tracks that are in sync mode?

 

[plateson]

well, I just tried it and yes, all tracks get over-written unfortunately. It's like the head isn't being given the right code..at least my best guess but I'm really quite unsure.

 

[hitchhiker]

well I'm no expert on these matters but i do own an M56 and i think they have much in common. My manual and schematics say that the sync mode gets 5 volts DC to trigger the sync relay but not the record relay.If set to record on that chan it would get the full 28 volts.
I would pull the main logic board and test diodes and transistors (IC's too) for shorts.
Mine had one shorted diode, but it was causing a different hell.

Lance

 

[plateson]

Thanks Lance. I'll give it a shot.
BTW, how does your M56 sound? Is it worth the hassle for us to keep our 79 in your opinion?

 

[hitchhiker]

I think its worth the extra work. I hope to one day have an M79 as well. My machine sounds really good and I look forward to many recordings on it. I've not heard the M79 but I understand it's a little more hi fi but still the same fat. I use GP9 tape at +6 with 3.5 db of over bias. I often talk with Dale Manquen , who is the designer of the M56 16 trk. Dale spent many an hour (via the phone) helping me to revive my machine and he's really a great guy. I'll relay your problem if we get stuck. Maybe an M79 user will come along soon!

Lance

 

[plateson]

as it turns out the pin in question only touches the logic board to pass thru. it runs back off and gets lost inside the 79 behind the audio cards. i'm going to take the audio card holder off and see what i find...

when i said help at any level of experience i didn't expect to get help from someone who knows the designer!

 

[hitchhiker]

Maybe don't get too much into dismantle mode. I've fixed quite a number of problems on my m56 and all the repairs were on the circuit boards ,relays, or wiring to the heads. All of its card cage wiring looks original.

 

[plateson]

point taken, but my problem is theat the only cards that i haven't seen yet are the ones in between the cards and the input output board. all the other cards look fine, but i'm not nescessarily "gifted" in electronics and am a little gun-shy about taking a multimeter to the cards.


i started to take out the audio cards, but i changed my mind about getting to the cards that way. instead i just gave them a good compressed air bath and cleaned the contacts and put them back to bed...

I think it would be easier to go thru the back and take the xlr panel out. i probably am done with it for tonight, but maybe i'll do more over the weekend. I am still open to any other thoughts, i really don't want to tear into my machine and ruin it somehow...

 

[hitchhiker]

I'll try to reach Dale tomorow. Do you have a manual? Are you able to
identify the circuit boards you have and have not reached?

 

[plateson]

i have a manual, actually 2, and the schematics, but i have no idea what circuit board the wires are going to without pulling that panel off...

 

[fazeka]

I have a M56 8-track currently being worked on by Doug Weeks out of Athens, GA. You could always try shooting him an email:

[email protected]

He's a 3M tech. Great guy. See what he has to say.

Lance, just curious but I believe you have a 16-tracker, eh?

Dale wasn't really around for the M79, something to consider. I'm sure he'd be willing to help whereever he can. I know he is the MAN when it comes to the M56.

Best,
Chris

 

[hitchhiker]

It might be the wires you speak of are going to the main logic board .I have no idea how the cards are situated in the M79. IIRC I removed my logic PCB from the back but it's a different arrangement in the M56. Let's see if I can find out from Dale how and if you should want to remove it. I understand how daunting the schematics would be. The M56 has 4 cards for each chan. An erase/bias, a record , preamp/overdub,and line driver. The M79 combines all four circuits onto a single circuit board for each chan. I believe both machines have a single main transport logic card.
See what you can suss out by looking about and tracing, just ot get familiar with the territory. If you don't want to be totaly dependent on techs you'll want to get very familiar with your machine. It's all very learnable.

Lance

 

[hitchhiker]

Hi Chris , congrats on the M56. Yes mine is 16 trk.

Hey Brett! Wouldn't hurt to shoot off an email to Mr Weeks, maybe he'll be happy to cut the chase. Does your other machine work? I'm wondering how easy it is to try a different logic board if that's the problem?

cheers!
Lance

 

[plateson]

no, my other machine doesn't work too many parts are gone including the main logic board. but it does have a 2" 16 track head assembly where mine has 24. I have a handful of extra cards, cables, solenoids, and power supply,etc. lots of miscellaneous parts. i tried using the link for mr. weeks but it told me there was no email address like that, so is there another way to reach him?

 

[Boswell]

Isn't this an erase head logic problem, not the record head?

 

[hitchhiker]

....the record and erase are switched in by the same relay on the M56.

Brett,
I tried to reach Dale but he doesn't seem to be there today. I do know that
he has some M79 machines that he's been going over lately, so I'd guess he will be able to be able to comment on your problem (sooner or later).
I'll try again this aft.

Lance

 

[fazeka]

Talk to Doug in person. The phone number's right there... :roll:

 

[plateson]

my buddy and i talked about pulling the plug on the erase head and doing a test record to see if it still erased all the tracks... you know, to see if it is the record head or the erase head acting up.

does this sound like a good idea or am i asking for trouble???

 

[hitchhiker]

That won't give any new info. If your material is being erased then the erase head is doing it. Without the erase head connected the record head is still working , and say ,if you had a signal going it would super impose that signal on top of the material previously recorded. When the relay trips that powers the erase head circuit it also powers the the record head circuit. Take a look through your schematics for a block diagram of the relays and electronic cards and if it's there you'll see a dotted line showing how one relay (for each chan) turns on both the erase and record. There is a main logic board that is likely mistakenly powering these relays when their chan are set to sync mode on the remote. Therefore the main logic board is where the problem likely is. I could be wrong but that's what I would expect to be told by Dale. I couldn't reach him today.
Maybe call Doug Weeks and send him the logic board to repair?

Lance