Audio Technica AT-RMX64 Story…

[sweetbeats]

Oh yeah it’s totally common. That’s how you calibrate the machine. Input level, meter cal, record level, bias level, playback level, and record and reproduce EQ adjust if it has it.

 

[sweetbeats]

Well the work on the transport and the new reel table drive tire and pinch roller has paid off…all transport modes function as they should. When I got the unit it wouldn’t FFWD, barely would REW, and would drop out of PLAY almost instantly. Fixed!

Next I can test record/reproduce functions.

 

[Robbie Stiefel]

Well the work on the transport and the new reel table drive tire and pinch roller has paid off…all transport modes function as they should. When I got the unit it wouldn’t FFWD, barely would REW, and would drop out of PLAY almost instantly. Fixed!

Next I can test record/reproduce functions.

Smooooth. What next?

 

[sweetbeats]

Smooooth. What next?

“Next I can test record/reproduce functions.”

I’ve not been able to do that since the transport never worked since I bought it. So now that it works I can see if it records and reproduces.

 

[sweetbeats]

Well my hopes are a bit dashed. I can’t get signal to print to tracks 1 & 2. Sort of.

All transport functions work flawlessly though, including the zero-stop function. But…worthless if I can’t get it to record and reproduce on all tracks. And the non-functional issues in the mixer section are pretty profuse. Will be quite a puzzle to sort out and I’m just not sure I have the time and space these days to tackle it without service documents. So I’m not sure what’s happening at this point…might just shelve it for awhile…I don’t know.

But at least maybe somebody can verify my line of thinking…the cables from the R/P head go to the playback amp card. They connect via two connectors carrying two tracks each. Here’s what I know:

1. I can get an input signal to propagate to all 4 SUB busses…meters display signal when monitoring the busses.

2. I can get signal from the 4 SUB busses to the recorder section input or record amp (i.e. when I switch the meters to monitor “TAPE”, arm the 4 tracks and put the transport into REC-PAUSE or REC mode all four meters show activity). So that’s all good.

3. When I play the tape back I only see activity on meters 3 & 4 and I can’t hear signal from track 2. I don’t know about whether or not I might be able to hear track 1 because I can’t get signal to pass through input channel 1 period.

4. When I flip the tape over I can now hear signal from track 2 (albeit playing backwards), and there is meter activity for both tracks 1 & 2 (again I don’t about being able to hear signal from track 1 because input channel 1 is dead). So I believe that tells me the R/P head is okay, and the cabling is okay, as well as the tracks 1 & 2 playback amp right? Like, I’m thinking it’s not the head, or head cabling or playback amp. It’s so funny when it’s somebody else’s PITA problem I can think through it and advise, but when it’s my problem I get all tripped up. Am I right?

5. I went an extra step and swapped the tracks 1 & 2 and tracks 3 & 4 head cable connectors on the playback amp card. When I did that, and played the recording back with the tape in the right way (not flipped), what I’d recorded was playing forward, I could hear it on track 2 and see meter activity for both tracks 1 & 2. This verifies the tracks 1 & 2 playback amps are good right? And then I did a new recording to all 4 tracks and played that back…nothing on 1 & 2 reproduces…3 & 4 are A-OK…I think this verifies the cabling is good. Then I did the same step I did before again and flipped the tape…same as first go-around, meter activity on 1 & 2, I can hear track 2 playing backwards…channel 1 is roached so I can’t hear that but assume if I could I’d hear something. This verifies tracks 1 & 2 are good on the head right?

6. So I was thinking I’ve at least got it narrowed down to something between the recorder section input amp and the output of the record amp to the head, and I was thinking it’s nothing related to the bias amp because it seems to be erasing across all 4 tracks. So I tried one more thing for shits and giggles…when you switch the transport speed on an AT-RMX64 to 1 7/8ips it shuts down tracks 3 & 4. The makers assumed if you were tracking at 1 7/8ips you were intending to record consumer format, so quarter-track stereo. I assumed if I did that I would just get nothing, since I couldn’t seem to record to tracks 1 & 2 at all. Nope. I got signal recorded to and reproducing from track 1…still nothing from track 2. No giggles from this guy. Just shits.

So I don’t know…I started looking for relays and I can’t find any so far…need to keep looking. I might just switch gears for a little and try and get the input channels working and do some verification signal is actually propagating from the 4 SUB channels to the recorder section input/record amp…that will unfortunately require mapping out some pinouts…and I’m just not sure I have it in me to go that deep right now and have this thing opened up on my operating room table forever…part of why that is is because I still have this stupid Pollyanna hope that schematics and/or a service manual will materialize for this thing…it takes so much more time to fix stuff like this when you have to reverse-engineer it. Anyway…shame on Audio Technica for not having those documents available. +1 for old Teac/Tascam stuff.

 

[sweetbeats]

Well…

Lol…

Okay. In the meantime while I half-waited for the replies I suspected I might not get, I fixed the mixer section. I think it’s all working now. Its a cryin’ shame they used phenolic material for the PCBs, especially the mother boards with plug-in card connections. The edge connectors really do get some oxidation on them and get stuck in place. I can’t underscore enough how important it is that if they’ve not been removed in a long time, even on a clean rust-free low-hours unit like mine, they will be tight that first time, and it is reasonably likely to damage boards, solder joints, etc. trying to extract them that first time. Watch my video earlier in the thread with my recommendations for extraction. I’ve been doing it a bunch and the technique works and minimizes the potential for damage. In some cases you have to hold the motherboard down while pulling up on the plug-in card to avoid damaging the motherboard and actually separate the connector. And do not skip the step of applying contact cleaner like DeoxIT D5 and exercising the connections…cleans, protects, and lubricated the metal-to-metal connections. Straight-up, that is how I fixed all of my intermittent or seemingly non-functioning mixer section functions. Just DeoxIT and exercising connections. If your AT-RMX64 has been messed with by somebody that’s not taken care with the extraction process, be prepared it may take more than that to resolve your issues because you may have damaged solder joints, or even a cracked board. And if you see evidence of moisture damage, and I’m not talking about submersion or fluids dripping on the unit, but just, like, evidence it was stored in a damp environment (look for any rust on screw heads or on the jacks), then you may be facing more oxidation on the card edge connectors and it will take more work to resolve that than I had to employ.

So the mixer is working, the transport is working, it’s just tracks 1 & 2 aren’t recording.

Another step I’m taking is to exercise the low insertion force ribbon cable connectors. There are white connectors into which the black ribbon cables terminate. Some may not realize these are really easy to unplug. There is a scissor type cover over the top of the connector that presses the sockets of the connector to the bare wires of the ribbon cable end. There is a small latch on the side of the connector…lift that with a small tool or spudger and the top pivots up, and you can easily remove the ribbon cable, or at least slide the cable in and out a little bit to break any oxidation. BUT…I’m super-interested now to do this to any of these connectors I can access, because there is one under the SUB 1 module that it appeared the insulation was stripped a bit too aggressively at the factory, and when I removed this ribbon cable from the connector one of the bare wire ends stayed in the connector socket. And upon inspection the two next to it were about ready to fall off. So I don't know if this is impacting anything, including my tracks 1 & 2 record issue, but certainly wasn’t right, so I trimmed and re-stripped the ribbon cable end…good as new now. I’m checking all of them. This is a pic of the faulty cable end and if you look close you might be able to see the wire end still stuck in the connector socket to the far left:

And here’s the next thing I’m going to check out as far as the tracks 1 & 2 record issue…there are three relays on the front motherboard under the SUB module section of the mixer. They are marked RL1, RL2 and RL3. I’m used to relays being marked with a “K” designation, so I wasn’t sure…but I thought maybe they were relays. So with the help of some friends I verified these are dual reed relays made by Hitachi. I don’t know what they do. Maybe they are record relays, but they are not located where I’d expect them to be, and I wouldn’t expect three, I’d expect two or four. Unless two are for high speed record and one is for low speed which only utilizes two tracks. At any rate, it’s the only set of relays I can find so far, and I’m going to test them to determine coil voltage and what functions energize them. Then we’ll know what they’re for. I’ll also be able to test if any are faulty. Here they are toward the front of the machine next to the transport section marked with yellow arrows:

Okay. More later on.

 

[RFR]

I knew you wouldn’t give up

 

[sweetbeats]

I knew you wouldn’t give up

Lol…grrr…can’t…stop…must…fix…

Here’s another YouTube video which captures a lot of what’s in my last post:

 

[famous beagle]

Thanks so much for documenting all this, Cory! If I were a betting man, I'd be betting on you getting this thing up and running again.

 

[sweetbeats]

Thanks so much for documenting all this, Cory! If I were a betting man, I'd be betting on you getting this thing up and running again.

Thanks @famous beagle !

FYI, the green relays, I believe the are muting relays. I clipped leads to the coil terminals and found that nothing I did would change the coils being energized with about +12VDC all the time. Then it dawned on me to see what happens at power up…and I find there’s no voltage for a couple-three seconds, and then it quickly ramps up to +12VDC. So the contacts stay open at power up for a short bit preventing the power thump from reaching the outputs, and then they close allowing signal to pass. I’m going to measure the contact resistance next, but I’m thinking these are not at fault for the tracks 1 & 2 record issue. I’ll test them out though and if all is okay with them then my next step is to make some extender cables, pop the record amp PCB out and hook it up to the extender cables, get out the scope and start probing.

 

[Robbie Stiefel]

Lol…grrr…can’t…stop…must…fix…

Here’s another YouTube video which captures a lot of what’s in my last post:

Dude. You can’t stop. I know bench space is valuable but you’re going where no man has yet gone. ha. You know I wonder if it’s possible to contact AT in Japan… I bet perhaps they’d have some documents. Probably in Japanese though. Ha. But you never know. Maybe they kept a set in English….

 

[sweetbeats]

Dude. You can’t stop. I know bench space is valuable but you’re going where no man has yet gone. ha. You know I wonder if it’s possible to contact AT in Japan… I bet perhaps they’d have some documents. Probably in Japanese though. Ha. But you never know. Maybe they kept a set in English….

I’d take the docs in Japanese. For the schematics that wouldn’t matter. And that’s what I’m most interested in.

Since my last post I was communicating with a friend of mine who, it turns out, has a friend who presently works for AT-USA, and has been there since the mid 1980s. My friend figured it was worth reaching out to him to see if he could look in the dark corners and rustle up some documentation. Unfortunately my friend’s friend is not aware of any possibilities and, in fact, doesn’t even remember the AT-RMX64…apparently nobody does…it is forgotten.

I have an email out to AT-Japan.

 

[Robbie Stiefel]

I’d take the docs in Japanese. For the schematics that wouldn’t matter. And that’s what I’m most interested in.

Since my last post I was communicating with a friend of mine who, it turns out, has a friend who presently works for AT-USA, and has been there since the mid 1980s. My friend figured it was worth reaching out to him to see if he could look in the dark corners and rustle up some documentation. Unfortunately my friend’s friend is not aware of any possibilities and, in fact, doesn’t even remember the AT-RMX64…apparently nobody does…it is forgotten.

I have an email out to AT-Japan.

Any word from AT Japan?

 

[sweetbeats]

Complete brick wall. Corporate policy. I even pleaded I’d sign a HHA, which I’ve done with other companies, but no dice. That doesn’t necessarily mean they have the documents. I didn’t get past the rep that answers emails, and their job is to regurgitate the policy.

 

[Robbie Stiefel]

Complete brick wall. Corporate policy. I even pleaded I’d sign a HHA, which I’ve done with other companies, but no dice. That doesn’t necessarily mean they have the documents. I didn’t get past the rep that answers emails, and their job is to regurgitate the policy.

What’s the policy exactly? That they don’t give out schematics?

 

[sweetbeats]

What’s the policy exactly? That they don’t give out schematics?

I asked for that, but that’s when they stopped responding, which suggests it was not somebody in authority with whom I was corresponding.

My assumption as to what the policy is, based on other processes I’ve been through with other companies, is it is primarily related to liability; risk management. They don’t want the risk of me holding them responsible when I zorch myself working on the equipment; work I execute pursuant to obtaining technical documentation from them. And they don’t want the risk of me sharing the documentation with others and those people holding the company responsible as a result of a mishap. Some companies, like Teac, are like “Sure! No problem! That’s $19.95 for the service manual and we’ll send it on out to ya!” Other companies are like “Sure! What’s your email address and we’ll email it on to ya!” Then you’ve got companies like AT that are, like, “No. Just no. Hell…no.” And then on the rare you have companies like TDK Lambda that have all the legal policy and guard their documentation, but under certain circumstances will share the documentation. But those processes take time and resources which means there’s an associate expense, AND an associated risk because ultimately it’s the honor system that I won’t share the information, or try to go after them. So that’s likely why AT is just a flat “no.”

I approached TDK Lambda a couple years ago because one version of the power supplies for my Studer mixing desk are actually loaded with high quality Lambda modular power supplies. Super-slick. But there are no schematics or parts lists or anything in the Studer service manual…only documentation relative to the chassis and the backplane PCB. And I don’t like that. What am I going to do if one of the regulated supply modules fails? So I reached out to TDK Lambda, and after some professional and congenial back and forth, they agreed to allow me to sign a Hold Harmless Agreement (HHA). It’s a lot of pages of legal-speak, but ultimately what it says is I will not go after them if I zorch myself or my equipment or somebody else is harmed in some way because of work I did on their products. And it further binds me to not share the documents with anybody. They have a copy, I have a copy. If I try to go after them or somebody else goes after them with whom I share the documents, they’ll pull out the HHA showing I breached the agreement and they are not liable. They were really nice to work with…very reasonable they want to protect their interests but were still willing to take the time to administer the HHA process and share the documentation. And of course I’m going to honor my agreement. It took a bit of time but it was worth it. Later I needed an additional set of documents for another module, and the process was much faster because they’d already vetted my intentions, and already had the HHA on file from me.

 

[Robbie Stiefel]

I asked for that, but that’s when they stopped responding, which suggests it was not somebody in authority with whom I was corresponding.

My assumption as to what the policy is, based on other processes I’ve been through with other companies, is it is primarily related to liability; risk management. They don’t want the risk of me holding them responsible when I zorch myself working on the equipment; work I execute pursuant to obtaining technical documentation from them. And they don’t want the risk of me sharing the documentation with others and those people holding the company responsible as a result of a mishap. Some companies, like Teac, are like “Sure! No problem! That’s $19.95 for the service manual and we’ll send it on out to ya!” Other companies are like “Sure! What’s your email address and we’ll email it on to ya!” Then you’ve got companies like AT that are, like, “No. Just no. Hell…no.” And then on the rare you have companies like TDK Lambda that have all the legal policy and guard their documentation, but under certain circumstances will share the documentation. But those processes take time and resources which means there’s an associate expense, AND an associated risk because ultimately it’s the honor system that I won’t share the information, or try to go after them. So that’s likely why AT is just a flat “no.”

I approached TDK Lambda a couple years ago because one version of the power supplies for my Studer mixing desk are actually loaded with high quality Lambda modular power supplies. Super-slick. But there are no schematics or parts lists or anything in the Studer service manual…only documentation relative to the chassis and the backplane PCB. And I don’t like that. What am I going to do if one of the regulated supply modules fails? So I reached out to TDK Lambda, and after some professional and congenial back and forth, they agreed to allow me to sign a Hold Harmless Agreement (HHA). It’s a lot of pages of legal-speak, but ultimately what it says is I will not go after them if I zorch myself or my equipment or somebody else is harmed in some way because of work I did on their products. And it further binds me to not share the documents with anybody. They have a copy, I have a copy. If I try to go after them or somebody else goes after them with whom I share the documents, they’ll pull out the HHA showing I breached the agreement and they are not liable. They were really nice to work with…very reasonable they want to protect their interests but were still willing to take the time to administer the HHA process and share the documentation. And of course I’m going to honor my agreement. It took a bit of time but it was worth it. Later I needed an additional set of documents for another module, and the process was much faster because they’d already vetted my intentions, and already had the HHA on file from me.

Are you gonna hold off on getting her working for the time being?

 

[sweetbeats]

No, no not at all. Me figuring this out doesn’t depend on having the tech docs. It would just be really nice. But I look at it like this…it’s all right there in front of me. It’s an “open-book” puzzle. There’s no wizardry to it. It’s an electronic device, relatively complex, yes, but signal is supposed to go certain places at certain times, and you just have to find a known place where the signal is supposed to be at a given time but isn’t there, and start tracing upstream until you have signal, and that’s usually where the fault is, whether a mechanical failure or a component failure. It’s all right there. It’s just a puzzle is all. And I have my $12 worth of breadboard jumper ribbon cables…took me 20 minutes to make up an extender cable set for one of the SUB modules:

I can start tracing anytime. But before I do that…

I have a couple theories as to why the record function is not working on tracks 1 & 2. And a quick and dirty way to test theories and narrow down the hunt is if you have a parts machine from which you can swap in assemblies.

Yes.

In the interim I bought AT-RMX64 #2. Spares are crucial. Here it is:

$300 shipped. It’s all there. It’s got more miles on it than my first one…it’s pretty filthy but oddly not so bad on the exterior…it’s dirty inside. All the plastic knobs and switch caps are way less discolored from sunlight than my first one though so that’s nice…but it’s been stored in something of a humid environment…fastener heads on the exterior are a little rusty in places…there’s a lot of scunge on the tops of the module sub-chassis with some surface corrosion…it’s weird…it’s, like, dirty under the dress panel:

Hm…maybe it sat in a dusty environment for a time with the dress panel removed. I bet that’s what happened. But there’s also this evidence of some moisture damage but it’s really isolated. It’s weird. Look here at the leading edge of the dress panel:

Seeing that I’d expect a disaster inside on top of all the dirt on the tops of the modules and some down on the floor of the chassis in between the modules, but the boards themselves and the connector pins look really clean. Here’s the SUB1 module. I’ve already removed the loose dust/dirt so what you see left is that surface corrosion:

But the board is clean and pins shiny:

Anyway I had this crazy idea the record functions would all work on this one and I could swap modules to narrow down if the problem with the recording was on the SUB module or further downstream. Lol. Just my luck…not only does it not record on tracks 1 & 2, it also doesn’t record on tracks 3 & 4…it doesn’t even erase on any tracks. I’ll re-seat the transport section cards and see where I’m at.

Most of the input channels weren’t passing audio but they woke up if I pushed on the TRIM and PAN knobs, so it’s just the typical need for re-seating and contact cleaning. 3 of 4 VU meter lamps are out. But all other lamps and indicators work. And the transport works…mostly. The FFWD button doesn’t work. But it pretty much does everything else although the pinch roller should be replaced and I’m sure it needs a new reel table drive tire. Anyway, what did I expect. Again, I’ll open up the transport section and re-seat stuff and see if I can get it to wake up.

But I had the same experience running audio through this second unit…I was just playing something on my mobile device. It sounds…really good…the mixing section. I can’t explain it because at least on what little looking I’ve done there’s nothing particularly remarkable looking about the components or anything…but it has a nice wide sound-stage, this nice natural mid range presence and clarity, and a lively punchy low-end. And I’m comparing to my experience with my Studer console. It just sounds surprisingly nice.

 

[sweetbeats]

Fixed the non-functioning FFWD control on unit #2…removed the keyboard PCB, cleaned it up, injected some DeoxIT into the switch, exercised it a bunch, now it works. It needs a little encouragement. Ideally I’d replace it. But this is small news. I just discovered something that kind of sucks.

I think unit #2 is probably from 1985. All the chips have 1984 date codes on them. Unit #1 I think is from 1986…chips have late 1985 date codes. Guess what? They revised the SUB PCBs somewhere along the way during the production run. You can’t plug early SUB PCBs into a later generation unit. There’s one less pin on the connector that handles the Dolby circuit on the early PCB version…17 pins on the early version, 18 on the later version. This is the connector that interfaces with the motherboard toward the rear of the machine. Which means the motherboard isn’t cross-compatible either. What a pain. So me thinking I had a parts machine is not entirely true. I wonder what else doesn’t match up…

 

[sweetbeats]

Okay…well, the input PCB assemblies are identical between the two machines…exact same connector pin count and assembly part and revision numbers. So that’s nice. It’s just unfortunate I can’t use the SUB modules from unit #2 in unit #1 for experiments and troubleshooting efforts.

FWIW all SUB modules have the 17 pin connector on the early version and all modules have the 18 pin connector on the later version. The additional pin is pin #12 and it is associated with some additional transistors and resistors…so for a quick visual on the differences, in the photo below the early version is at the top of the photo and the later version is at the bottom, and the area of additional components on the later version is encircled in a red oval: