Audio Technica AT-RMX64 Story…

So what I know so far is it’s not the Dolby chips. That was a suspicion I had, but I’ve ruled that out.

Each SUB module is essentially two almost completely separate circuit assemblies: the roughly half of the module toward the front of the machine is the SUB amp stages as well as other stuff depending on the module…the section of the module toward the back of the machine is the Dolby circuit with necessary amp stages, etc. The front half has essentially internal pre and post fade outputs to the motherboard. The pre fade out goes to the motherboard and through the guts eventually over to the bias amp PCB which has the record level pots on it. The output of those pots go back through the guts to the back half of the SUB module through all the Dolby stuff and then back out through the guts to (probably) the record amp PCB, which is the one with the record arming switches.

Pin 15 of IC10 (the Hitachi HA12058NT Dolby chip) is the output of the chip to the record amp. Pin 15 goes to output coupling cap C120, and then to pin 3 of connector 3 (CN3) of the module, which is the 18 pin card edge connector toward the rear of the module (17 pin if you have an early AT-RMX64). So I injected tone to an input module, assigned that to all four SUB busses, loaded a cassette, armed all the tracks, put the transport in RECORD PAUSE, adjusted levels accordingly and verified I’ve got good signal from pin 15 of IC10 all the way to pin 3 of CN3. It’s not the SUB module. Which I’m kind of happy about anyway because if the Dolby chips were bad that could be a problem…long since NLA unless you want to buy used or NOS, and in either case who’s to say the replacement wouldn’t be bad or go bad in the near future. It’s a complicated part and I’ve not been able to even find a datasheet on it. I have a jpeg of what looks to be an application of the part in some circuit, and at least the inputs and outputs to the chip are labeled in the jpeg, so that’s how I ascertained pin 15 was the record output from the chip.

So next I have to see where pin 3 of CN3 goes on the rear SUB section motherboard, and keep tracing. This will require removing the bottom cover panel and foam stabilizer blocks from the motherboard so I can visually follow the traces. This is where unit #2 will come in handy, because I don’t want to tear unit #1 apart just to put it together to test. I can leave #1 intact and use #2 as my road map.
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:

View attachment 127076
Any ideas yet what those extra components and pin are for?
Elimination is the fast way to fast way to track an error.
I aggree with your fault searching technique. Follow a bad signal until it gets good, or follow a good signal until it gets bad.
You probably already have all the data sheets on the chips. If not, they're all obtainable.
As for #2, a full forensic examination of all the fluff and muck may reveal what the previous owner had for lunch, while mixing.
Any ideas yet what those extra components and pin are for?
No clue. If I have time down the road, I might try to get an idea what they do just out of sheer curiosity, but at this point anything that doesn’t have anything to do with my tracks 1 & 2 record fault is not of interest, and those components don’t.
Elimination is the fast way to fast way to track an error.
I aggree with your fault searching technique. Follow a bad signal until it gets good, or follow a good signal until it gets bad.
You probably already have all the data sheets on the chips. If not, they're all obtainable.
As for #2, a full forensic examination of all the fluff and muck may reveal what the previous owner had for lunch, while mixing.
I do have datasheets for all the ICs, except for, as I mentioned, the Dolby chips…the Hitachi HA12058NT…can’t seem to find anything on that and even reached out to some contacts with technical libraries and nothing. There is a 28 pin version, and a 30 pin version with completely different pinouts. The AT-RMX64 uses the 30 pin variety. If anybody has the datasheet I’d love to have a copy.
Unit #2 is sufficiently disassembled in order to study and use as an exposed guide while tracing signal on the assembled unit #1…SUB modules removed along with the transport section amp cards:


Bottom panel removed and I’ve started peeling off select foam support blocks:


Here is the motherboard that’s getting the attention…motherboard “C” IIRC…you can see toward the right of the board the four vertical rows of solder joints…those are the board-mounted connectors that mate with the “CN3” connectors on the SUB modules. Then to the left you can kind of see three horizontal rows of solder joints…those are the transport section amp cards:


So after a quick game of dot-to-dot I know the record output of the Dolby chip IC10 on SUB module #1 goes to pin 1 of connector 1 (“CN1”) on the record amp PCB via pin 3 of CN3 on the sub module and the motherboard. I’ll gather a little more information before I start tracing signal on unit #1, but signal comes into the record amp board, goes to an input resistor, then to a trimmer (I’m reasonably certain the small trimmers located at the top of the record amp PCB, the ones you can access from the top when the board is installed, are the record level trimmers), and then an input coupling cap. That’s as far as I meandered. I just need to verify which input pins on the record amp PCB handle tracks 2, 3 & 4 and their related components so I can compare as I go. I think this is going to be pretty easy…
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So, just FYI to all you AT-RMX64-ophiles, I noted in an earlier post there was a significant revision to the SUB modules at some point in the production run, and it looks like there is also a significant revision to the record amp PCB. My unit #1 serial number is 652018, and I think it was manufactured in 1986. My unit #2 serial number is 520038. I think it was manufactured in 1985. I don’t know if Audio Technica’s serial number protocol is a simple sequential manufacture number, or if the digits represent something more like year/month of manufacture and unit number. But I can tell you my unit #1 was manufactured after my unit #2 based on the date codes of the installed ICs. SO…just be aware there is a 1st generation record amp PCB assembly p/n 7220136, and a second generation assembly p/n 7220136B that has some significant differences from the 1st generation assembly. They have the same number of pins in each of the two card edge connectors, but the screening on the boards is different, and there is an additional ribbon cable connector on the ‘B’ version. I don’t yet know where that goes. Here is a pic of the two…later generation revision ‘B’ assembly is the top one in the pic, earlier non-‘B’ version is on the bottom…note the two ribbon cable connectors on the ‘B’ version…one goes to the record-ready LED PCB just like the early non-‘B’ version…the other I have no idea where it goes…yet…


Anyway, I thought I’d let you AT-RMX64 minions know. FWIW the “OSC UNIT” PCB assembly (the bias amp board) and playback amp PCB assembly are identical between the two generations like the input modules.

So, to tally the early and later generation differences that I know so far, later version has some additional components on the SUB modules, an 18 pin “CN3” connector, and as a result a different “C Mother Unit” motherboard (the one that interfaces the rearward connector of the SUB modules and the transport section amp cards), as well as a ‘B’ revision record amp PCB. The early version lacks the additional components on the SUB modules and have a 17 pin “CN3” connector with corresponding C Mother Unit, and the record amp PCB assembly is the 1st generation non-‘B’ unit.

There may be other differences, but the above is what I know so far and I wanted to share.

FYI, the record feed from the SUB modules to the record amp PCB is this:

SUB 1 “CN3” pin 3 —> record amp PCB “CN12” pin 1
SUB 2 “CN3” pin 3 —> record amp PCB “CN12” pin 3
SUB 3 “CN3” pin 3 —> record amp PCB “CN12” pin 5
SUB 4 “CN3” pin 3 —> record amp PCB “CN12” pin 7

So pins 1, 3, 5 & 7 of the larger 18-pin connector “CN12” are respectively the tracks 1~4 inputs to the record amp PCB for both the 1st generation and revision ‘B’ versions.
Glad it was something really simple
Yeah me too…but to validate others who are struggling with these types of issues on an AT-RMX64, yes in some cases the solution may be simple, but finding the fault source can be a real challenge if you don’t have experience with this sort of thing, and the AT-RMX64 is relatively complex as far as interconnections. The semi-modular setup is nice with all the plug-in cards/modules, but there are lots of ribbon cables, and multiple motherboards and until you start to understand what circuits live where, and/or if you just lack a general understanding of how these things work electronically and what circuit blocks are typically present, it’s…hard. Like I’m at a point where I can approach it from the standpoint of “okay…X is not working…that’s related to the ___ circuit…so where is the ___?” As opposed to “How does this work?” Normally I have schematics and can learn a lot from that. With the AT-RMX64 and no schematics it takes more time because things have to be traced out on the device itself, but at least I have some inkling of what to look for. So I’m getting a bit more comfortable with what is where and hopefully can help others get their machines running again. It’s not always going to be bad solder joints. But maybe I can help narrow down a fault source having a decent idea of how things interconnect now. I will also say it was essential to make extender cables, and it was extremely helpful to have a second AT-RMX64 here to use as a reference; be able to have it opened up and partially disassembled and be able to see where things go and come from while the main unit was relatively assembled and powered and being tested.
Let there be light…


Yes…got the meter lamps replaced which, thanks to @famous beagle ‘s tutorial on the subject elsewhere on this very forum (just do a search for threads started by him with AT-RMX64 in the title and you’ll find it…), was a breeze…I’m polishing the meter bridge metalwork, and still need to clean all the knob and switch caps, but then I’ll put it back together and call it “done.”

I’ve been working on other things and also manhandling daily life, but I did do a bunch of other general investigative work into the guts of the AT-RMX64 because, unfortunately, I think until a miracle surfaces, I’ve lost all hope of obtaining tech docs for this thing…and I think with help from friends I’ve done a yeoman’s job trying. I wish the buyer of the full-size schematic set on eBay a couple three years ago would see this and do the AT-RMX64 community a solid, but that’s probably not happening…or Michael Dewenter of would respond…I’ve sent many emails through his website at this point with no response, and three years ago when I did connect with him via email he confirmed he had the documents, but had just moved and didn’t have his scanning equipment or documents available…totally respect that. But it’s been crickets since. So we know the documents are out there, but nobody is helping. I didn’t share this earlier but I have a good friend who has been in electronics for decades, super smart, worked for Tapco and Mackie among other companies designing consoles, still does quality sound reinforcement engineering to this day…connected…turns out he knows somebody at AT America that remembers the AT-RMX64 and was willing to go fishing…dead end. SO…I said fuck it and took to reverse engineering.

I have a complete electrolytic cap list by PCB with audio path caps identified for anybody that feels they need to recap. It will help you with your shopping. It also calls out differences between early generation and late generation AT-RMX64s, because there are some differences (limited) in the caps…

I also have almost every pin of the input modules mapped, and many of the sub modules…I figured out the signal flow through the record, bias and playback amplifier PCBs in the transport section.

I’m really busy so I haven’t put any thought to how to disseminate this information, but if you need it hit me up…I basically spent a big chunk of quality time mapping it out.

If nobody helps you, you have to help yourself.

Anyway, that’s the update. I’m on the cusp of having a fully operational 9/10 condition AT-RMX64 and have a butt-load of guts-knowledge now to support the device.

OH…and I think the main difference between the early and late generation units is a 555 IC timer circuit for muting when arming tracks. I should capture a video of the muting action on mine, but basically on the Mother C Unit of the later generation machines, which is the motherboard under the back connector of the SUB modules and the transport cards, there is an added 555 IC timing circuit, and related circuitry on the record amp PCB and SUB modules that mutes all output from any armed channels when you make changes to the track arming. That means there are differences in the record amp unit, mother C unit and SUB modules between early and late machines…not compatible. Test it…put in a cassette, send some signal to all 4 sub modules, arm all 4 tracks and turn up the record level…set all the meters to TAPE…put the transport in REC-PAUSE. You should now see level on the 4 meters. Take one track out of armed status. When you do this on a late generation machine, all tracks should go dead level-wise for a second or so, and then pop back. That’s the timer muting circuit working. I believe if you do this on an early machine you won’t see the muting action, and may hear switching artifacts in the outputs if monitoring. No big deal, and honestly it speaks to the relative fancy nature of the project that is the AT-RMX64 that they took the time to do this midstream in production, but it’s good for folks to be aware of. IMO I don’t think it makes late gen better than early gen, it still the same signal path and transport, etc., but it DOES mean incompatibilities in some of the PCB assemblies.


That’s all for now.
Sweetbeats, even though I don't have a multitrack cassette deck, I've been reading your whole proces. This has truly been a labor of love for you, hasn't it? It's like the people that find the hulk of a old car and rebuild and restore it back to glory.
Uh yeah I think it’s kind of like that…and there’s a little something additional with this particular project because the AT-RMX64 community has really been left in the lurch by Audio Technica with the lack of service documentation. And I don’t think most people are as nutty as I am as far as the sheer lack of sensibility about just diving in and tearing apart a relatively rare and pricier piece of vintage audio equipment without any documentation…but I’m just like “I gotta figger this out…”…and knowing that I figure nobody else is gonna do it and the community will continue to be without the information, the machines will continue to age and develop issues, so yeah it’s like the old car analogy, but in this case it’s almost like I feel a sense of duty to get at least some answers for other people that have one of these. Obviously the information I’m gathering and sharing is no replacement for a service manual, but it’s more than we’ve had up until now. So that feels good.
I am doing a series of videos regarding how to calibrate the AT-RMX64. The first step is to calibrate the meters…here’s the video related to that step:

The next step is to, using a bonafide calibration tape, adjust the playback levels…here’s the video for that step:

Next up will be one or more videos related to the next steps, which are setting the bias, and then the record level. I’m in the midst of an in-depth process for verifying the correct bias level, so it’s taking a little time.
This is what it looks like when A. your workspace still looks like stuff-vomit and nothing is setup because you’re still not settled from a move, but B. you really want to objectively set the bias on your AT-RMX64, learning how to do it as you go:


I had an idea for what I wanted to do and how I wanted to do it, and vetted the idea with a couple people leagues smarter and more knowledgeable than I am…I’m using my Tek SG 502 low-distortion oscillator to run tone to tape, and then analyzing the reproduced tone with my Tek AA 501A distortion analyzer, sweeping the bias level to identify minimum distortion while listening to the sweep to hear how the noise and distortion change…once I quantitatively know minimum distortion via the sweep and have an auditory sense of the gap between minimum distortion and minimum noise (both of which can vary significantly between tape formulations), I can record tone, reproduce, observe the distortion level, tweak the bias level trimmer, rinse and repeat until the track as at that sweet spot between minimum distortion and minimum noise…then do that for the remaining tracks. It’s a pain on a two-head machine, but it’s the only way to actually set the bias, and it’s critically interesting how much recording quality is impacted by small deviations from that bias sweet spot. What I’m doing here is a variation of the LF modulation “bias rocks” method, which is also an objective way for setting bias level…I’m doing that but using a higher frequency tone (700Hz instead of something subharmonic), and using the distortion analyzer in parallel to my ears. The common “overbias” method works, but it’s not truly objective…it’s relatively arbitrary. The worst method is setting bias by measuring the output of each channel’s bias amp. It has no regard for tape type and formulation. This is how the factory instructs the operator to set bias on a Tascam 388. But the instructions cause the operator to set a significant overbias condition unless using 457. I want to know what the correct bias level is on the AT-RMX64 so I’m doing it this way. Bias level has significant impact on noise level, distortion, and HF response as well as record level. I’m doing the exercise first with generic Cr02 tapes I have…have a whole flat of them…then will do it with TDK SM tape to determine differences.
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