Audio Technica AT-RMX64 Story…

[Slouching Raymond]

I would look at finding some long tool to lever up the cards from beneath.

 

[sweetbeats]

I would look at finding some long tool to lever up the cards from beneath.

But you’d be levering against the motherboard. I wouldn’t do that.

 

[Robbie Stiefel]

Thanks for feeding the animals.

I often wonder if anybody else has any interest in the same weird vein of “tear it apart and see how it’s made” that I do. And I know a lot of times my posts are long and too detailed and most of the young bucks move along because it’s too much to read. So I’m resigned to the fact 95% of the time my posts are just for me so I have somewhere to go to remember what I did and how I went about it. Because nobody else cares. But thanks for letting me know this stuff is of interest. At least to you lol.

oh I think more people thank you think are finding this extremely useful. Im seeing more folks talking about these units and virtually no one has worked on these. Or if they have they havnt posted about their experience. Like one post over ten years ago about changing the lamps in the meters. And that guy sold his way back.. haaaa...

 

[sweetbeats]

oh I think more people thank you think are finding this extremely useful. Im seeing more folks talking about these units and virtually no one has worked on these. Or if they have they havnt posted about their experience. Like one post over ten years ago about changing the lamps in the meters. And that guy sold his way back.. haaaa...

I know that guy…and he posted in this thread. Small world! BTW @famous beagle I’ve been referring to your repair thread and the pictures during reassembly, and will be referencing your tutorial on the meter lamp replacement. It looks pretty straight-forward relative to other machines’ meter lamps I’ve replaced from this era, but it NEVER hurts to reference the information gathered and presented by somebody that’s actually done the work. The gift that keeps giving.

Hey @Robbie Stiefel thanks for the feedback. I appreciate it.

I’ve got more updates but I’ll have to post later. But the transport and transport cards are back in…not ready to test yet, but it’s nice to have that back together.

 

[sweetbeats]

So, as I indicated in my last post, the transport, transport frame and cassette bay door retainer/release mechanism is all back in the main chassis. I’ve also applied DeoxIT D5 to the motherboard connectors for the transport amp cards (play, record and bias), and exercised those.

In addition to the refurbishing of the transport assembly detailed earlier in the thread, I also disassembled the cassette bay door retainer/release mechanism, cleaned all the old hardened lubricant off of it and applied new lubricant during reassembly. Prior to this the mechanism was difficult to operate, so you had to apply abnormal force to the door release button to get it to work, and the catch was slow to return to its neutral position afterward in order to retain the door when closed. So that’s all working slick now. And the other thing I did…and this is probably…no…it’s definitely…over the top…I replaced the hardware that fastens the transport chassis side panels to the transport assembly.

So earlier in the thread I posted pics of the transport assembly. That mounts to the main unit chassis via steel panels that screw to each side of the transport assembly. And the cassette bay trim and door also mount to those panels. Well, there are two screws on each side. And one was missing on one side of of mine. And it’s a bit of an oddball screw…like I know I could go to the hardware store and find a screw with a close enough diameter and thread pitch, but they’re these little Philips drive round head zinc-plated sheet metal screws…never gonna find one exactly like that. And…I can’t help it. They gotta match. Nobody is ever going to see it. But I’ll know it’s there. And then the other thing is I wasn’t really happy with those small sheet metal screws being responsible for holding the transport in the machine. Every time you put a cassette in you push down on the transport assembly…the screws are bound to loosen with time. Every other machine I’ve worked on uses machine screws here. The AT-RMX64 is the most robust all-in-one cassette multitracker, okay? But there are little things here and there that are not up to par. Now I know sheet metal screws can do a very good job of fastening and holding, but it all depends on the material, how thick it is, how big the screws are, etc. This wasn’t a good application for them…too small…anyway, so, yeah…I dumped out my bag of hardware I’ve collected from other period equipment, found four brand-new lookin’ matching zinc-plated pan head washer machine screws, reamed the holes in the transport chassis and tapped them, and drilled out the holes in the side panels. Here are the screws, factory little guy on the left, new screw on the right:



Now I know what you’re thinking…the new screw doesn’t look all that much bigger. But it’s bigger, and the machine thread gets a lot more threads in the body of the hole and that means better hold. So…it’s better. Here are the holes in the transport chassis before…actually this is after reaming…but before tapping and dressing…the holes in question are (going from left to right) the second and last holes:



And then here they are after tapping…I’d done the other side, so I just flipped the transport over for this pic, so the tapped holes are (going from left to right) are the first and last holes:



Now there is something to consider here…it’s good to have a rigid chassis, but because the transport assembly with its side panels fastens to another steel sub-chassis in the main unit, care has to be taken to make sure the transport assembly doesn’t rock at all when set down in the main unit. If it does, then when you tighten the screws that fasten it to the main unit, it will flex the transport assembly. And that’s not good. Teac/Tascam actually mitigated this in a number of there machines were the transport assembly actually floats in the main unit…there are three screws that hold it to the main unit, but they are shoulder screws that fasten tight before tightening the transport assembly chassis to the main unit…they just hold it in place. And then there are two screws that fasten the top of the transport to the cassette bay. That’s what keeps it from being able to rattle around. But because the shoulder screws at the bottom allow it to float, and furthermore because there are only three screws, there is no possibility of the sides of the transport assembly not being parallel at the bottom, and rocking like a chair with a short leg, and then when that gets tightened down that chair would be tweaked…think of the seating surface of the chair as the face of the transport, and one corner, one leg of the chair is short, but we force them all down to the floor and screw them down. The seating surface is now twisted. The Audio Technica doesn’t have as sophisticated a mounting system for the transport, and you can, just like with any other cassette transport chassis, twist it and flex it. This is fine under normal operation…like, playing a tape isn’t going to flex and stress the transport plate, but my point is it can flex and twist, and it will do so if it’s not true at the bottom with reference to the sub-chassis to which it mounts, so I feel it’s important to make sure it mates positively with the sub-chassis in the main unit, especially now that I put more robust fasteners in place. So what I did was assemble the side panels to the transport with those side panels sitting on a known flat surface, which in this case was a quartz kitchen counter-top…I have a granite surface block too, but the quartz slab is good enough in this case. So once the side panels are fastened to the transport assembly while sitting on a flat surface, I set the whole mess down in the main chassis where it will ultimately be screwed down, and careful press each corner to see if it rocks at all, even putting a thumb and finger diagonally at opposing corners and trying to rock it. It was fine. If it wasn’t I’d just loosen one screw of one side and press down on the transport chassis to shift one corner whatever way it needed to go to make the transport assembly “flat” with reference to the sub-chassis in the main unit. Because at the end of the day I don’t really care if it’s “flat” or true at the bottom…all I really care about is that the transport isn’t under any stress when it is fastened to the main unit, that it is “true” to the sub-chassis. Doing the initial assembly on the known flat surface just gives is a good starting point, and in my case that starting point was also the end point.

Anyway, that is all probably overkill. But that’s what I did.

Here’s the transport and amp cards back in the main chassis…I’m not putting the transport dress panel on yet, because the front panel/wrist rest is removed…still working on adhesive removal, and then polishing…so the dress panel wouldn’t sit right because the front lip of that sits on top of the front panel/wrist rest…so for now the keyboard PCB is just floating there as is the LED PCB toward the rear. I want to test the transport out, but not when stuff isn’t sitting right. Regardless, this is progress:



And I’ll leave you with an oblique shot into the cassette bay…looks good with the new pinch roller:

 

[Robbie Stiefel]

I know that guy…and he posted in this thread. Small world! BTW @famous beagle I’ve been referring to your repair thread and the pictures during reassembly, and will be referencing your tutorial on the meter lamp replacement. It looks pretty straight-forward relative to other machines’ meter lamps I’ve replaced from this era, but it NEVER hurts to reference the information gathered and presented by somebody that’s actually done the work. The gift that keeps giving.

Hey @Robbie Stiefel thanks for the feedback. I appreciate it.

I’ve got more updates but I’ll have to post later. But the transport and transport cards are back in…not ready to test yet, but it’s nice to have that back together.

Haaa. A small world indeed! Thanks for the info earlier re: my questions. Im VERY interested in your mod tapping channel strip post EQ. a "pre/post" switch/button would be ideal in the event you dont need EQ and would like to retain the shortest path possible... Curious is this possible? Re: the balanced and unbalanced sub-outs yeah a bit of a bummer that they cant be swapped to allow for balanced XLR on the pan usable outs is a bummer but not the biggest deal in the world. As you mentioned in your video there is definitely a reason the designers chose this and maybe one day we will hear from someone who has knowledge of the reason why they did this.... BTW, I have a few of those sales brochures... did you receive yours yet? Heres a copy in case anyone wants to look it over!.. Def useful info there. I had a whole bunch of them but I threw them out and kept only one. That was really stupid of me but I was moving studios and excess was driving me insane...

 

[sweetbeats]

Oh man I wish I’d known you had a pdf scan of it…I just wanted the info…spent a small pile of money to get the one I got.

 

[sweetbeats]

@Robbie Stiefel

Curious is this possible?

Your question regarding switchable pre/post EQ insert send…sure it’s possible, but one would have to identify where that post EQ point is and modify the modules. It wouldn’t be hard to trace the signal conductor from one of the send jacks on the backplane across the ribbon cable to the motherboard and identify the corresponding pin on the module…then trace that pin to an opamp…identify the EQ section output booster amp, cut trace and install jumper wire to EQ output booster amp. The same process then needs to be done for the return jack. So it’s just a matter of the time it would take to do that. The bottom panel comes off easy and you can see the motherboards, the meter bridge comes off easy so you can ID the correct ribbon cable(s)…from there it’s just spending some quality time with an input module…if you want it switchable you get a 2-pole switch. Or put a jumper on the PC board.

Okay so the front dress panel/wrist rest is all cleaned up and reinstalled, which allows me to put the transport dress panel back in place…not a whole lot to look at, but compare the front panel pictured below to the similar picture I took when it still had masking tape residue all over it in the first post. Better.

 

[Robbie Stiefel]

@Robbie Stiefel

Your question regarding switchable pre/post EQ insert send…sure it’s possible, but one would have to identify where that post EQ point is and modify the modules. It wouldn’t be hard to trace the signal conductor from one of the send jacks on the backplane across the ribbon cable to the motherboard and identify the corresponding pin on the module…then trace that pin to an opamp…identify the EQ section output booster amp, cut trace and install jumper wire to EQ output booster amp. The same process then needs to be done for the return jack. So it’s just a matter of the time it would take to do that. The bottom panel comes off easy and you can see the motherboards, the meter bridge comes off easy so you can ID the correct ribbon cable(s)…from there it’s just spending some quality time with an input module…if you want it switchable you get a 2-pole switch. Or put a jumper on the PC board.

Okay so the front dress panel/wrist rest is all cleaned up and reinstalled, which allows me to put the transport dress panel back in place…not a whole lot to look at, but compare the front panel pictured below to the similar picture I took when it still had masking tape residue all over it in the first post. Better.

View attachment 126014

Again very useful info. The deck is looking very good. Always a shame when someone uses duct tape on gear. Re: pre/post EQ points: I assume mods of this nature would be much speedier if you had the schematics and service manual?


-Robbie

 

[famous beagle]

That machine is looking beautiful! Your work never fails to impress!

I'm glad you found my thread useful in any fashion. I know it's way below your paygrade, but I hoped it might help others realize that fixing the lamps is not too big of a deal.

 

[sweetbeats]

Again very useful info. The deck is looking very good. Always a shame when someone uses duct tape on gear. Re: pre/post EQ points: I assume mods of this nature would be much speedier if you had the schematics and service manual?


-Robbie

I’ll take duct tape any day over masking tape. Dried masking tape adhesive turns into this stubborn mastic-type residue that requires a LOT of elbow grease to remove. Duct tape residue by comparison is relatively easy to remove.

And regarding the pre/post EQ insert send/return, yes absolutely…comparatively speaking it would be a cake walk to work up the modification if I had the schematics…don’t even need the service manual, just the schematics…would take minutes to figure out if I had those vs possibly hours otherwise. I’m not unfamiliar with having to reverse-engineer stuff…that’s been 100% the case with the “Tascam M-__” prototype console I have…it’s a relatively complex mixing console and there’s not one stitch of technical documentation available. So you just have to stare at it a bunch and make notes and sketch things out…eventually you do enough of it and if you study analog audio circuits enough pretty soon you start to recognize and anticipate typical circuit blocks and it goes a little easier, but it’s still a PITA and I’m not really very good at it.

 

[sweetbeats]

That machine is looking beautiful! Your work never fails to impress!

I'm glad you found my thread useful in any fashion. I know it's way below your paygrade, but I hoped it might help others realize that fixing the lamps is not too big of a deal.

Thanks, beagle!

And yes the thread was helpful…”pay grade” pshaw…you’re, like , the quintessential renaissance man…talented musician and recordist but capable repair tech too. That particular thread was nicely laid out…pics to boot. Should help anybody do the repair. In fact I’ll cross-link it here:

https://homerecording.com/bbs/threads/audio-technica-at-rmx64-cassette-4-track-repair-story.344695/

 

[sweetbeats]

So this is a pretty simple thing, and I expect anybody that’s opened up the transport section of the machine has or can figure this out, but in the guts kind of below the PAUSE button area are two little blue trimmers…these ones:





The one toward the rear is labeled 9.5, the one toward the front is labeled 4.75. Those numbers represent centimeters per second…those numbers are the metric equivalents of 3 3/4ips and 1 7/8ips. Those are the tape speed adjustment trimmers. So you could put a calibration tape in the machine with a known frequency tone, press PLAY, put a frequency counter on the output (many DMMs have that functionality), and do this in high speed and low speed and adjust the corresponding trimmer to dial the output frequency to the known frequency on the cal tape. So at least we can do that lol. There are trimmers all over the machine, down on the motherboard below the transport amp cards, on the amp cards, and on the SUB modules. I’m hoping over time to ascertain what’s what.

Oh and on a different subject, you can tell a lot about how much a machine has been used by the wear on the transport buttons…fingers kind of polish the plastic over time. Here’s some more evidence this machine is relatively low miles:



Very little wear there.

 

[Slouching Raymond]

Then, there's the worn off silver paint on the cassette door.

 

[sweetbeats]

Then, there's the worn off silver paint on the cassette door

I’m not sure what you’re talking about. There’s no silver paint on the cassette door from the factory.

 

[Robbie Stiefel]

So this is a pretty simple thing, and I expect anybody that’s opened up the transport section of the machine has or can figure this out, but in the guts kind of below the PAUSE button area are two little blue trimmers…these ones:

View attachment 126035

View attachment 126041

The one toward the rear is labeled 9.5, the one toward the front is labeled 4.75. Those numbers represent centimeters per second…those numbers are the metric equivalents of 3 3/4ips and 1 7/8ips. Those are the tape speed adjustment trimmers. So you could put a calibration tape in the machine with a known frequency tone, press PLAY, put a frequency counter on the output (many DMMs have that functionality), and do this in high speed and low speed and adjust the corresponding trimmer to dial the output frequency to the known frequency on the cal tape. So at least we can do that lol. There are trimmers all over the machine, down on the motherboard below the transport amp cards, on the amp cards, and on the SUB modules. I’m hoping over time to ascertain what’s what.

Oh and on a different subject, you can tell a lot about how much a machine has been used by the wear on the transport buttons…fingers kind of polish the plastic over time. Here’s some more evidence this machine is relatively low miles:

View attachment 126044

Very little wear there.

Is the frequent presence of trimmers like what you see on the AT common across other cassette “portastudios” of the period?

 

[Slouching Raymond]

I’m not sure what you’re talking about. There’s no silver paint on the cassette door from the factory.

I'm talking about the silver line on the edge of the door. The photos show it has worn off in parts.

 

[Slouching Raymond]

Hold that, I'm an idiot. Just looked again, and it looks like a reflected light.

 

[sweetbeats]

Is the frequent presence of trimmers like what you see on the AT common across other cassette “portastudios” of the period?

Do you mean the type of trimmer, or the presence of trimmers at all?

 

[Robbie Stiefel]

Do you mean the type of trimmer, or the presence of trimmers at all?

The presence of all trimmers.