DIY REPAIR Audio Technica AT RMX 64 4-Track Signal Loss

Cye3212

New member
Hi All,

This is my first post but I've researched this topic quite a bit and haven't found much info directly related to this machine - hoping the answers to this can help others as well as myself in solving this [and related] issue. I have a couple questions that actually encompass different topics but I'll try to consolidate them here as they are all fairly related.

Since I'm new I can't post a link to the schematics but you can find it here:

www (dot) studio250.fr/docs/divers%20enregistrement/atspecif.pdf

The Problem
So I have one of these amazing RMX 64 4-tracks but a couple channels do not appear to be passing signal properly. Channel 1 works perfectly as it should and I am getting full sound from both the Sub1 out and Direct channel out. Channel 2 does not pass the signal properly although I can just barely hear the signal thru the sub 1 out and direct out when I crank up the input gain/trim to the max; this issue is present for channel 6 as well.

When I first noticed this problem I found that when I firmly pressed down on the channels the signal would return but after releasing the pressure the signal would cut out again. Naturally I thought this may be some sort of connection issue and cleaned all pin connectors that attach to the mother units - this did not solve the issue. I recently purchased another rmx 64 for parts and it seems to be working fine as just a mixer (tape transport does not work) so I replaced the channels but the issue was still present. I even switched out the mother unit for the mix channels and the issue still persists. I am wondering if this may be a problem with eletrolytic caps in the pregain stage?

I am still very new to DIY audio repairs and electronics in general although I have a good grasp of basic signal flow and circuits.


Testing equipment (slightly different topic)
Currently all I own to analyze the issue is a multimeter, speakers and my ears.
From what I have read it is recommended that I purchase a tone generator and an audio analyzer capable of multitone signal generation. I am a musician by trade and my goal is ultimately to be able to accurately identify issues with my vintage audio gear and make repairs for fairly common issues - I am not looking to purchase a super fancy oscilloscope or audio analyzer for mucho $$$ at this time and would like some recommendations on what would be good to start with; I was thinking about a Whirlwind QBox but I am not sure how far that would get me, although it seems like it would help in this case.

Is this an issue I can probably solve given my basic tools on hand? I would imagine that I'd just need a tone generator and then follow the signal via multimeter thru the channels and see where the signal becomes lossy/cuts out to identify the problem component. If this is the case, would I just be poking around along the signal path looking for a 1khz tone with my multimeter? As I said I am very new to this so I am not sure how I'd go about all this.

Thank you all so much!
 
Greetings!

I can't post a link to the schematics but you can find it here:

www (dot) studio250.fr/docs/divers%20enregistrement/atspecif.pdf

Unfortunately there are no schematics in that linked document. There is a block diagram, but that is a pictogram of the signal flow, not a comprehensive schematic diagram showing detail of the device at a component level...there’s not much more than a high level hypothesis on the cause of your problem that can be generated from the block diagram.

The Problem
...a couple channels do not appear to be passing signal properly. Channel 1 works perfectly as it should and I am getting full sound from both the Sub1 out and Direct channel out. Channel 2 does not pass the signal properly although I can just barely hear the signal thru the sub 1 out and direct out when I crank up the input gain/trim to the max; this issue is present for channel 6 as well.

Just checking...does channel 2 pass appropriate signal to tape, or is it the same result as when monitoring the input via SUB 1 or the channel 2 direct out?

When I first noticed this problem I found that when I firmly pressed down on the channels the signal would return but after releasing the pressure the signal would cut out again.

What do you mean “pressed down on the channels”? Can you clarify what you are pressing on or where you are pressing and how hard?

Naturally I thought this may be some sort of connection issue and cleaned all pin connectors that attach to the mother units - this did not solve the issue.

That was reasonable logic. Obviously the cause is elsewhere, but that was good thinking.

I recently purchased another rmx 64 for parts and it seems to be working fine as just a mixer (tape transport does not work) so I replaced the channels but the issue was still present.

What do you mean by “replaced the channels”? Do you mean replaced the PCBs for the input channels? If so which ones? Not being familiar at all with the RMX-64, are the input channels on individual cards? Do they connect to a motherboard? If so, how? Cardedge connector? Ribbon cable? Pics would help.

I even switched out the mother unit for the mix channels and the issue still persists.

“...mother unit for the mix channels...” Again, not being familiar with the RMX-64, what is the “mother unit”? Are you talking about a motherboard? And “mix channels”...are you talking about SUB 1~4? Here again some pics would help.

If your problem is present at the direct out for channel 2, look at the block diagram. You see how that direct out jack comes right after the input preamp? It’s before the EQ, aux bussing, and any output bussing...so if the problem exists that far upstream, you won’t solve it by replacing stuff way downstream.

Study that block diagram before your next wholesale swap of guts. Swap guts when there is some reasonable assumption of the problem area. Make sense?

I am wondering if this may be a problem with eletrolytic caps in the pregain stage?

A bad cap is not the magic elixir to all audio problems. :)

You swapped the channel 2 PCB, yes? The input preamp is on that card? Then that’s not the problem.

How about this...how are the jacks interfaced with the channel PCB? Are they mounted on the channel PCB or are they mounted to the chassis and connectorized to the input PCB?

Testing equipment (slightly different topic)
Currently all I own to analyze the issue is a multimeter, speakers and my ears.
From what I have read it is recommended that I purchase a tone generator and an audio analyzer capable of multitone signal generation. I am a musician by trade and my goal is ultimately to be able to accurately identify issues with my vintage audio gear and make repairs for fairly common issues - I am not looking to purchase a super fancy oscilloscope or audio analyzer for mucho $$$ at this time and would like some recommendations on what would be good to start with; I was thinking about a Whirlwind QBox but I am not sure how far that would get me, although it seems like it would help in this case.

Is this an issue I can probably solve given my basic tools on hand? I would imagine that I'd just need a tone generator and then follow the signal via multimeter thru the channels and see where the signal becomes lossy/cuts out to identify the problem component. If this is the case, would I just be poking around along the signal path looking for a 1khz tone with my multimeter? As I said I am very new to this so I am not sure how I'd go about all this.

Thank you all so much!

Your concept of the basic troubleshooting is fairly spot on, but

1. What brand and model multimeter do you have and is the AC meter portion true RMS measuring?

2. Don’t spend your money on a tone generator. For your purposes, if you have a mobile smartphone (iPhone or some such thing) just install a tone generator app for free. That will work for what you are doing.

3. You really need the schematic diagrams before you start tracing signal. You won’t know where to poke next if you don’t have the diagrams, particularly around the active components (like the opamps), or unless you have a reasonable amount of familiarity with integrated opamp audio circuits and the power supplies that drive them. I’m gathering that’s not yet you, which isn’t a judgment, it just means it’s more important for you to have the schematics and be committed to trying to understand them...which is generally important for anybody at any skill level who is trying to diagnose a problem like this.
 
Hi there!

First of all, thank you so so so much for the thorough response. I understand that my post was rather long and I was asking a lot of questions but I have gained a wealth of information from your post - so again, you have my gratitude!

I can't believe I mixed up a block diagram for a schematic! haha, I suppose that and some of the other things I noted should tell you how green I am with all this - but I am eager to learn! Hopefully I can find a schematic for the RMX64 somewhere, I'll definitely ask around.

Just checking...does channel 2 pass appropriate signal to tape, or is it the same result as when monitoring the input via SUB 1 or the channel 2 direct out?
I am actually not sure...I suppose I should have noted that I have not tried to record anything as I intended to use the mixing capability of my 4 track. There appears to be some mechanical issues with the tape transport where the motor seems to be off and one side of the tape gets eaten up. I have not looked into this issue yet as I, at the very least, would like to use the mixing console function of the RMX64. So far, I have just hooked up the Sub 1 + 2 outs to a pair of speakers and listening through there as well as through the Direct Outs of each channel.

What do you mean “pressed down on the channels”? Can you clarify what you are pressing on or where you are pressing and how hard?

By this I meant I am physically and quite literally pressing down on the channel knobs. Imagine a downward force directly on the channel knobs. When left alone, the signal is cut but when I press down on the channel itself, the signal cuts in an out - as if the channel has not been seated properly on the motherboard. This occurs for only a couple channels, and even though channel 1 works it seems that if I wiggle the channel 1 knobs a bit the signal cuts out and back in as well, so I try not to be too rough with it.

What do you mean by “replaced the channels”? Do you mean replaced the PCBs for the input channels? If so which ones? Not being familiar at all with the RMX-64, are the input channels on individual cards? Do they connect to a motherboard? If so, how? Cardedge connector? Ribbon cable? Pics would help.

The RMX64 has modular channels, so I can simply pop individual channels out and have them replaced/serviced/etc.

https(colon)//photos.app.goo.gl/NEiSUHtciuVb35v59

*note: my apologies for the link but I do not have permission to post images on here yet.

As you can see in this pic I have a single [spare] channel on the right side of the RMX64. The channels are covered by a square plate that lifts up easily, giving you access to the channels.

https(colon)//photos.app.goo.gl/dKRdtbp3aKVR9mbs5

When I press down on the Send or Trim knobs the signal may cut back in as apparently is the case with Channel 6 but no amount of force will bring the signal back for Channel 2. I did not replace the PCB for the input channels themselves as I simply tried to replace the entire channel given that I have these spares (which seem to work alright in the For-Parts RMX64) although replacing the entire channel did not resolve the issue.

Again, not being familiar with the RMX-64, what is the “mother unit”? Are you talking about a motherboard? And “mix channels”...are you talking about SUB 1~4? Here again some pics would help.

The Mother Unit I suppose is technically the motherboard, on the actual board it says "Mother Unit B" so I just called it that. The motherboard is the only connection that the individual mix channels have to the rest of the machine. The input jacks, indicator lights, etc. are all connected to the motherboard(s).

https(colon)//photos.app.goo.gl/yAvXsDmuvfaDqnKG9

Here is a picture of the mother units. You can see Mother Unit B (which I have taken out) in the top right corner. I thought to replace that because of the whole when-I-push-the-channel-knobs-the-signal-cuts-out thing. It's very possible there could be an issue with Mother Unit A as well but as you surmised the issue may be way upstream. Also sorry for the confusion but I believe i meant "input channels" as I am only referring to the first 6 channels which have mic/instrument jacks. From what I can gather the sub channels 1-4 function properly.

A bad cap is not the magic elixir to all audio problems.

I suppose not! haha, I've been watching too many youtube tutorials...


You swapped the channel 2 PCB, yes? The input preamp is on that card? Then that’s not the problem.

I am not sure if the input preamp is actually on the channel module itself...Although I can have a closer look and see if I notice anything like an op amp near the front of the card...

Are they mounted on the channel PCB or are they mounted to the chassis and connectorized to the input PCB?

definitely connected. They are connected to the mother unit (B) which is then connected to the channel module.


As for my multimeter I have a Mastech MS8268 and I do not believe it has true RMS measuring.
 
...thank you so so so much for the thorough response. I understand that my post was rather long and I was asking a lot of questions but I have gained a wealth of information from your post - so again, you have my gratitude!

You’re welcome. Your post was long? Have you read some of my posts? :p

I can't believe I mixed up a block diagram for a schematic! haha, I suppose that and some of the other things I noted should tell you how green I am with all this - but I am eager to learn! Hopefully I can find a schematic for the RMX64 somewhere, I'll definitely ask around.

I don’t think it would take me long to find my post where I went to some lengths to upload and post links to the “schematics” for one of my machines or mixers as requested by a subject matter expert. What I posted was the PCB layouts. I didn’t know the difference between the two. I didn’t mean to embarrass, but hopefully teach...no dumb questions except the one that goes un-asked.

I am actually not sure...I suppose I should have noted that I have not tried to record anything as I intended to use the mixing capability of my 4 track. There appears to be some mechanical issues with the tape transport where the motor seems to be off and one side of the tape gets eaten up. I have not looked into this issue yet as I, at the very least, would like to use the mixing console function of the RMX64. So far, I have just hooked up the Sub 1 + 2 outs to a pair of speakers and listening through there as well as through the Direct Outs of each channel.

Try it. I suspect you will have the same problematic result, but it’s still useful info. And then if you have a known good tape, try playing back to see if playback is okay. We are trying to affirm your problem is on the mixer input side of the system.

By this I meant I am physically and quite literally pressing down on the channel knobs. Imagine a downward force directly on the channel knobs. When left alone, the signal is cut but when I press down on the channel itself, the signal cuts in an out - as if the channel has not been seated properly on the motherboard. This occurs for only a couple channels, and even though channel 1 works it seems that if I wiggle the channel 1 knobs a bit the signal cuts out and back in as well, so I try not to be too rough with it.

Thanks for the clarification. That makes sense. So, first of all, if channel 1 works, and channel 2 doesn’t, what happens if you move the channel 1 module to the channel 2 slot and vice-versa? Am I correct in my hunch the problem physically stays with the channel 2 slot (now populated by the module that was in the channel 1 slot)? If so, how handy are you with a soldering Iron? If it was me, I’d be reflowing the solder joints for the motherboard connectors on both the motherboard and module PCBs.

The RMX64 has modular channels, so I can simply pop individual channels out and have them replaced/serviced/etc.

Got it. Thanks.

As you can see in this pic I have a single [spare] channel on the right side of the RMX64. The channels are covered by a square plate that lifts up easily, giving you access to the channels.

Nice. Okay.

When I press down on the Send or Trim knobs the signal may cut back in as apparently is the case with Channel 6 but no amount of force will bring the signal back for Channel 2. I did not replace the PCB for the input channels themselves as I simply tried to replace the entire channel given that I have these spares (which seem to work alright in the For-Parts RMX64) although replacing the entire channel did not resolve the issue.

I suspect you need to reflow motherboard connector solder joints, but swap modules first.

The Mother Unit I suppose is technically the motherboard, on the actual board it says "Mother Unit B" so I just called it that. The motherboard is the only connection that the individual mix channels have to the rest of the machine. The input jacks, indicator lights, etc. are all connected to the motherboard(s).

Useful info. Thanks for the pics.

Here is a picture of the mother units. You can see Mother Unit B (which I have taken out) in the top right corner. I thought to replace that because of the whole when-I-push-the-channel-knobs-the-signal-cuts-out thing. It's very possible there could be an issue with Mother Unit A as well but as you surmised the issue may be way upstream. Also sorry for the confusion but I believe i meant "input channels" as I am only referring to the first 6 channels which have mic/instrument jacks. From what I can gather the sub channels 1-4 function properly.

Okay. Makes sense.

I am not sure if the input preamp is actually on the channel module itself...Although I can have a closer look and see if I notice anything like an op amp near the front of the card...

Don’t assume the opamp would be physically located at one end of the card. That’s logical and likely the case, but, really, you need the schematics to know. Actually, you could trace it from the input jacks, but if it was me I’d want the schematic in my hand.

I put my money on cold solder joints on the motherboard connectors either on the motherboard or on the input channel PCBs.

As for my multimeter I have a Mastech MS8268 and I do not believe it has true RMS measuring.

Yeah I’m thinking not. So, don’t worry about that for now. If you get into doing electronic alignment of the record/play electronics, or doing any frequency response checks across the audio band you’ll want something that it true rms measuring...and your meter is probably only relatively accurate around 400Hz, but if you are just tracing a 1kHz tone or something in order to troubleshoot, your meter will get the job done.

But do the module swap and report back...let us know what your soldering experience is.
 
Back
Top