Otari MX80 remote shenanigans

jpmorris

Tape Wolf
I am using an Otari CB140 remote unit with an MX80 24-track machine. It has been a little cantankerous, occasionally blowing the 2A internal fuse, especially if the unit was knocked. The other day, it threw a strop and now is blowing fuses continuously. I am still investigating, but I'm posting what I have discovered so far in case anyone else is interested or has a similar problem.

The CB140 takes power from the MX80 via the D-sub connector. Pin 1 is the chassis ground, pins 16 and 17 are an unregulated 24v DC supply (they are connected together). The other pins are 19 for the tach signal, plus a bunch of others to set up a balanced RS422 serial interface to the deck.

The CB140 takes the 24v supply and by some convoluted means converts it to a regulated 5v DC output to power the system logic. The guts of it is an STK772B chopper regulator of which there is very little information available, and almost the entire circuit board is taken up by support components. The regulator itself is attached to a massive slug of aluminium as a heatsink. The later output stages involve a triac, for reasons I do not comprehend.

The PSU board is thus:
DSC_5078-33.JPG

When we see the reverse side, things start to get ugly. Someone has been dicking around with it, cutting tracks off the board and attaching these strange globes, which I think may be some esoteric form of zener diode, but look like some kind of alien technology, or would if they glowed.

DSC_5085-33.JPG

I've replaced the electrolytics. The PSU board blows fuses even when disconnected from the motherboard which simplifies things a bit - going by the schematics the only other components after the fuse are the triac, a zener diode, and a mylar capacitor. I have sourced a replacement triac, but not the other suspect parts. In any case the board has some extra weirdness not on the schematics, and I have to wonder if the tacked-on globes or the cut traces are somehow shorting - the fact that it tended to blow fuses when knocked does make me wonder.

CB140_psu.png

At this point I'm starting to run out of 2A fuses as well as patience, so my current plan is to do away with the entire board, and replace it with a 5V DC-DC converter off ebay. As I write this I have actually managed to get the remote to boot and operate by patching a USB charger to the 5V line on the motherboard, but this is a complete bodge.

It may be of interest to note that if the remote is powered independently like this, with the remote booting up before the deck is powered, it will display Error 90, and then establish connection to the deck later once that has been powered up.

One thing that disturbs me is that using a continuity tester on the motherboard, the 5v and Ground test points appear to be shorted together. This persists unless CN5 is disconnected, which is the ribbon connector for the transport buttons on the remote control. Since I am more interested in recording my song than experimenting on the fragile innards of an already sick remote control I have left this disconnected for now, but I would be interested to know if anyone else with a CB140 can take the same measurements and let me know if that's normal (i.e. the Fluke is seeing ghosts).
 
Had you asked this a few weeks back, I could have turned around, unplugged my CB-140, opened it up...done. :)

Long story short....the CB-140 is packed away in storage and I'm back to using the original CB-124 that came with my deck...the CB-140 I picked up later, to have as a spare, and it had to get overhauled pretty heavily to get it fully functional.
It was working fine after the rebuild...I just wanted to bring the CB-124 back online since it was sitting in storage for a couple of years.
I didn't want it to get "stale". ;)

If you find you really need that one question answered (about the 5V and Ground shorting) before you can sort out your issues...let me know, and I will go dig the CB-140 out of storage, open it up, and see what I get with 5V and the Ground.
It's not a big deal...I just don't have it here in the studio at the moment. I don't think comparing the guts of the CB-124 would be of any value.
 
Yeah, if you get the chance, I would really appreciate that. And I am on the lookout for a spare remote, just in case. Since without one I am left with a tape player instead of a tape recorder.
 
Yeah, if you get the chance, I would really appreciate that. And I am on the lookout for a spare remote, just in case. Since without one I am left with a tape player instead of a tape recorder.

Hey...just getting back...I'll pull out the 140 today, and let you know what I get across the 5V and Ground.

That's the one thing I wish the MX-80 (and other pro-deck) would have provided the ability to "manually" engage the most basic I/O options at the deck too, and not just the remote...but I guess that would have added a lot more circuitry to the whole package, and a lot more cost.

Yeah...a spare remote is a must-have.
I had a 3rd spare at one point, a really beat up CB-124 that I found on eBay, but I promised it to the Otari tech as part of the rebuild he did on the 140 for me...not that it was part of the payment, I just offered it to him to have as a parts unit, figuring it would be a "pay-forward" if/when I ever needed his services again...and we developed a good rapport, though I haven't been in touch with him for a couple of years now.
One thing I learned from him...the remotes are really finicky to work on, and to get all "balanced" out so that all the functions work as expected. So I never wanted to try and fix my own, and preferred to keep a good rapport with him, just in case.
Much of the other stuff with the deck, I can deal with.

I'll have some info for you later on the 140....
 
Yeah, I'm glad it's just the power converter that's gone tits-up, debugging the logic is beyond my ability. (Curiously, the CPU has a manufacturing date of 1996 - looks like someone replaced it).

Truth be told I had a nasty scare when rigging the remote up to USB power. At first it did nothing at all, some of the lamps glowed and that was it. At one point a weird line appeared on the display. Then I realised that I'd swapped two of the ribbon cables over during earlier diagnostics. After that it was happy and I have been tracking with it today. (But without the transport buttons, I don't trust CN5, especially with no fuse!)

What would be really interesting would be attaching the MX80/remote combo to an RS422 analyser and reverse-engineering the protocol. That would allow for writing a software remote control, given a suitable USB-to-4-wire-RS422 dongle.
 
OK...here's the scoop...DO NOT mess with those funny looking jumpers/globes...my CB-140 has the exact same mods, so I am certain that was done by Otari during manufacturing to fix some issue. Maybe later models don't have it, or they all have it. :D
Considering that my CB-140 was a rebuild by an Otari tech, if that stuff was some weird hack done by a previous owner, the Otari tech would have sorted all that out...but it's all in place, and looks the same as yours....here's the pics:

Studio_CB140_June_16_2018_ (1).JPG

Studio_CB140_June_16_2018_ (2).JPG

AFA the +5V and Ground points off of CN1...they are not showing continuity. If I hold the tester for several seconds on the +5v and C10 + points, and juice up that cap...I'll get a continuity "beep" for a split second if I then move the probes to the 5V and Ground points, but it goes open instantly...so you should not have steady continuity between 5V and Ground.
I'm wondering if C10 is crapped on your board...that big 1000 uF 25V cap...???

The other thing I notice on your picture, is in the bottom right corner, with the comp side PCB right-side up...you have a large "glob" on that copper where the screw hole is. What does that connect with?
Mine is clean.
Of course, I doubt that's the issue, since your CB-140 was working, and then crap...so I think it's a component that went on you.

Hope that helps.
Good luck...and keep us posted. :)
 
Thanks. Definitely handy to know that that's a common mod - if only there was a schematic of the revised supply.

As for C10, that's of the caps I've replaced so I don't think that's it. C9 is a suspect but I don't have any suitable replacements. Interesting that you don't have the solder blob. Looking at it again, what they've done is scrape away the resist so that the blob links the ground plane to the chassis via the screw.

Just checking, did you measure continuity against the two test points in the bottom-right of the mainboard? Leaving the PSU aside (in fact, even with it disconnected) I get continuity between those two, but only while CN5 is connected. If you're not, I guess I'll have to pull the transport button PCB at some point and see what's happening.

I don't really want to open the remote at the moment while it's usable, so the underlying picture was borrowed from RevoxRemotes:

CB140_from_revoxremotes.png

Note how the first two ribbon cables cross over. They are not keyed, so if anyone out there is working on a CB140, be careful to ensure they're put back right. Mine have labels on them, be sure they match the number on the silkscreen.
 
Just checking, did you measure continuity against the two test points in the bottom-right of the mainboard? Leaving the PSU aside (in fact, even with it disconnected) I get continuity between those two, but only while CN5 is connected.

No...I only tested on the PSU, after I removed it. I thought that's what/where you meant about the 5V and Ground.
 
No...I only tested on the PSU, after I removed it. I thought that's what/where you meant about the 5V and Ground.

To be fair, the description was a bit rambling. I stayed up late to record while I knew the machine was working. But yes, there are two problems. Firstly, something is shorting in the power converter PCB itself, even when disconnected from the motherboard. But I'm not so concerned about that because I'm going to replace the PSU with a modern equivalent.

The second problem is that on the motherboard, even with the PSU disconnected, I appear to be seeing a dead short between the +5v test point and the 0v test point in the bottom-right. This only happens when CN5 - the transport control buttons - is plugged in. This is a bit more concerning because it would be nice to be able to use the transport controls on the remote. I don't know if this is a real problem (e.g. possibly the reason the PSU blew up in the first place) or merely a glitch on the fluke meter, like the capacitor effect you mentioned earlier.
If a working remote also does this, I know it's a glitch. If other remotes do NOT do this, it's probably not a good idea for me to reconnect CN5.
 
If a working remote also does this, I know it's a glitch. If other remotes do NOT do this, it's probably not a good idea for me to reconnect CN5.

Gimme about a 30-45 minutes...I'll pull out my 140 and check that. I originally assumed you were doing all the testing just on the PSU...and that's all I checked. :)
I'm just heading into the studio now...been watching the World Cup and doing other stuff today.
 
OK...this is not going to clear things up for you...well, maybe a bit.

With everything connected...I get a Short/continuity across +5V and Ground
With CN5 disconnected...I get an Open.

With CN5 connected and then removing one by one the three connectors on the PSU...I still get Short/Continuity for at each step....so it doesn't look like the PSU has any impact on the continuity when CN5 is connected.

I also tried it with both CN5 and the PSU disconnected, and it was Open, which I expected, since it's tied to the CN5 being connected or not.'

So...does that help you sort out why your 140 crapped out...mmm...probably not much, but at least it's not what you were thinking.

If it's not powering up and you're not getting any of the logic working...I'm still thinking it's something in the PSU...and that +5V and Ground thing on the other PCB and CN5 has nothing to do with it since mine does the same thing, and mine works OK. Then again, if you kinda replaced the PSU temporarily...things were still wonky, right?...but then you probably didn't have all the connections that are created by the three PSU connectors...plus, you have that "mod" that mod on the PSU to also consider.

You said you replace the big cap on the PSU...yes?...so why/when did you do that...have you had previous issues with this 140?

[EDIT]

You also said that the PSU fuse was blowing any time the 140 got knocked about...so could it be a loose/open connection somewhere that was shorting enough to blow the fuse, and then ended up in a permanent short with the fuses now blowing upon power up?
 
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OK...this is not going to clear things up for you...well, maybe a bit.
With everything connected...I get a Short/continuity across +5V and Ground
With CN5 disconnected...I get an Open.

With CN5 connected and then removing one by one the three connectors on the PSU...I still get Short/Continuity for at each step....so it doesn't look like the PSU has any impact on the continuity when CN5 is connected.

I also tried it with both CN5 and the PSU disconnected, and it was Open, which I expected, since it's tied to the CN5 being connected or not.'

So...does that help you sort out why your 140 crapped out...mmm...probably not much, but at least it's not what you were thinking.

Thanks, that's extremely helpful, since it implies it's normal behaviour (probably the Fluke being over-sensitive) and I can risk plugging the transport control panel back in without causing further damage. Right now the remote is running happily off USB, but because the fuse is on the PSU board (and the PSU board is sitting in the parts cupboard), the only short-circuit protection I have is whatever's built into the USB power unit. Just being cautious, really.

Right now it's working well enough to record on, but needs a more permanent fix since it's kind of held together with string, so to speak.

You said you replace the big cap on the PSU...yes?...so why/when did you do that...have you had previous issues with this 140?
You also said that the PSU fuse was blowing any time the 140 got knocked about...so could it be a loose/open connection somewhere that was shorting enough to blow the fuse, and then ended up in a permanent short with the fuses now blowing upon power up?

I replaced the electrolytics the other day to see if that helped the fuse-blowing problem at all, and also because they're several decades old. And yes, that would be my guess on the cause of the short, but I've so far been unable to track it down.
 
Can't help with the specifics but I have had quite a bit of experience with SMPSUs some years ago in TVs, VCRs and other kit. They ALWAYS have "afterthought" bits and mods on the back of the PCB! Most often CR 'snubbers' .

The modern digital test meter is a wonderful device (I have a much prized Fluke 83) but they can give weird readings, especially for continuity. Worth investing a few quid/$ on a 20k/volt fully analogue meter for this eventuality. "Avo" were dogs whatsits here but Simpson meters were pretty good.

Quite honestly, in your position I would buy a conservatively rated 'line lump' 5V SMPSU, crack it open and incorporate it in the chassis. You could look around for a 24> 5V SMPSU but the price might shock you.

Oh! And BTW,, the triac is a 'crowbar' protection device. You would not want 24V punching through to the 5V rail!

Dave.
 
The modern digital test meter is a wonderful device (I have a much prized Fluke 83) but they can give weird readings, especially for continuity. Worth investing a few quid/$ on a 20k/volt fully analogue meter for this eventuality. "Avo" were dogs whatsits here but Simpson meters were pretty good.

I may do that. Might ask what Dad is using these days, I remember him always using a Simpson when I was a kid.

Quite honestly, in your position I would buy a conservatively rated 'line lump' 5V SMPSU, crack it open and incorporate it in the chassis. You could look around for a 24> 5V SMPSU but the price might shock you.
I did wonder about that, to be honest - what a USB charger would do if it was fed 24V DC instead of 240V AC. Funnily enough there is no information about this online.

Oh! And BTW,, the triac is a 'crowbar' protection device. You would not want 24V punching through to the 5V rail!
Dave.

Ohh. That answers two questions. I've heard of them used for AC switching, but that's a new one on me. I was also wondering what I could do in terms of protecting the circuitry in case the DC-DC converter ever went rogue. Now I have something to look into.

EDIT: That also raises the question of whether the STK772B has gone bizarre and the fuses are blowing because the crowbar has kicked in.
 
A mains rated 5V SMPSU 'lump' will not work at 24V dc. I have had them work down to about 70V AC.

Crowbar circuits were common in PSUs years ago but I found THEY caused more trouble than they were worth due to spurious firing! Another option would be a nice efficient 60Hz 20VA toroid of about 8V and an L785S05CV linear regulator. You can go stupid with the bridge, 400V PIV+ and 15 amps, still cheap as chips and will never fail. 78 reg chips DO fail of course but extremely rarely and even there it is often due to penny-pinched heat sink design.

Dave.
 
EDIT: That also raises the question of whether the STK772B has gone bizarre and the fuses are blowing because the crowbar has kicked in.

Yeah...I was wondering too. There are some out there to be found:
STK772B - Google Search

AFA replacing the PSU...do you see any problems with rest of the connections. There seems to be a bit more there going on than just a 5V feed to the 140.
Do you see any other issues with that?
 
I am an old idiot! The feedback network R5/VR1. This is notorious for going high resistance/oc in such circuits (got one in 'our' HT pedals)

If VR1 wiper loses momentary contact..WOOOPS! Crowbar fires. Replace VR1 with a cermet trimmer and set Vout on a suitable load with the triac disconnected. Once set to 5V you can decide whether to put it back or not.

Dave.
 
Yeah...I was wondering too. There are some out there to be found:
STK772B - Google Search

AFA replacing the PSU...do you see any problems with rest of the connections. There seems to be a bit more there going on than just a 5V feed to the 140.
Do you see any other issues with that?

I assumed that at first, but having studied the schematics and got the remote working with the PSU board completely removed, no. Connection-wise, it does three things: Converts 24v to 5v for the logic board, grounds the chassis, and passes 24v back out to the external power connector on the back of the remote. I'm not even sure if there are any peripherals that can use the external power connector.

Literally everything else coming in from the D-sub port is routed around the power supply PCB with a cable loom, they go directly to the motherboard.

I am an old idiot! The feedback network R5/VR1. This is notorious for going high resistance/oc in such circuits (got one in 'our' HT pedals)
If VR1 wiper loses momentary contact..WOOOPS! Crowbar fires. Replace VR1 with a cermet trimmer and set Vout on a suitable load with the triac disconnected. Once set to 5V you can decide whether to put it back or not.
Dave.

That is a very good point. It might even explain how the unit seems to have blown fuses after being bumped while in operation. I'll have to take a look. I don't have any suitable replacements to hand, but it makes a lot of sense.
 
I assumed that at first, but having studied the schematics and got the remote working with the PSU board completely removed, no. Connection-wise, it does three things: Converts 24v to 5v for the logic board, grounds the chassis, and passes 24v back out to the external power connector on the back of the remote. I'm not even sure if there are any peripherals that can use the external power connector.

Literally everything else coming in from the D-sub port is routed around the power supply PCB with a cable loom, they go directly to the motherboard.



That is a very good point. It might even explain how the unit seems to have blown fuses after being bumped while in operation. I'll have to take a look. I don't have any suitable replacements to hand, but it makes a lot of sense.

Just short out the trimmer. The voltage will be low but you can 'tack in' various values until you hit 5V +-say 5%. In fact the only reason a trimmer was used I guess was to save this process in production? Much better to find a fixed value. Try 3 values, do a graph!

Dave.
 
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