Sequential Circuits Six-Trak MIDI OUT not talking

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Sequential Circuits Six-Trak MIDI OUT not talking

Postby cloudyskyle » Mon Oct 24, 2016 6:51 am

Hi there! I've got a Sequential Circuits Six-Trak with a MIDI out that isn't sending anything. I'm receiving MIDI data just fine. Just the MIDI OUT.

I've checked all the normal software configurations without any luck. Other synths are sending MIDI data with my same software configuration so I know it isn't something to do with software. I've even tried the "TRACK RECORD + 9" patch dump command without any signal response. I'm positive this isn't a software issue or an issue with connectivity.

I also checked out the service manual and started following traces, reflowing solder on major components dealing with MIDI (68850 UART, 7408 AND gate), but I don't know what I'm looking for with a multimeter to check to see what's wrong. I'm not sure if it's just my jack or what is happening. I'd love a little help or troubleshooting advice. I'm not very versed with schematics, but I am confident in my soldering/desoldering abilities. I'm ready to learn from anyone willing to provide a little insight. Thanks!
-keller
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby Rasputin » Tue Oct 25, 2016 2:50 pm

Haven't looked at the Six-Trak schematics but for MIDI out:

Pin 1 & 3 are not connected. Pin 2 is ground. Pin 4 should be +5V. Pin 5 should ultimately connect to the UART, but there are a few components that could be bad in between.

If you've got +5V and ground then try connecting a logic probe to the UART pin that connects to the MIDI out jack. Keep turning the mod wheel and see if that UART pin is pulsing. If it is then follow that upstream toward the MIDI out jack and see where the pulsing disappears.

Edit: Okay, got the schematics now, so there's one leg on 7408 (pin 6 on 7408) that connects to MIDI out (pin 5 on MIDI out jack), one (pin 4 on 7408) that connects to the UART (pin 6 on UART), and another pin (pin 5 on 7408). That last remaining pin should be +5V so check to make sure it's making it there. Pin 14 should be +5v as well (powers up the 7408), and pin 7 on 7408 should be ground, but if everything else on the Six-Trak seems to be working fine then 7408 is probably operating to some degree -- might as well double-check it though.

So to recap: Make sure there's continuity between 7408 pin 6 and MIDI out pin 5. Make sure 7408 pin 5 & 14 are +5v. Make sure there's continuity between 7408 pin 4 and UART pin 6. Make sure 7408 pin 7 is grounded.

If that all looks correct then look at pin 6 of the 68B50 UART. Make sure that UART pin 6 is sending a pulse out when MIDI Tx is active (notes being sent, etc.) If it is then you've got to make sure that the output pin of 7408 is sending out to the MIDI out jack in the same manner.

Side-note: Pins 1/2/3, 4/5/6, 8/9/10, and 11/12/13 on 7408 form four separate groups. Only three of those groups are actually in use. As mentioned before, 4/5/6 is for the MIDI out, but if the rest of 7408 is operating properly and there's a fault in 4/5/6 then you can reroute those pins like this:

Pin 8 to MIDI out, Pin 9 to +5V, and Pin 10 to UART pin 6.

That would save you having to replace 7408. See the diagram below for clarity.

Image
Last edited by Rasputin on Thu Oct 27, 2016 6:32 am, edited 1 time in total.
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby cloudyskyle » Wed Oct 26, 2016 3:28 am

Thank you for such a detailed response! If I get this thing working, I will definitely buy you a beer. I am beyond thankful for your help.

So far I've tested continuity and everything seems to be in order between MIDI OUT the 7408 and the UART.

7408 pin 6 is connected with pin 5 MIDI out and pin 6 of the UART. The 7408 is connected to ground via pin 7 and receiving 5V via pin 14.

I've misplaced my logic probe so I'll have to wait to get back to you on the pulse signal side of things.

I did try jumping UART pin 6 to MIDI pin 5 and still had no note signals. Not sure what that answered, but I tried it.

Supposing the 7408 is malfunctioning, if I try rerouting the pins with 8/9/10, do I need to break the traces from pins 4/5/6? Thanks!

*edit
I did notice that the 150 ohm resistor is cutting my voltage on the MIDI out jack (pin 4) down from 4.6V to about 3.9V. It looks like it is done on purpose. Does it matter?
-keller
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby Rasputin » Wed Oct 26, 2016 6:44 pm

cloudyskyle wrote:I did try jumping UART pin 6 to MIDI pin 5 and still had no note signals. Not sure what that answered, but I tried it.


The reason the 7408 AND gate is there is because MIDI is current (not voltage) based, and the UART pin 6 signal is too weak to drive the circuit by itself. Since the +5V at pin 5 on the AND gate is steady and constant, the input to 7408 pin 4 is the only thing that determines if pin 6 of the 7408 outputs a signal. In other words, it's an AND gate so both pin 5 *AND* pin 4 have to have a signal present before pin 6 turns on, but this is made a moot point by the steady +5V on pin 5. It's not really acting like a true AND gate because pin 5 isn't variable. Pin 4 is the only variable, and that connects to UART pin 6.

So the way it's wired up to the UART, 7408 really doesn't do anything "logical" (in a binary sense) to the signal. It's just an easy way to supply more current than the UART could do otherwise. Unless 7408 is broken or not being fed +5V on pin 5 (which would mean something is wrong) then UART pin 6 basically gets relayed straight to 7408 pin 6.

What I mean is: Jumping UART pin 6 to MIDI pin 5 is the correct signal path, just without the extra oomph it needs. So that's why your test wouldn't have worked anyway.

In theory, when MIDI is idle (no note events, etc.) the MIDI out pin 4 and 5 should present the exact same voltage. So I'd check to make sure that's the case. Unless there's data being sent, the UART Tx (and therefore 7408 pin 6) should always be stuck high, so MIDI out pin 5 should be stuck high as well. If there's no voltage at MIDI pin 5 then something is wrong.

Here's a little water metaphor:

The only time MIDI pin 5 should drop is when the UART pin 6 (Tx) goes low, causing 7408 pin 6 to go low, causing MIDI pin 5 to go low. This allows MIDI pin 4 (which is ALWAYS high @ +5V) to flow back downhill to MIDI pin 5. Because MIDI pin 5 is lower than MIDI pin 4 now, right? Keep in mind that the Six-Trak's MIDI out circuit is connected back to itself through whatever MIDI device you connect it to. When Six-Trak's pin 4 flows downhill to pin 5, the river flows through your external MIDI device, spinning the little hydroelectric dam in there which cause it to light up a lamppost. When Six-Trak starts and stops the flow of the river then the lamppost blinks on and off and then there's a dude on the opposite bank of the river who watches the pattern of the lamppost (the opto-coupler in the external MIDI device) and says "Hey guys, Six-Trak is signalling the notes to play!" This guy isn't even in the river, he doesn't ever get wet, he's just watching the pretty show.

The MIDI in of an external MIDI device is actually more part of the Six-Trak's circuitry than it is part of the external MIDI device (well, the initial portion, at least). The external MIDI device just monitors what the Six-Trak is signalling from a safe distance.
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby Rasputin » Wed Oct 26, 2016 7:02 pm

cloudyskyle wrote:Supposing the 7408 is malfunctioning, if I try rerouting the pins with 8/9/10, do I need to break the traces from pins 4/5/6? Thanks!


7408 should only be bad if you measure (at MIDI idle) a high on UART pin 6, a high on 7408 pin 4 & 5, and low on 7408 pin 6.

Or if 7408 pin 6 is always stuck high even when you measure a low on 7408 pin 4 and a high on 7408 pin 5.

To simplify: 7408 pin 5 really only needs to be measured once. It's either working or it isn't, and I believe you already verified that, so you want to watch for a discrepancy between 7408 pin 4 and 6.

If UART pin 6 is always stuck either high or low (despite MIDI traffic) then you've got a more significant issue. Finding something stuck low should be a dead give away, but it's harder to catch something stuck high even when it's supposed to be dropping low on occasion.
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby Rasputin » Thu Oct 27, 2016 6:23 am

I've been thinking about this more:

The UART has to be operating to a large degree if received data is working. That means power/ground are activating the chip, the data lines work, and the UART interrupt line is sending clock out to the flip-flop (74LS74). Since the flip-flop returns the clock back to TxC/RxC on the UART via the same pin then the flip-flop is at least outputting the clock and it's at least reaching the UART on the RxC pin. That means TxC (UART pin 4) could be disconnected or fried. I'm not sure if those pins are bridged or are separate traces somehow, so you might want to check that out. If they're separate traces then inspect the TxC trace carefully and check continuity between TxC and 74LS74 pin 12.

UART pins 4 & 6 are the ones that are most specific for Tx traffic. DCD, RTS, CTS, IRQ, +5V, GND and the data bus are all common to both Tx and Rx, so they should be good if receiving data works.

UART pin 6 is the one we've been looking the most at, so you definitely want to check that thoroughly. Getting a logic probe on the UART would be a good idea. I think both TxC/Tx should be pulsing when there's MIDI out traffic being sent.

The other idea is that the decoupling cap (probably a little ceramic disc by the +5V line on the UART) is bad. That could cause erratic performance, but one would think both Rx/Tx would be flaky or intermittent if that was the case.

Does received data ALWAYS work?

I'm hoping it's either 7408 or the decoupling cap. The Motorola 68B50 is only like $4, but an annoying 24 pins of delicate desoldering/soldering on a 30 year old PCB.
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby cloudyskyle » Fri Oct 28, 2016 8:00 am

Rasputin wrote:In theory, when MIDI is idle (no note events, etc.) the MIDI out pin 4 and 5 should present the exact same voltage. So I'd check to make sure that's the case.


My MIDI OUT pin 4 is registering at 3.9V due to 150ohm resistor ahead of the jack. MIDI OUT pin 5 is giving me 1.4V whether or not I trigger note events. Thus it's the same 1.4V I get from 7408 pin 6.

Rasputin wrote:Does received data ALWAYS work?


Received data works flawlessly. I've never even had any intermittent problems. Good sign?

Rasputin wrote:I'm hoping it's either 7408 or the decoupling cap. The Motorola 68B50 is only like $4, but an annoying 24 pins of delicate desoldering/soldering on a 30 year old PCB.


I'm hoping so, too. I have a new logic probe on order and I should be able to have it checked by Monday.

Thanks again for all of your help. This has been incredibly educational. Makes me less afraid to tackle larger projects.
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby madtheory » Fri Oct 28, 2016 3:51 pm

Rasputin, that is excellently written. This thread should be a sticky on old school serial interface trouble shooting.
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby Rasputin » Fri Oct 28, 2016 6:20 pm

cloudyskyle wrote:
Rasputin wrote:In theory, when MIDI is idle (no note events, etc.) the MIDI out pin 4 and 5 should present the exact same voltage. So I'd check to make sure that's the case.


My MIDI OUT pin 4 is registering at 3.9V due to 150ohm resistor ahead of the jack. MIDI OUT pin 5 is giving me 1.4V whether or not I trigger note events. Thus it's the same 1.4V I get from 7408 pin 6.

Rasputin wrote:Does received data ALWAYS work?


Received data works flawlessly. I've never even had any intermittent problems. Good sign?


If you're only getting 1.4V on pin 5 then I'd say that's the source of your problems. Seems like UART pin 6 might be floating. Is UART pin 6 the same voltage as 7408 pin 6? If it is then the UART is likely the problem. If UART pin 6 is 5V at MIDI idle and 7408 pin 6 is 1.4V then 7408 pin 6 is probably what's floating.

Really, 7408 pin 4/5/6 should all be approximately +5V at MIDI idle. If one of the pin voltages differs then that will pinpoint our problem.

Edit: Actually, thinking about this more clearly, if UART pin 6 was floating then 7408 pin 4 would either respond as on or off. If the 7408 saw it as an off then 7408 pin 6 should be 0V, and if it saw it as an on then 7408 pin 6 should be +5V. Since MIDI pin 5 is 1.4V then 7408 pin 6 should be 1.4V too which is an indeterminate output. The 7408 is almost certainly bad, but go ahead and check the voltages at 7408 pins 4/5/6 just to make sure I'm not missing something.

I'm sure you'll already understand this since you're familiar with logic probes, but for anyone following along: A voltage of 1.4V in a digital logic circuit is indicative of a problem because it's nonsense data, basically. It's not low enough to be a 0 and it's not high enough to be a 1. It's "floating" because it's in an indeterminate state. It's neither here nor there. Usually a 0 has to be 0.8V or under (ideally exactly 0V) and a 1 should be +2.0V at a bare minimum (ideally +5V [or sometimes only +3.3V]). In the real world, there's a little noise and margin for error, so a logic high might only be around 4.6V, but that's close enough.

So that's why 1.4V is a big red flag.

If Rx data always works then it's probably not the decoupling cap. I doubt the UART would be erratic in such a uni-lateral fashion, although I'm sure stranger things have happened.
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby Rasputin » Sat Oct 29, 2016 4:42 am

madtheory wrote:Rasputin, that is excellently written. This thread should be a sticky on old school serial interface trouble shooting.


Thank you for the praise. I'm a firm believer that anyone with an average intelligence or higher can eventually understand any concept when it's broken down into its fundamentals. I mean, that's how I get through my own life. Some of the stuff I always believed was super complicated and way out of reach turned out to actually be pretty simple once laid out in bite-sized chunks.

If nothing else, the MIDI circuit in the Six-Trak is (almost) identical to all the other early SCI stuff. It's pretty much 1:1 with the Prophet 600 and scarcely different from most other Sequential Circuit stuff of that vintage--the Drumtraks uses a 68A50 UART, for instance, but if you can fix one then you have a fair chance at fixing them all.

Another example is the Prophet 2000 and Prophet VS which have the UART Tx go to the base of a transistor instead of an AND gate (as the Prophet 600/Six-Trak does) but that's about the only difference (well, a MIDI Thru as well, but that works like MIDI Out).

Speaking of, in a case like the broken Six-Trak in this post: If the AND gate actually is bad (and you don't want to use the spare input) you could likely hook the UART Tx to the base of a PNP transistor, ground to the collector, and MIDI Out pin 5 to the emitter. Oh, and add a current limiting resistor between the Tx pin and transistor base pin too.
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby Rasputin » Sat Oct 29, 2016 7:53 am

Been thinking about this even more. Since 7408 is really suspect and you have a DMM but not a logic probe at the moment, consider this:

Set your meter to diode testing. With the synth power off, put your red probe on 7408 pin 7 (ground). Yes, red to ground, that's not a mistake.

Now touch the black probe to 7408 pin 4, 5 and 6 to see if they act like good diodes (0.4 - 0.6V or some such). If any of the three pins are short or open then you probably just found your fault.

7408 pin 6 is really what we want to focus on. I would not be surprised for the diode test on that to show open.
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby cloudyskyle » Mon Oct 31, 2016 5:25 am

Looks like I owe you a beer Rasputin! Thank you for your help and education. That, to me, is far more useful than taking this thing in to get serviced. I sincerely appreciate you.

The logic probe came in, and honestly, I think I ordered a faulty one because everything I was measuring was showing me only high with no pulse. I tried the diode test on my DMM, though, and sure enough pin 6 on the 7804 was open. I replaced the 7804 this evening, fired it up and my 5V was back. Now she sings. Thanks again! PM me your paypal email so I can send you a little something for your troubles.
-keller
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby cloudyskyle » Mon Oct 31, 2016 5:40 am

madtheory wrote:Rasputin, that is excellently written. This thread should be a sticky on old school serial interface trouble shooting.

I second the sticky. Rasputin provided some very useful information ultimately leading to a repaired synthesizer.

I found absolutely no results for troubleshooting MIDI output hardware/circuitry before posing here. All that turned up in my searches were an overwhelming number people asking how to get their MIDI controllers to work with their DAWs :facepalm:
-keller
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Re: Sequential Circuits Six-Trak MIDI OUT not talking

Postby Rasputin » Mon Oct 31, 2016 8:20 pm

cloudyskyle wrote:The logic probe came in, and honestly, I think I ordered a faulty one because everything I was measuring was showing me only high with no pulse. I tried the diode test on my DMM, though, and sure enough pin 6 on the 7804 was open. I replaced the 7804 this evening, fired it up and my 5V was back. Now she sings.


That's wonderful! :keys1:

Glad my yammering and a 75 cent part fixed your gear. It's quite a thrill and a relief when that happens, isn't it?
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