Chip indentification

Hello all,

I am new year, and newly exploring this world. I trained as an electronic and communications engineer many decades ago, but then changed careers. I'm looking forward to learning a lot here, in an area, I have little knowledge of.

And now for my question. I have a piece of equipment, and it has a chip which has a number written on the udnerside, but after some considerable online research I have no idea what it is. I am assuming it is a microcontroller or EPROM, but am a little in the dark. Written on the chip is TAIWAN 35C D07S 7340034T.

I am looking forward to some further information, and participating further in this forum.

Jonathan

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Comments

  • Welcome to the forums, jonathanl64

    The better any closeup pictures you can get, showing at least a good view of the marked side, and, perhaps, any other outstanding view of the unknown chip as placed onto your piece of equipment, the best chances someone else could help you finding any useful information.

    Hope it helps

    Henrique
  • Welcome to the forums!
    Top AND bottom pictures would help.
  • What is the piece of equipment this chip serves?
  • Here is a top and bottom view of the chip, and a view of the circuit board. The function of this board is a stop driver for a pipe organ, sending power signals to the drawstops to move them in or out. It comes from an existing system which is now not functioning fully. The company no longer exists and the designer, who is elderly is not contactable.

    The lockdown here is allowing me rather more time than I expected to learn how this works, and also learn all about microprocessors and EPROMs. Always a bright side.

    Many thanks in advance.

    2448 x 3264 - 1M
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    2448 x 3264 - 2M
  • I would say if some of the draw stops are not working it's because of one of those 16 transistors and not that chip which doesn't look like it goes on that board.

    Mike
  • Hello all,
    I am assuming it is a microcontroller or EPROM, but am a little in the dark. Written on the chip is TAIWAN 35C D07S 7340034T.

    Unlikely to be a micro of any description, there are not enough support components around it. And not an EPROM as it has not window (pedantic I know). It might be a PROM. And I *think* I saw some PAL type devices in a 0.6" DIP, maybe from Signetics? Something from the PLS range?

    How about plain old-fashioned logic? A 74154 decoder or 74116 decoder?

    Perhaps you could trace out the connections between the unknown chip and anything else on the board. It might give a clue as to the likely pinout.

    The numbers on the bottom aren't going to help. They'll typically contain a date code, a code for the plant that made them and something to identify the batch.

  • iseries wrote: »
    I would say if some of the draw stops are not working it's because of one of those 16 transistors and not that chip which doesn't look like it goes on that board.

    Those have all been checked, the board is one of several, and all discrete components have been checked. The chips have been swapped over, and the only think not working is the one I need info on.

  • Unlikely to be a micro of any description, there are not enough support components around it. And not an EPROM as it has not window (pedantic I know). It might be a PROM. And I *think* I saw some PAL type devices in a 0.6" DIP, maybe from Signetics? Something from the PLS range?

    How about plain old-fashioned logic? A 74154 decoder or 74116 decoder?

    Perhaps you could trace out the connections between the unknown chip and anything else on the board. It might give a clue as to the likely pinout.

    The numbers on the bottom aren't going to help. They'll typically contain a date code, a code for the plant that made them and something to identify the batch.

    Thanks Brian, that is very informative. I have transcribed the board into a schematic to get some idea (it took a while.) My experience in the early '80's was mostly with discrete components and basic logic chips. So this makes me feel out of my depth.

    I will do some more research on what you have suggested, Your last point is very helpful.

    I can post the schematic.

  • I'd love to see a picture of the reverse side of that board, but based on what I see on the top, I see 4 possible contenders, in order of likelyhood:

    74x154 - 4 to 16-Line Decoder/Demultiplexer
    74x4067 - 16-to-1 Line Analog Multiplexer/Demultiplexer
    74x4514 - 4-Line To 16-Line Decoder/Latch
    74x4515 - Same as 74x4514, but inverting

    YMMV.
  • Hi
    actually a picture of the board like the earlier one but with a light behind it to show through the reverse side tracks might be illuminating informative.
    Dave
  • tritonium wrote: »
    Hi
    actually a picture of the board like the earlier one but with a light behind it to show through the reverse side tracks might be illuminating informative.
    Dave

    I'll work on that! Thanks.
  • JRoark wrote: »
    I'd love to see a picture of the reverse side of that board, but based on what I see on the top, I see 4 possible contenders, in order of likelyhood:

    74x154 - 4 to 16-Line Decoder/Demultiplexer
    74x4067 - 16-to-1 Line Analog Multiplexer/Demultiplexer
    74x4514 - 4-Line To 16-Line Decoder/Latch
    74x4515 - Same as 74x4514, but inverting

    YMMV.

    I will take a photo later, it is a complex board as you can see from the first picture!
  • The board is not all that complicated, particularly if you already have a schematic. Even if you don’t have a schematic it should not be too hard to figure out if the 24 pin chip is one of the 4-16 decoders JRoark suggested by using a meter to see what transistor is connected to what pin on the chip. If it turns out that it is not one of the 4 to 16 decoders there may still be a way to create a schematic and repair the board. That would be tedious and time consuming but far from impossible. From what I can see of the board traces it looks like it is divided into three sections, the 4 logic chips, a block of 16 similar transistor circuits, and another block of 8 similar transistor circuits.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 22,731
    edited 2020-04-05 - 18:56:28
    And not an EPROM as it has not window
    There are OTP EPROMs that don't have windows. Same silicon, but opaque plastic package instead of windowed ceramic. And I'll bet that's what it is.

    -Phil
  • Here is a photo of the back, and one held up to the light. Also the realisation of the schematic, which I have checked several times, so hopefully correct!

    3264 x 2448 - 2M
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  • My first guess it is a PLA, ROM or OTP ROM. The board would suggest it's not a high volume product so a custom chip is unlikely.

    The pinout does not conform to the 2708/2716/etc as those chips had the D0-7 on pins 9-11,13-17, so I would say it is not a standard ROM or OTP ROM, but more likely a PLA. Try tracing (in one color) the inputs on your schematic, and then the outputs (another color).

    Your biggest problem will be to determine the fuse map, particularly if the chip is in fact dead.

    FWIW Customers with reasonable volumes were able to get standard chips with custom part numbers marked on them. Singer did this with their min-computers of the 70's and there was a lookup table for the 7xxxx equivalents provided to maintenance engineers.
  • kwinnkwinn Posts: 8,599
    edited 2020-04-06 - 03:00:51
    Here is a photo of the back, and one held up to the light. Also the realisation of the schematic, which I have checked several times, so hopefully correct!

    I took a look at your schematic and would suggest replacing the CMOS chips starting with the two 74HC03's if you have not done already, and then the 74HC273 if the problem is still there. That board has some open collector signals that will make it difficult to to troubleshoot at best, and not knowing what U2 is only makes it harder. Also there is at least one mistake on the schematic (U3 pin10 connected to U4 pin5). Both are inputs and not connected to anything else. There may be more but if that is the only one then you have done a great job.

  • There are schematic errors. Suggest you recheck further before we spend more time on this.

    Underneath shows U2-20 and U3-2,13,5,10 and U4-2,13,5,10 connected together.
    C3 goes to U4-12, not U4-13.
    Don't know if there are any more.

    The BXZ79C are input clamp protection zener diodes. There will be a voltage value on them (probably 5V1 or 4V7).

    Now looking at the unknown U2 we can see that...
    U2 4-11, 13-17 are all inputs from U1/U3/U4 outputs
    U2 1-3, 19, 22, 23 are probably inputs from connector J3 15-20 outputs, because J3 10-14 outputs are inputs to U1.
    What is interesting is that connections to U2 around pins 18, 20, 21 appear to have track cuts, and also 23.
    U2-20 is an output which drives U3 2,5,10,13 & U4 2,5,10,13 which enables these OC NAND gates.
    The schematic shows U2-18,21 as no-connect.

    Under what input conditions U2-20 output is high is of course the great unknown. And of course there is feedback from the this via U3 & U4 outputs.

    My guess now is that this is a PAL.

    BTW If any of those transistors Q1-Q8 are base-emitter shorted, or the zeners D1-D8 are shorted, then it's possible that U2 has faulty input values and locks pin20 low. So U2 could still be good.

  • ElectrodudeElectrodude Posts: 1,427
    edited 2020-04-06 - 05:30:00
    (Edit: Cluso99 covered much of this in his post, and much more concisely, too.)

    I love pipe organs, but, unfortunately, all the broken pipe organs at churches I'm involved with have far worse problems than bad electronics, e.g. bad leathers and such. I got really excited when I saw this thread was about a pipe organ, and a part of one that I might be able to help fix! I'm sorry this post is so long, but I hope it helps.

    Could you briefly describe this organ to me? In particular, how many stop presets are there for the manual or division that this PCB belongs to? I'm guessing it has six presets (the six traces that go under U2 from below). What else should I know that might be relevant for this board? What do all of the connectors on this board go to - in particular, can you tell what each of the lines on the upper-right connector (J3) go to?

    What parts of this board do or don't work? If you manually set some stops and then push the cancel button, do all the stops turn off? Do any of the preset buttons do anything? (This board only deals with presets, right? It doesn't actually directly influence what ranks sound, does it?)


    I took your top and bottom images, mirrored the bottom one horizontally, and more or less aligned them. They are attached. You should be able to overlay and switch back and forth between the two images on your computer screen to more easily trace traces. I just used these instead of your schematic, since I found a lot of errors in your schematic and the wires were rather tangled.

    I found several errors in your schematic. Here are a few:

    Both legs of C12 are tied together. This doesn't make any sense, and I don't see any trace connecting them together on the PCB.

    The two traces that your schematic show as connecting to pins 22 and 23 of U2 actually connect to pins 20 and 21. Pin 8 of U2 connects to pin 2 of U1, but to nowhere else. The branch to the leftmost stop driver circuit actually connects to pin 17 of U2 and then to pin 8 of U3, which goes through a NAND gate and ultimately comes from the bottom-most stop input.

    Pins 2, 5, 10, and 13 of U3 and of U4 are all connected together on the bottom of the PCB, and also connected to pin 20 of mystery IC U2. I will call this line the preset-enable line, as driving it low seems to prepare the board for loading a preset. (@kwinn brought up some problems in your schematic with this line.) Since pin 13 is already connected somewhere on the bottom, and pin 12 isn't connected on the bottom, and so therefore presumably connects to something on the top, the trace you have connected to pin 13 of U4 is probably actually connected to pin 12. It doesn't make any sense to leave the input of a NAND gate floating, and all eight circuits are otherwise generally identical.


    Here's some of how I think the board works:

    Since this is a stop driver board, U2 must be some kind of memory that remembers stop presets. The connector on the top-right of the board seems to read eight stop switches. These eight active-high lines are gated by the eight open-drain NAND gates and then proceed to the solenoid driver circuits to automatically flip the stop switches. Between the outputs of the NAND gates and the inputs of the solenoid driver circuits, the eight lines also connect to eight pins of U2 (pins 17, 9, 16, 10, 15, 11, 14, and 13). U2 can read them to save a preset, or can drive them to set a preset.

    When the preset-enable line is held high, each of the eight NAND gates pulls down its output when its other input is high and floats it when the other input it low - an open-drain inverter. But when the preset-enable line is low, they all float their outputs, since they're open-drain. If pin 20 of U2 is an input, then U2 drives its outputs instead according to the selected preset. However, pin 20 of U2 could be an input, in which case U2 must determine some other way that it's time to load a preset. The preset-enable line also connects to the third to bottom terminal of the topmost connector group of eight on the left side of the board (pin 6 of J3).

    When a preset button (or the cancel button) is pressed, the preset-enable line is driven low (either externally or by U2), disabling the outputs of the NAND gates, and then U2 drives those lines instead. Presumably, a related signal, which I suspect is the bottom of the three big horizontal parallel traces in the middle of the board, driven by the transistor Q25 in the can package, is then triggered, causing each of the eight stop driver circuits (via a lot of diode logic that I didn't really try to figure out) to drive its stop according to the signal asserted by U2.

    If I knew more about external connections to the board, I could probably say something about why U1 is needed to latch some signals, or how U2 is told to store a preset. Are you an organist? What do you push to actually store a preset on an organ? If you don't know, I can ask an organist tomorrow.
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  • Cluso99Cluso99 Posts: 16,580
    edited 2020-04-06 - 06:37:52
    Interesting info Electrodude. I know nothing about pipe organs.

    As a further comment that I forgot...
    There are no bypass caps on the ICs, excepting three big orange 0.1uF 250V. And the one near the ICs seems like the +V track is broken??? Not a very good design :(

    But, there is light at the end of the tunnel. With Electrodudes input, and perhaps with a better understanding, the chip (or all of them) could be replaced with a prop (Propeller Microprocessor) and a 3V3 regulator :wink:
  • I think the schematic points to U2 being a standard pinout RAM, something like a 6116. Pin 20 on most early 24-pin chips of that type was /OE which appears o go off board along with pin 21 which was /WE (write). Likewise you'd expect to find the I/O on pins 9-17 and they go towards the transistors.
  • Cluso99Cluso99 Posts: 16,580
    edited 2020-04-06 - 08:52:34
    Brian, did you read my comments above. It looks like they are all inputs with only pin20 as an output.
    The circuit diagram is wrong as pin 20 enables all of the NAND gates in U3 and U4.
    Perhaps U2 is used to disable the organ in case of errors???
  • Cluso99 wrote: »
    Brian, did you read my comments above. It looks like they are all inputs with only pin20 as an output.
    The circuit diagram is wrong as pin 20 enables all of the NAND gates in U3 and U4.
    Perhaps U2 is used to disable the organ in case of errors???

    I did. But I am confused about the diagram. If I look at the photos that Electrodude tweaked, I can convince myself that pins 21 and 20 are connected to tracks on the top side of the board and that they go to the edge connector.

  • Cluso99 wrote: »
    My first guess it is a PLA, ROM or OTP ROM. The board would suggest it's not a high volume product so a custom chip is unlikely.

    The pinout does not conform to the 2708/2716/etc as those chips had the D0-7 on pins 9-11,13-17, so I would say it is not a standard ROM or OTP ROM, but more likely a PLA. Try tracing (in one color) the inputs on your schematic, and then the outputs (another color).

    Your biggest problem will be to determine the fuse map, particularly if the chip is in fact dead.

    FWIW Customers with reasonable volumes were able to get standard chips with custom part numbers marked on them. Singer did this with their min-computers of the 70's and there was a lookup table for the 7xxxx equivalents provided to maintenance engineers.

    Ok, that helps greatly, mainly by elimination, but it is step forward. There are multiple boards, and only one has failed, hence being able to check that this is the non-working chip.

    This would have been a small production run, so knowing it might be an off the shelf chip is also very helpful.

    Thanks.

  • kwinn wrote: »
    Here is a photo of the back, and one held up to the light. Also the realisation of the schematic, which I have checked several times, so hopefully correct!

    I took a look at your schematic and would suggest replacing the CMOS chips starting with the two 74HC03's if you have not done already, and then the 74HC273 if the problem is still there. That board has some open collector signals that will make it difficult to to troubleshoot at best, and not knowing what U2 is only makes it harder. Also there is at least one mistake on the schematic (U3 pin10 connected to U4 pin5). Both are inputs and not connected to anything else. There may be more but if that is the only one then you have done a great job.

    Excellent, thank you for spotting that, I must admit I was somewhat dazed after doing the schematic! I have amended it, rechecked the 74HC03's and made a few adjustments. There was more than one mistake, and still some more. I will check it all again in a bit. I am somewhat rusty, my electronics career ended in about 1985!
  • Cluso99 wrote: »
    There are schematic errors. Suggest you recheck further before we spend more time on this.

    Underneath shows U2-20 and U3-2,13,5,10 and U4-2,13,5,10 connected together.
    C3 goes to U4-12, not U4-13.
    Don't know if there are any more.

    The BXZ79C are input clamp protection zener diodes. There will be a voltage value on them (probably 5V1 or 4V7).

    Now looking at the unknown U2 we can see that...
    U2 4-11, 13-17 are all inputs from U1/U3/U4 outputs
    U2 1-3, 19, 22, 23 are probably inputs from connector J3 15-20 outputs, because J3 10-14 outputs are inputs to U1.
    What is interesting is that connections to U2 around pins 18, 20, 21 appear to have track cuts, and also 23.
    U2-20 is an output which drives U3 2,5,10,13 & U4 2,5,10,13 which enables these OC NAND gates.
    The schematic shows U2-18,21 as no-connect.

    Under what input conditions U2-20 output is high is of course the great unknown. And of course there is feedback from the this via U3 & U4 outputs.

    My guess now is that this is a PAL.

    BTW If any of those transistors Q1-Q8 are base-emitter shorted, or the zeners D1-D8 are shorted, then it's possible that U2 has faulty input values and locks pin20 low. So U2 could still be good.

    Thank you for this very useful information, and the corrections. I have made these and am now going to completely recheck the board. It is 30+ years since I did anything like this, so it is stretching the brain cells, actually building mechanical pipe organs is a lot easier!
  • To me, the 6 tracks that go under U2 bottom on topside (looking at underside shadows), 3 go to 1,2,3 and the other 3 are less clear but look like 23,22 and either 18 or 19.
    If you look at the schematics (which do have errors) it shows these 6 pins going to U2 pins 1,2,3,19,22,23 which concurs.

    It is clear that U2 20 goes to each gate in U3 and U4 as an enable. I don’t think 20 goes/comes from anywhere else. If this is so, then pin 20 is an output. Therefore IMHO it’s not a RAM/ROM chip. But see below for the alternative...

    Next schematic error...
    U2-2 does not go to Q1 base resistor. That comes from U2-17 and U3-8.
    Now this makes mores sense. U2 and U3 are quad 2 input oc nand gates, and their outputs (8 of them) go to U2 and the 8 base resistors of Q1...8. These pins going to U2 do in fact go to pins 9,10,11,13,14,15,16,17 which would be the data pins on a RAM/ROM.

    If you assume that its a ROM, then pin 20 would be the /OE pin which is an input. If this pin is connected to somewhere else that is driving it, then the ROM -or- the gates would be active, but not both. So this could work then.
    Therefore, we need to know positively if pin 20 goes anywhere else. Fortunately its easy to check with a meter as there is no solder resist on this pcb.
    Now if this is so, then next we need to know if pins 21 (/WE on a RAM or another /CE on an EEPROM) and 18(/CS) go anywhere. There were variations of these 3 pins ( including pin 20).
    However, if it’s a RAM/ROM/EEPROM/OTP then what is it’s purpose? Perhaps to play a tune. But the fact it is in parallel to the gates indicates that it should be possible to play manually by disabling U2.

    I don’t know what damage could be done to the organ if transistor drivers were switched on??? If it’s only possible that pipes could be played, then i suspect cutting the track from pin 20 and connect the U3 and U4 end to +5V might permit the organ to play.
  • Cluso99Cluso99 Posts: 16,580
    edited 2020-04-06 - 11:57:24
    Looking at those pics, particularly the underside, it looks to me that those 6 topside tracks entering under U2 from the bottom, 3 look like they go to pins 1,2,3 by looking a
    Cluso99 wrote: »
    My first guess it is a PLA, ROM or OTP ROM. The board would suggest it's not a high volume product so a custom chip is unlikely.

    The pinout does not conform to the 2708/2716/etc as those chips had the D0-7 on pins 9-11,13-17, so I would say it is not a standard ROM or OTP ROM, but more likely a PLA. Try tracing (in one color) the inputs on your schematic, and then the outputs (another color).

    Your biggest problem will be to determine the fuse map, particularly if the chip is in fact dead.

    FWIW Customers with reasonable volumes were able to get standard chips with custom part numbers marked on them. Singer did this with their min-computers of the 70's and there was a lookup table for the 7xxxx equivalents provided to maintenance engineers.

    Ok, that helps greatly, mainly by elimination, but it is step forward. There are multiple boards, and only one has failed, hence being able to check that this is the non-working chip.

    This would have been a small production run, so knowing it might be an off the shelf chip is also very helpful.

    Thanks.

    OK so you have a number of these pcbs and this one does not work.

    Can you explain the symptoms of what happens when you move the faulty pcb into a different position?
    Is it that you miss a group of notes being played?
    Are the tracks that we see on the pcb around U2 21, 20, 18, 12 and U1 3,4,7,8,10,11 and the orange cap top pin actually cut/missing? Or have they been cut to find the problem?

    Also I presume you cannot get easy access to any of these pcbs while the organ is being tested?

    If you are sure U2 is the problem, are you competent to desolder the chip? That will show us the tracks underneath.
    Since you quoted the part numbers came from the underside of the chip, you've obviously removed it from the pcb. Can you post a photo of the pcb with the chip removed. The photo looks like they may have sanded the part number from the chip.

    Is the organ controlled by a microprocessor, or is it just manually?
  • (Edit: Cluso99 covered much of this in his post, and much more concisely, too.)

    I love pipe organs, but, unfortunately, all the broken pipe organs at churches I'm involved with have far worse problems than bad electronics, e.g. bad leathers and such. I got really excited when I saw this thread was about a pipe organ, and a part of one that I might be able to help fix! I'm sorry this post is so long, but I hope it helps.

    Leatherwork is easy, as is working with the actual pipes. I am principally a woodworker (although was a professional organist and choir director in a former life!) My electronics background was up to the end of university when I changed course.

    Could you briefly describe this organ to me? In particular, how many stop presets are there for the manual or division that this PCB belongs to? I'm guessing it has six presets (the six traces that go under U2 from below). What else should I know that might be relevant for this board? What do all of the connectors on this board go to - in particular, can you tell what each of the lines on the upper-right connector (J3) go to?

    Some details, there is at least one board per division and four divisions, Pedal, Great, Swell and Choir. The Swell division has two boards as there are more than eight stops.

    There are also piston boards, which are separate. Six pistons per division and six generals. There is a setter and general cancel. The stop reversers are completely separate.

    The connections are as follows, J3 (top left) and J4 (bottom left) go into the sockets on the master board on the rack. J1 (top right) connections from the stop reed switches to read if the stop is on or off. J2 (bottom right) outputs to the magnets that drive the stops in and out.

    The level settings are through a bespoke board. Photo attached.

    What parts of this board do or don't work? If you manually set some stops and then push the cancel button, do all the stops turn off? Do any of the preset buttons do anything? (This board only deals with presets, right? It doesn't actually directly influence what ranks sound, does it?)

    I have checked the outputs and checked through the board. Eventually I swapped the chip U2 and the board then worked normally, hence my belief the U2 chip has failed on one of the boards. Now I cannot check specifically what is and isn't working as the unit is in the workshop, which I can access during the lockdown, but the church isn't. At present the swap enabled the failed board (great) to be swapped so the Choir division is off but everything else is working.

    The company who made the boards went out of business about twenty years ago, and the designer, who is well into his 80's as disappeared off the face of the earth, and may well have passed on.

    So this lockdown time is an opportunity to try to fix this issue!
    I took your top and bottom images, mirrored the bottom one horizontally, and more or less aligned them. They are attached. You should be able to overlay and switch back and forth between the two images on your computer screen to more easily trace traces. I just used these instead of your schematic, since I found a lot of errors in your schematic and the wires were rather tangled.

    I will try that, I'm sure it is easier than trying straight from the board. 30 odd years out of the game means I am very rusty. As for the schematic, it was a challenge to draw, and I agree needs tidying.

    I found several errors in your schematic. Here are a few:

    Both legs of C12 are tied together. This doesn't make any sense, and I don't see any trace connecting them together on the PCB.

    The two traces that your schematic show as connecting to pins 22 and 23 of U2 actually connect to pins 20 and 21. Pin 8 of U2 connects to pin 2 of U1, but to nowhere else. The branch to the leftmost stop driver circuit actually connects to pin 17 of U2 and then to pin 8 of U3, which goes through a NAND gate and ultimately comes from the bottom-most stop input.

    Pins 2, 5, 10, and 13 of U3 and of U4 are all connected together on the bottom of the PCB, and also connected to pin 20 of mystery IC U2. I will call this line the preset-enable line, as driving it low seems to prepare the board for loading a preset. (@kwinn brought up some problems in your schematic with this line.) Since pin 13 is already connected somewhere on the bottom, and pin 12 isn't connected on the bottom, and so therefore presumably connects to something on the top, the trace you have connected to pin 13 of U4 is probably actually connected to pin 12. It doesn't make any sense to leave the input of a NAND gate floating, and all eight circuits are otherwise generally identical.

    I am checking these through now.

    Here's some of how I think the board works:

    Since this is a stop driver board, U2 must be some kind of memory that remembers stop presets. The connector on the top-right of the board seems to read eight stop switches. These eight active-high lines are gated by the eight open-drain NAND gates and then proceed to the solenoid driver circuits to automatically flip the stop switches. Between the outputs of the NAND gates and the inputs of the solenoid driver circuits, the eight lines also connect to eight pins of U2 (pins 17, 9, 16, 10, 15, 11, 14, and 13). U2 can read them to save a preset, or can drive them to set a preset.

    When the preset-enable line is held high, each of the eight NAND gates pulls down its output when its other input is high and floats it when the other input it low - an open-drain inverter. But when the preset-enable line is low, they all float their outputs, since they're open-drain. If pin 20 of U2 is an input, then U2 drives its outputs instead according to the selected preset. However, pin 20 of U2 could be an input, in which case U2 must determine some other way that it's time to load a preset. The preset-enable line also connects to the third to bottom terminal of the topmost connector group of eight on the left side of the board (pin 6 of J3).

    When a preset button (or the cancel button) is pressed, the preset-enable line is driven low (either externally or by U2), disabling the outputs of the NAND gates, and then U2 drives those lines instead. Presumably, a related signal, which I suspect is the bottom of the three big horizontal parallel traces in the middle of the board, driven by the transistor Q25 in the can package, is then triggered, causing each of the eight stop driver circuits (via a lot of diode logic that I didn't really try to figure out) to drive its stop according to the signal asserted by U2.

    That is useful, I will try to get my brain around it!

    If I knew more about external connections to the board, I could probably say something about why U1 is needed to latch some signals, or how U2 is told to store a preset. Are you an organist? What do you push to actually store a preset on an organ? If you don't know, I can ask an organist tomorrow.
    Yes I am an organist, I am now based in France and play at a catholic church on a rather nice neo-Baroque entirely mechanical instrument, which I love, but we aren't allowed to travel more than 2km during the lockdown and the church is closed for the duration of the confinement. My company is based in the UK and I work all over the world, currently a good deal of time in West Africa.

    Thank you for all your input so far, it is much appreciated!

    3264 x 2448 - 1M
    3264 x 2448 - 2M
  • Cluso99 wrote: »
    Interesting info Electrodude. I know nothing about pipe organs.

    As a further comment that I forgot...
    There are no bypass caps on the ICs, excepting three big orange 0.1uF 250V. And the one near the ICs seems like the +V track is broken??? Not a very good design :(

    But, there is light at the end of the tunnel. With Electrodudes input, and perhaps with a better understanding, the chip (or all of them) could be replaced with a prop (Propeller Microprocessor) and a 3V3 regulator :wink:

    Thank you. Anything to improve it may be very valuable. A replacement system would cost thousands and is not within the church's funds.



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