VFD display problem (PFAFF 1473 CD) (FIXED!) - Page 1
Forum section: electronics / repair

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 22, 2023, 04:23:53 am
content: Another repair project. This PFAFF sewing machine (1473 CD) seems working, except that the VFD display shows all 'A', which is incorrect (the photo captured not showing all 'A' is due to the refresh rate). The VFD module consists of 16 characters, 12 segments each. This is the behaviour when powered up without any human input. There is another display mode by pressing a button ("display"). In this mode, it's better - showing what's supposed to be shown except for the first 4 characters - again all 'A's, which is incorrect.The VFD module is on a separate daughterboard along with buttons and LEDs (it's invisible, but most likely there is a driver IC under it). This board communicates with the motherboard through a 12-wire flat cable.After a close look, there are residuals of battery leak around the battery holder, causing corrosion (left-hand side of the motherboard). Broken traces of pins due to corrosion were not found. But it was likely the oxides might short-circuit (to some degree) adjacent traces or pins. I gave the affected area a careful clean-up with vinegar, IPA, and flowing water, then dried it thoroughly. However magic did not happen - no improvement.What do you think most likely the culprit might be? Thanks.[Edit: By the way, this machine is beautifully designed and made (in West Germany). My hat's off to PFAFF (not existing since 1999)!]

author: cruff
author_rank: Regular Contributor
timestamp: December 22, 2023, 04:55:18 am
content: The solder connections of the VFD itself look messy, have you cleaned those up and reflowed them?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 22, 2023, 05:01:55 am
content: Thanks. I checked closely. Despite looking messy, it must be residuals of original flux, without any signs of defect. So I didn't reflow them.From the condition of the thing, the problem probably is not on the daughterboard. Everything looks tidy except the mess caused by battery leakage.My guess is that these messy-looking solder points were all manually done in the factory. These are not signs of someone attempting to repair something after it leaving the factory.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 22, 2023, 05:53:17 am
content: There are a few 22uF 6.3V electrolyte caps across the 5V rail and presumably GND, for logic chips. One next step might be to replace all of these (but I need to order new ones). Are they likely to be the cause of this behaviour though? The 5V rail is okay.Since there are only 12 wires between the two boards (which include at least GND, filament voltage, VFD grid voltage), clearly there is a VFD controller under it, which is invisible (and I don't intend to get into it by risking breaking it). I will need to look into the common VFD controller to find out the communication protocols between it and the MCU(s) on the main board, and how this kind of problem can possibly happen. I suspect (assume, or hope) the display side is okay.Also, another look at the back of the small board shows that there are clearly three chips under the VFD module, two 16-pins and one 40-pin. At least one of these (probably 16-pin) must be a driver, the big one might be another MCU?Is there any 16-pin VFD driver IC popping up in your mind?

author: cruff
author_rank: Regular Contributor
timestamp: December 22, 2023, 02:05:14 pm
content: The VFD can be removed using a desoldering tool with care, it's obviously a tedious task but reasonably safe. The only VFD that I've replaced had on-board driver chips and used a command/data interface to interface to the remainder of the system. I'm not familiar with the types of high voltage driver chips used for VFDs.If your device uses an off-VFD driver chip, then higher voltages will obviously be present and contamination on the circuit board could potentially cause this symptom. Easy enough to clean up the board and see if it makes a difference.

author: inse
author_rank: Super Contributor
timestamp: December 22, 2023, 03:09:18 pm
content: At first, I would check the signals on the interface cable if there is any sign of life when you would expect the display to change.If so, the suspicion narrows down to the actual display control.Without removing the VFD tube you won’t be able to inspect what is hidden beneath.The communication interface can be anything from 4/8 bit parallel to SPI or even UART.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 23, 2023, 07:16:07 am
content: Quote from: cruff on December 22, 2023, 02:05:14 pmThis would be the last thing I would like to do and only if absolutely necessary. After all the machine seems to be working well, only with a display problem.Quote from: cruff on December 22, 2023, 02:05:14 pmThere is 32V DC voltage present on the board, supplied through the black wire of the interface cable. On the photo of the back of the board, it's the thin trace on the left edge of the board. It looks like the designer discreetly minimised its interference with other signals.Quote from: inse on December 22, 2023, 03:09:18 pmGood point. Having had another look at the interface cable, there are several wires the functions of which I now know. Starting from the Black (1): 1: 32V DC (this should be for the VFD anode); 2: 5V - this is probably for one or more logic IC on this board (under the VFD); 3: GND; 7: NC; 11/12: 4.5V AC for filament. There are only 6 wires left for signals.Probed some of these 6 wires with a scope, but didn't get a clue yet.Not I think the problem is very likely with those chips underneath. It's unlikely for the detail of display segments to be determined by the MCU(s) on the main board and signal transferred through this 12-pin cable. Very unlikely. And it's very likely the 40-pin IC is an MCU for the VFD (and potentially for the buttons and LEDs). Now my guess is that the two 16-pin's are logic ICs. The culprit is probably something around them or one of these two ICs themselves.It looks like now I have a hard decision to make, whether to desolder the VFD module, risking making things worse. (This would not be a hard one if I had a desoldering station that can suck when the head is on heating, such as the one used by Mr Carlson's Lab - this desoldering job would be a piece of cake!)But among the things that should be tried before this move, I will replace all the 22uF 6.3V electrolyte caps (and another 1uF 63V) first.

author: picburner
author_rank: Frequent Contributor
timestamp: December 23, 2023, 12:12:25 pm
content: The DIP-40 under the VFD is most likely the vfd driver, it could be an SN75518 or HV518 or similar. If it had partially failed it could very well highlight the defect shown in the pictures.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 23, 2023, 07:57:02 pm
content: Quote from: picburner on December 23, 2023, 12:12:25 pmThank you. This is a plausible theory. Both of these drivers have a 32-bit register, which looks like something that suits this VFD module very well. What the other DIP-16 ICs (or their function) do you think are likely to be?I'm now considering buying a desoldering station for this job because I really don't want to ruin this machine (and if possible get it fully funcitoning), which I value highly. I saw once in Mr Carlson's repair videos a nice kit, with which this job is just easy as. Does anyone have a recommendation on this?

author: inse
author_rank: Super Contributor
timestamp: December 23, 2023, 08:56:00 pm
content: You don’t need to buy a desoldering station for just one job.A good quality desoldering pump, fresh (leaded) solder and some extra flux will do the job if supported by a decent soldering iron.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 23, 2023, 10:21:25 pm
content: Quote from: inse on December 23, 2023, 08:56:00 pmNot just for one job of course. It looks like only for one, but the fact is that often one job triggers the idea of buying some tool that has been on the wish list for long and just been waiting for a proper/final justification, and surely will be useful later on. Another thing is that I’m hopelessly pedantic!

author: inse
author_rank: Super Contributor
timestamp: December 24, 2023, 04:24:50 am
content: Thats an acceptable answer

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 24, 2023, 09:08:00 am
content: Have done a bit more trace tracking and got this table for the 12-wire interface cable. The DIP-40 on the display board is almost certainly a VFD driver, such as SN65518 or HV518.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 26, 2023, 01:56:21 am
content: All electrolytic capacitors are replaced (except the big ones in the power supply stage). The old ones do not look too bad (or not bad at all, which is surprising). Unfortunately, no miracle happened. The same.The next step is probably to get a proper desoldering tool and tackle the bigass VFD module. One problem with that though, is that when it is off, there is no display available as a handy indicator of what the problem is, or when it's back working properly, without soldering it back. So I guess before doing the big job, I need to do a bit more in understanding what voltages it should give on some critical pins/pads, if it is working properly.

author: sarahMCML
author_rank: Regular Contributor
timestamp: December 26, 2023, 02:22:06 am
content: Above the green connecting strip at the bottom of the first pic, some way up is a thick trace. Just above this, in the middle are a pair of solder joints, one of which looks to be barely, if at all, soldered. There's another similar poor joint over to its left which needs re-doing as well!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 26, 2023, 03:34:18 am
content: Quote from: sarahMCML on December 26, 2023, 02:22:06 amThanks for your spotting. I will have another close look at these, but very likely they are good. These messy-looking joints I strongly believe were done manually at the factory. There are the LEDs that stand up and fit with the right height in the holes in the face panel. This machine was made in West Germany so before 1989. Since they are only LEDs, even if they were out, normally it would not affect the rest of the circuit.One obvious thing to take note of before taking off the VFD module is perhaps the voltages on its 32 pins in the middle. If replacement of any of the chips under it is necessary and when it’s done, I can check if there is any change to these voltages. If there are, it’s likely fixed and I can proceed with soldering the VFD back.I had another look at these soldering points of the LEDs. Despite looking messy in the photo, they are perfectly fine (just because they were done manually with an abundance of natural rosin as flux. Its residue is not harmful.)

author: picburner
author_rank: Frequent Contributor
timestamp: December 26, 2023, 10:24:59 am
content: If you have a scope you can check the problem with the help of the SN75518 datasheet.I reported on your pic the position of the main pins and their function (at least what I can understand based on the pic).If it was indeed the DIP-40 that was faulty (and I think so) and you have no experience with a desoldering station, I advise you to gain experience on another faulty PCB first.Try desoldering many components without breaking them or damaging the PCB before moving on to desoldering a delicate glass VFD!

author: inse
author_rank: Super Contributor
timestamp: December 26, 2023, 09:58:23 pm
content: I would not be so much afraid of the VFD, it has long legs, so thermal stress shouldn’t be a problem.If all pins are properly desoldered and are moving free in their holes, the display should be easily removable.It will come off with that plastic holder, just make sure to unclip all clips, sometimes plastic feet will be deformed by wave soldering.Edit: should not be a problem as the display is hand soldered, obviously.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 26, 2023, 10:24:05 pm
content: Quote from: inse on December 26, 2023, 09:58:23 pmI believe this is 100% true, the same for the stand-off LED's.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 26, 2023, 10:26:35 pm
content: Quote from: picburner on December 26, 2023, 10:24:59 amThat's sensible advice, and was exactly my worry -- I didn't want to make it worse! After all the machine is now working except only the display (or one of the two display modes - or a little bit more than that), and it's a beautiful machine!Yes, I need to look a bit closer at what's going on around the DIP-40, which is almost certainly an SN75518 or similar. Only if I'm sure that the problem is indeed with it, or the DIP-16's, should I start to remove the delicate display module.[And obviously, I need to source a replacement chip before doing that. It does not seem easily available now (the DIP version)!]My soldering skill is not too bad, desoldering not as good.

author: inse
author_rank: Super Contributor
timestamp: December 28, 2023, 06:07:55 am
content: Once the VFD is removed you could solder in a socket terminal strip or a IC socket split in half for debugging.

author: jchw4
author_rank: Regular Contributor
timestamp: December 28, 2023, 07:33:52 am
content: Quote from: max.wwwang on December 26, 2023, 10:26:35 pmI would strongly recommend buying a dedicated tool for that purpose.HakkoFR-401FR-301 is a bit expensive but it's worth having. If you damage the glass the cost of repair will easily outweight the price of the pump.There are many alternatives too, you just need to have this kind of tool even if you use it for this project only and sell later.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 28, 2023, 08:09:43 am
content: Quote from: inse on December 28, 2023, 06:07:55 amGood idea. Thanks.Quote from: jchw4 on December 28, 2023, 07:33:52 amVery true.Quote from: jchw4 on December 28, 2023, 07:33:52 amThanks. I was considering Hakko 301 and was not aware of the FR401. Yes, buy one, use for one project and sell is also a possibility. Just keep your mind open!

author: jchw4
author_rank: Regular Contributor
timestamp: December 28, 2023, 08:15:31 am
content: Quote from: max.wwwang on December 28, 2023, 08:09:43 amOuch! You are right. 301. Or any other soldering pump.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 28, 2023, 08:18:56 am
content: Had a little bit more gazing at the thing and got the connectivity of the VFD and the (presumably) SN65518 (mounted upside down). VFD pin # counting from left to right, ignoring the outmost pair. Segment pinout is yet to be figured out.One observation. There are 16 letters in the display, each with 17 segments. Based on my understanding of the workings of VFD so far, there will need to be at least 33 pins, other than the 2 for filament, if we want to make full use of the segments. There are only 32 pins in the middle. The answer I have is that probably one segment is never used (which is strange)? Another possibility is that, there are two segments that are only visually separate but logically as one combined (i.e. can only be on or off at the same time).Also probed some legs of the VFD with a scope. Shown is for the 4th leg (for one segment, which is unknown yet). As I said, there are two display modes, let's say mode 1 and 2. 1 is when you don't do anything whereas 2 is when the "display button" is pressed and held. In mode 1 all letters shown are "A"; in mode 2 what's shown is in photo. Another two photos of the scope for the two modes, of leg 4. The scope setting is 0.2ms/div and 1V/div.Does this give any clue of what's going on and what's going wrong? Any insights are appreciated.My guess was, since all letters are "A" in mode 1, the segment lines do not even need to have pulses - they can stay either constant high or low. My guess was apparently wrong, because the scope does not give me a horizontal straight line.

author: inse
author_rank: Super Contributor
timestamp: December 28, 2023, 08:27:20 am
content: As per photo, the driver is able to address separate segments and separate digits, I would tend more to suspect that the uC decoding the display data has got corrupted (degraded (E)PROM)Or is receiving garbage from the mainboard.Which would be hard to fix in both cases unless you are willing to reverse engineer the firmware.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 28, 2023, 08:54:31 am
content: Quote from: inse on December 28, 2023, 08:27:20 amThat's likely, because the RAM/ROM chips are close to the area affected by battery leak. That's worrying. But for now, at least I would like to confirm the display board is working just fine, or otherwise. Reverse engineering the firmware can be a possibility (if ever possible), only a bit distant for now.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 28, 2023, 09:04:07 am
content: Just to digress, as I said earlier, this machine was (beautifully) made in West Germany.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 28, 2023, 11:42:27 pm
content: Thinking of reverse engineering the firmware again. It's almost impossible, unless I purchase another working one and clone its content, or its dump is available somewhere, because I only know what's shown is not right (because not making sense) but don't know what should be shown!Anyway, have I now catched the tail of a bad guy? I'm probing the clock signal going into NS65118 (pin 21, coming from pin 11 of one of the two uCs, MAB8031, which TxD/P3.1 serial output), which is obviously critical, regardless whether the input data is correct or is garbage. My question here is, why does it look so messy? I think it should be very close to a square waveform? Thoughts?The scope setting is 0.5us/div and 1V/div. So the clock signal is about 1MHz with p2p voltage 4V-ish.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 29, 2023, 12:27:59 am
content: Caught another guy that is certainly bad, the closest to the battery leak affected area, that is the T74LS03B1 quad 2-input NAND gate. When probing its 1/2/3 pins, I got all constant HI (5V), which is surely wrong - with 1(1A) and 2(1B) both HI, 3(1Y) as NAND(1A, 1B) surely should be LOW (0V)!This is an easy fix. Hopefully this is all that's wrong! Anyway, eventually I've caught something!My question in the above post remains to be answered.

author: inse
author_rank: Super Contributor
timestamp: December 29, 2023, 02:52:30 am
content: Regrading the clock signal:Did you compensate the probe capacitance?Make sure to use a good, short ground connection for the probe.The signal doesn’t look nice but might be acceptable for the receiver depending on threshold and setup/hold timing.The supply voltage quality at the uC might also be worth checking.What is the function of the suspicious NAND gate?I had a look at the mainboard photo and was wondering why they used two microcontrollers.The LS03 is the one close to the RAM chip?Probably used to interface it to the 8051/8031 data bus.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 29, 2023, 04:24:14 am
content: Quote from: inse on December 29, 2023, 02:52:30 amNo, I didn't make any compensation. What is it about?That's probably the reason of the signal looking strange; the ground lead is long and through another wire.Quote from: inse on December 29, 2023, 02:52:30 amTrue.Quote from: inse on December 29, 2023, 02:52:30 amVery clean 5V DC.Quote from: inse on December 29, 2023, 02:52:30 amNot easy to tell. Tracking the trace is very difficult. Many turn sides via through holes under the chips, as if the designer made an effort to make it difficult for reverse engineering.Yes, it's very intriguing that two uC's of the same family, one 8051 the other 8031, are used side by side here.Quote from: inse on December 29, 2023, 02:52:30 amYes, another photo added. It's the top left dirty one.

author: inse
author_rank: Super Contributor
timestamp: December 29, 2023, 06:43:34 am
content: Probe compensation matches the probe impedance to the scope input impedance.Decent scopes have a square wave signal available at the front panel and the probe has a tiny trimmer to adjust it to make the signal look as good as possible like a square wave.I notice the fine traces, make sure none of them has been interrupted by the corrosion you mentioned.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 29, 2023, 07:58:11 am
content: Quote from: inse on December 29, 2023, 06:43:34 amMy scope does come with a test signal terminal and the probe has a small trim screw for adjusting the overshoot of the test square waveform.Yes, interruption of traces by corrosion was my concern. I have closely inspected but have not noticed one.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 29, 2023, 11:39:34 pm
content: More investigation when waiting for the part to arrive (it will take a good time). Fortunately, this is much easier than having to deal with the chips under the display module.I know that 3(1Y) of the NAND gate chip is erroneous. This also goes to 5(2B) of another gate, so 6(2Y) is ruined, which goes to the RAM chip's (SRM2064) 27(!WE, write enable). It is constant LOW when testing, i.e. write enabled all the time. I guess garbage information is probably written when it should not.Also interesting is that the RAM's pin 27(!OE, output enable) is constant HI, so output is never enabled.It comes from 8031's pin 17(!RD/P3.7). This means its content is never read by the 8031 uC.It's a complex spider net obviously. Perhaps wait and see after the replacement of the NAND gate.

author: inse
author_rank: Super Contributor
timestamp: December 30, 2023, 06:17:36 am
content: Very good circuit debugging!I wonder what they need the RAM for anyway - is it backed up by the battery?What kind of information could be stored in there?WAIT - one idea just came to my mind: I had a look at the 74LS03 dataset and it’s an open collector NAND.There has to be a pull-up resistor and it needs to be supplied.Did you put a fresh battery in already?I propose to desolder the LS03, _test all traces for continuity_ and solder a socket for further investigations.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 30, 2023, 07:12:31 am
content: Quote from: inse on December 30, 2023, 06:17:36 amThanks!Good question. Yes, this machine requires two AA batteries to keep the memory (that's what caused the mess around the battery compartment), which I now think is probably what is stored in this RAM, though I cannot be sure yet.As far as I understand, this machine can let you customise your own stitch patterns and stored in its memory for use whenever you want (so far as it's not lost).Yes, I tried multiple times putting in batteries and taking them out. It should work just fine without the batteries.This train of thought makes me think even if this one is replaced, the problem of the display might not be fixed? I don't like this thought!Surely when taking it off, with more spaces exposed, I will have another good check and clean before putting the new one back.

author: inse
author_rank: Super Contributor
timestamp: December 30, 2023, 07:27:58 am
content: If the obvious failure was the battery leakage, I would thoroughly test all the fine traces in the affected area for continuity.(electrically, not optical)I have seen cases where corrosion went along the leads into components, Idk if this may also have happened at the LS. But the leads don’t look so bad on the photo.A second failure on the machine is certainly possible but less likely.We don’t know how the firmware reacts to issues with the RAM.Don’t give up

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 30, 2023, 07:36:31 am
content: Quote from: inse on December 30, 2023, 07:27:58 amIn this case, it's almost certain that the corrosive alkaline has managed to get into the LS chip (probably along the pins) and failed a part of it. Fingers crossed and hope it will get well fully after a new NAND gate chip!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: January 11, 2024, 08:16:50 am
content: The original NAND gate DIP-14 IC to be replaced is T74LS03B1. I was unable to find the exact IC but found SN74LS03 on Aliexpress. Ordered 5 and arrived very quickly (which was a pleasant surprise). Also to my surprise though, with a quick test before soldering one on, they do not seem to work! This is really a surprise because I bought many electronic parts from Aliexpress before, and never had such experience.[Edit: I was wrong. Pull-up resistor is required on the output pin of each NAND gate. The chips are all good.]Now searching with only "quad 2-in NAND" again, there appear to be many (more than I thought) that all look like good as replacements. From the local store Jarcar alone, I got:74LS00 Quad 2-Input NAND Gate Schottky IC;74HC00 Quad 2-Input NAND Gate CMOS IC;4011 Quad 2-Input NAND Gate CMOS IC;74LS01 Quad 2-in NAND Gate Low power Schottky ICThese all seem ok as a replacement? The last one looks the closest to me (from the name of it). Can someone please help me confirm? Thanks.

author: picburner
author_rank: Frequent Contributor
timestamp: January 11, 2024, 10:12:18 am
content: How did you test them?The 74LS03's are open collectors (outputs), you cannot replace them with the 74LS00's which are Totem Pole if the circuit does not allow for it.

author: inse
author_rank: Super Contributor
timestamp: January 11, 2024, 03:25:13 pm
content: Open collector, because they might be used to interface the battery backed up RAM.Analyze the circuit, maybe a LS00 with a diode can be used as replacement (botch).As picburner suggested already, did you have a pull-up at the output for your test?One more tip: solder an IC socket, so you’re free to swap forward and backwards as you wish

author: Martinn
author_rank: Frequent Contributor
timestamp: January 11, 2024, 08:01:12 pm
content: Quote from: max.wwwang on December 28, 2023, 09:04:07 amWhen I some years ago bought a new sewing machine, I wanted something robust, purely mechanical without unobtainium chips that would fail on me.It turned out that Pfaff sold off their Tipmatic production line and they are to date still manufactured in Iran, trade name "Gritzner". I have a tipmatic 1037 which has full cam control, no electronics besides lamp and motor.Great machine, however in hindsight I really would love to have needle positioning, which of course only the newer electronic machines have.Reading your post however tells me that decision was not so bad after all.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: January 11, 2024, 08:18:57 pm
content: Quote from: picburner on January 11, 2024, 10:12:18 amThank you. Lucky I asked here! They took all identical to dumb me!I didn't use any extra components testing the 74LS03's - no pull-up's or pull-downs or anything else. I just plug it on a small breadboard, feeding the 5V and GND rail to the chip's Vcc and GND pins, mock one gates input with Vcc and/or GND (for high or low) and see what I get from its output pin.So I suppose they are probably ok just not suitable as replacement?Quote from: inse on January 11, 2024, 03:25:13 pmThanks! IC socket is a great idea, though I need to check if there is enough (head) room for it.Knowing the above, and getting back to square one (looking for an identical replacement), does anyone know where I can find T74LS03B1 or anything that is equivalent? Does this mean none of the NAND gate chips I listed above is a straightforward equivalent replacement for T74LS03B1? (Datasheet of each can be found under the tab "download".)Will take time to look closer into the datasheets, and the circuit about the T74LS03B1, and digest the information I got from you guys.

author: inse
author_rank: Super Contributor
timestamp: January 11, 2024, 10:47:35 pm
content: The last posting confuses me- just drop in the LS03 you received and see if it works or behaves differently

author: max.wwwang
author_rank: Frequent Contributor
timestamp: January 12, 2024, 01:57:23 am
content: Quote from: inse on January 11, 2024, 10:47:35 pmNot like those experienced experts, I tend to do enough 'mental exercises' before action just to avoid ruining things by repeated soldering and desoldering unnecessarily, particularly with things having more than 3 pins. I have not even pulled the bad guy out.As pointed out by other people above, obviously I missed an important piece of information about pullup resisters in the datasheet when testing the new chips. I guess they are probably good (based on my experience with Aliexpress). Will test again. It looks like they should be ok as direct replacements based on the TI's referenceUnderstanding and Interpreting Standard-Logic Data Sheets(page 15). They are both 74LS03, where 74 means commercial grade (54 military grade), LS means "Low-Power Schottky Logic" (though the meaning of 03 is still not obvious to me). It was these magic codes that puzzled me.

author: inse
author_rank: Super Contributor
timestamp: January 12, 2024, 04:49:35 am
content: There is a certain level of logic behind the naming scheme of logic ICs as you have already started to understand.The last digits code the function of the IC, starting from 00 - in the past entire data books used to be common.So don’t worry, any 74LS03 will work in your sewing machine unless the mainboard is designed to the edge of technology like some kind of supercomputer.If you are careful while desoldering and don’t use force, everything will be fine

author: max.wwwang
author_rank: Frequent Contributor
timestamp: January 12, 2024, 07:12:09 am
content: BINGO! FIXED ! after popping in the new chip.As usual, abundant care was taken. To avoid stress to the board and the pads/traces, I cut break the legs of the chip with a sharp knife before desoldering them - now a piece of cake! It's not super easy but much easier than you might think. Some cleaning (not the best or the tidiest but reasonably good) ... I'm satisfied with the outcome.Thanks heaps for all the help and support along the way, particularly by reminding me of the need for pullup resistors (so I didn't judge the good chips as faulty!)... I apologize to Aliexpress, you rock! Of course, before using the new chip, I test it again - this time with a pull-up resistor on the output pin of a NAND gate. It worked as expected!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: January 12, 2024, 07:17:43 am
content: Quote from: Martinn on January 11, 2024, 08:01:12 pmWhatever your decision was, if it was a PFAFF, that's the right decision. Yes, as soon as you have tried the electronic needle up/down feature (one of the very first new features compared with the pure mechanical machines that the early electronic machines offered), you feel you cannot live without it.

author: inse
author_rank: Super Contributor
timestamp: January 12, 2024, 07:40:33 am
content: Congratulations

author: max.wwwang
author_rank: Frequent Contributor
timestamp: January 12, 2024, 07:42:40 am
content: Quote from: inse on January 12, 2024, 07:40:33 amThank you!

author: lasseo
author_rank: Contributor
timestamp: January 12, 2024, 07:57:26 am
content: Quote from: max.wwwang on January 12, 2024, 07:12:09 amCongratulations, well done! Another piece of quality appliance back in its original glory.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: January 12, 2024, 10:01:20 am
content: Quote from: lasseo on January 12, 2024, 07:57:26 amThanks. And a piece of history!