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Swtpc pr40

On the PR-40, the input strobe signal (from the computer to the printer) arrives on J4-3. It is pulled up to +5V via resistor R12 and feeds the inputs of three (3) IC input gates (IC14/10, IC14/12 and IC15/6).

It should not be driven by anything!

Check the 5V rail for a stable 5V +/- 5% and with minimal ripple.

After that, look for solder splashes etc. shorting out the PCB track attached to J4-3.

Are IC14 and IC15 in sockets? If so, you could remove them from their sockets to see if the 'pulsing' on J4-3 goes away. These ICs are the correct type and have been installed the correct way round haven't they?!

Regarding the SWTPC with the MP-LA card, if you disconnect the printer from the parallel card and pull the A-SIDE C1 input high (to +5V) via a 1 kOhm resistor this should result in the printer test program (when run) stopping after it has output a $0D character to the A-SIDE parallel port.

Check the A-SIDE parallel outputs and ensure this is what you observe ($0D). If not, the test program (or the human driving the test program) is not working correctly :)!

You can also patch the printer test program as follows:

Address $A020 patch the data byte from $2A to $53. Change the instruction BPL into COM B.
Address $A021 patch the data byte from $FC to $53. Change the operand for BPL into COM B.

This will have the effect of telling the printer test program to ignore the handshake back from the printer and carry on regardless. If you use a logic probe / oscilloscope you should see continuous activity on the A-SIDE data and strobe line from the MP-LA. Don't have the printer connected at this stage though!

Dave
 
Last edited:
daver2 said:
On the PR-40, the input strobe signal (from the computer to the printer) arrives on J4-3. It is pulled up to +5V via resistor R12 and feeds the inputs of three (3) IC input gates (IC14/10, IC14/12 and IC15/6).

It should not be driven by anything!

Check the 5V rail for a stable 5V +/- 5% and with minimal ripple.

After that, look for solder splashes etc. shorting out the PCB track attached to J4-3.

Are IC14 and IC15 in sockets? If so, you could remove them from their sockets to see if the 'pulsing' on J4-3 goes away. These ICs are the correct type and have been installed the correct way round haven't they?!

Regarding the SWTPC with the MP-LA card, if you disconnect the printer from the parallel card and pull the A-SIDE C1 input high (to +5V) via a 1 kOhm resistor this should result in the printer test program (when run) stopping after it has output a $0D character to the A-SIDE parallel port.

Check the A-SIDE parallel outputs and ensure this is what you observe ($0D). If not, the test program (or the human driving the test program) is not working correctly :)!

You can also patch the printer test program as follows:

Address $A020 patch the data byte from $2A to $53. Change the instruction BPL into COM B.
Address $A021 patch the data byte from $FC to $53. Change the operand for BPL into COM B.

This will have the effect of telling the printer test program to ignore the handshake back from the printer and carry on regardless. If you use a logic probe / oscilloscope you should see continuous activity on the A-SIDE data and strobe line from the MP-LA. Don't have the printer connected at this stage though!

Dave
Click to expand...

Thanks Dave.. I will work on this tonight. I'm heartened to see my own guesswork on where to go next is (mostly) aligning with yours. That shows I've learned something.. sort of.. along the way. :)

I had good +5V (about 4.95V) and no noticeable ripple.

Regarding accidental solder bridges, etc - this was a working printer - it is evident from the marks on the platen it was used quite a bit in its heydey - so I think we are safe from mistakes as clearly it was used. I *will* check anyway especially for IC substitutions.

Regrettably nothing is socketed (of course), so no quick swapping/removal of anything. I kind of get the demerits of socketes but compared to the difficulty you face if a direct soldered IC fails..
 
Alright.. had to take a breather as I had my wisdom teeth pulled and let's just say vintage computing wasn't top of mind. :) Plus, I had to dig around for a 1K resistor for the test program.

So getting back on track, it appears in addition to suffering from wisdom tooth problems, I was also battling a chronic case of MORONICITY, because I didn't notice the ENORMOUS cut in the track between IC7 pin 11 and port A C2. I'm not sure exactly what effect that would have or why it was done - apart from the B side having 8835s instead of 8833s.. I'm guessing since they had port A configured as an input, maybe they didn't need C2 for what they were doing. I don't see any other major changes to the board.

I'm assuming I'll want to restore that track before doing much else..

Regarding the printer, I think after probing things more I will carefully desolder the possible suspect chips. If I had to lay money on it, I'd bet we have a faulty 7400 on our hands.. the failure mode seems familiar.
 
I would check the schematic before modifying anything (https://deramp.com/downloads/swtpc/h..._schematic.pdf). IC7 pin 11 shouldn't be wired to anything at all...

Perhaps a close-up of the top and bottom tracking so that I can check?

Perhaps having your wisdom teeth out actually increased the wisdom and didn't decrease it?! Ouch anyhow...

Can you have a 'poke' around the two ICs (IC15 and IC16) on the printer for the pins that are 'clocking' - and tell me which pins are permanently HIGH and LOW and which pins are CLOCKING on each. There must be a reason for the clocking (before you start desoldering things).

Hope you feel a bit better soon. When I had some dental surgery - I needed something to take my mind of the darn thing!

Dave
 
Thanks Dave. I don't work well when I'm in pain.. I did try but yeah.

Here are a couple of pics. It looks like they cut the track to IC7.. and I can see the remnants of copper wire on the c2 pin as well as the third ss30 pin in from the A side. Looks like it connected to -12v??

https://drive.google.com/file/d/1VKDb8VLLjDOTXltiWFfwvhUhmObJsI8b/view?usp=drivesdk

https://drive.google.com/file/d/1VGueYG8RhaY0ah0j6Te8n1n5matQuZ9N/view?usp=drivesdk

I also goofed in my pin count.. ic7 is 16 pin not 14.. so the disconnected pin is pin 13, not 11.
 
Dave I think the moronicity is real and acute. When I was testing the printer handshake wires that had been removed from the MPLA, I had my logic probe powered off the computer, not the printer. With the printer fully separated and the probe powered off the printer.. there is now no activity whatsoever on either data accepted or data ready lines, and all the data lines are low.
 
A couple pages ago I made the semi-serious suggestion of just using some pull-up/pull down resistors to assert a carriage return onto the printer’s data lines and wiring a push button as a strobe, I’m going to repeat that suggestion. If you really want to see if it will show signs of life without the complications of trying to make the old 6800 computer work at the same time…
 
Yeah I'm thinking you are correct on that. However - I've no idea exactly how to wire up what you described there.

One way or another I suppose we have to get into the computer anyway, since my goal is to actually drive the printer off a real SWTPC 6800. But it would be nice to sorta separate the two and make sure each is working in its own right before marrying them together.
 
Woohoo! It works! Sort of.

So I consulted the schematic again, checked to make sure that track to C2 should be connected, and then reconnected it. Also checked around for other defects but couldn't find any. So after reconfirming the printer didn't have any ringing (with the logic probe connected to it rather than the computer), I wired it back up, entered the test program, and this what we get:

https://youtu.be/h4QcmLdvlWg

It's all scrunched up for some reason! Also, the ribbon advance doesn't seem to be working properly.. it's not pulling the ribbon from one spool to the next. On the very first go, the printer seemed to jam a bit on the rotating drum that moves the printhead. I freed it up, and then let it go. It managed to print one correct line amongst the jumble, but otherwise it seems to be printing everything slammed together, and then shuddering as it moves the rest of the page width. I did try adjusting trimmer R17 but that seemed to make no difference. Here's a sample of the printout.. maybe someone can make sense of it.

https://drive.google.com/file/d/1GIs4h3Zj9LII1vB6T_TG7Pa6k5AGuN2-/view?usp=sharing
 
It lives!!

https://youtu.be/ZnTo2r36StM

I noticed it seemed to be binding a little bit in the drum channel for the print head as well as the slider bar. I added some lithium grease (not sure if that's ideal but it's what I had) and now it prints just fine!

Weirdly.. it prints even without a ribbon(?!). The paper seems to have some kind of blue carbon copy like stuff embedded in it. When the print head hits it you get a light blue print.

I wonder if any SWTPC software like BASIC had the ability to print listing, etc. Would love to find more for this thing to do.

Anyway, special thanks to Dave and Eudimorphodon for the help!

We aren't done with this MPLA though.. there are two more projects to go.. one is my PPG joystick.. and then the more ambitious one is my GT 6144 graphics board.
 
Good job - you must be feeling better today!

I have been quickly reading your posts - but haven't had time until now to respond.

Yes, the broken/removed PCB track on C2 would have caused the strobe signal from the computer to the printer to not get there... No printing as a result.

I notice quite a lot of old flux residue on these cards. Could I suggest that you get a bit of IPA and some small stiff-bristled brushes and clean the flux residue off? This should be well worth the investment - as some flux residues can become conductive over time and cause some weird failure modes to occur. I would also check (and touch up if necessary) any poor solder joints. Some of these 'kits' were not constructed well to start off with - and the ravages of time and all that...

It looks like this parallel card has been repurposed by the original owner for some bespoke application (hence the cut track and the non-standard buffers). Always good to do a bit of 'due diligence' on cards when you get them - and compare them with 'stock' manuals and schematics to identify modifications like this.

When I saw your later post, I was going to suggest that either the lubrication had dried up or has even clogged the mechanical mechanism. Cleaning off the old lubrication and reapplying the correct lubrication on something mechanical would always be the first step.

More projects - keep them coming (it breaks the boredom of working for real)...

Just out of interest I checked ePay Canada and you get a much better selection of equipment over your side of the water than we see over this side!

Dave
 
Many thanks Dave.. I especially appreciate your help as you take the time to explain things in a language I can understand. Yes I will clean this card up, I have learned the value in that after solder Flux caused some weird issues with my TVT.

I suppose Canada gets a better selection due to its proximity to the biggest original producer of vintage equipment. I seldom see stuff locally here I'm BC.. usually have to do ePay and pay a fortune for shipping.

I'm trying to decide on next project.. I have to wait for parts to build my PPG.. unless I use the original SWTPC kit..

I feel like I want to tackle one of my four non working terminals next. I just scored an ADM-1 (may work.. hasn't arrived yet), I have that no name TTL based terminal, a Keyedit 100 terminal (not sure if it could be made functional in the absence of the rest of the keyed system), and then an OSI rebadged 1420 terminal that blows fuses and smokes from somewhere on the board (think voltage regular) when you try to power it on.

So many.. heh!
 
Congratulations on getting it to work! In the end I guess I'm actually not too surprised it didn't take much more than some cleaning and grease to get the printer itself to work, it looks like that mechanism was built to survive Armageddon. (In particular I love that cylindrical dingus it uses to horizontally position the printhead. Takes up a lot more room than a belt or whatever but hey, it works.)

falter said:
I suppose Canada gets a better selection due to its proximity to the biggest original producer of vintage equipment. I seldom see stuff locally here I'm BC.. usually have to do ePay and pay a fortune for shipping.
Click to expand...

I do have to salute your fortitude (financial and otherwise) for taking on this sort of hardware. My "vintage" interests are largely convergent, IE, older the better, but I just die of sticker shock when I see what most pre-1980 hardware sells for these days.
 
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