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-   -   Zx81 (https://www.vintage-radio.net/forum/showthread.php?t=164648)

John Earland 21st Apr 2020 10:29 am

Re: Zx81
 
I've just tried cleaning it and on the one pin hole it appears to be burned. Luckily the connection is not the other side which is intact. Don't know if that will make a difference?

Slothie 21st Apr 2020 10:32 am

Re: Zx81
 
The problem with nail varnish remover is that it is acetone based which attacks plastics and unless you were very quick to remove it i imagine it will dissolve screen printing and solder resist. It also usually contains. Oils to mitigate drying of the skin which might be annoying to get off. You could try meths or surgical spirit which are similar to IPA and more likely to be lurking in a cupboard or the local chemist (although ive not tried buying meths since the 70s when I had a mamod steam engine!).
Looking at the photos theres not too much flux residue and i'd probably leave it if I didnt have anything to hand. The main risk from rosin flux residue is that in a damp environment it can absorb moisture and become corrosive and possibly conductive - but hopefully this board wont be kept in a damp place! I have circuits i built in the 80s with flux residue without any detectable corrosion because they have been kept dry.

SiriusHardware 21st Apr 2020 11:33 am

Re: Zx81
 
I take it the burned pad is one which has no tracks connected to it on the lower side of the PCB? Those are the ones most prone to being burned off, because there is no track to conduct some of the heat away.

There are three aspects to each solder hole on a double sided PCB - a pad on the top side of the PCB (which is OK here) a pad on the underside which is (presumably the burnt one) and a hollow tubular conductive sleeve running through the hole, connecting the bottom side pad to the top side pad.

As long as the top side pad and the tubular sleeve are intact you should be able to run solder onto the pin and it will run down inside the tubular sleeve and connect the pin to the top side pad and anything which is connected to that.

Slothie 21st Apr 2020 12:06 pm

Re: Zx81
 
Ive repaired connections to ICs where the damaged pad is on the top by wrapping a very fine wire (one strand from a multistrand wire) around the pin on the socket, carefully soldering the wire to the pin, then fitting the socket and soldering the 'tail' of the fine wire to the track. This works even if the through-hole plating has come away. You can even connect to both sides of the board with a little care if required.

John Earland 21st Apr 2020 12:10 pm

Re: Zx81
 
Thanks for the suggestions. There is a hollow tube. The conection on top runs into this hole and through to the bottom and it is the bottom which is burnt - there isn't any obvious underside connections to that hole.

SiriusHardware 21st Apr 2020 12:25 pm

Re: Zx81
 
In that case I think you will be OK just running solder onto the pin and down inside the hollow sleeve.

John Earland 21st Apr 2020 5:00 pm

Re: Zx81
 
1 Attachment(s)
Ok guys-I’ve soldered in the socket! I would appreciate your comments regarding the soldering?

SiriusHardware 21st Apr 2020 5:49 pm

Re: Zx81
 
Is that an unsoldered pin, seventh from left on the bottom row in your photo? Also clean / flick off the splash leading away from the leftmost pin, upper row.

Otherwise, not bad if that is the first time you have ever soldered a socket in.

John Earland 21st Apr 2020 6:13 pm

Re: Zx81
 
1 Attachment(s)
Thank you but I’m not out of the woods!
Okay so I put in the replacement ULA and I got a picture! It was very fuzzy and grainy with very thin strips/horizontal lines-and was flashing colour so I fiddled with the switches on the ULA which stopped the flashing and in some configuration stopped the picture i.e. the cursor disappeared. So I guessed it might be something to do with the RF mod I did-so I’ve taken it out and restored it (all in the picture). Sadly, now I can’t get anything at all. I’ve checked the 5v and video connections into the module and there is a good connection between going in and through the components. So I’m thinking of putting the mod back in but not sure where to go from there. It’s highly likely of course that my first time socket soldering has not been a success!

SiriusHardware 21st Apr 2020 6:28 pm

Re: Zx81
 
Can you post a link to the instructions which came with your replacement ULA?

One of the things I remember about it is that the output is switchable between composite suitable for input to the modulator / composite suitable for direct output to the composite input on a monitor or TV.

In other words, this ULA does not require the transistor buffer, you can do the simpler straight-through mod as on your Spectrum if you have the ULA output configured the right way.

Did we not come to the conclusion that the RF modulator was also faulty in that machine? I would suggest the simplest configuration with this new ULA is to set the output for direct composite out and do a straight-through mod from modulator input to output.

SiriusHardware 21st Apr 2020 6:32 pm

Re: Zx81
 
Just found it, reading it now.

https://vdrivezx.files.wordpress.com...ual_v1.3-1.pdf

SiriusHardware 21st Apr 2020 6:39 pm

Re: Zx81
 
I would say the correct settings for your case would be all switches OFF, which equates to

SW1 off = direct 75R output to composite video input
SW2 off = 'Back porch' enabled, like on 2C210 ULA
SW3/4 both off, as you are not using any special memory modes.

Leave all the ULA switches in the OFF position and make a straight through connection from modulator video IN to modulator OUT, bypassing the modulator. Do not use or include a transistor buffer, as the new ULA already includes one.

John Earland 21st Apr 2020 6:52 pm

Re: Zx81
 
Here is the link.
https://retrorevivalshop.co.uk/index...roducts_id=560

As I said I did get a picture but not now returned to original setting. I will modify it as I did with the Spectrum and try that. Thanks again.

John Earland 21st Apr 2020 6:54 pm

Re: Zx81
 
Here’s the manual:
https://vdrivezx.files.wordpress.com...ual_v1.3-1.pdf

julie_m 21st Apr 2020 6:56 pm

Re: Zx81
 
Well, that's certainly not bad for a first attempt! Well done :clap: If the damaged pad was not connected to anything on its side of the board, don't worry too much about it. Just make sure that there is some solder on the side where it is connected to something. If any of the holes has lost its through-plating, be sure to solder the socket pin on both sides of the board so as to complete the circuit. Start by soldering just two opposite corners of the new socket, and double-check it is sitting level and the right way around (the "pin 1/pin 40" end will look different from the "pin 20/pin 21" end somehow, depending on the manufacturer) before continuing. Been there, done that, grown out of the T-shirt ..... If any more pads break off when soldering in the new socket, cut the track cleanly with a sharp knife blade to limit the damage, solder a single strand taken from some hookup wire to the end of the track and wrap it around the pin. Use long-nosed pliers to shape it. If you grip the wire between the track and the pin, the pliers should conduct some of the heat away so as not to melt the first joint undone. You probably won't need to do this, but forewarned is forearmed.

I've been thinking about getting a "proper" desoldering station, rather than :censored:ing about with de-solder braid, plunger pumps that need priming each time or even just deft flicks of the wrist. I mean, just because you can do a good job without an expensive, special tool is no good reason not to get one, is it?

EDIT: Oh, too late, I see! Never mind, it's a great job.

SiriusHardware 21st Apr 2020 7:44 pm

Re: Zx81
 
I managed for decades without an assisted (electric) desoldering iron and then I went to a job where every bench had one as standard, and that ruined me. I wouldn't consider not using one now. I have an oldish Weller at home, and an Oki / Metcal at work.

I'm not sure what it is that John has, the simplest type of 'electric' desoldering iron is very similar to the manual teflon tipped plunger type, except that the nozzle is metal, and heated.

John Earland 21st Apr 2020 7:57 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1237528)
I managed for decades without an assisted (electric) desoldering iron and then I went to a job where every bench had one as standard, and that ruined me. I wouldn't consider not using one now. I have an oldish Weller at home, and an Oki / Metcal at work.

I'm not sure what it is that John has, the simplest type of 'electric' desoldering iron is very similar to the manual teflon tipped plunger type, except that the nozzle is metal, and heated.

I have a manual desoldering device with a plunger which has to be reset everytime. I'm doing the mod now.

John Earland 21st Apr 2020 10:27 pm

Re: Zx81
 
3 Attachment(s)
Well, going for the simple mod worked. However there are still some problems. The cursory looks like it crackles with tiny pinpoints of light (photo 1-though don’t know if it shows properly) then intermittently I get the second picture! Mmmm. The picture improves when I set the ULA to the configuration shown in the third picture-though I still get the colour image. However that was earlier. I’ve left the ZX81 for a couple of hours and it is stone cold now and I’ve just switched it on and it seems to be behaving itself! I shall put it through it’s paces in the morning and have a good look at the manual!

John Earland 22nd Apr 2020 8:18 am

Re: Zx81
 
2 Attachment(s)
Just switched it on this morning - this is what I’ve got from cold. Apart from this, the output is really crisp once it settles down! There is definitely colour trying to get through though. The switches on the ULA are set to (I think) ON OFF OFF ON.

I‘ve had it running for about 10 mins and has seemed to settle though it’s not terribly stable! Colour continues to deep through into the characters. As you can see from the second picture, when it does stabilise the image is crisp! Maybe I need to adjust the switches. Back to the manual.

John Earland 22nd Apr 2020 9:22 am

Re: Zx81
 
Just a quick update: I've changed the switch settings to ON ON OFF OFF - doesn't seem to make a difference - though it should! Switch 2 is for the back porch which I needed with the chip I had which was 2C210E. However, I have turned the colour off on the LCD and that has done the trick it seems! Not really a solution but it appears to work. It could also be that as the machine warms up that affects it too!

SiriusHardware 22nd Apr 2020 10:23 am

Re: Zx81
 
The ZX81 doesn't have the fairly complex circuitry needed to generate a colour signal for the set to 'see', but in the absence of a colour carrier from the source, a circuit called a 'colour killer' should really come into play on the TV.

Maybe this particular TV is a little bit taken aback when confronted with a monochrome source. (Being an LCD set, it will have been made many, many years after mono broadcast transmissions and other monochrome-only sources were common).

You could try letting the fault clear and then try cooling various things including the regulator, electrolytic capacitors, etc, to see if the colour noise comes back when you do that. Don't blast the electrolytics too hard, they contain liquid inside which needs to remain liquid in order for them to work...

John Earland 22nd Apr 2020 10:35 am

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1237684)
The ZX81 doesn't have the fairly complex circuitry needed to generate a colour signal for the set to 'see', but in the absence of a colour carrier from the source, a circuit called a 'colour killer' should really come into play on the TV.

Maybe this particular TV is a little bit taken aback when confronted with a monochrome source. (Being an LCD set, it will have been made many, many years after mono broadcast transmissions and other monochrome-only sources were common). - I think you might be right here! I have another older LCD which I could try.

You could try letting the fault clear and then try cooling various things including the regulator, electrolytic capacitors, etc, to see if the colour noise comes back when you do that. Don't blast the electrolytics too hard, they contain liquid inside which needs to remain liquid in order for them to work...

I will need to get some more 'poop spray' from B&M so might take a while! And, I hesitate to say this(!), but the machine switches off when I try to load in a program! Yes - Bermuda loading triangle again. I have tried a sound setting which works on my working ZX81 but this mchine simply switches off everytime I load something. I shall be patient though and try various volume settings.

I have had it running for over an hour now though and it is still working. The black characters remain a little grainy - though no colour now because I've switched it off! At least the replacement ULA works. Could this issue be caused by my soldering efforts!?

SiriusHardware 22nd Apr 2020 11:31 am

Re: Zx81
 
May I draw your attention to the paragraph "Mic Levels" on the first page of the manual?

The video and tape signals are somewhat interconnected on the ZX81 so any change to the video output circuit can mean that you may need to set the loading volume to a substantially different level to the one which worked previously.

John Earland 22nd Apr 2020 12:02 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1237713)
May I draw your attention to the paragraph "Mic Levels" on the first page of the manual?

The video and tape signals are somewhat interconnected on the ZX81 so any change to the video output circuit can mean that you may need to set the loading volume to a substantially different level to the one which worked previously.

Yes - I saw that. I will keep trying. I'm sure something will happen. However, the ZX81 switches off now even if there is nothing plugged into the EAR. Just typing LOAD "" switches the machine off and I have to reset it. It wasn't doing this this morning but it is now.

SiriusHardware 22nd Apr 2020 1:22 pm

Re: Zx81
 
Unless you have a specific reason not to, I would try putting switches 3 and 4 to OFF and leaving them there, since your '81 does not have anything unusually exotic in the way of memory arrangements. (The external RAM pack does not count as 'unusual'.).

John Earland 22nd Apr 2020 4:09 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1237751)
Unless you have a specific reason not to, I would try putting switches 3 and 4 to OFF and leaving them there, since your '81 does not have anything unusually exotic in the way of memory arrangements. (The external RAM pack does not count as 'unusual'.).

Yes, I've tried all combinations. At the moment only the mod switch is on but it's still crashing when I type LOAD "". It is strange because it wasn't doing that this morning - hey ho. The joys of vintage computing!

julie_m 22nd Apr 2020 7:05 pm

Re: Zx81
 
Quote:

Originally Posted by John Earland (Post 1237535)
I have a manual desoldering device with a plunger which has to be reset everytime. I'm doing the mod now.

:o Then that's an even better job than I first thought!

John Earland 22nd Apr 2020 7:52 pm

Re: Zx81
 
Quote:

Originally Posted by julie_m (Post 1237861)
Quote:

Originally Posted by John Earland (Post 1237535)
I have a manual desoldering device with a plunger which has to be reset everytime. I'm doing the mod now.

:o Then that's an even better job than I first thought!

Thank you:thumbsup:

John Earland 22nd Apr 2020 8:15 pm

Re: Zx81
 
1 Attachment(s)
Quote:

Originally Posted by julie_m (Post 1237861)
Quote:

Originally Posted by John Earland (Post 1237535)
I have a manual desoldering device with a plunger which has to be reset everytime. I'm doing the mod now.

:o Then that's an even better job than I first thought!

Here is the desoldering device!

SiriusHardware 22nd Apr 2020 10:58 pm

Re: Zx81
 
Yes, fantastic job if that was your first time doing a major desoldering job, and with a manual solder sucker as well. I confess I thought you had bought an 'electric' one.

As always, try the simple stuff first. Carefully remove the new ULA from the socket and make sure none of the pins went heading off in the wrong direction, ie, got folded over, when the device was inserted. It's very easily done, still happens to me occasionally even after 40+ years of doing this kind of thing. Is the IC socket you fitted an 'ordinary' socket, or a turned-pin type? Turned-pin sockets are great but the IC pins all have to be arrow straight otherwise it can be very difficult to get the IC in.

If all looks OK we'll come up with a list of things to measure from and to to see if there are any missing connections from the ULA socket to anywhere else.

Sorry to have to ask: But you did solder that apparently unsoldered pin that I pointed out? Something like that would definitely cause a problem like this.

Also - I'm assuming you have already tried this - does it behave any differently when the RAM pack is not plugged in, so it's just the bare 1K machine?

John Earland 23rd Apr 2020 9:21 am

Re: Zx81
 
3 Attachment(s)
Hi, the socket is the ordinary type not the turned in one. The replacement ULA has very strong pins – in fact, it looks like they are already in a turned in pin ‘frame’, the type of connector that comes in strips where you can snap them into any length you want. No pins were bent and the IC is very securely seated.

I attach two photographs of the soldering from different angles. I have looked at them carefully and I really can’t see that the soldering is incomplete but I may be missing something. It’s not terribly clear and even when you’re looking at it it’s not clear but I think the soldering is sound.

The machine works perfectly with and without the RAM pack. I can save programs onto tape without the machine crashing. It’s the loading that is definitely the problem. Interestingly, I switched it on this morning, typed in LOAD and I got the stripey lines, and the machine didn’t crash.However I didn’t get the broader stripey lines you see when you are loading a program. After about half an hour this stopped completely and every time I typed in load the machine crashed. i’ve now just left it for half an hour to cool down. And tried again and I get the screen in picture 3 but it won’t load.

John Earland 28th Apr 2020 5:55 pm

Re: Zx81
 
Hi folks,
I've checked the soldering on the socket again and all pins seems sound. I have checked each pin and there is no resistance between top and bottom of the pcb and I have checked the pins to the components on the board and they are all connecting. Any ideas to what I could be checking next? Any help would be gratefully received.
J

SiriusHardware 28th Apr 2020 7:25 pm

Re: Zx81
 
Quote:

Originally Posted by John Earland (Post 1239954)
I have checked the pins to the components on the board and they are all connecting.

That is quite a general statement so I'm not sure precisely what you have and have not done.

For example, you've checked out the connections from the socket contacts to the pins on the underside of the board which is certainly a valid test / task, but really you need to check that there is a connection from every one of the 40 pins of the ULA to every other place that each of those pins should be connected to. That will be difficult to do in the purely physical trace-by-eye sense because tracks disappear under ICs, zip through holes from one side of the PCB to the other and back again somewhere else, making them very hard to follow.

What we need now is for you to start to get more familiar with reading circuit diagrams, so that you yourself can see, from looking at the diagram, which connections need to be checked out.

Lucky this is a ZX81 and not your PET, where each data line alone would probably swing past around a dozen ICs.

I don't know how you want to proceed from here - you are tremendously single minded and diligent when in pursuit of a problem so I have no problem with trying to walk you through testing all the connections but it will take a lot of time and numerous posts.

As a final resort, if you wanted to send it (just the PCB) up to me I could take a look, I have an 'open plan' ZX81 where the ULA is in a socket, so the first thing I would do would be to put your super-ULA in my ZX81 to test that out and make sure the ULA has not somehow suffered damage before looking around elsewhere. I could also read out the ROM and compare it with one of my known good ROMs or with a known good ROM image from the net. I have all the other required accessories including a keypad so I would not need the power supply, video lead, RAM pack, etc.

I don't have a listing of the ZX81 ROM but I am wondering if your problem (freezing when trying to invoke load from cassette) points to the unit having problems when trying access a particular part of the ROM or a particular address range within the ROM. It could even be the ROM itself faulty (one or more corrupt bytes) but if so it is odd that you didn't seem to have this problem earlier... or did you?

I have a feeling this one is not going to be easy, as it seems to have behaved badly and intermittently at every stage even before you changed the ULA - it seemed pretty clear that the original ULA had a thermal fault so it is disappointing that you are still having problems after doing such good work to replace it.

John Earland 28th Apr 2020 9:36 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1239998)
Quote:

Originally Posted by John Earland (Post 1239954)
I have checked the pins to the components on the board and they are all connecting.

That is quite a general statement so I'm not sure precisely what you have and have not done.

For example, you've checked out the connections from the socket contacts to the pins on the underside of the board which is certainly a valid test / task, but really you need to check that there is a connection from every one of the 40 pins of the ULA to every other place that each of those pins should be connected to. That will be difficult to do in the purely physical trace-by-eye sense because tracks disappear under ICs, zip through holes from one side of the PCB to the other and back again somewhere else, making them very hard to follow.

What we need now is for you to start to get more familiar with reading circuit diagrams, so that you yourself can see, from looking at the diagram, which connections need to be checked out.

Lucky this is a ZX81 and not your PET, where each data line alone would probably swing past around a dozen ICs.

I don't know how you want to proceed from here - you are tremendously single minded and diligent when in pursuit of a problem so I have no problem with trying to walk you through testing all the connections but it will take a lot of time and numerous posts.

As a final resort, if you wanted to send it (just the PCB) up to me I could take a look, I have an 'open plan' ZX81 where the ULA is in a socket, so the first thing I would do would be to put your super-ULA in my ZX81 to test that out and make sure the ULA has not somehow suffered damage before looking around elsewhere. I could also read out the ROM and compare it with one of my known good ROMs or with a known good ROM image from the net. I have all the other required accessories including a keypad so I would not need the power supply, video lead, RAM pack, etc.

I don't have a listing of the ZX81 ROM but I am wondering if your problem (freezing when trying to invoke load from cassette) points to the unit having problems when trying access a particular part of the ROM or a particular address range within the ROM. It could even be the ROM itself faulty (one or more corrupt bytes) but if so it is odd that you didn't seem to have this problem earlier... or did you?

I have a feeling this one is not going to be easy, as it seems to have behaved badly and intermittently at every stage even before you changed the ULA - it seemed pretty clear that the original ULA had a thermal fault so it is disappointing that you are still having problems after doing such good work to replace it.

Hi,thank you for this. To be clear, I have managed to check that there is a connection from each of the pins of the ULA to every other place that each of those pins should be connected to on the underside of the pcb. As far as I can see they all show no resistance so is therefore a good connection. As you say tracing the ones on top is a little trickier but I'll track down a circuit diagram.

I don't recall having a loading issue with this one and I can certainly save a program.

I am happy to do this rather than send it to you but I am all too aware that I have taken a lot of your time (and others) so I would not expect you to carry helping me. You have been the biggest help I could have hoped for and so I am already in your debt.

Slothie 28th Apr 2020 10:02 pm

Re: Zx81
 
John, I can't recall if you have replaced or tested the power supply electrolytics and any on the board? Poor smoothing can cause all kinds of intermittent and temperature related problems.

SiriusHardware 28th Apr 2020 10:07 pm

Re: Zx81
 
Well, let's go the middle way and give you a hand to do it yourself.

Here is a redrawn circuit for the ZX81.

http://www.mainbyte.com/ts1000/good_schematic_hi.jpg

If you have the circuit diagram, you don't need to literally follow the tracks from A to B to C because the circuit diagram tells you where they go to.

Look at the circuit diagram, find the ULA, which is IC1. Now find pin 1 (Marked 'A7'), which is on the lower edge of the drawing of the chip. Follow the black line leading away from it first down, then right, then up, to where it comes to a junction. At the junction, wires lead left and right. The left one goes to pin 1 on IC2, the 8K ROM. the right one goes to one end of resistor R21, a 1K resistor. There are no further wires going from this line to anywhere else. So now, you can check (with your meter) to see if IC1 (ULA) pin 1, IC2 (ROM) pin 1 and one end of R21 are all connected together because the circuit diagram says they should be.

Move on to the ULA pin 2, marked 'A8'. Follow the line on the circuit diagram leading away from pin 2 down, right, up and to another junction, this time going to IC2 (ROM) pin 23 and one end of R22, another 1K resistor. Check, with your meter, to see if IC1 (ULA) pin 2, IC2 (ROM) pin 23 and one end of R22 are all connected together.

And basically, repeat this process for the other 38 pins of the ULA.

John Earland 28th Apr 2020 10:10 pm

Re: Zx81
 
Quote:

Originally Posted by Slothie (Post 1240057)
John, I can't recall if you have replaced or tested the power supply electrolytics and any on the board? Poor smoothing can cause all kinds of intermittent and temperature related problems.

Hi, no I haven’t tested that yet. I don’t have access to an oscilloscope etc-minimal equipments really. :-(

John Earland 28th Apr 2020 10:11 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1240059)
Well, let's go the middle way and give you a hand to do it yourself.

Here is a redrawn circuit for the ZX81.

http://www.mainbyte.com/ts1000/good_schematic_hi.jpg

If you have the circuit diagram, you don't need to literally follow the tracks from A to B to C because the circuit diagram tells you where they go to.

Look at the circuit diagram, find the ULA, which is IC1. Now find pin 1 (Marked 'A7'), which is on the lower edge of the drawing of the chip. Follow the black line leading away from it first down, then right, then up, to where it comes to a junction. At the junction, wires lead left and right. The left one goes to pin 1 on IC2, the 8K ROM. the right one goes to one end of resistor R21, a 1K resistor. There are no further wires going from this line to anywhere else. So now, you can check (with your meter) to see if IC1 (ULA) pin 1, IC2 (ROM) pin 1 and one end of R21 are all connected together because the circuit diagram says they should be.

Move on to the ULA pin 2, marked 'A8'. Follow the line on the circuit diagram leading away from pin 2 down, right, up and to another junction, this time going to IC2 (ROM) pin 23 and one end of R22, another 1K resistor. Check, with your meter, to see if IC1 (ULA) pin 2, IC2 (ROM) pin 23 and one end of R22 are all connected together.

And basically, repeat this process for the other 38 pins of the ULA.

Thank you. I will get onto this in the morning and report back.

SiriusHardware 28th Apr 2020 10:18 pm

Re: Zx81
 
Quote:

Originally Posted by Slothie (Post 1240057)
if you have replaced or tested the power supply electrolytics and any on the board?

The power supply on these is remarkably uncomplicated - just a 7805, and the only associated electrolytic is a relatively small one on the input - could be worth changing though. On the regulated side it's all ceramics.

The only other electrolytic I see is a 1uF on pin 26 of the Z80 which the originator of the drawing has neglected to mark, but it is the _RESET pin of the Z80 so that's the reset capacitor, obviously.

I'm afraid there are disappointingly few electrolytics for recappers to go at on these. The PSUs do have a bigger electrolytic in them and that will have been roasted by the heat from the transformer but John has two power supplies which I think power his other ZX81 OK.

Slothie 28th Apr 2020 10:25 pm

Re: Zx81
 
Quote:

Originally Posted by John Earland (Post 1240060)
Quote:

Originally Posted by Slothie (Post 1240057)
John, I can't recall if you have replaced or tested the power supply electrolytics and any on the board? Poor smoothing can cause all kinds of intermittent and temperature related problems.

Hi, no I haven’t tested that yet. I don’t have access to an oscilloscope etc-minimal equipments really. :-(

If you can find a 20uF or larger capacitor and put it temporarily across C3 (or just replace C3) that would show if you have a problem. Just be sure to connect them the right way round. Since there are so few on the ZX81 and power supply, RAM pack it might just be worth replacing them all since they are likely to be failing after 40 years even if they appear to be functioning.

John Earland 29th Apr 2020 5:17 pm

Re: Zx81
 
[QUOTE=John Earland;1240064]
Quote:

Originally Posted by SiriusHardware (Post 1240059)
Well, let's go the middle way and give you a hand to do it yourself.

Here is a redrawn circuit for the ZX81.

http://www.mainbyte.com/ts1000/good_schematic_hi.jpg

If you have the circuit diagram, you don't need to literally follow the tracks from A to B to C because the circuit diagram tells you where they go to.

Look at the circuit diagram, find the ULA, which is IC1. Now find pin 1 (Marked 'A7'), which is on the lower edge of the drawing of the chip. Follow the black line leading away from it first down, then right, then up, to where it comes to a junction. At the junction, wires lead left and right. The left one goes to pin 1 on IC2, the 8K ROM. the right one goes to one end of resistor R21, a 1K resistor. There are no further wires going from this line to anywhere else. So now, you can check (with your meter) to see if IC1 (ULA) pin 1, IC2 (ROM) pin 1 and one end of R21 are all connected together because the circuit diagram says they should be.

Move on to the ULA pin 2, marked 'A8'. Follow the line on the circuit diagram leading away from pin 2 down, right, up and to another junction, this time going to IC2 (ROM) pin 23 and one end of R22, another 1K resistor. Check, with your meter, to see if IC1 (ULA) pin 2, IC2 (ROM) pin 23 and one end of R22 are all connected together.

And basically, repeat this process for the other 38 pins of the ULA.

I have now logged and tracked each ULA Pin in a spreadsheet, took me a little while to map it all but at least I have all 40 of the pins and their intersections and terminals! So I will now start to go through each one methodically to test connectivity. It may take me some time! :-)

John Earland 29th Apr 2020 7:13 pm

Re: Zx81
 
I have traced a problem. PIN 20 on the IC1 (the underside I originally burned). Although the top part still connects ok to where it should, the metal ‘tube’ running through the hole as it were is no longer there thus no connection. I’ve removed the socket because try as I might I wasn’t able to rectify the problem. The top part remains in good condition with all contacts in place. My problem is now remaking some sort of tight connection with the pin of the socket and the socket hole connector.

SiriusHardware 29th Apr 2020 8:14 pm

Re: Zx81
 
Assuming the IC socket is out and all the holes are clear:-

Carefully scrape the 'resist' (the red stuff) off around 5mm - 10mm of the track leading away from the pin 20 pad on the top side of the PCB. Ideally, pick a track which will normally be under the IC so that the repair will not be visible when the IC and socket are in place.

Take a 1" to 2" long strand of very fine uninsulated wire, feed most of it through the pin 20 hole from top side through to bottom side and fold the very end of it over, flat, so it lies along the bared bit of track, and solder it to it. Keep the soldering as low profile / flat as possible so it does not prevent the socket from being refitted afterwards. Be careful not to let the pad / hole fill up with solder otherwise you'll have to remove the wire, clear the hole and start again.

If you've done it right, you now have a fine strand of wire soldered flat along the track on the top side, turning sharply at 90 degrees and dropping through the pin 20 hole and emerging out of the hole on the underside of the PCB.

Now refit and resolder the socket, leaving pin 20 until last. When you get to pin 20, you should have the IC socket pin and the thin wire strand both coming through the hole side by side.

What you do next is determined by how pretty you want to make the job look, but the simplest compromise is to wind the thin wire several times snugly around the IC socket pin and then solder it, cutting off any excess length afterwards. By doing this, you reinstate the missing connection between the IC socket pin and the track on the top side.

A good source of very fine strands of wire is a piece of thin insulated multistrand wire - strip about two inches off the end and fan out the individual stands of wire, select one strand only and cut it off. Essentially, the wire you choose has to be fine enough for both it and the IC socket pin to fit through the hole side by side.

SiriusHardware 29th Apr 2020 8:18 pm

Re: Zx81
 
1 Attachment(s)
By the way, did you notice that pin 20 is the 'Tape In' pin on the ULA? ;)

John Earland 30th Apr 2020 8:10 am

Re: Zx81
 
Hi, thank you for that advice. I will have a go. I have some wire as part of an electronics starter kit-the type that’s used on a ‘cheese board’, there are fine stands in those I believe, if not I’ll have something I can use I’m sure. I have telephone wire but they may be too fine?

Yes, having gone through the circuit diagram bit by bit I spotted that PIN 20 was connected to the EAR. There is no red covering on the top only underneath the pcb. On top all the connections are exposed. I’ll have a go this morning. Many thanks.

John Earland 30th Apr 2020 11:13 am

Re: Zx81
 
Quote:

Originally Posted by John Earland (Post 1240569)
Hi, thank you for that advice. I will have a go. I have some wire as part of an electronics starter kit-the type that’s used on a ‘cheese board’, there are fine stands in those I believe, if not I’ll have something I can use I’m sure. I have telephone wire but they may be too fine?

Yes, having gone through the circuit diagram bit by bit I spotted that PIN 20 was connected to the EAR. There is no red covering on the top only underneath the pcb. On top all the connections are exposed. I’ll have a go this morning. Many thanks.

The connections seems to work - eventhough I've not soldered anything yet! I have decided to wait until some new sockets arrive. I have ordered some turned in pin ones because the new IC has that type attached to it so I thought it might be better than the flimsy cheap ones I have. I wish I had ordered better ones for the PET now!

I'll see how the installation goes!

On another matter - I have different types of solder with flux in it. However, (and maybe this is something I am doing wrong!) it seems quite 'sticky' - doesn't seem to flow as I would expect. I sticks to the iron too much and so when I pull away from the joint, it either doesn't stick terribly well at times. I have the iron at between 230 and 350 degrees - does that sound about right?

SiriusHardware 30th Apr 2020 11:39 am

Re: Zx81
 
What sort of solder is it? (What sort of metals is it made from?)

John Earland 30th Apr 2020 11:50 am

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1240652)
What sort of solder is it? (What sort of metals is it made from?)

It just says 'Rothenberger Industrial Electronic solder - 1mm / 20g flux core', the other was given to me by my friend whos a whizz at soldering and the other reel I have is 'Silverline Solder, 60% tin, 40% lead' don't think that has flux. I have a little tub of flux to use with that one.

SiriusHardware 30th Apr 2020 12:22 pm

Re: Zx81
 
With regard to turned pin sockets, they are good but they have one drawback, they are much, much harder to desolder from a double sided PCB because the round shape of the 'fat' upper part of the pin completely seals the hole on the top side, making it nearly impossible for a solder sucker to draw air and solder through the hole.

Bottom line: If you fit a turned-pin socket, first be absolutely certain that you will never need to remove it again.

John Earland 30th Apr 2020 12:26 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1240671)
With regard to turned pin sockets, they are good but they have one drawback, they are much, much harder to desolder from a double sided PCB because the round shape of the 'fat' upper part of the pin completely seals the hole on the top side, making it nearly impossible for a solder sucker to draw air and solder through the hole.

Bottom line: If you fit a turned-pin socket, first be absolutely certain that you will never need to remove it again.

That's a very good point! I hadn't thought of that! Mmmm-rethink required!


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