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

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 Ė in fact, it looks like they are already in a turned in pin Ďframeí, 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 canít see that the soldering is incomplete but I may be missing something. Itís not terribly clear and even when youíre looking at it itís 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. Itís 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 didnít crash.However I didnít 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. iíve now just left it for half an hour to cool down. And tried again and I get the screen in picture 3 but it wonít 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 havenít tested that yet. I donít 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 havenít tested that yet. I donít 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 thatís used on a Ďcheese boardí, there are fine stands in those I believe, if not Iíll have something I can use Iím 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. Iíll 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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