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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 3rd May 2021, 11:25 pm   #1
Timbucus
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Default Heathkit Computers and the UK especially H8

I have alluded on some other threads that I may have picked up a major restoration project. I am going to post this as a quick update here and a plea for some opinions and help from the extensive knowledge here.

Here is the recent acquisition:

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- you can see why I have been a bit preoccupied the last week or two and have not built my SOFTY/INS8073 up or repaired my BBC yet.

Anyway - I do now have a very basic three card H8 running checking out 8K of RAM as OK. The H9 (terminal) and the H17 (disk drive) have some significant faults - this is one of the drives for example:

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So did the H8 itself but, that is a tale I may expand on in the thread - Many Tantalums, Cleaning and reseating chips, replacing many LS240, RAM fault - etc.

My main ask though is if anyone remembers seeing these advertised anywhere in the UK Electronics and Computing press - I was sort of vaguely aware of the Heath company and perhaps these machines in the late 70's but not sure how. I have come up short on much material that could explain how this unit was purchased and built by someone in the Monmouth area - the single disk it came with is dated 1979 so I would assume that it was not built all in one go (the chips on the H8 are all 1978) so it was around then it was constructed - and very well I will say.

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All the units have a lot of chip leg rot and rusty parts so this will be a long job.
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Old 4th May 2021, 12:51 am   #2
Mark1960
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Default Re: Heathkit Computers and the UK especially H8

Not sure what you already found, but seems to be quite a lot on the net.

I have seen a few heathkit adverts in eti and practical electronics, but they were mostly adverts for the heathkit catalog. I had heard of the H8 but not sure if this was in a review or advert from the time or something I have seen more recently.

http://www.decodesystems.com/heathkit-h8-ad-1.gif

Schematic for ram and crt here:
https://www.vintage-radio.info/heathkit/

http://www.oldcomputers.net/heathkit-h8.html

https://wiki.theretrowagon.com/wiki/Heathkit_H8

https://museum.syssrc.com/artifact/118/
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Old 4th May 2021, 10:14 am   #3
Timbucus
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Default Re: Heathkit Computers and the UK especially H8

Thanks Mark a few I had not found there - that CRT is the later H19 but, the 4k/8K RAM card is the one I have that has proven faulty and I do not have the schematic as they are supplied as a separate booklet I think which I do not have. I was lucky my 16K one was working as there is very little online but, I did find enough in this video to work out how to change the jumpers...

https://www.youtube.com/watch?v=zXcPWgkNLqo&t=590s

You are suffering the same thing as me it seems with adverts - any specific ads from the electronic mags that people have spotted would be great - it just seems that the fair number of issues I have on paper do not have any news or adverts - although each previous search including online I was not specifically looking. I may trawl the worldradio archive when I have a few hours to while away!

Anyway so this thread is useful to any other Heathkit computer owners you will rapidly come across the very active user group who have a sort of online archive:

https://sebhc.github.io/sebhc/

and an active google group:

https://groups.google.com/g/sebhc

I am going to see if I can work out why my 4K RAM is not working today -probably one or more of the 4Kx1 4044 RAM chips (it has new LS240 and each logic IC has been removed, cleaned and tested) they are helpfully called 443-832 by Heathkit but, they do list traditional numbers in the parts lists and in the pinout diagrams. Of course among the tens of memory chips I have there are none of them as spares...

I will say the the actual assembly Manuals are excellent - I was lucky to get quite a few with the machine but, not for all parts so lucky so many are online.

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It was a real joy last night to get it running as the split octal display on the front panel is unusual - it shows high and low bytes of the address / registers as two groups of three with a third group of three for the memory contents. For its time the Front Panel monitor is really well thought out and super easy to use say compared to the MK14 - but then again it was six times the price...

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They are not messing about with the 8v PSU capacitor either... the three rails (Other two are +/- 18v) are only half wave rectified and smoothed, cleverly the voltage regulators are on each card using the mounting rail as the heatsinks....

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Over the last week or two - I actually took the big Cap out and proved it out on a bench supply slowly - the smaller two I just checked for no short. As it was linear I removed and unplugged all boards and then just powered it up with a Dim Bulb and checked the rails were all stable for an hour or so prior to trying each panel in turn.
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Old 31st May 2021, 8:56 am   #4
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Default Re: Heathkit Computers and the UK especially H8

Well I am making progress on my restoration at least as far as the main unit and disk drive - spoiler alert - it was faulty RAM chips... I can now boot HDOS 1.5 but, you will need to wait for part 2 for that and what was involved.

https://youtu.be/QD1V3uBg4Iw

Hopefully the background on the system and its UK availability will interest some readers.
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Old 31st May 2021, 9:44 am   #5
SiriusHardware
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Default Re: Heathkit Computers and the UK especially H8

Thanks Tim, I LIKEd it. Another informative and historically interesting piece, especially nice that you were able to provide a little bit of background on the original owner / builder. You must feel, as I probably would, that you now owe it to him to get it working again. Looking forward to the next episode.

Watching your videos, I do actually wonder if a series like this, or one more broadly about retro-electronics including computers and game consoles will eventually find a home on some TV channel like 'Yesterday', as they already do various series about museums and restoration of things such as aircraft.
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Old 31st May 2021, 11:14 am   #6
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Default Re: Heathkit Computers and the UK especially H8

Thanks Sirius for the support - yes it was lovely that Mark was prepared to share a little bit about his Dad and it does give an extra push to do the restoration justice. I really think all of these early hobby systems are rooted in their builders and how the world was then - it is hard to remember how important those magazines, specialist shops and the catalogues were to do anything before the internet.

It is possible a specialist channel may pick it up I suppose, as some of the big YouTube trash to treasure electronics and computer vloggers do attract a lot of views. I watch a few myself and would make the effort to tune into a professional program with good production facilities. In the end it would likely need to be more superficial than even my 20 minute skims are.

I make them as they act as a useful quick index into the more detailed knowledge that the resources I link too like this site provide and will hopefully act to encourage the remaining rare machines to be preserved rather than sent to landfill as they emerge from attics and sheds across the world.
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Old 31st May 2021, 11:12 pm   #7
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Default Re: Heathkit Computers and the UK especially H8

I do remember this system, saw a demo of it at a computer show in the late 70s and saw it in the Heathkit catalogue, which was full of many delights neither I or my parents could afford, but I nonetheless drooled over!
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Old 31st May 2021, 11:21 pm   #8
Slothie
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Default Re: Heathkit Computers and the UK especially H8

Page 14 of this gives some info of the H8
http://www.g4kfk.co.uk/Heathkit-Cana...2018-01-24.pdf
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Old 1st Jun 2021, 12:32 pm   #9
John KC0G
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Default Re: Heathkit Computers and the UK especially H8

There were articles about the construction of the Heathkit H8 in the February and March 1981 issues of ETI. The Microbasics column, of which these two article were part, started in the September 1980 issue.
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Old 1st Jun 2021, 4:52 pm   #10
Timbucus
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Default Re: Heathkit Computers and the UK especially H8

That is really useful thanks guys, I would have seen those articles as we were still getting ETI in 1981 so is probably where I was aware of the machine from - for those who want to read the issues they are of course online at:

https://worldradiohistory.com/UK/Ele...ETI_Feb_81.pdf

and

https://worldradiohistory.com/UK/Ele...ay-1981-03.pdf
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Old 31st Jul 2021, 7:18 pm   #11
Timbucus
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Default Re: Heathkit Computers and the UK especially H8

OK second attempt at a long post - just closed the TAB arghhh

It has been suggest that an update may be a good idea here - and I have an ulterior purpose. I have just pushed out my usual Monthly video which actually uses the H8 and H17 disk to play the earliest available version of ADVENT on a home computer...

https://youtu.be/P7XvHifN78k

To get there I have had a variety of events from fixing blown bullet tantalums on a very old 5.25 drive PCB to having to use a HP Logic Analyser to find out what is wrong with the IRQ system when booting from disk. At the moment I have a switch that allows it to be disabled to get to stage 1 and press a key then back on to continue. Glenn in the user group kindly built an identical config system and proved it did not do it for him - which means I need to look again.

That of course is after I had to spend some hurtful sums on unusual RAM chips to repair those boards.

The other reason for the update is I am about to start looking at the H9 video terminal and although I have the original Assembly and Operation manuals I do not have the traditional Heathkit separate schematics booklet... The two I have are online here if you are interested in this fascinating machine which is pretty much all TTL no CPU like the ADM-3A and most earlier GlassTTY. Who knows what questions I may have if I have to puzzle out its operation - the PET thread may dwarf in comparison.

https://sebhc.github.io/sebhc/docume...l#Heathkit_H-9

For those coming to the thread later and/or want to watch the videos mentioned I add new videos on this system restore to my playlist.

https://www.youtube.com/playlist?lis...ig9vm2YgCgz99x
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Old 31st Jul 2021, 10:45 pm   #12
Timbucus
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Default Re: Heathkit Computers and the UK especially H8

Yay - if anyone wants to see the circuit diagrams it has been pointed out by a club member they are hiding under the CRT directory here... now do I stay up late and study or try to sleep thinking about it

https://bitbucket.org/HeathkitGuy/he...c/master/CRTs/
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Old 31st Jul 2021, 11:11 pm   #13
Timbucus
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Default Re: Heathkit Computers and the UK especially H8

The Jim Austin Collection also has a H8 which it seems came from my neck of the woods originally - Wales must have been a hotbed of Heathkit H8 users...

http://www.computermuseum.org.uk/fix...athkit_h8.html
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Old 31st Jul 2021, 11:21 pm   #14
SiriusHardware
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Default Re: Heathkit Computers and the UK especially H8

So which of those terminal diagrams applies to your specimen, or do you not know yet (having only just found them).
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Old 1st Aug 2021, 10:22 am   #15
Timbucus
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Default Re: Heathkit Computers and the UK especially H8

They are the H9 ones at the end - I have been reading them this morning.

The one with the overview of operation which I am reading on paper is the operation manual:

H9 Video Terminal Operation Manual Part 595-2017-03.pdf

Which refers to the diagrams in its partner booklet...

H9 Video Terminal Operation Manual Schematic Part of 595-2017-03.pdf

It is quite large as a PDF so I am trying to cut out smaller diagrams but, I think as before I will need to stitch together several A4 sheets to get readable printouts. The two block diagrams are here for others to wonder at the complexity...

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For a laugh the TPU one is here but, you will need the original to read it - I suspect that the PLA and or one of the three fused PROM's may be the first area to investigate although there is no equivalent model number for the PROMS which are 256x8 so I may need to puzzle through from the pinout on Page 84 - at least the contents of the PLA and ROMS are on the next pages...

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Old 1st Aug 2021, 10:33 am   #16
Timbucus
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Default Re: Heathkit Computers and the UK especially H8

I should also state that the current condition of the unit is as follows - the picture shows how easy it is to get to the logic boards from the bottom - the IO board and Video boards are on the top section.

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It seems the Video circuit is running as I get a stable screen with what I assume is a cursor in the top left - now that is static so at least the timer that triggers it blanking is not running. This originates from the TPU and ends on pin 10 of IC201C which is located on the Character Generator Board. So that is where my scope will be looking in a minute...

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In other testing I have been able to, with some switch fiddling, get the keyboard to make clicks although nothing appears on the screen so I assume I can put the small analyser on the ASCII bus and see if it makes it there or never leaves the Keyboard - if it does not then again the TPU arbitration onto the ASCII bus will be suspect.

I have not tried injecting characters into the parallel or serial ports yet but, that will be worth a try as well while looking at the ASCII bus - I will modify the Baud rate to something I can generate from a modern machine (it is 600 at the moment) - but, I will scope each of the master clocks first to make sure they are coming out of the character generator board.
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Old 1st Aug 2021, 3:45 pm   #17
SiriusHardware
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Default Re: Heathkit Computers and the UK especially H8

All interesting stuff - 600 Baud was a fairly standard baud rate, I'm surprised that you now find it unsupported by modern utilities. Just checked though, and surely enough 600 is a missing baud rate in the drop down list on Windows Hyperterminal at least.
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Old 1st Aug 2021, 5:37 pm   #18
JohnBHanson
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Default Re: Heathkit Computers and the UK especially H8

The keyboard and crseen will not normally be connected data wise. So I would
not expect characters typed on the screen to appear on the keyboard

To check that either select half duplex (hdx) if you can or use a 25 pin
connector with the following connected together
2,3
4,5
6,8,20

From memory that should work as a loopback connector - then what is
typed on the keyboard will appear on the screen
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Old 1st Aug 2021, 5:50 pm   #19
Timbucus
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Default Re: Heathkit Computers and the UK especially H8

Thanks John - looking again it is actually the Half Duplex key on the keyboard that generates the ticking noises when you hold Repeat but, sadly displays nothing... I have of course not had five minutes today to look at the unit!
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