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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 Oct 2022, 6:52 pm   #161
SiriusHardware
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Default Re: 1978 Commodore PET project

Some soldering iron families offer(ed?) a range of tips which included shaped 'block' tips designed to exactly match up with all of the pins of a DIP IC at once, precisely for the purpose of desoldering DIP ICs. I've always felt they are more about removing the ICs intact than they are about preserving the board, as it would be very easy to burn the board by applying so much heat to so much of the PCB at once. They are probably best used for salvaging ICs from PCBs considered to be scrap IMO.
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Old 3rd Oct 2022, 6:59 pm   #162
Mark1960
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Default Re: 1978 Commodore PET project

Its also possible to get a tip for the soldering iron thats designed to heat every pin of a 14 or 16 pin dil ic. Not sure they go a far as 20 pin for the 74ls244. On an old pcb like the PET there is still a risk of damage to top side pads or traces if the solder is not properly molten on the top side when the IC is pulled. I would possibly try with a temperature controlled heat gun from the top side of the board, but this has a similar risk, which is why we haven’t recommended this for Brackenfix.
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Old 3rd Oct 2022, 7:14 pm   #163
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Default Re: 1978 Commodore PET project

I have found in the past that the bonds / glue holding the tracks and pads onto old 'fibre glass' boards of this age aren't very heat tolerant and are easily overheated and cooked right off the board, plus it's very easy to brown or blister an area of the PCB by applying heat to one place for a little bit too long. I definitely would not recommend the use of a hot air wand to try to remove ICs on an old PCB like this, however, pre-warming - warming, not baking, the area to be worked on just before using a desoldering gun does help.
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Old 7th Oct 2022, 2:22 pm   #164
BrackenFix
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Default Re: 1978 Commodore PET project

I replaced UE7 and had the same so moving onto the next chip suggested now

For info, if I remove UE7, I get the attached

NEXT!!!
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Old 7th Oct 2022, 2:41 pm   #165
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Default Re: 1978 Commodore PET project

It may not seem like it, but removing / replacing an IC and seeing no change is actually good in one respect - it means you've successfully carried out the work without introducing any further faults.

Sorry it didn't turn out to be UE7, as Tim said the 74LS244 buffers (UE7) have a bad reputation for failure and as Tony said, 2114 RAMs (UF7) are basically just regarded as electronic fuses...

...So it'll probably turn out to be UF9 after all!

How are you taking the ICs out, in bits or in one piece?
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Old 7th Oct 2022, 3:00 pm   #166
BrackenFix
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Default Re: 1978 Commodore PET project

The results are in and..............

UF9 was the fault!!!!!
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Old 7th Oct 2022, 3:15 pm   #167
BrackenFix
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Default Re: 1978 Commodore PET project

Quote:
Originally Posted by SiriusHardware View Post
It may not seem like it, but removing / replacing an IC and seeing no change is actually good in one respect - it means you've successfully carried out the work without introducing any further faults.

Sorry it didn't turn out to be UE7, as Tim said the 74LS244 buffers (UE7) have a bad reputation for failure and as Tony said, 2114 RAMs (UF7) are basically just regarded as electronic fuses...

...So it'll probably turn out to be UF9 after all!

How are you taking the ICs out, in bits or in one piece?
Took them all out in one piece with no damage to the chips or the board

Miracle!!!!!!
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Old 7th Oct 2022, 3:19 pm   #168
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Default Re: 1978 Commodore PET project

Odd result as the input of UF9 seemed to work when pulled down by the resistor. Maybe the 2114 is not strong enough to pull down the input of the old UF9. See what happens when its run for a few hours.
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Old 7th Oct 2022, 3:50 pm   #169
BrackenFix
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Default Re: 1978 Commodore PET project

See, now I am wondering if I'm right

I am all in a pickle as I actually socketed UF7 and UF9 at the same time for ease. I got a bit excited and put the new chips in both. I then realised that was silly so i replaced UF7 with the original and left the new UF9 in and that didnt work so put the new UF7 in.

That means what worked as UF7 and UF9

Numpty
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Old 7th Oct 2022, 4:15 pm   #170
SiriusHardware
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Default Re: 1978 Commodore PET project

It's great that you've got it working and believe me, that was no mean feat with only a meter to diagnose the fault with - you were a little bit lucky with the area where the fault lay, which lent itself to investigation with a meter. For problems in most other areas you would have needed a scope to get anywhere.

Your original idea was that it was the display memory which was certainly in the right ball park area.

If you removed the original ICs intact and fitted sockets you can always try the original ones back in one at a time to see which one reinstates the fault, and keep the other (working) original ICs as potential spares.

So how's that keyboard working now?

Last edited by SiriusHardware; 7th Oct 2022 at 4:31 pm.
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Old 7th Oct 2022, 4:39 pm   #171
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Default Re: 1978 Commodore PET project

Aw is that it (well done for fixing it so quickly)?

Colin.
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Old 7th Oct 2022, 4:39 pm   #172
BrackenFix
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Default Re: 1978 Commodore PET project

Keyboard is working very well. Every key seems to work perfectly.

Going to service the tape deck (i have a new belt etc) then clean everything and get it running some tapes etc.

Although i have done a video, I need to forewarn everyone that I did but a desoldering tool but only a cheap one. That said, it seemed to work perfectly!

https://uk.farnell.com/duratool/d018...oss_price=true
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Old 7th Oct 2022, 4:41 pm   #173
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Default Re: 1978 Commodore PET project

Quote:
Originally Posted by ScottishColin View Post
Aw is that it (well done for fixing it so quickly)?

Colin.
Lets see if it actually computes first
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Old 7th Oct 2022, 5:14 pm   #174
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Default Re: 1978 Commodore PET project

There is also the question of whether the IEEE-488 port works as it seems to be a very common secondary problem which comes up on repaired PETs, but it only really matters if you intend to use either a real Commodore disc drive unit or something which pretends to be one, like the SD2PET.
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Old 8th Oct 2022, 7:22 am   #175
BrackenFix
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Default Re: 1978 Commodore PET project

If you fancy seeing how I did removing the chips

https://www.youtube.com/watch?v=N_7VKa866yI
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Old 8th Oct 2022, 9:15 am   #176
SiriusHardware
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Default Re: 1978 Commodore PET project

Well done - those 'simple' electric desoldering irons can potentially be quite hard on the workpiece so you have to have quite a deft touch to be able to use one of those without doing any damage.

If you ever get to the point where you feel you can afford an electric pump assisted one you should still consider buying one - the main difference obviously is that an electric pump assisted desoldering iron keeps on sucking for as long as you hold the button down so you have a much better chance of clearing each hole at the first attempt.

It will be interesting to see how you progress with the tape deck. Very good going so far.
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Old 8th Oct 2022, 9:59 am   #177
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Default Re: 1978 Commodore PET project

Excellent progress. I'm impressed!

Alan
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Old 8th Oct 2022, 10:09 am   #178
ScottishColin
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Default Re: 1978 Commodore PET project

The code I used to test the IEEE488 port is in this book:

http://www.primrosebank.net/computer...8_Bus_text.pdf

The testing Appendix starts at page 181 of the book and the code is on page 184.

It'll be easier if your tape drive works so you can type this in and save it on a cassette rather than having to type it in each time, so the focus on the tape drive is probably the right thing at the moment.

Colin.

Last edited by ScottishColin; 8th Oct 2022 at 10:21 am.
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Old 8th Oct 2022, 11:16 am   #179
BrackenFix
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Default Re: 1978 Commodore PET project

Quote:
Originally Posted by SiriusHardware View Post
Well done - those 'simple' electric desoldering irons can potentially be quite hard on the workpiece so you have to have quite a deft touch to be able to use one of those without doing any damage.

If you ever get to the point where you feel you can afford an electric pump assisted one you should still consider buying one - the main difference obviously is that an electric pump assisted desoldering iron keeps on sucking for as long as you hold the button down so you have a much better chance of clearing each hole at the first attempt.

It will be interesting to see how you progress with the tape deck. Very good going so far.
Will definitely be getting an electric one, I can completely see why the manual ones can damage boards as they are quite aggressive to say the least.
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Old 8th Oct 2022, 11:17 am   #180
BrackenFix
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Default Re: 1978 Commodore PET project

Quote:
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Excellent progress. I'm impressed!

Alan

Thank you, that's very kind
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