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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 12th Feb 2020, 4:12 pm   #101
julie_m
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Default Re: Commodore PET 3016

Quote:
Originally Posted by dominicbeesley View Post
Are we sure the keyboard is playing up and not the video character RAM - I'm not sure how it works on the PET but a BBC Micro was showing similar behaviour in text mode (7) which turned out to be a dodgy DRAM - just a thought.
It's an interesting thought; but if that were the case, you would expect the relationship between the "wanted" and "actual" characters to be different. Faulty memory would manifest as characters with similar bit patterns but one bit always set or never set; for example, if bit 4 were stuck "on" then you might get @→H, A→I, B→J, C→K ..... P→X, Q→Y, 0→8, #→: and so forth. When the relationship is that the "actual" character is in the same row or column of the keyboard matrix as the "wanted" one, it's more likely that the fault is at the stage of reading the keyboard.
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Old 12th Feb 2020, 9:06 pm   #102
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Default Re: Commodore PET 3016

Fair play. It was just a thought

D
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Old 12th Feb 2020, 9:45 pm   #103
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Default Re: Commodore PET 3016

It is by no means certain that the column driver IC is to blame although the nature of the fault, in which column N and N+1 of a row produce the same letters, then column N+2 and Column N+3 produce the same letters, then column N+4 and N+5 produce the same letters, etc, makes that an obvious thing to eliminate at the earliest opportunity.

It sounds like the 'KEY A' (ones bit) line between the 6520 and the 74LS145 is either stuck in one state or being read as such by the 74LS145, resulting in the column select line advancing by two, not one, and only on every second step of the four bit count being output by the 6520.

We can only wait and see.
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Old 13th Feb 2020, 11:11 pm   #104
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Default Re: Commodore PET 3016

Hi everyone,
Unbelievably the guy has already repplaced the IC for me and he has done a magnificent job. And I have the PET working! All keys work! Pressing the SHIFT key produced the graphics! I was euphoric! Except, and this is strange, when I switched the machine on again the following happened:

When I pressed !QAZWSX nothing happened. All the other keys worked! Even the @ symbol far left! Those two columns = nothing. When I pressed the reset button on the Tynemouth Software add on everything worked!

However, now when I try those things it remains the same no matter what I do. Pressing !QAZ@WSX produces nothing and the SHIFT has stopped working

How could it have worked so briefly and then stoppped??
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Old 13th Feb 2020, 11:27 pm   #105
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I'be just swapped the 6520 and all seems to be working again! I shall have to give it a thorough going over tomorrow morning - I will keep you posted!
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Old 13th Feb 2020, 11:37 pm   #106
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Default Re: Commodore PET 3016

Phew, what a relief that replacement of the 74LS145 does seem to have made a difference but I am wondering if the socket that the 6520 is fitted in is causing you problems as well - TonyDuell did say that in his experience the IC sockets in PETs can be troublesome.

If it keeps on working, just leave well alone (if it isn't broken, don't try to fix it). But if you do have further problems you may need to consider having the 6520 socket changed and have the solder joints on the keyboard connector resoldered as well.
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Old 13th Feb 2020, 11:41 pm   #107
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Default Re: Commodore PET 3016

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Originally Posted by SiriusHardware View Post
Phew, what a relief that replacement of the 74LS145 does seem to have made a difference but I am wondering if the socket that the 6520 is fitted in is causing you problems as well - TonyDuell did say that in his experience the IC sockets in PETs can be troublesome.

If it keeps on working, just leave well alone (if it isn't broken, don't try to fix it). But if you do have further problems you may need to consider having the 6520 socket changed and have the solder joints on the keyboard connector resoldered as well.
Yes-it might be worth doing! The guy I asked is a genius at soldering!!
Thank you so much for everything. You guys are the best! I will give it a thorough testing tomorrow morning and get back to you-fingers crosses and all that!!

Last edited by John Earland; 13th Feb 2020 at 11:42 pm. Reason: Mistyped sentence
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Old 14th Feb 2020, 9:46 am   #108
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Hi guys - right update!

I've just switched the PET on and nothing! Nothing on the screen at all! I then decided the replace all the chips with newer ones (the ones that are socketed of course) and it now seems to be working!

Last night the older chips were working fine and now this morning not. I'm wondering whether the board is affected by the cold?? Or, as you've pointed out the board's soldering is being temperamental!

Are there any checks I could be doing? It seems to be loading software from tape although no longer automatically plays them (as it once did). I have to type in RUN but at least it seems to load them.

I am going to keep the PET on for about an hour or so to see what happens. However, I've already noticed that some of the keys that were working have stopped e.g. ! and " top left
:-(
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Old 14th Feb 2020, 10:30 am   #109
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Default Re: Commodore PET 3016

I've just pressed the keyboard connector more firmly and then rubbed the decoder chip and hey presto - the keys are working again! I think there needs to be resoldering done!
J
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Old 14th Feb 2020, 11:23 am   #110
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Default Re: Commodore PET 3016

If you take the machine back to your soldering guru I would definitely ask him to replace the IC sockets, especially the one the keyboard 6520 sits in, but ideally all of them. There have been several occasions now when taking one lot of socketed ICs out of the machine and putting another lot in have changed the behaviour of the machine, sometimes for the worse, sometimes better, but even then only temporarily.

I strongly suspect that it is the sockets rather than the ICs which are causing your intermittent problems, but the keyboard connector has also been stressed quite a bit lately (unplugged / plugged in / unplugged / plugged in) and maybe the solder joints around the connector pins are starting to crack so get those checked and redone as well.
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Old 14th Feb 2020, 4:45 pm   #111
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Default Re: Commodore PET 3016

I should probably qualify the suggestion about replacing ALL of the sockets - to be more precise, I meant 'All of the sockets which currently have ICs in them'. Unless you plan to populate the empty sockets at some later stage, there is no need to replace those at the moment.
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Old 14th Feb 2020, 10:20 pm   #112
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Default Re: Commodore PET 3016

Quote:
Originally Posted by dominicbeesley View Post
Fair play. It was just a thought

D
Probably worth mentioning anyway, for the benefit of anyone who may come along in future, searching for help with a similar problem .....

Anyway, replacing the column decoder IC appears to have fixed one fault (key presses detected as wrong column) only to show up another (one row responding only intermittently). So, welcome officially to the world of vintage electronics restoration!

This behaviour positively screams "poor connection" -- something is making contact, but only just, and sometimes coming apart and going back together. It could be a cracked PCB trace or a broken wire, a dry solder joint or a fractured crimp, but it's most likely to be a mechanical connection -- a socketed IC or a plug and socket connection. Try probing about with a wooden cocktail stick to see if you can find where brings it in and out of contact. The schematic diagram will show you which connection the faulty keys have in common, and which connectors it passes through on the way to which pin of which IC.

It seems, from watching a few restoration videos, that the IC sockets on Commodore motherboards have a bit of a reputation for unreliability, and are best replaced with modern, turned-pin versions.
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Old 15th Feb 2020, 10:24 pm   #113
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Default Re: Commodore PET 3016

Thank you for all of your suggestions guys. I completely see where you are coming from and I will ask my friend to do the soldering on all the used sockets. I’m sure he won’t mind.

I have to say a massive thank you for e dry thing. I have learned loads! I feel so much more empowered now to tackle other projects!!

My aim now though is to try to install a piezo buzzer so I can hear sound when it plays space invaders and seeing how I can attach the PET to the internet! I’ve seen it being done on YouTube but I’d like to have a go!!

Best wishes
John
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Old 15th Feb 2020, 11:31 pm   #114
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Default Re: Commodore PET 3016

The easiest way of getting any 8-bit machine onto the Internet is with a Raspberry Pi; you can just use the PET as a dumb terminal, and send and receive characters over a serial link to the RPi.

Purists aren't going to like it, because the more powerful machine is really doing all the work. But when it's an effort just to get data into and out of a machine, I think you deserve a little slack
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Old 16th Feb 2020, 11:27 am   #115
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Default Re: Commodore PET 3016

I'm glad to see you are already knitting socks for your baby, John, but I must stress that those used sockets need to be replaced, not just resoldered. It might be prudent to ask if your colleague already has some sockets and if not, to provide him with some.
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Old 16th Feb 2020, 11:40 pm   #116
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Default Re: Commodore PET 3016

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Originally Posted by SiriusHardware View Post
I'm glad to see you are already knitting socks for your baby, John, but I must stress that those used sockets need to be replaced, not just resoldered. It might be prudent to ask if your colleague already has some sockets and if not, to provide him with some.
Thank you-yes, Iíve already ordered some from eBay and waiting for them to arrive. Heís already agreed to replace them for me! I feel really lucky to have such support!
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Old Yesterday, 6:31 pm   #117
John Earland
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Default Re: Commodore PET 3016

Hi guys, Whilst I wait for the sockets to arrive I thought I might experiment with putting sound on. The connections seem simple enough but my attempts doesn't work!

I've followed the instructions which says to use the user port. Connect the GND to the N and the 'live' to the speaker via a 50-500k resistor at M. Admittedly I have only used a 10k resistor but I expected this just to be loud - but maybe it's this that's causing no sound!

Schematic is here:
http://www.6502.org/users/andre/petindex/pinouts.html ( I think)

1 2 3 4 5 6 7 8 9 10 11 12
# # # # # # # # # # # #
A B C D E F H J K L M N

I've also seen this:
CB2 (the 2nd to last pin on USER port) -----> 20 ohm resistor ------> (+) PIEZO BUZZER (-) ---------> GROUND (last pin on the USER port)

Which is what I've done though not the same resistor.

Any advice?
Best wishes,
John
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Old Yesterday, 10:53 pm   #118
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Default Re: Commodore PET 3016

Attached, a snippet from the same diagram we used when looking at the keyboard problem, only this is the upper half and it shows the user port and its connections to the 'other' 6520 - the one which is not used for the keyboard.

If you want to be sure you are using the right contacts on the user port, use your meter-fu to check which of the user port pins is directly connected (zero ohms) to pin 19 of the second 6520. That user port connection will be 'M'/CB2.

You can use any user port pin which is directly connected to 0V as the return connection, but 'N' will be the most convenient as it is next to 'M'. Again, use your meter to check that the user port connection you believe to be 'N' is directly connected (zero ohms) to any known 0V point.

We need a little more info about the sounder / buzzer you are trying to use, either a link to where you bought it from or, if it's one you already had, a few clear images.

The diagram shows CB2 coming directly from the CB2 pin (pin 19) of the second 6520 and if that's true the CB2 pin alone will not have enough strength to drive a low or medium impedance speaker. It may be able to drive a high impedance transducer such as a passive piezo sounder, although there are potential pitfalls when using those too.

Let us know what sort of sounder you actually have.
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Old Yesterday, 11:06 pm   #119
John Earland
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Default Re: Commodore PET 3016

Hi, thank you-I will go through the checks tomorrow-the buzzer is a cheap on from eBay-12v DC though it is quite loud from a 9v battery. Thanks for getting back to me.
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Old Yesterday, 11:26 pm   #120
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Default Re: Commodore PET 3016

Ah, that sounds like you have an 'active' buzzer which consists of an oscillator and a sounder combined. The CB2 pin on the 6520 probably doesn't output enough voltage and current combined to run that, and even if it did it would output a single fixed tone whenever it was powered. If it makes a loud continuous tone when powered, it's the wrong sort.

What you really needed was something which was just a bare high impedance sounder with no included electronics. Something like these: (This was the first example I found - similar devices are available from other suppliers).

https://www.amazon.co.uk/sourcingmap.../dp/B077GL6HMN

The key word is 'Passive' as opposed to 'Active'. 'Active' means the unit has a built in fixed tone generator circuit. 'Passive' means the unit only contains the transducer and your circuit - in your case your PET, has to provide the pulse waveform to make it produce sound.
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