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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 25th Apr 2021, 9:39 pm   #1761
ortek_service
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Default Re: Non-working Commodore PET 3016

Quote:
Originally Posted by ScottishColin View Post
>>
>>
At rest after powerup,
UC7/6 (datasette 1) - 4 v
UC7/7 (datasette 2) - 5v

Pressing play (without any LOAD commands)
UC7/6 - 0.23v
UC7/7 - 5.1v (this noticeably changes on the meter from 5v)
>>
>>
That's some interesting results......

UC7/7 needs to go close to 0V to work.
- So if the Datasette on Port 2 is all OK, then there must be a break in the connection of UC7/7 from J6 (Port 2) Pin F-6.

And the prime suspects would be someone has removed one of the cassette jumper links
Or that UC7's IC socket is dodgy on pin 7.

So a bit of tracing with the DMM on continuity-test should find the fault. And hopefully, no parts need to be bought.
- Especially if you've already got some spare DIL40 sockets
(Although if UC7's socket is at fault, then may be able to make original one work for a bit, with some cleaning of that pin)
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Old 25th Apr 2021, 9:57 pm   #1762
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Default Re: Non-working Commodore PET 3016

Well that turned out to be a pretty simple answer that I should have thought of beforehand.

The motherboard connector for datasette 2 was filthy. A quick rub down with a rubber to shine the pins up - all working fine now thanks.

I do wonder why the external ports at the back of the PET are gold plated and datasette2 is silver plated - but it's all working now thanks.

Last thing to test is the user port. I have a couple of connectors and will knock up a speaker tomorrow for Space Invaders to test it.

Colin.


Quote:
Originally Posted by ortek_service View Post
Quote:
Originally Posted by ScottishColin View Post
>>
>>
At rest after powerup,
UC7/6 (datasette 1) - 4 v
UC7/7 (datasette 2) - 5v

Pressing play (without any LOAD commands)
UC7/6 - 0.23v
UC7/7 - 5.1v (this noticeably changes on the meter from 5v)
>>
>>
That's some interesting results......

UC7/7 needs to go close to 0V to work.
- So if the Datasette on Port 2 is all OK, then there must be a break in the connection of UC7/7 from J6 (Port 2) Pin F-6.

And the prime suspects would be someone has removed one of the cassette jumper links
Or that UC7's IC socket is dodgy on pin 7.

So a bit of tracing with the DMM on continuity-test should find the fault. And hopefully, no parts need to be bought.
- Especially if you've already got some spare DIL40 sockets
(Although if UC7's socket is at fault, then may be able to make original one work for a bit, with some cleaning of that pin)
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Old 25th Apr 2021, 10:54 pm   #1763
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Default Re: Non-working Commodore PET 3016

Whew, at least the 6522 is OK.
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Old 25th Apr 2021, 11:17 pm   #1764
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Default Re: Non-working Commodore PET 3016

Quote:
Originally Posted by ScottishColin View Post
Well that turned out to be a pretty simple answer that I should have thought of beforehand.

The motherboard connector for datasette 2 was filthy. A quick rub down with a rubber to shine the pins up - all working fine now thanks.

I do wonder why the external ports at the back of the PET are gold plated and datasette2 is silver plated - but it's all working now thanks.

Last thing to test is the user port. I have a couple of connectors and will knock up a speaker tomorrow for Space Invaders to test it.
That's good to hear. And quite surprising that connecting and disconnecting the Datasette plug a few times didn't clean the PCB's edge pads (which will be Tin / Solder plated, rather than silver - who's oxide is apparently actually conductive, as some watch batteries use that) contact enough - particularly if a standard rubber did work, and didn't need an abrasive version or IPA / contact cleaner.

But it seems it was just years of muck that had accumulated on these quite exposed connectors, and many PET's may have been badly stored for years due to their size making them difficult to keep in a box etc!

IIRC there's actually pads on both top & bottom sides of PCB, as they number & letter these, so I thought that would increase connection reliability - But looking inside a Commodore Datasette plug, the contacts are only on one side!

I presume they didn't think Gold plating was necessary for so few connections. And many computers did generally work OK without Gold plating on their edge connectors (even though the mating connectors sometimes were). Plus it seem that unless both parts are Gold plated, then it doesn't always make it help


I couldn't remember if you'd been able to test the User port Data I/O lines, from the details previously given. The User Port article did also give a test for reading CA1 input & Outputting to CB2 I/O line (But it seems reading from CB2 may not work in BASIC, due to Kernal's interrupts accessing registers).
Although, if CB2 is mainly used for providing a Sound Output, then that should probably test it good enough for most purposes.


I did also see this article a short while ago, featuring a new clone design, new PCB based where they combined the Cassette's Read and Write lines with PB2 so you could listen to the cassette's data as well https://www.eevblog.com/forum/projec...programmer/75/
(I think debugging that Breadboard (Not SiriusHardware's favourite construction method, it seems), could take a while!)

As all are on User port, you could do something similar - and might be able to do it with just a few high-value resistors, to save on some complexity (but ideally would need to feed into a small linear amplifier stage, rather than direct to a speaker-switching transistor)

Last edited by ortek_service; 25th Apr 2021 at 11:30 pm.
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Old 25th Apr 2021, 11:30 pm   #1765
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Default Re: Non-working Commodore PET 3016

I like the principle of breadboard, but not the practice - too many component and jumper lead pins with highly variable shape and thickness all going into the same size / shaped holes in the breadboard is a recipe for unreliability.

The problems that I (and Colin) recently encountered were mainly to do with the jumper leads we used initially being a very poor / slack fit in the receptacles they were plugged into. I did subsequently find that the 'Dupont' type with their square pins and square connector bodies were a lot better.
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Old 25th Apr 2021, 11:47 pm   #1766
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Default Re: Non-working Commodore PET 3016

Yes, I do remember using old thick resistor leads in breadboard. Then trying to insert polystyrene capacitors leads (A bit like with IC sockets and wrong pins types)
But with breadboard, you did at least tend to be able to feel when the leads were too loose, and could remove the bottom and the contact strips to crimp them back up again (or remove snapped-off wire ends!). The real problem is when a complex built circuit stops working, due to a wire getting dislodged or breaking and you've got to go through it all. Might also find that some of the recent cheaper versions don't grip as tight, even when new.
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Old 26th Apr 2021, 9:11 am   #1767
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Default Re: Non-working Commodore PET 3016

I suspect this will be nearly the last post on this mammoth thread - I want to take the PET apart to be able to clean the monitor board and to be able to de-rust and respray the case.

I am vaguely aware that poking at CRTs even while switched off may not be a good idea. I have just turned the PET off; is it easiest for me to wait until everything is discharged or is there something I should do to discharge things?

The PET is not plugged into any 240V socket right now.

Thanks.

Colin.
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Old 26th Apr 2021, 10:03 am   #1768
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Default Re: Non-working Commodore PET 3016

You should ideally discharge the tube first, I'll leave it to the CRT experts to tell you how to do that safely and correctly.

If / when you remove the tube be as protective towards it as you know how, especially with the tube neck and tube end where the connections are. Also at least wear protective eyewear when handling / moving it because if you fumble it and drop it there will be glass everywhere.
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Old 26th Apr 2021, 12:49 pm   #1769
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Default Re: Non-working Commodore PET 3016

Forgive me for butting in but I have recently been given a number of old computer chips and there were quite a number of devices specific to the Commodore Pet (various vintages from what I can gather) and Commodore 64 - which I am looking to get hold of at some point but that's another story.

So if you do find one of the less common ICs at fault - I may have one.

On the subject of discharging the tube - be warned - it's like a VERy big capacitor with the anode at several thousand volts. Even when discharged they have a habit of somehow attracting a charge back again! I found out the hard way when I was fixing a monitor many years ago at the BBC and it bit me!

Thanks,

Brian
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Old 26th Apr 2021, 1:27 pm   #1770
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Default Re: Non-working Commodore PET 3016

The 2nd Cassette port on the 3000 series was likely only there for the case when the mainboard was used to upgrade/replace the mainbourd from a 2000 with built in cassette, which is probably why its not gold plated. The 3000 series were mainly aimed at schools and businesses and by the time it was released the floppy drives were available, and the need for 2 cassette ports had gone (early PET software was made that processed data from 1 tape to the other mainframe style, but no-one would seriously do this once a floppy drive was available). The internal cassette connector on the 3000 series was pretty much vestigial.
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Old 26th Apr 2021, 2:32 pm   #1771
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Default Re: Non-working Commodore PET 3016

An epic repair with 1770 posts. Having watched this thread from near the start and having been in electronic design and programming using early 8 bit CPU's back in the 80's, you almost had me longing for a pet...but the number of faults has thankfully cured me of that.
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Old 26th Apr 2021, 4:46 pm   #1772
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Default Re: Non-working Commodore PET 3016

Surprised no-one has jumped in with information about how to discharge the tube. A search on Youtube gets you some hits of varying quality but all showing the same basic method.

All the ones I found advocate connecting one bare end of an insulated wire to one of the four corner fixing nuts of the tube, attaching the other bare end of the wire to the metal shaft of a longish screwdriver and - (holding the screwdriver by its insulated handle!) - slide the tip of the screwdriver under the rubber cap on the tube until it makes contact with the high voltage contact. You might hear a 'zap' or 'click' when the tip of the screwdriver gets close to the contact and the stored high voltage jumps across the gap - even so, keep going until the screwdriver tip definitely makes direct contact with the connection under the rubber cover.

To disconnect the high voltage connection after discharge, don't just try to pull it straight up and off and definitely do not try to lever it off - to release it, you need to squeeze the centre portion of the rubber cap from the sides, easing the whole rubber cap from side to side as you do so.
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Old 26th Apr 2021, 6:24 pm   #1773
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Default Re: Non-working Commodore PET 3016

Right. I'll get my wellies on and some rubber gloves and only handle things with one hand.

My will is up to date. I'll let you know how it goes.

Colin.


Quote:
Originally Posted by SiriusHardware View Post
Surprised no-one has jumped in with information about how to discharge the tube. A search on Youtube gets you some hits of varying quality but all showing the same basic method.

All the ones I found advocate connecting one bare end of an insulated wire to one of the four corner fixing nuts of the tube, attaching the other bare end of the wire to the metal shaft of a longish screwdriver and - (holding the screwdriver by its insulated handle!) - slide the tip of the screwdriver under the rubber cap on the tube until it makes contact with the high voltage contact. You might hear a 'zap' or 'click' when the tip of the screwdriver gets close to the contact and the stored high voltage jumps across the gap - even so, keep going until the screwdriver tip definitely makes direct contact with the connection under the rubber cover.

To disconnect the high voltage connection after discharge, don't just try to pull it straight up and off and definitely do not try to lever it off - to release it, you need to squeeze the centre portion of the rubber cap from the sides, easing the whole rubber cap from side to side as you do so.
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Old 26th Apr 2021, 6:48 pm   #1774
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Default Re: Non-working Commodore PET 3016

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My will is up to date
Are we in it? If so, we may have some last minute changes to add to the advice...
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Old 26th Apr 2021, 6:57 pm   #1775
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Default Re: Non-working Commodore PET 3016

On a serious note, if you do this you must be sure that the 'corner nut' end of the wire is securely connected and there is absolutely no chance of it slipping off.
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Old 26th Apr 2021, 7:06 pm   #1776
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Default Re: Non-working Commodore PET 3016

So I've taken the back off and this doesn't look like what I thought it might - there's no rubber cap on the end of the tube for example, and the corner screws are a long way back (ie right at the front of the monitor) and will require me poking into there with a long screwdriver.

Any good ideas

a) where to place the screw-driver to short it?
b) where I can connect the wire as I can't see how I can safely get a screwdriver all that way past the screen, unscrew the screw, not drop the screw, fashion some kind of connection with the wire and then screw the screw back in.....

Colin.
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Old 26th Apr 2021, 7:14 pm   #1777
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Default Re: Non-working Commodore PET 3016

Not the best angle as it happens, but do I not see a rubber cap on the centre RHS of the tube as you look into the rear of the monitor?

I'm afraid you are going to have to get a secure connection to the outside frame of the tube (in other words, one of its fixing nuts) as that is probably the only safe place to discharge the tube to.

If you are going to attempt this, disconnect the monitor from the mainboard and swing the connector and cable loom well out of the way so that if you do have some kind of mishap there's no risk of you sending the tube discharge onto the mainboard.
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Old 26th Apr 2021, 7:16 pm   #1778
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Default Re: Non-working Commodore PET 3016

the rubber cap is on the right hand side half way down the tube and towards the front
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Old 26th Apr 2021, 7:34 pm   #1779
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Default Re: Non-working Commodore PET 3016

I'm being dense. Would it be possible for someone to take these two photos and have a look and draw on them where the screwdriver goes?

Thanks.

Colin.
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Old 26th Apr 2021, 7:35 pm   #1780
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Default Re: Non-working Commodore PET 3016

Yes, I think Colin may had misunderstood the comment about the cab being on the 'end' of the tube, whereas it's actually on the side.

As the CRT on these is relatively small and mono, then the EHT it uses will be rather less (Maybe 10kV), compared to the 25kV that was used on larger Colour CRT's.

I recall I used a DMM probe, sliding it under the cap, although strictly speaking many of these are only rated to around 1kV (continuous).
But you may find that there will be a small arc to the probe, once it's quite close to the metal connection before it actually touches it, so not quite the full voltage (And CRT may have self-discharged a bit so probably won't be at full operating voltage)

If there is a lot of years of muck inside the PET there, you might want to actually remove the EHT Cap and give the outside of the CRT a bit of a clean to prevent future conduction away from the cap.
But take care when trying to disconnect the cap, as whilst some just click-in, others have a sprung-hook on each side, and must unhook one side first by pushing down on cap or using an insulated screwdriver.
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