1978 Commodore PET project

[BrackenFix]

Originally Posted by kan_turk

If we were to exclude anybody who did anything silly this would be a very, very quiet place - well done so far

Thank you so much

[ScottishColin]

I can vouch for this.

Some remarkably patient people on here.

Colin.

Originally Posted by kan_turk

If we were to exclude anybody who did anything silly this would be a very, very quiet place - well done so far

[ScottishColin]

If it comes to it, with my newly found EPROM reading/programming skills, I'm happy to check and/or burn ROM replacements if we get to that.

Colin.

[BrackenFix]

Originally Posted by ScottishColin

If it comes to it, with my newly found EPROM reading/programming skills, I'm happy to check and/or burn ROM replacements if we get to that.

Colin.

Amazing, thanks so much

Ill put it all back together one i get the chip fixed and we will go from there

[SiriusHardware]

I'm happy to check and/or burn ROM replacements

Colin has the advantage over me in that he not only has EPROMs and a suitable programmer for them, he also has a PET to test them in.

[ScottishColin]

Looking at your video, the CPU seems to be working and you have power to the cassette.

Others can help you with the fix to the current problem.

You will probably need a belt for the datasette as it may have perished - search for a C2N belt and you should find one easily enough. This video at about the 15:30 mark shows how to take the datasette apart and which cable to get:

https://www.youtube.com/watch?v=WMKECAQjCH8

The same video shows how to take the chiclet keyboard apart - the keyboard will need taking apart and a good clean. There's a possibility that some of the traces on the keyboard PCB may have failed (mine had) but you won't know that until you take it apart and do some continuity testing. Let me know when you get to the keyboard as I did mine and can help there.

Colin.

[ajgriff]

I admire your enthusiasm BrackenFix but please don't start taking anything else apart until you've fixed the basic problem. Softly, softly, catchee monkey.

Alan

[BrackenFix]

Will do

[Timbucus]

Good luck with the repair and the channel. I would also suggest you obtain an antistatic wrist strap and make sure the deskmat is grounded when handling chips - some can be easily destroyed.

A WARNING: Please, until you have extensively studied the safety needs, do not handle the mains side of the PSU or Video/Tube areas of the machine as they can both be fatal. If in doubt seek advice here ahead of any planned works.

You are doing the right thing watching Adrian he covers a lot of ground - you will also find several of the series by JM Precision excellent as a really experienced repairer of vintage technology (and lots of other things). https://www.youtube.com/c/JerryWalker-JMPrecision

[BrackenFix]

I am very wary of the PSU etc, I'm am hoping I can leave them well alone but will absolutely seek advice before doing anything

[ajgriff]

The mains wiring is hidden away underneath the transformer and the exposed connections on top are all low voltage. The monitor seems to be working fine so there’s no need to remove the back and delve inside.

FOR FUTURE REFERENCE ONLY, the machine featured in the video quoted by Colin (post #66) is an earlier version of the PET. The board layout is quite different and it uses different RAM (static rather than dynamic) and ROM chips. Even the PSU connector is not the same and nor is the tape drive. Also note that the chiclet keyboard is more delicate than the business style keyboard Colin dismantled and it’s very easy to break a keypad from its plunger. Notice that the presenter in the video didn’t attempt to remove the keys.

As a general point a lot of jargon and acronyms are being used here so you’d be well advised to spend time ‘googling’ so that you understand what a PIA (for example) is and does.

Let us know when you’ve got back to square one.

Alan

[SiriusHardware]

We haven't mentioned solder yet - all the solder in this PET will be lead/tin solder and so any repairs you make to this ideally need to use the same sort of solder - lead/tin, not lead-free.

There's also another reason to favour lead-tin solder, lead-free solder is absolutely awful. When you get your desoldering gun one of the things you will notice quite quickly is that they work best if there is a generous amount of solder on the pins to be desoldered, so very often the first step in desoldering is, counter-intuitively, to put more solder onto the joint first, before you try to desolder it.

Lead/tin solder has a lower melting point than lead-free solder so it is a standard technician's trick to top up joints which are to be desoldered with lead/tin solder even if the unit or PCB was built with lead-free solder, as the lead/tin solder lowers the melting point and makes desoldering easier.

So - if you don't already have some lead/tin solder in your toolbox, it would be a good idea to get hold of some.

[BrackenFix]

Originally Posted by SiriusHardware

So - if you don't already have some lead/tin solder in your toolbox, it would be a good idea to get hold of some.

Awesome, ill order some now

[BrackenFix]

Originally Posted by SiriusHardware

Could you please measure the DC voltages on the component leads indicated on the attached sketch? In case it's not clear from the diagram, we are asking for measurements on the right hand ends of diodes CR10, CR11, CR12 and on the lowest lead of regulator VR6. If you need somewhere to put the black probe while making these measurements, the -Ve (right hand) end of C62, the large electrolytic with the cable tie around it, is connected to 0V.

Hi, sorry for the delay

I have got it back together and it still has the same issue. I guess the silver lining is that I didn't break it any further!!

I will be honest though, soldering new legs to the damaged UD7 chip didn't go well and so I had to adapter socket it. I tested continuity and all seems ok. Full disclosure, I have messaged ScottishColin about getting a replacement. God knows what damage I have done to it

I have tested the voltages as above and here are the figures

CR10 = 5.11v
CR11 = 5.09v
CR12 = 11.75v
VR6 = 11.16v

[SiriusHardware]

No problem, good that you are back to where you were. If the symptoms are exactly the same as they were before the legs fell off then you've done it right, as you wouldn't get the same result if the ROM was not being read correctly.

Can we just ask you to report the voltages on the other two legs of VR6 (Regulator) before proceeding?

[BrackenFix]

The OUT leg of the regulator (VR6) was actually -11.3v. Same with the IN.

Ground was 0

[SiriusHardware]

Hmm, that's not quite right, as VR6 is the minus FIVE volt regulator.

You may or may not know, but the pinout on the 79-series (negative) regulators is not the usual IN-GND-OUT arrangement that you find on 78 series regulators. Instead, it's GND-IN-OUT.

Either way, one of those three VR6 regulator pins should be at minus five volts with respect to a definite 0V / GND point such as the -ve end of C62.

If you really don't have minus five volts on one of the pins of VR6, that's your next problem to sort out.

[BrackenFix]

Sorry, ROOKIE MISTAKE!

I do indeed have -5v on the OUT pin. I was accidentally reading the pad below as had the probe too far down

Embarrassing!!

[SiriusHardware]

That's a lot better than there really being -12V on the -5V rail, as that could have done some damage further downstream - so don't worry, we'll take that.

Assuming you are back where you were in post #1, could you make voltage measurements from the following IC pins and report back? I'm at work today so I don't really have time to go into why I'm asking you to do this, but I will try to expand on it this evening or maybe someone else can explain in the meantime.

What voltages do you see on:

UF8 Pin 14
UF8 pin 13
UF8 pin 12
UF8 pin 11

UF7 pin 14
UF7 pin 13
UF7 pin 12
UF7 pin 11

?

[BrackenFix]

I cant find a pin out diagram for the MPS2114 chip

I have found my board is the 380351 as expected