Commodore PET 4016

[ScottishColin]

I've managed to pick up the PET and the disk drive that I posted about a few weeks ago.

The case is going to need some proper rust treatment this time - not just a rub down and a respray.

Interestingly when I opened it, there is a Supersoft High Resolution Graphics Board (HR40B) installed. My plan is to remove that and get the PET working first and then start on the graphics board.

The board seems to have been pretty thoroughly documented and reverse engineered here if anyone wanted to do some light reading:

https://github.com/sjgray/SuperSoft-HR

https://github.com/InsaneDruid/SuperSede-HRx

This has previously been discussed on vcfed and there are some links there to flickr which show the capabilities of the board.

https://forum.vcfed.org/index.php?threads/supersoft-hr-40-b-hi-res-graphics-board.58668/

I'll get everythng disconnected and check volatges first and I'm sure I'll be back here for help.

Colin.

[SiriusHardware]

Good work finding the info about the third party graphics PCB although I agree that if you can extricate it and return the machine to as-built in a non-destructive way that will probably make it easier to work on.

[ortek_service]

That Supersoft High Resolution Graphics Board (HR40B) is a little strange, in that it looks more of a memory-expansion board, being mostly 4x 6116 (2KB SRAM)? and a couple of EPROM's - Only one with a protective label over the window, but maybe the other one's label fell off.

It's not exactly-clear from the photo's, but it looks like it mainly connects over one of the soldered-in DIL-40pin IC, with a DIL24? Header, in the blue plastic housing? But not clear if this is pushed over to top or whether it has been soldered to the DIL-40pin IC which would make removal rather more involved.

I can see its RAM could be to provide more VRAM, that would be required to store higher resolutions screen. But wondered what the EPROM's are for / how they are mapped - Maybe character ROM's for text to higher-res graphics?
I presume some of the other (logic?) circuitry is to change the timing, to get higher-resolutions? Not sure if this PET has a 6545/6845 that could be just re--configured as long as there's enough VRAM.

[ScottishColin]

There's two EPROMs - one to replace the character ROM and one to replace the ROM at E000.

https://mikenaberezny.com/wp-content/uploads/2017/05/superso...alog_spring_1982_graphics_board.pdf

I've popped a sticker back onto the exposed EPROM and will take a look at it once the PET is working.

There is a HD46505SP on this board which I believe is functionally equivant tp the 6545 used in later PETs.

Colin.

[ScottishColin]

Quick update.

It's a 4016 which has been upgraded to 32 K. Also one of the 6520s has previosuly been replaced.

On the upside, the monitor and the monitor PCB work fine as I popped in another 4032 motherboard and all is good.

Back to the PET in question.

I have discovered that the 6502 and the 6522 are duff and have been replaced. Having doen that, the frequencies are all fine and pop out on UD13 and UD14 as pre previous testing with a NOP generator installed.

However, no chirp and nothing on the screen.

I took a quick squizz at the CRTC chip on here and on a working PET, the frequency is 50hz on pin 40. On this PET, it's about 11ish so there's something not right here.

Any suggestions before I take out the CRTC chip?

Colin.

P.S. the motherboard is an 8032080 with no factory fitted amendments (which my last 4032 did have). Schematics here:

https://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/univ/index.html

[ortek_service]

Originally Posted by ScottishColin

>>
>>

There is a HD46505SP on this board which I believe is functionally equivant tp the 6545 used in later PETs.

Colin.

I presume that this HD46505SP is actually on the PET's main board rather than the Supersoft High Resolution Graphics Board? - As there wasn't and DIL 40pin IC's on the top, unless it is hidden on the underside (It did appear like there might be some DIL 24pin? footprints on the top that had pins coming from the underside).
And the (HD4)6505 is pin and software compatible with the 6545, if that is normally used in most PET's? I presume the 6505 is original and not swapped instead of an original 6545 for the Supersoft HRG board to work?
- I don't recall ever seeing the 6505 used in anything before, so could be difficult to obtain.

And was the 6505 the IC on the PET main board that the blue header cable from the Supersoft High Resolution Graphics Board fitted over, and presumably now removed?

I wonder if anything on the main board needs to be isolated for the Supersoft High Resolution Graphics Board to function and restored again if you've removed it?
Although if this board is more of a RAM / CharGen / Display ROM upgrade, then I presume the video signal itself still comes from the Mainboard circuitry with H & V timing from the (HD4)6505 CRTC ?

[ortek_service]

Originally Posted by ScottishColin

Quick update.

It's a 4016 which has been upgraded to 32 K. Also one of the 6520s has previosuly been replaced.

On the upside, the monitor and the monitor PCB work fine as I popped in another 4032 motherboard and all is good.

Back to the PET in question.

I have discovered that the 6502 and the 6522 are duff and have been replaced. Having doen that, the frequencies are all fine and pop out on UD13 and UD14 as pre previous testing with a NOP generator installed.

However, no chirp and nothing on the screen.

I took a quick squizz at the CRTC chip on here and on a working PET, the frequency is 50hz on pin 40. On this PET, it's about 11ish so there's something not right here.

Any suggestions before I take out the CRTC chip?

Colin.

P.S. the motherboard is an 8032080 with no factory fitted amendments (which my last 4032 did have). Schematics here:

https://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/univ/index.html

Well with two ? IC's found to be faulty so far and the amount of rust inside this PET / on the TO3 regulators cases (I presume these are all functioning OK, and the DC supply rail voltages have all been measured?), then the (rather-bad for these) storage conditions could well have damaged many IC's - As Chris was seemingly finding on his latest 'Barn find' acquisition. And multiple IC failures seems rather-common on the various other faulty PET's you've obtained!

But maybe the various diagnostic tools you've now got may be of assistance?
- Although I presume these all rely on video output to see the results, and the CRTC IC needs the CPU system to be running correctly to work, making it harder to fault find on these CRTC models when there's no video output.

I presume the 'Chirp' doesn't rely on the CRTC working, so that may be an indication that something's not right with the main CPU system at least.
I wonder of there is a diagnostic tool that wiggles port lines to show it is running OK / if you can normally generate a 'Bell' etc from the keyboard?
Are there any other socketed IC's you can check like the ROM's - reading out / trying in other PET's
There's also the multitude of (buffered by the 'Commodore fuses' 74LS244 buffers, so several of these may be faulty) buses that complicate things.
- Maybe looking on your 'scope on either side of these 74LS244's to check for any buffer outputs that do not pulses / follow their inputs?

[SiriusHardware]

Coming to this a bit late but the fact that you have totally wrong V_Drive frequency does not necessarily mean the CRTC is dud, it may just mean that the CPU subsystem is not functional enough to set the chip up to generate the right V_Drive and H_Drive frequencies.

So if you remember we came up with a trick to see if the CPU was attempting to write something to the CRTC, and that was to install Daver's Pettester and look at the CE pin of the CRTC to see if the CRTC was being written to a precise number of times immediately after switch-on or reset - I think it was something like nine, or possibly eighteen pairs of CRTC chip select pulses that you would expect to see if the CPU is attempting to set up the CRTC.

You used the one-shot 'capture' mode of your scope to capture the CRTC CE pulses so you could examine and count them.

[ScottishColin]

The Supersoft board is removed and the 6505 is the CRTC chip on the main PET motherboard in UB13.

The installation instructions require that the Character ROM and the EDIT ROM be removed so I have created new ones, tested them and installed them on this 4016. Apart from that there are a couple of clips to be installed for the Supersoft board and three connections to J4 and J9 that I noted before removing them.

But yes - I believe the video signal still comes from the main motherboard even when the Supersoft board is in place.

Colin.

Originally Posted by ortek_service

I presume that this HD46505SP is actually on the PET's main board rather than the Supersoft High Resolution Graphics Board? - As there wasn't and DIL 40pin IC's on the top, unless it is hidden on the underside (It did appear like there might be some DIL 24pin? footprints on the top that had pins coming from the underside).
And the (HD4)6505 is pin and software compatible with the 6545, if that is normally used in most PET's? I presume the 6505 is original and not swapped instead of an original 6545 for the Supersoft HRG board to work?
- I don't recall ever seeing the 6505 used in anything before, so could be difficult to obtain.

And was the 6505 the IC on the PET main board that the blue header cable from the Supersoft High Resolution Graphics Board fitted over, and presumably now removed?

I wonder if anything on the main board needs to be isolated for the Supersoft High Resolution Graphics Board to function and restored again if you've removed it?
Although if this board is more of a RAM / CharGen / Display ROM upgrade, then I presume the video signal itself still comes from the Mainboard circuitry with H & V timing from the (HD4)6505 CRTC ?

[ScottishColin]

It's a good point below re the chirp.

to test this, I removed the CRTC from a working 4032 and powered on and although I got no display, the chirp still works so I think we're pointing at something more fundamental.

I know the transformer is fine as it powered a different 4023 board successfully. I did check the voltages on the 4016 board but I will re check them for certainty.

Colin.

Originally Posted by ortek_service

Well with two ? IC's found to be faulty so far and the amount of rust inside this PET / on the TO3 regulators cases (I presume these are all functioning OK, and the DC supply rail voltages have all been measured?), then the (rather-bad for these) storage conditions could well have damaged many IC's - As Chris was seemingly finding on his latest 'Barn find' acquisition. And multiple IC failures seems rather-common on the various other faulty PET's you've obtained!

But maybe the various diagnostic tools you've now got may be of assistance?
- Although I presume these all rely on video output to see the results, and the CRTC IC needs the CPU system to be running correctly to work, making it harder to fault find on these CRTC models when there's no video output.

I presume the 'Chirp' doesn't rely on the CRTC working, so that may be an indication that something's not right with the main CPU system at least.
I wonder of there is a diagnostic tool that wiggles port lines to show it is running OK / if you can normally generate a 'Bell' etc from the keyboard?
Are there any other socketed IC's you can check like the ROM's - reading out / trying in other PET's
There's also the multitude of (buffered by the 'Commodore fuses' 74LS244 buffers, so several of these may be faulty) buses that complicate things.
- Maybe looking on your 'scope on either side of these 74LS244's to check for any buffer outputs that do not pulses / follow their inputs?

[ScottishColin]

As per post 10, I get a chirp on a working 4032 with the CRTC chip removed. I don't on this 4016 so there's something more fundamental going on I think.

I don't think any of my PETs have displayed anything at first power on so this is no exception.

I removed the CRTC 6505 from the 4016 successfully and put a socket in. I then tried the 6505 in another PET that was socketed and it worked fine. I tried the 6505 from that donor PET in the 4016 and no change so I have put the original 6505 back into the 4016 in the socket as a tested and working chip.

I will try to capture the pulses as below and report back.

Colin.

Originally Posted by SiriusHardware

Coming to this a bit late but the fact that you have totally wrong V_Drive frequency does not necessarily mean the CRTC is dud, it may just mean that the CPU subsystem is not functional enough to set the chip up to generate the right V_Drive and H_Drive frequencies.

So if you remember we came up with a trick to see if the CPU was attempting to write something to the CRTC, and that was to install Daver's Pettester and look at the CE pin of the CRTC to see if the CRTC was being written to a precise number of times immediately after switch-on or reset - I think it was something like nine, or possibly eighteen pairs of CRTC chip select pulses that you would expect to see if the CPU is attempting to set up the CRTC.

You used the one-shot 'capture' mode of your scope to capture the CRTC CE pulses so you could examine and count them.

[SiriusHardware]

Even though you've ruled out the CRTC controller, looking at start-up for that train of 18/36 pulses on the CRTC controller CE pin (with Pettester EPROM installed) is good predictable behaviour to look for which will tell you whether the system is currently able to run any code at all.

[ScottishColin]

Sorry - I've looked at what I did before and that looked like it was pin 25 - is that right?

https://bitsavers.trailing-edge.com/components/motorola/_dataSheets/6845.pdf

Colin.

[SiriusHardware]

Indeed, active-low chip select on pin 25. If you can see that telltale blurp of chip select pulses just after switch-on or forced reset, you at least know that the machine is able to run code from EPROM, as that's one of the first things Daver2's test code does, and it does it without using system RAM.

Regarding the 'chirp', I think the code to generate that is in the EDIT ROM / EPROM and I can't remember where that is in the initial sequence of things. I think in one of the past threads I may have partly disassembled the ROM code up to the point where the screen gets cleared and the 'chirp' gets generated. If I can't find that thread I might have to have another look at and this time write it down somewhere.

[ScottishColin]

I have manged to capture the pulses on the working 4032 but not on this 4016.

Colin.

[SiriusHardware]

OK, so probably not running code.

Check the CPU is not being held in reset, and if it is not then - because I know you already have a NOP rig - try running a NOP test to check out the address lines, the buffered address lines and the chip select lines. As you may remember you should get a particular set of steady frequencies on the unbuffered and buffered address lines with the highest frequency on A0 / BA0, halving each time you go up one address line all the way up to A15 / BA15.

[ScottishColin]

OK - progress of a kind.

I removed UE12 which was a Mitsubishi M53354P and tested that on the working 4032 and that's also duff. I've swapped in the 75154 from the working 4032 into the 4016.

I plugged in the Romulator which is set to use ROM and RAM from itself and ignore those on the motherboard and I now get a chirp.

And the display looked like this which I'm pretty sure we can work through.

However, subsequent boots just give me a blank screen again with no chirp.

Colin.

[ScottishColin]

All frequencies present and correct using the NOP Generator.

Colin.

[ScottishColin]

OK - I think we have a duff 6502 40 pin socket as some judicious nudging/twisting of anything inserted into there makes it work.

I'll replace that.

Colin.

[SiriusHardware]

Maybe just let out a resigned sigh and replace ALL of the white sockets, it will probably save you some time later.