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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 22nd Apr 2021, 6:15 pm   #1721
ajgriff
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Default Re: Non-working Commodore PET 3016

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Originally Posted by ScottishColin View Post
None of the three datasettes get any power from the PET port 2 - where can I start checking for power please?
Power is supplied to the datasette via the two contacts either side of the key slot on the PET board. One supplies the motor and the other the electronics. Check the voltages with respect to ground in each case and report back.

Alan
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Old 22nd Apr 2021, 6:46 pm   #1722
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Default Re: Non-working Commodore PET 3016

OK.

Working port (port 1) - pins reading from right to left from the top of the motherboard

PIn 1 - 0.33V
Pin 2 - 5.3V
Pin 3 - 0.28V
Pin 4 - 0.08V
Pin 5 - 5.21V
Pin 6 - 5.25V

Non working port (port 2) - pins reading from right to left from the top of the motherboard

PIn 1 - 0.36V
Pin 2 - 5.38V
Pin 3 - 6.03V
Pin 4 - 5.18V
Pin 5 - 5.21V
Pin 6 - 5.26V

So Pins 3 and 4 look strange to me.

Also note I retested Pin 3 on port 2 and it had climbed to 7.16V

Colin.
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Old 22nd Apr 2021, 8:24 pm   #1723
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Default Re: Non-working Commodore PET 3016

The schematics (page 3) indicate that pins 3 & 4 go through "Cassette Jumpers" but having traced it on the board, they're not jumpers but four points in a square on the motherboard (marked in green on the attached photos).

Anyone happen to know anything about these?

Colin.
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Old 22nd Apr 2021, 9:01 pm   #1724
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Default Re: Non-working Commodore PET 3016

Please note the voltages in 1722 were taken with the PET switched on but no LOAD instructions typed in.

Colin.
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Old 22nd Apr 2021, 9:04 pm   #1725
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Default Re: Non-working Commodore PET 3016

Are any of those cassette jumper pad pairs linked / joined in ether of the two groups, either on the top side of the PCB or the underside?

Edit: Just realised there are actually THREE groups of those cassette jumper pads on the diagram.

Last edited by SiriusHardware; 22nd Apr 2021 at 9:16 pm.
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Old 22nd Apr 2021, 9:26 pm   #1726
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Default Re: Non-working Commodore PET 3016

Yes - they're linked and I get continuity all the way to UC5/18 from pin 4 for example.

I'm interested why they're called jumpers though.

Any views on the higher voltages on pins 3&4 of the second cassette port?

Colin.


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Originally Posted by SiriusHardware View Post
Are any of those cassette jumper pad pairs linked / joined in ether of the two groups, either on the top side of the PCB or the underside?

Edit: Just realised there are actually THREE groups of those cassette jumper pads on the diagram.
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Old 22nd Apr 2021, 10:02 pm   #1727
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Default Re: Non-working Commodore PET 3016

They may have been intended to be jumpers like the ones commonly seen on PC cards and PC motherboards, but maybe someone realised that just offered more opportunities to mess things up so they started hard-linking them in a fixed configuration at the build stage.

Plug a datasette into cassette port 2 and look again at the voltage on cassette port 2 pin C-3. I suspect that variable high voltage you are seeing there will vanish when there is a load connected to pin C-3.

You may also find that the voltage on pin D-4 (CASS READ) will be different when you do have a datasette plugged in / don't have a datasette plugged in.

With a datasette plugged into port 2, what voltage / logic level do you see on UC5 pin 14...

-When cassette port 2 is idle (not doing anything)?
-When cassette port 2 is trying to load something?

A reminder - do not connect anything or disconnect anything while the PET is turned on.
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Old 23rd Apr 2021, 12:00 am   #1728
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Default Re: Non-working Commodore PET 3016

As far as I can make out, the Commodore Datasette does not use any bias, but drives the head firmly into saturation when recording. During playback, the waveform is badly distorted, but the computer only really cares about how much time elapses between successive crossings of the zero line.

The cassettes can be played on a standard cassette deck; the head will see no difference between saturation and the very ends of the linear region. A faithful reproduction should read fine to the computer as long as you have a strong enough signal.
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Old 23rd Apr 2021, 6:38 am   #1729
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Default Re: Non-working Commodore PET 3016

Yes I would expect that normal tape-to-tape copying would still work for this system, else the original software duplication-houses etc. would have required special equipment to do Commodore tapes.

And IIRC, there were some cassettes with Commodore on one side and another computer systems on the other. Or maybe those bargain compilation C90's etc. cassettes with lots of different formats on.
There was also the BASICODE (I have some cassettes for this, that I think were produced by the BBC / Radio 4 transmitted these?) system, where once you'd loaded the computer-specific interpreter, the BASICODE program cassettes would then load on any platform that supported BASICODE.

In terms of preserving original recording, I think this is best done using a digital format file, that can be stored on any media / 'net, rather than just creating more analogue cassette backups.
And ideally produced digitally, by re-saving the program from the PET, rather than trying to convert old cassettes in case these have faults.
But may need a bit of extra circuitry from the PET's cassette port digital output to the line input of a PC, to capture in audacity etc.

However, there are a few Commodore cassette interface replacements, based around the Arduino-nano microcontroller that use a memory card and have a small OLED screen to be able to load (and save I believe) tape files. I do have one of these that's nearly complete, that a friend who was getting his own customised PCB's made for these had given me. These were for the C64/VIC20, but I assume would also work with the PET: https://www.c64-wiki.com/wiki/Tapuino_C64


I do recall various Spectrum add-ons that tried to 'regenerate' the audio signal, by feeding through digital gates with an in and out connection.
Or the Maplin 'Easy saver / loader' analogue filtering unit I once built (but stripped-down, as I don't recall it really worked that well)
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Old 23rd Apr 2021, 7:07 am   #1730
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Default Re: Non-working Commodore PET 3016

I think I'm well and truly overwritten as regards duplication of PET cassettes, and whether it can be done by analogue means.

As Owen suggests, it is better to capture it in the form of a digital file which can be written back to a tape at some point, although this is an interesting point: Colin's tapes and the hardware to replay them are still working after 40+ years - can we honestly be sure that we will still have something which will play .WAV or .MP3 files in 40 years? For most of us here, this is fortunately not something we are likely to need to worry about.
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Old 23rd Apr 2021, 7:09 am   #1731
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Default Re: Non-working Commodore PET 3016

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Originally Posted by ScottishColin View Post
Yes - they're linked and I get continuity all the way to UC5/18 from pin 4 for example.

I'm interested why they're called jumpers though.

Any views on the higher voltages on pins 3&4 of the second cassette port?

Edit: Just realised there are actually THREE groups of those cassette jumper pads on the diagram.

Jumper links were originally just a soldered hard-wired link on the PCB, that jumped across tracks - Not only for (re)configuration of the connections: https://www.harwin.com/connectors-ha...horting-links/

But if done with a 0.1" (or sometimes 2mm on smaller more recent boards) spacing, then they did allow the use of removable shorting-links that became widespread on later PC's and FDD's / HDD's etc.
The BBC computer also had some soldered ones, as well as lots of removable shorting links on headers (although on the Acorn System 1 etc they had originally often had provision for (non 0.1") wire-links which also had a PCB track across. So you didn't need to fit the link, unless track had been cut, to reconfigure and then wanted to change again)

On the PET cassette interface, they do seem just to make it more confusing to follow the schematic, as if you changed which port-lines controlled the interface, then you'd have to have a different version of the Kernal etc. ROM.
So I can see why they would have been permanently soldered-in. (Although I don't recall Commodore ever having removable links, and I'm sure the first IBM PC's just had DIP-switches for memory-size setting and didn't have any links)
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Old 23rd Apr 2021, 7:19 am   #1732
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Default Re: Non-working Commodore PET 3016

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Originally Posted by SiriusHardware View Post
I think I'm well and truly overwritten as regards duplication of PET cassettes, and whether it can be done by analogue means.

As Owen suggests, it is better to capture it in the form of a digital file which can be written back to a tape at some point, although this is an interesting point: Colin's tapes and the hardware to replay them are still working after 40+ years - can we honestly be sure that we will still have something which will play .WAV or .MP3 files in 40 years? For most of us here, this is fortunately not something we are likely to need to worry about.
Yes, retention in many forms of non-magnetic tape Digital media is another issue. DVD-R's could be less than 2 years it seems (rather less than old VHS tapes), with very few committed to giving an expected lifetime. And FLASH memory based memory-cards minimum is only around 10years (Although so are many EPROM's), with maybe higher-density multi-level ones being more vulnerable. However, it seems many old floppy disks are not faring quite so well after a few decades, and have heard the some old media needs to be baked first to prevent it falling to bits when played after being stored a long time.


But hopefully, with copies distributed around the 'net, then that should ensure there's always one available somewhere (There was a rule of backups, of needing something stored in at least 3 different places - including off-site - to ensure it really was).
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Old 23rd Apr 2021, 8:12 am   #1733
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Default Re: Non-working Commodore PET 3016

Quote:
Originally Posted by ScottishColin View Post
Working port (port 1) - pins reading from right to left from the top of the motherboard
PIn 1 - 0.33V
Pin 2 - 5.3V
Pin 3 - 0.28V
Pin 4 - 0.08V
Pin 5 - 5.21V
Pin 6 - 5.25V

Non working port (port 2) - pins reading from right to left from the top of the motherboard
PIn 1 - 0.36V
Pin 2 - 5.38V
Pin 3 - 6.03V
Pin 4 - 5.18V
Pin 5 - 5.21V
Pin 6 - 5.26V

So Pins 3 and 4 look strange to me.

Also note I retested Pin 3 on port 2 and it had climbed to 7.16V

Colin.


Quote:
Originally Posted by SiriusHardware View Post
Plug a datasette into cassette port 2 and look again at the voltage on cassette port 2 pin C-3. I suspect that variable high voltage you are seeing there will vanish when there is a load connected to pin C-3.

You may also find that the voltage on pin D-4 (CASS READ) will be different when you do have a datasette plugged in / don't have a datasette plugged in.

With a datasette plugged into port 2, what voltage / logic level do you see on UC5 pin 14...

-When cassette port 2 is idle (not doing anything)?
-When cassette port 2 is trying to load something?

A reminder - do not connect anything or disconnect anything while the PET is turned on.
Yes, Pin 3 is the crucial CASS MOTOR supply to the Datasette, and assuming 7.5V zener diodes are reasonably-accurate (I have encountered some zener's even when new that were quite a bit different by > 5% from nominal) then should be either:
(7.5V - 1.2V = approx. 6.3V) when enabled/ around 0V when disabled.

And a fault with Q1, Q2 or Q3 is most likely if all port lines on the UC7 6520's / the 6522 are all OK.
Or maybe even VR1, if voltage is going much-higher than it should be.

At least with a working cassette port 1, then you can use that one to compare voltages on its Q4, Q5 & Q6 with the same ones on port 2.
- But do need to ensure that it's with the same enable / disable status as the other port.


Pin 4 should probably be pulled-high to +5V by the internal 6520 PIA / 6522 VIA pull-ups, with no Datasette plugged-in.
And I guess (working) Port 1 had one plugged-in, for you to measure 0V on this.

Last edited by ortek_service; 23rd Apr 2021 at 8:30 am.
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Old 23rd Apr 2021, 8:16 am   #1734
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Default Re: Non-working Commodore PET 3016

I would say that the C-3 voltage needs to be measured with a load on it (ie, with datasette connected) because otherwise that connection is just the open emitter of Q1 and whatever is leaking through it - bearing in mind that the meter doing the measuring will itself have a resistance of many Megohms, so it won't exert any significant pulldown.
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Old 23rd Apr 2021, 8:43 am   #1735
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Default Re: Non-working Commodore PET 3016

Yes, I had thought at first that it may leak a bit. But I did see that Q3 should clamp Q2 base at around 0V.
So really just (very-low nA's bipolar) leakage current of Q2 multiplied by (often quite low for many power transistors) hFE of Q1.
And therefore shouldn't be too-much leakage.

If not got a 10k etc. resistor to hand, to connect across DMM as a slight-load, then maybe using a 'scope (with usual 1M rather than 10M input resistance) with a x1 probe might help getting a better idea.

The voltage on the working Port 1 pin 4 did read around 0V, when disabled
- But it does look like this probably had the Datasette connected as Pin 4 (CASS READ) was also around 0V, that should normally be pulled-high internally on VIA/PIA.
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Old 23rd Apr 2021, 12:23 pm   #1736
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Default Re: Non-working Commodore PET 3016

I guess everyone has already worked this out, but the "jumpers" on the board allow the 2 cassette ports to be "switched". On the original PET interface #1 was on the lhs next to the transformer, and interface 2 was on the back. On later models interface #1 was on the back and the 2nd cassette was inside the case on the lhs. I imagine when laying out the board for the 2nd revision and on, they put in provision for jumpers so a "new" board could be put into an "old" PET with built-in cassette (for repair purrposes). As is often the case, the "standard" configurations are pre-wired with thin tracks since the majority of "new" boards would be going into "new" computers. If the main board needs replacing in a PET with built in cassette then the procedure presumably was to cut the tracks and fit actual jumpers.
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Old 23rd Apr 2021, 12:28 pm   #1737
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Default Re: Non-working Commodore PET 3016

Sounds reasonable, so now I have another question: Why are the CASS READ and CASS WRITE signals of the two cassette ports also connected to the user port? Just so that those lines can be used as extra user lines? Or...?
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Old 23rd Apr 2021, 1:00 pm   #1738
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Default Re: Non-working Commodore PET 3016

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Originally Posted by SiriusHardware View Post
Sounds reasonable, so now I have another question: Why are the CASS READ and CASS WRITE signals of the two cassette ports also connected to the user port? Just so that those lines can be used as extra user lines? Or...?
I think that was to allow the onboard diagnostics to work. There were connectors to plug into the user port and keyboard that pulled the 'diag' pin low, and I think the read and write pins were connected to the i/o port pins on the user porrt so that the function of the cassette circuits could be tested.
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Old 23rd Apr 2021, 2:54 pm   #1739
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Default Re: Non-working Commodore PET 3016

That makes lots of sense - thanks. I had forgotten about the internal tape drive on early models.

Colin.


Quote:
Originally Posted by Slothie View Post
I guess everyone has already worked this out, but the "jumpers" on the board allow the 2 cassette ports to be "switched". On the original PET interface #1 was on the lhs next to the transformer, and interface 2 was on the back. On later models interface #1 was on the back and the 2nd cassette was inside the case on the lhs. I imagine when laying out the board for the 2nd revision and on, they put in provision for jumpers so a "new" board could be put into an "old" PET with built-in cassette (for repair purrposes). As is often the case, the "standard" configurations are pre-wired with thin tracks since the majority of "new" boards would be going into "new" computers. If the main board needs replacing in a PET with built in cassette then the procedure presumably was to cut the tracks and fit actual jumpers.
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Old 23rd Apr 2021, 2:56 pm   #1740
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Default Re: Non-working Commodore PET 3016

UA7 and UA8 replaced and the test program now runs through without error - thanks all.

Cassette port 2 next.

Colin.
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