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Old 23rd Apr 2021, 8:12 am   #1733
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Location: Northampton, Northamptonshire, UK.
Posts: 629
Default Re: Non-working Commodore PET 3016

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


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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