UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Non-working Commodore PET 3016

ortek_service · 23rd Apr 2021, 9:43 am

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.

23rd Apr 2021, 9:43 am	#1735
ortek_service Octode Join Date: May 2018 Location: Northampton, Northamptonshire, UK. Posts: 1,440	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.