Commodore Pet 8032SK with monitor not working

[ilpaninaro]

That's OK. I wanted to see that at least one side has the +9V on it...

What about the UB12 pin 19 test results? These results will affect the voltage reading on the other side of R48.

Dave

Ok, on UB12 PIN19 i find 2V but if i press Play i have 0V!!!

[daver2]

>>> but if i press Play i have 0V!!!

Don't sound surprised, this bit seems to be working OK!

Can you next check the voltage on pins 13, 14 and 10 of UE16 (Q2T2222 transistor array) relative to 0V with no dataset keys pressed first and then with the PLAY key pressed.

Hopefully, this will identify the bit of the circuitry responsible...

Dave

[ilpaninaro]

>>> but if i press play i have 0v!!!

Don't sound surprised, this bit seems to be working ok!

Can you next check the voltage on pins 13, 14 and 10 of ue16 (q2t2222 transistor array) relative to 0v with no dataset keys pressed first and then with the play key pressed.

Hopefully, this will identify the bit of the circuitry responsible...

Dave

ue16:

13:0,71v
14:0,005v
10:0v

with play pressed:

13:0,01v
14:7,35v
10:6,83v

[daver2]

So, because UE16 pin 10 is changing state based upon the state of the PLAY key on the dataset, I suspect Q1 is possibly dead.

With no keys depressed, can you measure the voltages on the c, b and e of transistor Q1 (TIP29).

Repeat the Q1 c, b and e voltage measurements with the PLAY key pressed.

The collector should always be +9 Volts.

The base should go positive when the PLAY key is pressed (this is the same point as UE16 pin 10).

The emitter should go positive when the PLAY key is pressed (this is the same point as J3 pin 3).

Dave

[ilpaninaro]

Q1:

C:9,7V
B:0,12V
E:0,63V

With Play pressed:

C:9,6V

B:6,8V

E:0,42V

[daver2]

Transistor Q1 appears to be dead.

Either that, or something on the PCB is dragging the signal down to 0 Volts.

Remove transistor Q1 and test it on your multimeter with the hFE function. It is an NPN transistor.

The pinout can be found by looking up the datasheet on the internet (but you should already know that as you have measured the voltages on the e, b and c pins...). The minimum and maximum hFE is also specified on the datasheet, so you can compare that with your multimeter value.

I think you said that neither cassette ports worked didn’t you?

Which makes me wonder if the same fault is on cassette port 2. You can move the datasette onto port 2 and repeat the same voltage measurements on that port if you wish (see the schematic for the PET to help you identify the components and pins you would need to measure).

Dave

[ilpaninaro]

Dave on the other port if i measure on Q2, i have:

C: 9,7 V
B: 0,11 V
E: 0,53 V

Same results with Play pressed...

[ilpaninaro]

Thank you so much Sir Dave Roberts!!!
Now tape works on both ports
I changed q1 and q2 transistors !
Now seems that cursor don't freeze again....i hope that this pet is finally ok
20191110_181824.jpg

[daver2]

Teamwork!

Nice picture...

It is interesting that both Q1 and Q2 had failed... almost as though someone had used a faulty datasette and tried it on both cassette ports on the PET - bang, bang...

I don’t think these transistors should have had any effect on your lack of flashing cursor though...

I am on a business trip this week, so any more problems I am sure someone else will chip in to help.

Dave

[dave_m]

Now tape works on both ports
I changed q1 and q2 transistors !

Il Paninaro,
I hope you are taking good notes.

Our Sir Dave is turning you into a first class PET Technician. Congratulations!