Desperado
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- Nov 25, 2017
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No! When checking resistance the power should almost always be off; it's also generally a good idea to short out the terminals you're measuring (C17 to ground in this case) before you measure in order to discharge any capacitors. You might also measure the resistance across Q13 collector to emitter (i.e. junction of Q13 and T1 to ground).well!but when i check resistance on C17 i must turn on ?
Folks, when R34 on a PET monitor smokes, it's usually a problem with Q13 (also probably shorted by now) and generally Q11/Q12 circuit not working properly. Yes, it might be C17 shorted, but in my repair history it's the least common occurrence.
In these cases, you must first of all check Q13, if it's shorted, remove it from the circuit, then check Q11 and Q12 for correct B-E and B-C junctions with the diode probe function of the multimeter. If all looks good, without installing a replacement for Q13, look at the horizontal sync pulses that go through Q11 and Q12 with an oscilloscope and check where they go missing or have bad levels (so that maybe resulting in Q13 being constantly on). Only when Q11/Q12 circuit works fine, one can install a replacement for Q13.
Another thing that can happen is Q14 B-E short, but that usually takes more time for Q13 to give up.
Tip for the newbie repairer: Never power a PET monitor before checking all transistor and diodes junctions with a multimeter.
Frank
PS: you can't test reliably C17 and Q13 for shorts while in circuit since if either is shorted, the short will be "visible" on both C17 and Q13. You must lift either C17 or Q13 to be sure where the short is (I bet on Q13).
Unfortunately Pet show black screen
Without the reset signal, it is highly unlikely that the CPU will start-up correctly.
You have checked the reset pulse at the NE555 pin 3 (device A2), but what about pins 1 and 2 of the 74LS04 at A3 and pins 13 and 12 of the 7417 at A10 (i.e. Follow the reset chain through the logic). The signal will probably fail at one point, where?
I would have thought you would have had 'garbage' on your screen if the CPU was faulty, so you may be chasing multiple faults with this PET.
As Hugo has stated, either check the video, horizontal synch and vertical synch signals from the PET main board to the monitor with a scope if you have one, or measure the d.c. Voltage on these signals with a multimeter set to indicate 5V full-scale and post the results.
Dave
J7 pin 1 (video), and pins 3 and 5 (synchronising pulses).
I think those readings indicate no video and/or synch signals.
Need dave_m to weigh in here to confirm my suspicions.
How many of your chips are in sockets? I am thinking of pulling things like the PIAs and VIAs as they can be a source of problems and aren't needed for normal operation.
Do you have access to an oscilloscope? If you do, it will be much easier to look at the clock divider chains and signals.
Dave