Help with Reset circuit

[Doomguy42]

Hi, I'm a novice with Electronics.

I'm working on a pcb repair. The (old) board has worked great for a year. It's slowly been getting more and more difficult over a time... takes a few goes to get it to boot up. Now it won't boot at all.

I've checked clocks, bus lines and now I'm looking at the reset circuit for clues. It does pop out of reset but instantly goes back in a cycle.

I'm looking at what the trigger is. Seems it can be the software (tr2) or 12v unregulated at 100hz (tr3). I'm measuring 10.5v from the psu on this line. It goes through c6 (10nf cermaic) and comes out at 0.2v to the base of TR3 (ztx450 npn). I don't understand how this can 'turn on' this transistor?
The last paragraph explains that tr2 and tr3 which are linked emitter to collector respectively need to be switched on to drain to ground to stop a cap charging to keep the system out reset

 

[Doomguy42]

Here's the pics of the description and schematic. I've written on what voltages I'm recording.

 

[Doomguy42]

The 12v MM 100hz unsmoothed goes through some diodes in the power supply so I'm thinking that's why it's slightly down at 10.5v. It comes from 11.5v windings?

I have checked the mentioned wdog pins on the timer. They do appear to be high, with a tiny drop then high

 

[Doomguy42]

I don't quite now what the threshold is for when the ztx450 (tr3) gets switched on?
Thanks for any help + teachings

 

[rjenkinsgb]

It goes through c6 (10nf cermaic) and comes out at 0.2v to the base of TR3 (ztx450 npn). I don't understand how this can 'turn on' this transistor?

The capacitor, diode and transistor act as a kind of rectifier; each AC cycle, current from the AC in to the cap goes through the diode on one half cycle and the transistor base-emitter junction on the other half cycle.

The average voltage should be low, but it will be pulsing positive and negative 0.6V or so.

Just temporarily short C7 if you want to know if the watchdog is causing a reset?

Or each transistor collector-emitter to eliminate the AC sense or software controlled watchdog pulses.

 

[Doomguy42]

Thanks for the reply. I linked C7 pins and it does stay out of Reset. Its not working though which means it must be a software fault if im bypassing the watchdog? I will try to find out more