Not quite accurate:
I think Q2503 is dead. Your measurement of 0.3V between base and collector is a strong indication. A silicon junction have about 0.6 - 0.7V forward voltage. Measuring a substantially lower value means a shorted device. There could be a short elsewhere. But this is unlikely.
RobII, I'm a little confused where you're probing the negative of your DMM in all these measurements, so my apologies in advance if there's a reading that disputes what I'm about to say.
Although it may be Q2503, I think it could also be the output of U2523, the latch that outputs the scan signal. I would check one more thing before you start pulling out transistors.
With your DMM set on *ACV* and negative lead to a ground (chassis is usually good), check the output of U2523 pin 15. That is the output that drives Q2503. You should see some reading there, indicating there's a changing waveform. You can compare this to the other scan outputs on U2523, pins 12, 9, 6, 16, 5. Those should definitely have an AC voltage on them since those buttons are working.
If you have no AC volts on pin 15, or very little (like < 0.1V), switch to DCV mode. If you get a constant voltage close to +5V or 0V, I would say that output on U2523 is dead.
If you do see a comparable AC voltage on pin 15, check to see if you see an AC voltage on the other side of R2536, which is also Q2503's base. You should also see an AC voltage on the emitter of Q2503.
I think Q2503 is dead. Your measurement of 0.3V between base and collector is a strong indication. A silicon junction have about 0.6 - 0.7V forward voltage. Measuring a substantially lower value means a shorted device. There could be a short elsewhere. But this is unlikely.
Why would you say that? In circuit, it's entirely possible to have less than 0.7V between the base and collector since that junction is not forward biased. And these are darlingtons, so you're going to see a double drop measuring between base and emitter, so it's not always 0.7V.
None on hand. I am going to ask one of the repair techs where I work. I am excited about this!!
A silicon darlington has ~1.4V between base and emitter. But when it measures 0.3V then it's dead. (Ok, BIG silicon semiconductors can measure 0.3V with the little current of a DMM. But this is not the case here).
About further measurements. The best would using a scope and look at the waveforms. Then the fault could be narrowed down easily.
Agree completely. This would take about 2 seconds with a scope, but I'm assuming all we have is the DMM.
You can't conclude it's dead. It could simply be off (Vbe < 1.4V).
It's difficult to conclude anything with average DC volt readings taken on dynamic digital signals.
You can't conclude it's dead. It could simply be off (Vbe < 1.4V).
It's difficult to conclude anything with average DC volt readings taken on dynamic digital signals.I talking about testing it with the DMM diode function. Not measuring the voltage while the scope is running.
I'm sorry - you are right. I missed that you were referring to diode test mode.
With your DMM set on *ACV* and negative lead to a ground (chassis is usually good), check the output of U2523 pin 15. That is the output that drives Q2503. You should see some reading there, indicating there's a changing waveform. You can compare this to the other scan outputs on U2523, pins 12, 9, 6, 16, 5. Those should definitely have an AC voltage on them since those buttons are working.
If you have no AC volts on pin 15, or very little (like < 0.1V), switch to DCV mode. If you get a constant voltage close to +5V or 0V, I would say that output on U2523 is dead.
If you do see a comparable AC voltage on pin 15, check to see if you see an AC voltage on the other side of R2536, which is also Q2503's base. You should also see an AC voltage on the emitter of Q2503.
Do you have a soldering iron? If so, are you comfortable in using it?
As previously suggested, you could swap it with another one of the transistors and see if it fixes the AS3 line.
For such an inexpensive component, I'd just replace it and see if it changes anything. If not, back to the drawing board.
Quick edit. I didn't notice there was more than one 74HCT374. Which one is U2523? I checked the one on the right of the pic.
Checked pin 15 (and the others you suggested) all had an AC voltage bouncing around 1.5vac. Also checked resistor R2536 and others near it with voltage about the same. Is it safe to assume transistor Q2503 is the fault or is there something else I should be checking first?