Mappy reset circuit bad

MajorHavoc

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So after spending a couple hours swapping boards and parts and then chasing down signals (sound CPU never coming out of reset, main CPU active but only accessing highest EPROM, nothing useful coming out of PAL @ 2B), I finally lifted the reset pin on the main CPU and activated it manually. What do you know! The board works 100% ... except for the reset circuit.

So how is the reset circuit supposed to work? The resistors and diode check out OK. Having a hard time measuring the caps (one is electrolytic, the other looks like a resistor) and have yet to pull the transistor.

I could just shotgun parts but I am trying to get better at diagnosing and resolving problems on the analog side. Thanks!

Mappy_Reset.jpg
 
Test Q1 for shorts... Check pins 1/2/3 on 3A and 11 on 3D for shorts to ground. Make sure the reset switch isn't stuck on. Check the C5 and D1 for shorts. Beyond that, look for physically damaged components or traces.

It's rare, but possible that the resistors and C4 could be bad, but that's pretty far out there as odds go.

RJ
 
Test Q1 for shorts... Check pins 1/2/3 on 3A and 11 on 3D for shorts to ground. Make sure the reset switch isn't stuck on. Check the C5 and D1 for shorts. Beyond that, look for physically damaged components or traces.

It's rare, but possible that the resistors and C4 could be bad, but that's pretty far out there as odds go.

RJ

Thanks Manic. So I finally get back to checking this and Q1 tests open where it shouldn't. Don't have the same transistor so I just stick in another NPN (2N3902) I have. No joy.

But while examining the PCB some more, I find the R14 connected to +5V and the CENTER pin of Q1. But the schematic shows R14 connected to the collector (see the image in my OP). So either the schematic is wrong or Q1 (2N3391A) has the collector on the center pin.

I have searched but so far can not determine the pinouts on these 2 guys. Anyone know if these two have the same pinout? If so, can I use 2N3902 in this circuit? If not, what common transistor can I use?

Thanks!
 
It's a simple switch so you can use many different types of transistors there. It's not handling much current either. Try a simple 2N2222.

This is where a scope or a logic probe comes in handy. Put it on the base of Q1 and you should see it sitting low for about 50 to 100 or so milliseconds then flip high. Similarly, if you hold that reset switch closed it should stay low then switch high when you let off of it.

That causes the transistor to switch the current to ground. The resistor on the collector just provides enough current to tie the line high if the transistor isn't conducting.

It's that momentary low followed by the transition to high that causes the CPU to start executing code at the address it starts at once it starts running. (Different CPUs start code execution in different places after the reset) If the low is there but not long enough then replace the 22uf capacitor.
 
If you want to bring the board by tomorrow we can check it out on the bench. I should be home all day.

What transistor are you looking for? I cant make out the number on the scat.
 
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Just wanted to post a follow up on this for the archives. The problem was a bad Q1 transistors. Replaced and the board booted right up.
 
Just wanted to post a follow up on this for the archives. The problem was a bad Q1 transistors. Replaced and the board booted right up.

Nice to know it was the first thing I suggested. ;)

Thanks for the follow up!
 
I can't trade this one...a buddy gave it to me to troubleshoot..its really his..I'd have to see if he wants to trade it to me for something...I'd like to get it working..first..I'm sure its something simple..I really haven't worked on it..other than to plug it in to see what it was doing...I thought you wanted a 640K Eprom board as a trade..?
 
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