help with troubleshooting

[algorithm]

Sorry if this is a broadened sort of question, as well as any stupid questions I might ask.
Lately I have found myself having trouble trying to figure out what is wrong with a PCB.
When I got started with electronics, I got lucky with most repairs, but when it gets past a PSU recap or bad bridge rectifier it gets confusing for me. Right now I have quite a few projects in this sorta boat.

For instance, recently I got a circuit board that has four 7 segment displays, a z-80, a transistor array, etc.
It would freeze up a few seconds after powerup, or when any button was triggered. Figured it was the z-80 causing the freezing, so I got a new one, replaced it. When I tested the board right after the replacement, it worked a treat! the display lit up, and no freeze.

I put it all back together carefully, and then no display, but 2 single LEDs lit up.
I dont really know what to be looking for. I have an oscilloscope, but I have no idea what I should see on the z-80 pins, or many other IC's on this board for that matter.

I also notice people testing a trace with an ohm meter and being able to see if something is not "right" on that trace sometimes. Is there a standard in ohms that people are looking for on traces in question, or is there more to it? (surrounding components, etc.)

Also, I am confused on some through hole boards... on the empty inlets connecting both sides in places, I noticed some boards seem to have these filled with solder. Sometimes they are not filled. I see this, and think its a cold solder joint. Is this incorrect? Will an inlet that is empty still work for both sides? Are some empty inlets manufactured that way?

Again, I am sorry if these questions are poor, but I am truly puzzled with this stuff mentioned. Thanks ahead of time.

[lapm]

Try poking pcb with plastic pen/stick carefully. Sounds to me like there could be bad connection or ic socket with bad contacts... Have you been able to verify crystal clock circuit working properly?

[algorithm]

What would poking it with plastic do? Hmm, that is a thought. Although, if it was, would that have prevented the display from working after the z-80 replacement?

It tested good after the replacement, it only stopped working after it got screwed back together.

I have not tried to test the crystal yet. I will look into how to test that.
After looking at the z-80 datasheet, it seems like there should be differentiating square waves on certain signals. I should be able to test with a scope to see...

When I said inlet solder joint, I meant the through hole solder point to join traces, not for a component...
Sometimes I see them empty, sometimes not. Would some boards be designed to have certain holes filled with others not?
Could a soldering iron get so hot it would melt some of those tiny points to join both sides?

[PedroDaGr8]

Those holes are called vias. They don't have to have solder because the sides of the hole typically have a metal foil layer that connects the traces. Any solder fill is likely a side effect of the soldering process used to make the board. Those are not your issue.

The stick would cause the board to flex, showing any intermittent/bad joints.

[Bukurat]

The z80 needs a program to tell it what to do so there will be rom, prom or EPROM  memory there and possibly some RAM  for variable storage as well.  If you have socketed chips you should check to ensure the chips are making good contact.

There should be some activity on the address and data bus.

When you say "freezing" do you mean it stops completely or stops responding?  Is there a reset button?, if so what happens if you press it?

It's been quite a few years since I used the z80 but I seem to recollect that it can be put into a tight loop under some circumstances associated with faulty memory.

[mayor]

Long shot, but if you have access to an oscilloscope, probe the crystal inputs to the cpu. I had something similar to what you describe, and it turned out the crystal needed to be resoldered.

[algorithm]

Thank you all very much for the answers everyone.

Ok, a whole bunch of confusion. Someone keeps saying that this is not working because of q101 being a 2n3904. Here is the service manual. http://www.synfo.nl/servicemanuals/Sequential/420-TOM_SERVICE_MANUAL.pdf
From what I gathered, it seemed to me that it was being used as a switch for the LEDs?

Ok, great, I will test the crystal.

Thanks again everyone.

It seems the chips are getting input voltage across the board..

[algorithm]

The z80 needs a program to tell it what to do so there will be rom, prom or EPROM  memory there and possibly some RAM  for variable storage as well.  If you have socketed chips you should check to ensure the chips are making good contact.

There should be some activity on the address and data bus.

When you say "freezing" do you mean it stops completely or stops responding?  Is there a reset button?, if so what happens if you press it?

It's been quite a few years since I used the z80 but I seem to recollect that it can be put into a tight loop under some circumstances associated with faulty memory.

There is a reset circuit, I really do not know how it works from looking at the schematic though.

[techricky]

I remember having a Z80 in the toolbox with the data pins carefully bent up and all connected to the GND pin.
This would cause it to run through its whole address range by always reading NOPs,  thereby allowing testing of every address line buffer and decoder in the system being tested. Well for simple systems anyway...