Pcb has no pulse from clock when checked with logic probe

dyno

Well-known member
Joined
Apr 11, 2009
Messages
1,644
Reaction score
81
Location
Lower Mainland, Britsh Columbia
Pcb has no pulse from clock when checked with logic probe

Working on a game board and I am not getting any pulse from the clock. I thought it might be the oscillating crystal but I swapped that out and still no pulse when I test with my logic probe. It was working at first, stopped working for a bit, then worked again and now nothing at all. Any ideas what to check? I am having some voltage issues on a few ic's on the board, them being low.
 
Working on a game board and I am not getting any pulse from the clock. I thought it might be the oscillating crystal but I swapped that out and still no pulse when I test with my logic probe. It was working at first, stopped working for a bit, then worked again and now nothing at all. Any ideas what to check? I am having some voltage issues on a few ic's on the board, them being low.

If you're checking at the crystal instead of after it's buffered, you likely won't get enough of a swing to trigger a logic probe.
 
On all the ic's I am not getting a pulse at all, I am not even gettting a pulse of the microprocessor ic where the clock goes into.

What's the board you're working on ? No clock at all on the board is most likely the inverter used for the oscillator is dead. Just try swapping that out.

- James
 
What kind of +5 are you getting across that board? You mentioned some low voltage readings. Your going to need to address that problem first before I would worry about anything else. How low are your readings, less than 4.5v?
 
I replaced the inverter for the oscillator and still no pulse.

Looking at the schematics when you tested at Pin 2 of D1, you get no oscillation ?
Also for the master oscillator when you replaced the crystal are you using a series resonant crystal ?

- James
 
Power on the board is good, it is over +5 at most spots with the lowest being +4.99 at a few spots. Pin2 at D1 I am not getting any oscillation and the power output at that pin is only +2.15v which I believe is to low. As for the for the crystal I am not sure if it is a series resosnat crystal. Also at Pin2 at D1 when I check it with the logic probe I get no pulse, and neither the low or high light on the logic probe comes on but when I test for power at pin2 it shows +2.15v so I know there is power there but it isn't showing on the logic probe.
 
As for the for the crystal I am not sure if it is a series resosnat crystal. Also at Pin2 at D1 when I check it with the logic probe I get no pulse, and neither the low or high light on the logic probe comes on

Might be a silly question but.. can your logic probe handle the 12MHz clock. Also try putting the original crystal back in and test. Plus check at E1 pin 5 or 6, or E1 pin 8 or 9.

- James
 
I think my logic probe can handle 12mhz because it was reading a pulse earlier so I assume it is okay. I put the original crystal back on the board and still have no pulse. When I check the pins on E1 this is what I got, pin 5 I got a low reading on the logic probe with a voltage of 1.85, pin 6 I got a low reading but the green light on my probe was very faint and the voltage was 2.03, pin 8 got a low reading on the logic probe with a voltage of 1.88 and pin 9 I got a low reading also witha voltage of 1.87. Is it possible that the ic at E1 is bad?
 
I think my logic probe can handle 12mhz because it was reading a pulse earlier so I assume it is okay. I put the original crystal back on the board and still have no pulse. When I check the pins on E1 this is what I got, pin 5 I got a low reading on the logic probe with a voltage of 1.85, pin 6 I got a low reading but the green light on my probe was very faint and the voltage was 2.03, pin 8 got a low reading on the logic probe with a voltage of 1.88 and pin 9 I got a low reading also witha voltage of 1.87. Is it possible that the ic at E1 is bad?

It is possible that E1's gone bad, I've personally seen things like a bad 7474's on williams boards actually pull the clock down, so it's not impossible.

Just to be on the safe side check the discretes in the clock circuit make sure there is no physical damage, like knocked off capacitor, and that resistors R3, R4 are still in tolerance and C5 is still good.

- James
 
I have checked the pcb over a few times and there doesn't seemto be anything damaged, I replaced the R3 and R4 just to be on the safe side and have also changed C5. The power readings from the resistors to D1 show in +1 voltage range. In the schematic it shows the E1 ic being a 74s74 which I have ordered and am waiting for but I do have some 7474's here but I am not sure if I can try to use one of those. Isn't a 74s74 just a schotty ic so it just uses less power.
 
Isn't a 74s74 just a schotty ic so it just uses less power.

It was the 'LS' that is the lower power IC, the 'S' series wasn't lower power but a lot faster than a regular 74 series TTL, for the 12MHz clock then a 74LS74 should work fine in there too.

- James
 
Still no pulse with a different ic in there but I think my logic probe is the problem because I tested it on my working galaga and got no pulse on it still. I am still getting low voltage readings on some ic's on the board though so any help with that would be greatly welcomed. Thanks
 
Back
Top Bottom