~RESET signal problem on Vectrex logic board

[dmuntz]

Problem: ~RESET (6809 pin 37) is always low.

All the chips in this system work fine in another system.
R208, C228, C229, D201, and R201 are all good.
Everything in the schematic that is supposed to be on the RESET circuit is connected, and I've done a thorough search for shorts to anything in the RESET circuit.
The RESET switch has been temporarily replaced with a known-good switch.
So... If I pull the 6809 and power up the logic board, RESET behaves as expected although seems to be at 4V (need to compare this to another machine to see if that's expected--5V supply is a steady 5.0V). When the 6809 is present, RESET is "magically" driven low. I say "magically" because I can't find any frickin' way the presence of the 6809 should cause that. This CPU works in another machine, and I can see a valid 6mHz clock input, and 1.5mHz clock from the CPU. Maybe I've missed something in the 6809 specs?
Any suggestions appreciated!
(also posted on /r/Vectrex on reddit, but I think there are more technical people here)

 

[daver2]

My initial suggestion would be to pull the CPU and measure the voltage on CPU socket pin 37 (/RESET) when you power the machine up. You will need to use a digital oscilloscope and trigger it on the +5V supply.

Look at the voltage on pin 37 as C228 charges up and see if the time constant is consistent with the capacitor and resistor values.

It is bedtime in the UK, so I will dream up something overnight!

Dave

 

[dmuntz]

My initial suggestion would be to pull the CPU and measure the voltage on CPU socket pin 37 (/RESET) when you power the machine up. You will need to use a digital oscilloscope and trigger it on the +5V supply.

Look at the voltage on pin 37 as C228 charges up and see if the time constant is consistent with the capacitor and resistor values.

It is bedtime in the UK, so I will dream up something overnight!

Dave

Hi Dave,

Looks like about 160ms from 5V turn on to pin 37 hitting TTL 1. With the 22k resistor,160ms seems reasonable (RC = 484ms)--at least not off by orders of magnitude.

 

[dmuntz]

Hi Dave,

Looks like about 160ms from 5V turn on to pin 37 hitting TTL 1. With the 22k resistor,160ms seems reasonable (RC = 484ms)--at least not off by orders of magnitude.

One other thing I had done after I wrote the original post was I lifted 6809 pin 37, and ~RESET was still driven low.

 

[daver2]

Hmm, that is interesting...

Dave

 

[daver2]

That one has got me beat as well!

The /RESET signal not only goes to the 6809 CPU but also to the 6522 VIA and the AY3-8192 PSG. Both of these pins are also inputs.

The /RESET is also an input on the 6809 CPU.

The only thing I can think of as a diagnostic tool is to leave the CPU /RESET pin bent out and then test each side of each component hung off the /RESET line to see what voltage values you obtain.

This should be an easy test for things that are pulled up to +5V (e.g. R201 and D201).

The only thing I could then think of is to start removing one lead of components connected to 0V (e.g. C224) and remove R208 to see if anything downstream of this resistor is to blame.

Before you do that, have you tried to remove the 6522 VIA and AY3-8192 PSG? I know you have stated that the devices work (presumably tested elsewhere) but we are grasping at straws here...

Dave

 

[dmuntz]

Yeah, I tried pulling the 6522 and the AY3-8192 to no effect. Removing the 6809 is the only thing that lets /RESET go high. Also tried removing R208 and C224 (great minds... ).

One thing I noticed last night... the scope was reporting ~80-120mV on grounded pins, but ~200mV on 6809 pin 37.

There's also some noise on pin 19 (A11), but that might just be a side-effect of being used in the NAND/OR logic.

> The only thing I can think of as a diagnostic tool is to leave the CPU /RESET pin bent out and then test each side of each component hung off the /RESET line to see what voltage values you obtain.

I believe I tried this, but I'm not certain, so I'll revisit.

Thanks for the input so far! I'll update with any new info.

 

[dmuntz]

This is interesting. Lifting pin 33 of the 6809 allows /RESET to hit +4V (consequently lifting pin 36 allows it to hit +3.3V, as 33 and 36 are connected via 2x 3.3k resistors).
However, I'm still not finding a short (or even low resistance) between pin 33 and anything that seems important. Will do a more thorough search for shorts to pin 33 later.
Also note that pin 33 is tied high for anther wtf moment.

Hmm... voltage at pin 33 is +4.8V, but at pin 36 is +4.3V. R210 (measured in circuit) appears to be out of spec. Going to pull R209 and R210 to see if they're good, or if something else is going on...

 

[dmuntz]

Well... I lifted 6809 pins 33 and 36, and connected each via 3.3k resistors to +5V rail. Video is screwy (possibly unrelated problem--this machine is a mess), but audio goes through startup and minestorm. I _still_ can't find a short/low-resistance connection between these pins and anything they shouldn't connect to, but it _must_ exist. I'm tempted to pull the 6809 socket to see if there's anything visible that I'm somehow missing with the meter/scope.