Just following it through ,
From the 74hct139
Pin8 goes to pin 1 oe on both the 74hct573's to the 80c32's right and pin 20 on the eprom.

Pin 4 goes to another 74hct573 Which I think is associated with the motor drive

 

I think it's 3 layers, some pins look like they have no connection but still go somewhere!

 

Just following it through ,
From the 74hct139
Pin8 goes to pin 1 oe on both the 74hct573's to the 80c32's right and pin 20 on the eprom.

Pin 4 goes to another 74hct573 Which I think is associated with the motor drive

Look again.
Pin-8 of 74HCT139 is GND.

 

Scan with the ohmmeter to see if /RD pin-17 of the MCU goes to any of the 74HCT573 and any pin on any other IC (with power OFF, of course).

 

I was concerned it was beer goggles last night but can confirm,
Pin 20 of the eprom and oe on both latches between the 80c32 and the eprom are to ground when power off.

I'll have another check but I can't find pin 17 rd goes anywhere else but oe on the 74hct139

 

The only other place I can find rd is pin 20 of the 'missing' ic d12.

I checked the working board, the same connections are to ground so I guess those latches are just operated using the le pin?

 

As it seems pin 17 is only connected to pin 1 of the 74hct139 might the best test be to snip pin 1?

If the signal returns there's the problem if not it's probably the 80c32?

Cheers Geoff

 

Hi Geoff,
Itching to help you, but it could cause confusion in Chips debugging procedure.

E
OT: what's the weather like at your end.?

 

Soggy!
Thanks Eric,
Mr chips debugging is teaching me a lot ! I'd have a mess of solder and burnt varnish by now if was just up to me.
No staff today so not so much time to look at it.
Cheers Geoff

 

When we have to diagnose a stuck node, one option is to cut PCB traces.

How comfortable are you with cutting PCB traces and having to rejoin them later?

 

Hi,
I would suggest, before you cut the track, scrape away the varnish layer at the planned cutting point and cut the track at a slight diagonal.

E

 

Your best case scenario is a bad 74HCT139.

Snip each pin high up on the 74HCT139 and unsolder the pins one at a time.
Replace with a 16-pin DIP socket and a new 74HCT139.

What desoldering tools do you have?
A solder sucker, solder wick, and flux pen would be handy to have.

 

Yes I'm happy to do that, the only place to get to that track is on the under side of the board, have all the tools and am happy replacing IC.

Will put a dip socket in there.
Hopefully get to it today, haven't got any 74hct139's for sure so will scope rd and let you know.

Thanks Geoff

 

I've snipped pin1 on the 74hct139 and rd remains high, no pulse.

I'll check again for another connection and scope rd on the good board again (laptop went flat!)

I guess this points towards the 80c32?

Thanks Geoff

 

Fortunately the processor I ordered from eBay and was expecting on next Wednesday has come in post this morning!

I guess the last bit of diagnosis to do is cut pin 17/rd and scope the stump before I do anything rash!

Cheers Geoff

 

If you have to replace the MCU, get a 40-pin DIP socket.

 

After disconnecting the driver of a node (pin-17 of the MCU), you can double check for stuck inputs.
A 10kΩ resistor to GND must be able to pull the node LOW.
The 10kΩ resistor to Vcc must be able to pull the node HIGH.
(Power has to be ON.)

 

Quick update.

I removed the old processor, soldered in a socket got all excited and plugged it in!
It's doing something now but not quite what I'd like.

Seems to the untrained eye (mine) like it's trying to go but is send data to the wrong place.

I took the processor back out the checked the 74hct139, oe can be pulled high or low with a resistor.

I resoldered the socket, still much the same, I'm wondering if the 74hct139 is squiffy so I have some on the way.

The reset led is blinking about every 3 seconds now instead of 1.6 so some code must be running!

Cheers Geoff

 

With the original MCU, the symptom was /RD was always HIGH.
With the replacement MCU, is there a proper /RD signal as shown in this timing diagram?

 

There is a signal , but not as well defined as I expected.
Looks more like a spike to 0v, I'll get some pictures of it later.
Having said that it does look similar to rd on the good board.

Cheers Geoff