MP-1 no display or LED's

[MarshallJMP]

I mean measure it with the scope 

[andtimebegins]

Oh. Whoops! Apologies. Haha.

Do I measure these with the same settings on the scope you had me do the last test with?

Not sure if this is correct? I just took this pic:

[MarshallJMP]

Is this the eprom or the ram?

also set the voltage knob on 0.2 so I have a bigger image on the scope.

[andtimebegins]

First pic is the eprom

Second pic is the RAM.

[MarshallJMP]

RAM CS is not ok, should look a bit like the eprom.

Check U44 (74LS20)
pin 9 ==> should be pulsing
pin 10 ==> should be high (5V)
pin 12 ==> should be high (5V)
pin 13 ==> should be pulsing

[andtimebegins]

First pic is pin 9 of u44
Second is pin 13 of u44

Neither seemed to pulse like pin 16 of the z80 did.

Both pin 10 and 12 were 5.1v

[MarshallJMP]

Ok signals are there, looks like the 74HC20 is busted. On pin 8 you should have a signal like this. (see attachment)
My scope's timebase was set to 1µs. Check it and post a pic of it.

[andtimebegins]

This is what I got on pin 8 of the u44

[andtimebegins]

So I tested it again.. now it does this everytime I test it.

Hmm...

[andtimebegins]

So, I've noticed that there is something up with that pin 8 of the U44. When I power the unit on again after waiting, I'm getting what I saw in the first picture, then it fades into what I show on the second picture.

Definitely seems like there is an issue there or something.

Just to be sure, I attached a picture of the chip we are talking about.

[Iperfungus]

Eating my popcorn while reading.
I bet there will be another working MP-1 in short times here.  :thumb-up:

[MarshallJMP]

Yep I think so too, I'm almost 100% sure that this is your problem. I would try to replace it. You need a 74HC20 not a 74LS20 like I mentioned before. They used an 74HC (CMOS) because the power consumption in static mode is far less than a 74LS (TTL) (up to a 1000 times). This 74HC20 is permanently under power,even when the MP-1 is powered down, that's why it's connected to the battery (the reason why they used a CMOS version). So make sure you use a 74HC20 !! or your battery will drain a lot faster.

So the RAM is only selected (pin 8 ) when pin 9 (Memory request) and pin 13 (Address 14) is high.

See attachment. Logic analyser pin 8,9,13 , yellow is actual pin 8 signal.

When pin 9 and 13 is high pin 8 is low. (RAM Chip Select is active low).

[MarshallJMP]

Yes that's the one, I see it's in a socket so easy to replace. Just desolder the resistor, replace it and resolder the resistor. (without the resistor it won't boot)

These fail sometimes and yes some MP-1's came with these in a socket.

So it seems it works for a few seconds and then the Z80 crashes due to the bad CS of the RAM. That's why the leds work for a few seconds.

[Iperfungus]

So it seems it works for a few seconds and then the Z80 crashes due to the bad CS of the RAM. That's why the leds work for a few seconds.

So, basically, the Z80 is failing/crashing not because it's broken but because of a busted chip related to RAM access?
Lucky guy: the broken chip is socket mounted.

[MarshallJMP]

Yes, it's trying to read from RAM and it gets garbage in and crashes.

I've seen these fail before that's why I wanted to see the signals because I was already pretty sure this was the problem.