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MP-1 no display or LED's

Started by andtimebegins, November 30, 2020, 11:17:42 PM

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MarshallJMP

I mean measure it with the scope  ;D

andtimebegins

#16
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

#20
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

#23
So I tested it again.. now it does this everytime I test it.

Hmm...

andtimebegins

#24
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:
On the run again!

MarshallJMP

#26
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

#27
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

Quote from: MarshallJMP on December 06, 2020, 01:25:44 PM
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.
On the run again!

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.