MP-1 no display or LED's

[andtimebegins]

I recently got an MP-1 (rev B,  1.38 eprom)  which is not working from a friend who pointed me in this direction for help.

The screen flashes on for about a second when I power on. LED's cycle on as well, but completely dissappear after about a second.

Wall voltage: 118v

1. C131 reads 11.9 volts with tubes in, with tubes out 13.2v.
2. Pins 11, 17,  24 and 25 of u29 reads 5.1v with tubes in or out.
3. Pin 16 of u29 reads 3.2v with tubes in or out.
4. Pin 6 of u29 reads 0.65v with tubes in or out
5. Pin 27 and 28 of the u30 reads 5.1v with or without tubes
6. VR4 reads 5.1v

I'm not sure how to test for pulses on pin 16 of u29. Its voltage is consistent with or without tubes.

I swapped out D38 and 39 with new ones.
Swapped out c136 for a new .047 cap.

No changes...

I'm starting to think it could possibly be the crystal? I have a new one on order(hope it's the right one, 12MHz). Should be here in a few days.

I'm wondering if these readings stand out as anything obvious? I don't own an oscilloscope.. but if need be, I could pick one up.

Thank you in advance for any help on this.

[andtimebegins]

So, I got the new crystal installed... still no go. Same results as with the old crystal.

hmmm....

[andtimebegins]

Figured I'd drop a picture of the power section.

[Iperfungus]

Hi!

The unit seems to have the battery mod...
Has it any other mod inside?

It could be a problem related to failing Z80 CPU or EPROM, maybe.
Such failures can originate issues like that (I had a similar experience with another preamp, where the CPU broke and I had to change it: once the job was done, the unit came back 100% healthy).

You definitely need the help of MJMP.
Let's wait for him.

[MarshallJMP]

So the leds in the buttons go on from left to right (one by one)?

If so then the display should show 8 from left to right, and then in your case 1.38, this does not happen?

BTW if you could get a scope that would be great -, this way we can troubleshoot it better.

[andtimebegins]

Thanks for the replies guys....

When I turn it on I do see an 8 sometimes real quick then it fades away to nothing on the screen.. Half of the push button LED's flash on and off, and then the other half does the same, all happens in about a second.  I think I've seen it also say ADA and I've seen it flash the 1.38. Maybe even looked like it was doing a count down or something?

I'm going to see if I can grab a scope asap.

Is there any chance though that it could be one of the IC's? Maybe the eprom? I'm not getting anything on pin 26 of the eprom. I've noticed you mentioned to others that you should get a reading on pin 26 and 28. I get a reading on pin 27 and 28. Is that normal?

[MarshallJMP]

Can you make a video of the startup and mail it to me so I can have a look?

BTW you are correct , it should flash one half of the leds then the other half, (mistaked it with the mp-2 which turns on one at a time).

It seems it wants to startup but something happends to prevent it, usually this is a power supply problem. Try to start it without the tubes.(just remove them).

Don't think it's a Z80 or eprom problem.

BTW does it restart or does it starts up and then a blanc display?

[andtimebegins]

MarshallJMP. I'll do that asap!

I just got a 2 channel 20 MHz scope, its a bit old, but works good.

Oh the screen goes completely blank, to answer your question.

I really appreciate the help!

[rnolan]

Also check that the ribbon cable between display and PCB is seated properly in the PCB socket, I suspect MJMP is right and it may be power supply related but sometimes they can play up when the cable isn't connecting properly.  If you remove the EPROM and put it back it will force a full boot check where it runs through a series of self diagnostics (Maybe even looked like it was doing a count down or something) this maybe what you saw happening.

[MarshallJMP]

I think maybe he saw the C 1 to 128 count up? andtimebegins, could it be it was counting up?

Ok you got a scope, nice  , begin with the clock input on the Z80, should be 6 Mhz ( a period of 0.166µs )
Then on the interupt input you should see pulses.

[andtimebegins]

So this is what I got for the clock input.

I admit, I'm not too savvy with the Oscilloscope.

To be honest, I'm not sure I did this correctly.

Any chance you could tell me how to set this scope to read the interupt signal for pin 16?

[MarshallJMP]

Just put the timebase knob on 0.5 ms, the rest can stay like it is.

The clock period looks good as far as I can see, you have around 0.8 x 0.2 µs which is 0.160 µs, close enough.

[andtimebegins]

So I got this.

Its moving from left to right across the screen. Should I send you a video over of that? Or can you tell from this picture? Its a little blurry.

*dunno why the pic is coming in upside down? Haha

[MarshallJMP]

Ok looks good. Next I would check the CS lines of the eprom (pin 20) and RAM (pin 20).

[andtimebegins]

Eprom (pin 20) = 1.7v with or without tubes.

RAM (pin 20)  = 4.64v with or without tubes.