MTX 512 (debugging thread)

[GozdniJezek]

Hello,
all of the measurements were done with disconnected keyboard.
Immediately after turning the machine on the values are:
voltage across R16: 0.003 V
Pin1 and GND on 9C (4070B): 4.873 V
across C11: 4.912 V

I presume everything looks as expected and this matches the symptoms (stable picture for couple of seconds).
However after some time when the picture gets lost the values are:
voltage across R16: 0.218 V
Pin1 and GND on 9C (4070B): 3.5 V
across C11: 3.0 V

If I understand correctly than the issue could be either C11 (voltage is lower than on 9C) or there is a unwanted connection.
But the symptoms are the same like they were before I changed out C11 with a new one, would it be fair to say that lowers the chances of two bad capacitors in a row . I'll check for any shorts around this circuitry, however the couple of seconds of working order before going bad, sounds like 9C is more likely culprit, does that sound reasonable?

Thank you for your help.

[1024MAK]

That’s very strange! So at power on, C11 is charging up (normal and too fast to see with a meter) and then slowly being discharged, and at a rate faster than R18 can recharge it.

If C11 has already been changed, it’s unlikely but still possible that the replacement is faulty.

Given the small voltage across R16, I don’t think the chip (9C) is the problem. If the chip was faulty and discharging C11, the voltage across R16 would be much higher.

So the next thing to look at is resistor R18. When the symptom is present, do you have a nice steady +5V supply voltage at one end? What voltage do you get at the other lead? Is this similar to the voltage across C11?

Do you have a spare resistor, preferably a 10k resistor, but any value between 1k and 22k will do. Find a different +5V supply point to the +5V supply feeding R18 (any +5V supply point will do, for example pins 12, 13 or 14 of 9C logic gate), and connect this to your spare resistor with a wire. Then connect the other end of the resistor to pin 1 on 9C logic gate (4070B). Do confirm with your meter that the +5V supply point you use is actually at +5V.

Measure the voltages as before (across C11, R16 and pin 1 of IC 9C (fiddly I know).

Mark

[stephen_usher]

If you've got a multimeter which can measure current, pull the fuse and clip the multimeter between the two sockets and see how much the machine is drawing. It should be pulling about 0.85 amps. Anything significantly more than that and something is taking too much.

[GozdniJezek]

Hello, time for a new report.

My friend removed the C11 and measured it with a tester and multimeter and he got:
tester:
C11 ( 10uF 50V electrolyte)
10.68 uF
ESR 1.1 Ohm,
Vloss 1.4

Multimeter:
10.03 uF

Judging by the measurements this should also be OK.

1.JPG (77.27 KiB) Viewed 672 times

0.793A measured across the fuse. I think this is in spec.
The yellow channel is the voltage on C11, the blue is the output from 4070B (both values correspond to the 5V, they are just offset). All was measured without the keyboard.

The voltage on R18 is a stable 5V.

But he changed the C11 nevertheless for another brand new one and this resulted in stable reset (yaay ) (for now at least ) , so Mark was spot on once again! (thank you Mark)
The new C11 is 10 uF 25V, ESR 1.5 Ohm

Now the image was stable for at least 20 minutes as far as he tested.

2.JPG (73.27 KiB) Viewed 672 times

Now this looks like another bad 4116, which I can handle as they are all socked now. (the image is black and white, because we've changed the link on the video board to B (bypassing the mixing chip) during the testing. Does this sound reasonable? As we've tested all of the video ram connections for continuity and for shorts.

Any thoughts? Was it all because of a possibly weird C11? He did report, that he got a stable picture with the old C11 after he soldered wires to it for measurement (it's possible that he fixed a cold join with this? Or changed some properties of C11 with scope probes attached to it?), but after manually resetting on the keyboard connection circuit exempted the old symptoms again. He doesn't want to try the reset again with the new capacitor so this is it for now

Any other ideas and suggestions?


As always thank you for your help.