C64 ASSY 326298 ToD issues

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cmherndon79
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C64 ASSY 326298 ToD issues

Post by cmherndon79 »

Hi,

I've got a strange issue regarding the Time of Day clock on this C64. The value seems to float from a few seconds over a 24 hour period to something hours off. Sometimes the minutes and seconds are the same, but the hours off.

Background on the board: This is a 326298 Rev A board. The motherboard has had all the electrolytic capacitors replaced, the capacitor at C38 replaced with a 4700pF value to make the Restore Key work normally. R10 was swapped from a 300 ohm to a 120 ohm to make the video less dark. A 1K ohm resistor was installed at R36 and pin 9 of the 556 cut, leaving the factory bodge in place to fix the Reset circuit.

What I've looked at and tried:

Desoldered and installed sockets at U1 and U2 and installed different CIAs
Checked R37 and R5 and found different values installed from the factory than what is called out in the BOM and schematic. Checked another 326398 board and found the values matched the BOM and schematic. Replaced the resistors with a 2.7k and 560 ohm per the schematic.
Swapped out the 74LS08 at U27 with a new TI made part.
Tried an Electroware and NuBrick PSU (since this is derived from the 60Hz AC from the PSU)
Ran a bodge wire between pins 19 of both CIAs
Swapped out the 6510 with another

I guess I might try running a bodge between U27 pin 11 and U2 pin 19. Other than that though, I'm about out of ideas. Anyone have any idea of what the cause of this strange behavior may be?

Note: The pictures below show a failure on the 6581 for the Control Port, which is a false positive and expected on this board revision per the diagnostic documentation.

Any ideas are appreciated. Thanks!
IMG_20220228_191200234_HDR.jpg
IMG_20220227_074621846_HDR.jpg


banman
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Re: C64 ASSY 326298 ToD issues

Post by banman »

Hi cmherndon79,


Wow, that is odd!

I noticed that you are running very long testing times.

I was wondering if there maybe a bug in this particular diagnostic program.

Have you been able to get this to occur on other versions or even completely different diagnostic programs?

Maybe just setting up a very simple BASIC program to poll these TOD registers and get them up a the screen.

Let it run for a few hours and compare your results.

I was just checking around and found this very interesting article.....


https://retrocomputing.stackexchange.co ... 0-hz-power


It may explain some discrepancies.

I have one over those watt meter things that you plug in the wall and check the power consumption of anything plugged in to it.
I live in Oz. It has a mains cycle readout amongst other things. I just checked it now. Our standard is 220v @ 50Hz.
This meter I have is fluctuating anything from 220 -230v and I also note the frequency is not steady 50Hz. It is actually a little bit up or down depending on how long you watch it.
Interesting. I would definitely say that the TOD clock would go out based on my very crude observations just now. By how much I could only speculate.
banman
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Re: C64 ASSY 326298 ToD issues

Post by banman »

Hi cmherndon79,


I was just browsing YouTube and found this very interesting video from Noel's Retro Lab.



Have you investigated the clock circuit on your C64 mainboard?
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