c64 - ASSY 250425 Black Screen

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banman
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,



I was just browsing the internet and found this C64 service manual PDF file. It's got nice clearly readable schematics that we can use as a reference.

https://www.google.com/url?sa=t&source= ... p-Ffu2IChm



That blue memory IC chip socket is really giving me OCD. Are you able to check all the connections on this one to see if they are connected correctly with the other memory IC sockets in that row.

I would use the continuity mode on your multimeter and check the connections between the memory IC's of your good C64 motherboard and correlate those readings to the C64 motherboard under test.
Test left to right in the top row of 4 memory IC's and then check the bottom row of 4 memory IC's.
Do it with the power off.

Page 40 of the above service manual has the memory wiring diagram for the ....425 board. I'll check my board as well.

What brand of memory IC chip sockets are those. I can make out made in the USA on some of these? Are the memory chips making good electrical contact to the mainboard?

Apologies for the time delay. I'm in Oz.


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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,


Here is my 250425 board..
I have got an over voltage protection unit connected to the 5 Volt line. I do recommend using one the other things are your discretion.

Be aware that I have a Swinsid sound chip and and OTP EPROM PLA substitute installed on this 250425 board so my power draw on this machine is about 720 milliamps. Considerably less than a stock machine. Because of this you will see a ZTE 700 milliamp USB 5V charger in the photos. I have a regular 9v AC wall wart transformer style (US Robotics brand I believe) suppling the 9v AC component to the C64.
IMG_20220624_131015.jpg
IMG_20220624_131041.jpg
IMG_20220624_131149.jpg
banman
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Infact, a completely stock factory C64 should be drawing around 1amp, here.....

I am using an Asus USB 5V charger at 2amps as opposed to the smaller 700 milliamp in the above photos.
If you can measure the current draw on your machine and confirm that around 1 amp on the 5 Volt line is being used.
IMG_20220624_132855.jpg
IMG_20220624_133006.jpg
IMG_20220624_132751.jpg
banman
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

CM220624-133105002.jpg



Here is a picture of the current draw across the 9V AC glass fuse. It should be around 50 milliamps.

IMG_20220624_132113.jpg

Page 48 of the above mentioned C64 service manual PDF has the power switch wiring diagram.

I imagine if you didn't want to make a wiring harness up one might be able to check the current draw from pin 4 or 5 of the power connector to the ,+ side of C91.

The only issue with testing this way is the power switch is not being able to be checked. I know from personal experience the power switch is a real potential failure mode.
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Here is a test of the under powered ZTE USB mobile phone charger on a stock factory C64 motherboard.

The charger is unable to deliver the 1 amp current draw. The C64 power LED goes very low.

IMG_20220624_132643.jpg
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Just to illustrate this point.

Here's a video from Dave Jones.

dpalmeira
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hi, thank you for your messages, I will review them...

On the other hand, I've created a PDF with all my scopes around the CPU.



I can see some Address pins only high, I dont know if it's normal...
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,


I have taken some more basic readings of a properly working 250425 PAL style C64 mainboard.

Did you look at your Capacitor c91, what is the value? Did you also reset the PAL jumper pad?

I noticed that increasing the oscilloscopes frequency range increased the jaggedness of the signal image. You will notice that I dropped the range down to 20mHz from 150mHz. This made a smoother image, however the values were a little lower on the Voltage readings. I am not sure if that is an issue or not.

The PAL crystal should be 17.73442mHz (pin 21 VIC II chip).
Pin 22 of the VICII chip should be 7.88mHz
PIN's 1 and 39 of the CPU need to read .985mHz for any type of operation of the C64 to happen. Do you get these readings?

I have included readings from the voltage levels coming out from the 8Pin video connector. Are you getting these readings.


I am not too sure regarding some of the reading on your CPU. Pins 9,10, 11, 12 and 15 look they are signalling however they don't seem to be going true low, maybe this is valid I'm not sure.
I will compare these signals with my setup and get back to you.


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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,

I have been lurking about on the internet...

The multiplexing is done by U13 and U25, 74LS257 Multiplexors. They are connected to the CPU lines I mentioned that seem to be off in your images. The 74LS257 Multiplexors are responsible for decoding the memory address lines, I think. Maybe follow the lines from your CPU 9,10,11,12 and 15 and check that there is proper continuity to U13 and U25 multiplexor chips. Put your oscilloscope on these corresponding multiplexor chip pins and see if you are getting the exact same signals as you see on the CPU.

I will do the same and report back to you.
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Answers

- U23 MEM (blue socket): I've tested all lines with the multimeter, pin to pin between chips. It connects perfect with the other memories, chips and component as expected.

- C91 is OK, 100uF approx. remember that I've replaced all electrolytic capacitors with new ones, and I measured them first with my capacitor instrument.

- Jumper PAL is again short as I found originally: G-PAL

- "The PAL crystal should be 17.73442mHz (pin 21 VIC II chip)." - Correct, it's OK as expected. On the other hand if you see the clock received in the CPU seems to be ok too.

Crystal:
crystal.png
VICII-Pin21
VICII_pin21.png
VICII-Pin22
VICII_pin22.png

- CN5 is dead, no signals in scope or multimeter voltages (DC and AC).

- U13 and U25 74LS257 working well, because I have tested them with my IC 40x/70x tester.

- CPU A9, A10, A11, A12 and A15 have correct continuity to U13 and U25.
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