Jump to content
IGNORED

Commodore 64 - black screen.


Recommended Posts

Picked up a c64, black screen. I tested the power supply, I'm getting the correct voltages for AC and DC. I put a meter on the 7805, got 5v. Tested the 7812, got 12v. Power LED does NOT light up, but I get a solid black screen on channel 3 when hooked up via RF. I can also get the black screen to go to channel 4 by switching the 3/4 switch. What voltage is the power LED?

 

CPU and VIC chips are both socketed, but that's it, everything else is soldered.

 

Tried a cart, no dice.

 

I don't have another C64 for chip swapping, at least not yet.

 

What are my next troubleshooting steps?

Link to comment
Share on other sites

Have you tried taking the composite signal from the video DIN connector at the back of the computer? It's on pin 4 - or one just right of bottom centre - which is ground.

 

Measure voltage on 7406 (u8 or u22 depending on board) between pin 7 and 14. If it's higher than 5.2v then your power supply needs binning - those old Commodore bricks will all die sooner or later.. If it's below 4.8v then the psu could be faulty, but if the volatge is in between then strange the LED isn't lighting - probably worth checking for dry solder joints there, and the board over.

Link to comment
Share on other sites

There are a few troubleshooting guides around, Ray's being the best so far as I know. The black screen can be caused by a number of failures, power being one of them. The PLA, VIC, and CIA (#2, IIRC) can also cause a black screen, again among other problems.

Link to comment
Share on other sites

I tried tapping into the composite output, same black screen.

 

Checked voltage on the 7406 at U8, I get zero volts between 7 and 14 (black lead on 7, red lead on 14). Am I checking that right? I get .436v when checking both pins with the red lead on either pin 7 or 14 and black lead on ground.

 

I double checked the output of the 7805 regulator, and got 5.05v.

Link to comment
Share on other sites

Here is the link to Ray Carlsen's site (referenced to by OLD CS1 above) in case you don't have it:

http://personalpages.tds.net/~rcarlsen/cbm/c64/

 

He has got both troubleshooting tips and service manual at the same location, though I don't see anything about lack of power LED. Which board revision are you working with? While it is not socketed, a very common cause of fault would be the PLA chip on the earlier models.

Link to comment
Share on other sites

I've been doing some reading on Ray's site, and it appears the next logical step for me would be to get a known working power supply and test from there. If the machine doesn't fire up with a known working power supply, then the next logical steps would include having another known working C64 with all socketed chips to start testing for suspected failed chips, one at a time.

 

Any reason why I couldn't fabricate my own test power supply by tapping in a 9vac power brick for the 9v line and a 5vdc power supply for the 5v line? Or should I hold out for a replacement PS?

Link to comment
Share on other sites

As far as I can tell, you can make your own PSU exactly as you describe. Many people find it clumsy for longer term use but for testing it should be OK. But if you get 5V and 12V after the voltage regulators, isn't that signs that your current PSU should be OK enough?

 

Sometimes it is said that one can try to kickstart the computer: rapidly cycling on-off-on-off-on and leaving it on to see if it brings the PLA temporarily to life. I don't know how well that works or if that chip is your fault but generally it is very common to fail on older C64's, more so than CPU or VIC chip IMHO.

Link to comment
Share on other sites

I tried the rapid-fire on-off method, no dice.

 

Looks like the 5v drift is the major concern on the OEM power supplies, so I'll temporarily disconnect the 5v rail from the OEM PS and pipe in my own 5v from an arcade switching power supply I have. That way I can be sure I'm feeding a nice stable 5v into the machine.

 

What are the best options for a replacement PLA, should I need to go that route?

Link to comment
Share on other sites

Some replacements:

 

PLAnkton: https://www.polyplay.xyz/PLAnkton-PLA-Replacement(also overpriced on Amazon, eBay, sometimes AtariAge etc)

realPLA: https://github.com/EddyBeaupre/realPLA(not sure if it exists for sale anywhere)

SuperPLA: https://icomp.de/shop-icomp/en/shop/product/superpla-v3.html(the chip that started it all, though it might have minor issues)

 

There is one called PLAtinum too, but I don't know if it is hit or miss. You might find some original PLA chips too, 82S100 / PLS100 though I don't know how long they last anyway.

Link to comment
Share on other sites

I poked around a bit more on the board this morning before going to work. The VIC does get pretty warm the longer the unit is left on.

 

Measuring voltage around various places on the board, I'm getting .408v pretty much at pin 1 of every chip on the left side of the board: ram, rom, CPU, logic, all of them have .408v. This is the same voltage I see at the LED as well.

 

The VIC chip socket has 5.04v at pin 1, and the VIC shows input voltage of 5.04v when inserted, as do the surrounding logic chips inside the cage.

 

Removing the VIC, I get nothing on the TV when turning the board on and off. None of the chips on the left side of the board have any voltage then either. Inserting the VIC, I get "black screen," and all of the chips have .408 v again.

 

I'm thinking at this point the issue lies with a failed VIC chip. Something has to be dragging down the 5v line to .408v, and I think this might be it. Is there a way to check this with a multimeter?

 

I'll check into buying the PLAnkton (EDIT: may not be needed....)

 

I really wish I had a working unit nearby for chip swapping. I'd feel a little less blind then.

Edited by smalltownguy2
Link to comment
Share on other sites

Oh, good thing that the PLA is socketed. You previously wrote that only the CPU and VIC-II chips were socketed. I don't know if it would affect voltages in the way you describe, perhaps if it has an internal short? It normally runs hot though, probably the chip that is hottest inside the entire C64 which is why people who still have a working old model PLA chip make sure to put on a heat sink for as long as it will last. Heat sinks can be useful on other chips too, I've read though I've never installed those myself.

Link to comment
Share on other sites

Ok. It also will run a little hot, though not as hot as the PLA and perhaps SID do. It is not surprising that you get nothing on the TV without the video chip, but that very low voltage you measure when it is plugged in is beyond my skills.

 

I found a Reddit thread from 2017 which sounds very much like what you're experiencing:

https://www.reddit.com/r/c64/comments/6cam2r/no_video_output_broken_vicii_chip/

Link to comment
Share on other sites

Some people are brave an order them from Chinese surplus sellers. However it should be noted that very often you get fake or broken chips from those sellers. It has become a "hobby" to wipe out the old markings and paint more desirable numbers on the chips. Sometimes the size of the chip and number of pins doesn't even match. Officially it is all a big mistake after the print on the chips has faded in the sun, all chips were mixed in a pile and they did their best to sort them up and paint new text on them, but generally buyers don't believe that story.

 

Still, I have read success stories about people ordering VIC-II or CIA chips and found that 5/5 or 10/10 of the ordered chips are genuine and working, while others ordering from the same seller have found that 0/10 chips are correct or fine.

 

You can also try eBay and other refurb parts resellers, perhaps you would be best off finding another partly broken C64 with working VIC chip but other faults.

Link to comment
Share on other sites

A couple of good sellers from whom I have picked up C64 chips recently:

 

eBay Seller: basicwayne1?ff3=10&pub=5574883395&toolid=10001&campid=5336500554&customid=&mpt=[CACHEBUSTER] (6526 CIAs)

eBay Seller: computer_preservation1?ff3=10&pub=5574883395&toolid=10001&campid=5336500554&customid=&mpt=[CACHEBUSTER] (8562 VIC-IIs)

 

When replacing the VIC-II, without scrolling up I think you will need the 6567 which is a different voltage than the 8562 I bought.

Link to comment
Share on other sites

Replacement VIC chip was swapped in, and issue persists. Same black screen. I only left the machine powered on for about a second or two, because I didn't want to risk burning up the new chip if there was some sort of voltage problem.

 

At this point I only have 1 option left currently: try swapping in external power. I will fire up an external 5v power supply and tap it in to see if that changes anything. Based on how I was measuring voltages before I don't believe the power supply is bad, but testing with another known working power source will rule that out completely.

 

If it doesn't boot with externally provided 5v, then my path forward will have to involve a known working C64. I will need to begin swapping chips to narrow down where the failure is. I DO have a local friend who has a C64; I'm just not sure if he'll loan it to me for troubleshooting or not. There is a swap meet coming up next weekend that could be used as a common meeting point though, so maybe I could make some progress there. If not, I may need to purchase a known working C64 to finish the repairs and then re-sell it once I'm done.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...