C64 black screen. 12V reading 13.2V

Got an issue?
Borat
Member
Member
Posts: 18
Joined: Sat Oct 31, 2015 5:55 pm
Contact:

C64 black screen. 12V reading 13.2V

Post by Borat »

I just picked up a faulty C64 (250425) with the infamous black screen fault. During my initial tests I found the 12V line is actually 13.2V. The input to the 7812 is just over 20V so I was suspecting possibly C88 being faulty.

However my main question - is this high voltage a red herring in my search for the black screen solution or is it likely to be part of the problem? I've read several threads already saying this could faulty RAM chips or PLA. I just want to rule in/out this 13.2V first.

Unfortunately only the kernal and SID are socketed (both are OK).


Borat
Member
Member
Posts: 18
Joined: Sat Oct 31, 2015 5:55 pm
Contact:

Re: C64 black screen. 12V reading 13.2V

Post by Borat »

I've de-soldered the PLA and tested it - all OK. I've now replaced it in a socket. However I'm seeing CHARREN, LORAM and HIRAM all sitting at 0V on the 6510. I'm suspecting the 6510 is faulty, so just waiting for some 40 pin sockets and a replacement to arrive.
User avatar
eslapion
Active Member
Active Member
Posts: 1215
Joined: Mon Jul 20, 2015 10:11 am
Location: Canada
Contact:

Re: C64 black screen. 12V reading 13.2V

Post by eslapion »

I checked the 7812 here on my 250466 and the input is 21V so your value seems fine.

The 13.2V on the output seems a bit high at 10% over. Should not be more than 5% over or 12.6V. Did you check for ripple on the 12V line? Could indicate a faulty regulator.

DRAM and PLA are the "usual suspects" but you've already checked the PLA.

If the SID is OKAY, leave it out during your investigations as it can get damaged by an excessive or rippling 12V. Is the VIC-II on a socket too? It too uses the 12V. If you can take out both the SID and the VIC-II then check on a scope what comes in and out of the 7812 as well as the 7805.

The 3 memory mapping control lines may be low because the 6510 never manages to start properly - not because its faulty. I rarely see a faulty CPU in a 64.

Touch the DRAM chips and see if one will dissipate a lot of heat - if so then its faulty.

If the 7812 still heats a lot with both the SID and VIC-II removed then its more than likely faulty.
Wealth, like happiness, is never attained directly. It comes as a by-product of providing a useful service. -Harland D. Sanders
Borat
Member
Member
Posts: 18
Joined: Sat Oct 31, 2015 5:55 pm
Contact:

Re: C64 black screen. 12V reading 13.2V

Post by Borat »

Thanks for the reply.

I had a 6510 faulty recently where Pin 29 (Loram) of the 6510 was measuring 1.2V instead of 5V. After bending the pin out of the socket it came good. That's what made me think this was another suspect 6510 but all 3 lines faulty??? Maybe not.

I have already removed the SID to protect it (working) and the VIC also is socketed. I've ordered a new set of capacitors and a new set of regulators.

Unfortunately I don't have a scope to measure ripple, it's just me and my trusty meter. I didn't check the DRAM chips for high temps, I'll try that now.
Borat
Member
Member
Posts: 18
Joined: Sat Oct 31, 2015 5:55 pm
Contact:

Re: C64 black screen. 12V reading 13.2V

Post by Borat »

Well all the DRAM chips are cool and after removing the VIC so also are both regulators. However the 6510 is very hot to touch just (in the centre) 30s - 60s after power on, as is the VIC.

The investigation continues...
Borat
Member
Member
Posts: 18
Joined: Sat Oct 31, 2015 5:55 pm
Contact:

Re: C64 black screen. 12V reading 13.2V

Post by Borat »

So what does the 6510 need to "start properly" i.e. bring the 3 lines up to 5V? The only thing I can see between the 6510 and PLA (on those 3 lines) are the 3 pull-up resistors R43, R44 and R45.
User avatar
eslapion
Active Member
Active Member
Posts: 1215
Joined: Mon Jul 20, 2015 10:11 am
Location: Canada
Contact:

Re: C64 black screen. 12V reading 13.2V

Post by eslapion »

The CPU requires a valid duration reset at start-up and access to the kernal.

However, if it warms up like a toaster as you describe, I can only see 2 possibilities:
- It is trying to pull-up something that's pulled low by an external (faulty?) component or the other way around
- Itself is faulty

If possible try the VIC-II on another 64.
Wealth, like happiness, is never attained directly. It comes as a by-product of providing a useful service. -Harland D. Sanders
Borat
Member
Member
Posts: 18
Joined: Sat Oct 31, 2015 5:55 pm
Contact:

Re: C64 black screen. 12V reading 13.2V

Post by Borat »

I've tried moving the VIC around into another C64 but it checks out OK. I'm waiting for delivery of some 40 way sockets and a replacement 6510. Once I've swapped it I'll know whether to start looking elsewhere.

How could I find out what the power-on state of the main chips are? I find myself checking pins and wondering if they are in the correct state. Also, what voltage range is a "low" and a "high" i.e. 0V to ?V is low and ?V to 5V is high?

Thanks
User avatar
eslapion
Active Member
Active Member
Posts: 1215
Joined: Mon Jul 20, 2015 10:11 am
Location: Canada
Contact:

Re: C64 black screen. 12V reading 13.2V

Post by eslapion »

This should answer a couple of questions.

http://www.everythingcommodore.com/down ... p?view.777

More here:
https://computerarchive.org/files/comp/books/commodore/

Check for Sams Computerfacts CC4 and CC17.
Wealth, like happiness, is never attained directly. It comes as a by-product of providing a useful service. -Harland D. Sanders
Borat
Member
Member
Posts: 18
Joined: Sat Oct 31, 2015 5:55 pm
Contact:

Re: C64 black screen. 12V reading 13.2V

Post by Borat »

Thanks very much! Just what I needed.

I received my spare 6510 today and some sockets so I got straight to it. Turns out it was the 6510 that was faulty. I tried it in a known good C64 and got the same symptoms.

Ever since I started out with the C64 in 1983 I have never seen a faulty unit... until the last month or two, where I have seen two faulty CPUs. My first had pin 29 LORAM, stuck at 1.2V (approx) giving a blank screen with border. I now use it a a test CPU as I can pull pin 29 out of the socket and it partially works.

Thanks for your help with this BTW ;)

Forgot to also mention, I replaced all the electrolytics on the board to see if it would sort out the 13.2V issue but no luck. I thought I may as well replace the 7812 and that improved it some. it's now 12.1V :)

Changing the capacitors on this board was difficult to say the least (compared to 250407 which I also did recently). So much heat is lost into the PCB.
Post Reply Previous topicNext topic

Who is online

Users browsing this forum: No registered users and 8 guests