Two sick 64's

[KW42784]

Hello, I have 3 64's and I recently set aside space for them to live again, but have concluded today that two of them are sick.

The first one, a 1984 brown breadbox, works great, but has no sound only a continuous low hum, that could just be line noise, not sure. Also, one of the ceramic disc capactitors (C25) is phsyically broken.

The second, a newer C64C, loads some games and programs just fine (genuine Battle of Britain and pirated Sim City), but some have garbled text (genuine Ghostbusters) and one (genuine Ace of Aces) won't load past the title screen.

The third also a C64C loads them ALL fine, so that tells me the others are busted and it's not a software issue.

I have repaired PC's since I was a teen, but never Commodores, where do I start?

[LoneWolf33]

Hi,

On the '84 brown breadbox which output was connected to the TV: modulator (RF) or the A/V (Baseband)?
If you have sound when the TV is connected to the A/V output of the C64, but no sound when using the
modulator output, then it's probably a modulator problem. If no sound on both A/V and modulator outputs,
you can try to:
- check the presence of 9V (12V) supply on the collector of Q3, and pin#28
- check the presence of audio signal at pin#27 of the SID (U9). It may require an oscilloscope due the low signal level.
- check the audio signal at the emitter pin of Q3, this is connected to R12 (an oscilloscope or a high impedance headphone might come in handy too)
If all of these are OK, probably C77 capacitor is dead.

The C64C may have a memory (RAM) problem.

I hope this helps.

[KW42784]

So far I had only tried the single RCA jack on the back as it is going to a RCA to coax adapter then to an old lcd TV. I will hook it up with the commodore video cable and see what happens, then I will probe it.

...where is the best spot to put the black lead of the meter on the 64?

... If A/v does work, are those Commodore to s-video adapters on eBay any good?

...and for the C64C where is a good source of 25 yo ram chips? also eBay?

[LoneWolf33]

Hi,

The black lead can be put on the screening of the expansion port or the screening of the RF modulator.

Regarding the A/V cable do you need the four pin (+screening) S-Video ending or SCART ending is also good?
I guess you can find DIN to SCART cable more easily on ebay.
There's a third way: luminance and chrominance signals on separate RCA connectors (these are also available
on ebay), but not all TV supports this (chrominance on RCA).

I'd suggest to execute a RAM test software before the final diagnosis, just to make sure.

[KW42784]

I'm American, so for s-video I only know of the US standard 4 pin s-video... Never heard of SCART

I don't know if I have a C64 RAM test programs, I only know on the one C64C, some games load, some glitch, and some fail to load, but work fine with same 1541-II on the other C64C.

[LoneWolf33]

SCART is also known as EIA Multiport in U.S. that may sounds more familiar.
Most of the TVs are equipped with this type A/V connector.
The C64 DIN to SCART / EIA Multiport cable looks like this:
http://www.ebay.com/itm/Commodore-64-C6 ... 19e5960207

In case of S-video you can try something like this:
http://www.ebay.com/itm/NEW-DUAL-Compos ... 6677&rt=nc

Regarding the RAM test the following forum might provide useful info:
http://www.forum64.de/wbb3/board2-c64-a ... d-problem/
It also contains a small type-in memory test program that you can save to disk (so you don't have to type it in again)
and the result can be compared between your C64 machines.

[KW42784]

Ok thanks, can't remember ever seeing that connector on anything...

I entered the program from the link into my "good" machine and got " 0" which is a pass, I will try the other 2 after work (Should be around +4 hours after this post.)

[KW42784]

All three C64's passed the test with result " 0".

[LoneWolf33]

I've compiled back the machine language program from the data lines,
and it seems this program scans only a small 256 byte RAM area (from $A000 to $A0FF).
So the "0" result confirms the correct operation of this small RAM portion only...

[KW42784]

Ok, so back to square #1...

Are there any other programs or were any retail RAM testers made?

Also, the 1984 brown C64 did not have sound output with the Commodore Chroma/Luma/Audio cable. I will have to probe the board.

[BenjaminLumen]

KW, You may not be back at square 1:

If you compiled the assembly, you should just be able to go in and change where the register is counting memory, and which address that is being pointed to, you should
be able to modify the program to look at more then the 256 byte segments....

You should find the addressing schema in a variable or actually being loaded immediate mem style into the accumulator..

I haven't looked at the program but that is something akin to what it should be doing..

[KW42784]

I'm not sure how to modif the program...

But, I see some "diagnostic cartridges" on ebay, would they assist with this issue?

[BenjaminLumen]

I am not really up to snuff on 6502 uProc Assembly Language, ........

Vaguely.... Very Vaguely it would go something like...

Wow so vague that I can even conjure up an op-code right at the moment....

You could try getting a sticky thermometer and see if any of the chips are running hot... the thin film thermometer may tell you something..

But memory wise you have 256 segments of 256 bytes, so you would need two nested counters one counting one every time the first counter counts to 256, and then repeating until the second counter
is up to 256 also...

YOu can look at the Memory but that will not necessarily tell you if it is incorrect, unless you fill a segment with 256 bytes of zero's then read it back and see if it is difference then the zero's you loaded into
it... SO you could take that approach..

or you could just find a junk shop and try to find good memory and swap them all out in one shot and see if it works..

Fry's Electronics should have a garbage dump behind every one of their stores.... Because that would be where all the cool stuff ends up...

To bad its not profitable to run a junk shop , because those things are my dream computer shops.....

"There is no such thing as an obsolete Calculating Device." Is what I would name my Computer junk shop..

The diagnostic cartridge will work most likely as it is running a program not unlike what the one above does but most certainly with more bells and whistles..

[LoneWolf33]

YOu can look at the Memory but that will not necessarily tell you if it is incorrect, unless you fill a segment with 256 bytes of zero's then read it back and see if it is difference then the zero's you loaded into
it... SO you could take that approach..

This is what the previously linked RAM tester routine does (to be accurate almost): it tests the mentioned RAM area with
two different byte patterns so IMHO it does tell if that area has problem. (It's too bad that the routine not covers more than 256 bytes)