C64 questions

Tony Duell ard at p850ug1.demon.co.uk
Fri Aug 20 12:51:59 CDT 2010

> I've got an application that depends on high precision timing that I 
> originally wrote for the C64.  I decided to use it to acquire more data and 
> had very bizarre behavior from the C64 in that at first the standard screen 
> didn't come up but then it did and the letters faded and the C64 was 
> unresponsive to keyboard input or just showed garbage characters.  I happen 
> to have a spare C64 and it demonstrated exactly the same behavior.

Hmmm.. While you might think it unlikely for 2 machines to develop the 
same fualt, if this is, say, due to dried-up capacitors, it could well 
happen. So my first suggestion is to forget you have a second machine, 
and to debug the first one 'properly'. 

> The 5 V supply reads 4.94 V under load and has little ripple.  There is 9 

Sounds fine.

> VAC coming out of the power adapter and I get +5 V and +12 V at the outputs 
> of the internal C64 regulators.  There seems to be little ripple on these 
> supplies also (about 20 mv) but this check was done with the frequency 
> measuring setting of my DVM so I don't know if there are short HV spikes on 
> any of the supplies (have to try to remember where I put my scope after the 
> last move).

I really think you need to find that 'socpe and check thsoe supplies more 
carefully... I have the advantage of owning a 'scope I can't easily lose 
-- a Tekky 555 :-). I wm wondering just what the DMM is displaying -- 
you say it's a 'freqeuncy measuring' range, but you're giving a voltage. 
Some DMMs will display the ripple voltage if set to ACV and conencted to 
the DC power eail (they ignore th standard DC level), but you want to 
make sure it's doing what you think it's doing before you trust the readings.

> The C64 that I want to use is 27 years old and worked just fine when I last 
> fired it up about 10 years ago.  Anyone have any experience with this type 
> of behavior?  Is it worthwhile to replace the internal power supply 
> electrolytics or should I build a new power supply for it?  Have lots of 5 
> VDC supplies but don't seem to have a 9 V transformer in my junk box.

If you think it's the power brick (and it might be), you could test 
things by using the 9V AC otuput from that 'brick' (that is unlikely to 
fail in an odd way, it's just the secondary wining of a transformer), 
along with a separate 5V supply connected in placeof the 5V output from 
the brick (in other words, desolder the 5V wire at the C64 conenctor and 
connect a 5V PSU to the C64). If it works, then you know ehre the problem 

If it's not PSU problems, my next approact would be to try freezer spray 
on the ICs in the C64.


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