Hello everyone! 

Having never used a Commodore computer before, I took the opportunity of
grabbing a second-hand Commodore 64 at a car boot sale (a bit like what
would be called a Flea Market in the US), in order to discover some of the
delights I have been missing out on all these years. Unfortunately, the
thing does not seem to work :-(

Basically what I get is a pure black screen on the TV. Sync pulses are
present, but no picture, and the Power LED is not lit. Doing the Google
thing, I found this page:
http://personalpages.tds.net/~rcarlsen/cbm/c64blank.txt but this does not
seem to cover my exact symptoms.

I have checked the voltage lines, and both +12v and +5v are present and
correct at the regulator outputs, but the LED is only receiving 0.4v,
which is not enough to light it up. I tried a replacement LED, and that
did not light either, so I strongly suspect a power problem.

So, my questions are: How many volts should I see on the LED, and what
kind of power fault would cause the TV screen to remain black, but still
have sync pulses?

I'm in the UK, so this is a PAL model C64 :-)

Any advice would be very gratefully received! 

Tim. 

-- 
Please remove all-your-clothes before replying. 

Tim,

> I have checked the voltage lines, and both +12v and +5v are present and
> correct at the regulator outputs, but the LED is only receiving 0.4v,
> which is not enough to light it up. I tried a replacement LED, and that
> did not light either, so I strongly suspect a power problem.

First, look at the backside of the power supply, to see if the
electrical specs are noted. Commodore US power supplies are clearly
marked for 5V DC and **9V AC** (not 12V!!!!)

Watts = Volts x Amps, therefore 8.5 watts output on the 5 volt line =
1.7 amps at the power connector (values are from the C64 power supply
I just happen to have handy).

Take a look at another Carlsen article for more hints about the power
supply: http://personalpages.tds.net/~rcarlsen/cbm/pstester.txt

James

On Sat, 2 Jun 2007, James @ cbm264 wrote:

> First, look at the backside of the power supply, to see if the
> electrical specs are noted. Commodore US power supplies are clearly
> marked for 5V DC and **9V AC** (not 12V!!!!)
<snip>
> supply: http://personalpages.tds.net/~rcarlsen/cbm/pstester.txt

Thanks James. 

The Carlsen article is very useful, but my power supply is good, 
unfortunately. The pinouts on my unit differ slightly from Carlsen.s 
diagram. I have the following: 

Pin 1 - No Pin
Pin 2 - 0v
Pin 3 - No Pin
Pin 4 - +5VDC
Pin 5 - No Pin
Pin 6 - 9VAC
Pin 7 - 0v

Off load, my voltages measure 10.7V and 5.13V. 

Inside the C64 there are two voltage regulators - an AN7812 (12 volts) and 
an AN7805 (5 volts). Both of these are fed from the 9VAC input - there is 
a voltage doubler circuit to create an 18v input to the 12v regulator, and 
a rectifier/smoothing cap to feed the 5v regulator. The outputs of both of 
these regulators are correct - I measure 11.74 and 4.98 volts 
respectively. 

The C64's Ground is pulled up to some potential with respect to the power 
supply Ground. This makes the absolute voltage at the Power LED w.r.t. the 
supply 4V, but the potential difference across the LED is only 0.4V. The 
LED is fed from the 9VDC rectifier output, before it reaches the 
regulator, but the 0V side is sourced from the C64 Ground, which is at 
about +3.5V w.r.t. the supply Ground. Presumably this is done to effect a 
dual-rail supply (+5v and -5v), but how I have no idea. Or could that be 
the fault?

I'm no expert on power supplies, so at this point I.m totally out of my 
depth. Can anyone give me any further pointers? 

Before anyone suggests it, the answer is no, I.m not going to give up and 
throw it away ;-) That's just defeatist. 

Tim. 

-- 
Please remove all-your-clothes before replying. 

On Sat, 2 Jun 2007 16:25:32 +0100
Tim Fardell <tim.fardell.all-your-clothes@virgin.net> wrote:
> I'm no expert on power supplies, so at this point I.m totally out of my 
> depth. Can anyone give me any further pointers? 
> 
> Before anyone suggests it, the answer is no, I.m not going to give up and 
> throw it away ;-) That's just defeatist. 

First, just because that power supply is in spec when you checked it doesn't mean it's always in spec. It may go in and out of spec.

Right?

Second, that may not be the same power supply that fried the motherboard in the first place.

You seem to know more about electronics than me, so it would be great if you could fix it - I just wouldn't trust that power supply on giving a consistent voltage.

Reseating the socketed chips may do something (it's never worked for me).

Good luck!

Christian

> First, just because that power supply is in spec when you checked it doesn't mean it's always in spec. It may go in and out of spec.
> Second, that may not be the same power supply that fried the motherboard in the first place.
> You seem to know more about electronics than me, so it would be great if you could fix it - I just wouldn't trust that power supply on giving a consistent voltage.

I agree with Christian. This is why I sent you a link to that page....
the power supply really needs to be tested under a load.

Here's a link to a C64 service manual. Maybe you'll find more hints in
there. http://www.devili.iki.fi/Computers/Commodore/

James

On Sat, 2 Jun 2007, James @ cbm264 wrote:

> I agree with Christian. This is why I sent you a link to that page....
> the power supply really needs to be tested under a load.
> 
> Here's a link to a C64 service manual. Maybe you'll find more hints in
> there. http://www.devili.iki.fi/Computers/Commodore/

Thanks for your help guys! 

Yes, sorry I forgot to say I have tested the power supply on load, i.e. 
connected to the C64 with the power switched on. Both 9V and 5V were 
arriving in the C64. 

Anyway, I have got the thing working now! The annoying thing is, I am not 
quite sure exactly *how* I got it working! I was in the process of testing 
all the electrolytic capacitors across the supply lines, and all of a 
sudden, the +5V line sprang into life! I connected up the LED, and sure 
enough, it glowed brightly! Whether it was a dodgy capacitor and I 
disturbed it back into life, or some other problem, I don't know - but it 
is now working :-)

At least, I think it is. I grabbed a tape at random (Hunchback) and put it 
in the cassette player, and pushed SHIFT+RUNSTOP. It said PRESS PLAY ON 
TAPE, but on doing so, the screen blanked. A few seconds later, up came 
FOUND HUNCHBACK, then a message saying it would take 6 minutes to load, 
then it blanked again. After about 6 minutes, up came the game, and it ran 
OK. 

I have since tried a few more games, and they all blank the screen during 
any loading. 

Is it normal for the screen to be completely blank during loading, or have 
I still got a fault? 

Thanks! 

Tim. 

-- 
Please remove all-your-clothes before replying. 

On 2007-06-03, Tim Fardell <tim.fardell.all-your-clothes@virgin.net> wrote:
> Is it normal for the screen to be completely blank during loading,
> or have I still got a fault?

Yes, this is normal for tape. Some disk speeders also blank the screen.

-Pasi
-- 
"I know, I know what it's like to lose someone, only to find her and
 then lose her a second time. I wouldn't wish that on anyone, not even
 you, as much as I might want to."
	-- Sheridan to Bester in Babylon 5:"Rising Star"

On Sun, 3 Jun 2007, Pasi Ojala wrote:

> On 2007-06-03, Tim Fardell <tim.fardell.all-your-clothes@virgin.net> wrote:
> > Is it normal for the screen to be completely blank during loading,
> > or have I still got a fault?
> 
> Yes, this is normal for tape. Some disk speeders also blank the screen.
> 
> -Pasi

Ah, ok thanks! That is good to know. 

-- 
Please remove all-your-clothes before replying. 

> FOUND HUNCHBACK, then a message saying it would take 6 minutes to load,
> then it blanked again. After about 6 minutes, up came the game, and it ran
> OK.

It was coureous of the programmer to give you an esimated load time.
Don't expect that from all tapes.

In fact, I don't think I've EVER had a program tell me how long it
takes to load, unless I'm reading the manual.

James

"Tim Fardell" wrote ...
>
> Anyway, I have got the thing working now! The annoying thing is, I am not
> quite sure exactly *how* I got it working! I was in the process of testing
> all the electrolytic capacitors across the supply lines, and all of a
> sudden, the +5V line sprang into life! I connected up the LED, and sure
> enough, it glowed brightly! Whether it was a dodgy capacitor and I
> disturbed it back into life, or some other problem, I don't know - but it
> is now working :-)

I would guess it was either the capacitor, or a cold solder joint that you 
disturbed.  In either case, should it go out again, you have an idea where to 
start looking for the problem.
-- 
Best regards,

Sam Gillett

Change is inevitable,
except from vending machines!