Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.sys.apple2 > #3029 > unrolled thread

AppleColor RGB all-red display

Back to article view | Back to comp.sys.apple2

Contents

  AppleColor RGB all-red display dog_cow@macgui.com (D Finnigan) - 2011-10-15 17:38 +0000
    Re: AppleColor RGB all-red display dog_cow@macgui.com (D Finnigan) - 2011-10-15 17:41 +0000
      Re: AppleColor RGB all-red display Grent <grent3@gmail.com> - 2011-10-15 17:33 -0700
        Re: AppleColor RGB all-red display dog_cow@macgui.com (D Finnigan) - 2011-10-17 20:47 +0000
          Re: AppleColor RGB all-red display dog_cow@macgui.com (D Finnigan) - 2011-10-18 14:19 +0000
            Re: AppleColor RGB all-red display Michael J. Mahon <mjmahon@aol.com> - 2011-10-18 12:05 -0500
              Re: AppleColor RGB all-red display dog_cow@macgui.com (D Finnigan) - 2011-10-18 17:41 +0000
                Re: AppleColor RGB all-red display Grent <grent3@gmail.com> - 2011-10-18 23:27 -0700
                Re: AppleColor RGB all-red display Alex Freed <alex_news@alexfreed.com> - 2011-10-19 02:10 -0700
                  Re: AppleColor RGB all-red display Michael J. Mahon <mjmahon@aol.com> - 2011-10-19 16:08 -0500
                    Re: AppleColor RGB all-red display Kevin Dady <kevin@hackaday.com> - 2011-10-19 18:16 -0700
                    Re: AppleColor RGB all-red display Alex Freed <alex_news@alexfreed.com> - 2011-10-20 01:25 -0700

#3029 — AppleColor RGB all-red display

I was fiddling with my AppleColor RGB (for the IIgs) yesterday, and I
managed to break it.

I was trying to figure out why the voltage to the screen was increasing,
thus making the intensity slowly ramp up as the electronics got warmer over
about 10 minutes.

I measured voltage across a 4.7k Ohm resistor on the CRT board (the one that
connects to the neck of the CRT) and it was about 50 volts. I then measured
across the identical (?) 4.7k Ohm resistor on the left side of the same
board. This is where things went wrong. I have an analog multimeter, and
when I connected the leads, the needle went back, like negative voltage. I
assumed this meant that I had the leads the wrong way around, so I reversed
them. I got a spark, a zap, and a nasty smell. A second later, the display
went all red. I turned off the monitor.

I searched the Apple II FAQs, and found in the Monitors, question 008-
"Suddenly my monitor has an all-blue (all-red, etc.) screen! How do I fix
this?"

Well, I measured the resistance across all three inductors, and they were
all about 10k Ohms. I powered on the monitor and measured the voltage drop,
which was negligible (the needle barely moved).

So, now I must assume that some other component blew up and made the nasty
smell. Any ideas? I'm not really good at doing electronics, so I'm going to
have a graybeard help me with the repairs.

-- 
]DF$
Mac GUI Vault - A source for retro Apple II and
Macintosh computing.
http://macgui.com/vault/

[toc] | [next] | [standalone]

#3030

Oh, yeah. To add: I do get recognizable video. I can see the IIgs desktop,
icons, menubar, etc. It's just that everything is a shade of red. And it's
all intense red.

[toc] | [prev] | [next] | [standalone]

#3033

On Oct 15, 1:41 pm, dog_...@macgui.com (D Finnigan) wrote:
> Oh, yeah. To add: I do get recognizable video. I can see the IIgs desktop,
> icons, menubar, etc. It's just that everything is a shade of red. And it's
> all intense red.

Usually it was a choke that would cause the color shift that you were
talking. Some how I want to say 47microheneries. There was one for
each of the 3 colors. Not sure these green and black things that look
like caps are them or if they look like a resistor that has a coil
wrapped around it. Then again you could have fried something else.

Take Care
         George

[toc] | [prev] | [next] | [standalone]

#3067

Grent wrote:
> On Oct 15, 1:41�pm, dog_...@macgui.com (D Finnigan) wrote:
>> Oh, yeah. To add: I do get recognizable video. I can see the IIgs
>> desktop,
>> icons, menubar, etc. It's just that everything is a shade of red. And
>> it's
>> all intense red.
> 
> Usually it was a choke that would cause the color shift that you were
> talking. Some how I want to say 47microheneries. There was one for
> each of the 3 colors. Not sure these green and black things that look
> like caps are them or if they look like a resistor that has a coil
> wrapped around it. Then again you could have fried something else.
> 

I stopped in with the local graybeard and showed him the schematics, plus
told him the testing that I'd done. His best guess was that the NPN
transistor that controls the red beam has failed. This is the transistor
that's near the inductor and 4.7k resistor on the CRT board.

I'm going to test it this evening (or tomorrow morning if it's too late) and
report back.

[toc] | [prev] | [next] | [standalone]

#3094

The story continues:

Yes, it was a bad transistor for the red channel. It tested differently
compared to the blue and green transistors. Using better light, I could also
see scorch marks on the PCB from where I'd apparently made a short circuit
the other day.

Next step now is to find out what kind of transistor it is, and get a
replacement part.

-- 
]DF$
Mac GUI Vault - A source for retro Apple II and
Macintosh computing.
http://macgui.com/vault/

[toc] | [prev] | [next] | [standalone]

#3107

D Finnigan <dog_cow@macgui.com> wrote:
> The story continues:
> 
> Yes, it was a bad transistor for the red channel. It tested differently
> compared to the blue and green transistors. Using better light, I could also
> see scorch marks on the PCB from where I'd apparently made a short circuit
> the other day.
> 
> Next step now is to find out what kind of transistor it is, and get a
> replacement part.

Glad you found it. 

I'd guess a high voltage (50-70v) medium beta NPN. After replacement you
may need to adjust the red brightness ("screen") and contrast ("gain") to
get the colors right again. 

-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon

[toc] | [prev] | [next] | [standalone]

#3109

Michael J. Mahon wrote:
> D Finnigan <dog_cow@macgui.com> wrote:
>> The story continues:
>> 
>> Yes, it was a bad transistor for the red channel. It tested differently
>> compared to the blue and green transistors. Using better light, I could
>> also
>> see scorch marks on the PCB from where I'd apparently made a short
>> circuit
>> the other day.
>> 
>> Next step now is to find out what kind of transistor it is, and get a
>> replacement part.
> 
> Glad you found it. 
> 
> I'd guess a high voltage (50-70v) medium beta NPN. After replacement you
> may need to adjust the red brightness ("screen") and contrast ("gain") to
> get the colors right again. 
> 

I just researched the transistor number, and it appears to be a standard
part: C2688.

<http://search.digikey.com/us/en/cat/discrete-semiconductor-products/transistors-bjt-single/1376376?k=c2688>

[toc] | [prev] | [next] | [standalone]

#3134

On Oct 18, 1:41 pm, dog_...@macgui.com (D Finnigan) wrote:
> Michael J. Mahon wrote:
> > D Finnigan <dog_...@macgui.com> wrote:
> >> The story continues:
>
> >> Yes, it was a bad transistor for the red channel. It tested differently
> >> compared to the blue and green transistors. Using better light, I could
> >> also
> >> see scorch marks on the PCB from where I'd apparently made a short
> >> circuit
> >> the other day.
>
> >> Next step now is to find out what kind of transistor it is, and get a
> >> replacement part.
>
> > Glad you found it.
>
> > I'd guess a high voltage (50-70v) medium beta NPN. After replacement you
> > may need to adjust the red brightness ("screen") and contrast ("gain") to
> > get the colors right again.
>
> I just researched the transistor number, and it appears to be a standard
> part: C2688.
>
> <http://search.digikey.com/us/en/cat/discrete-semiconductor-products/t...>

Yes thank you for posting the information

[toc] | [prev] | [next] | [standalone]

#3137

On 10/18/2011 10:41 AM, D Finnigan wrote:

> I just researched the transistor number, and it appears to be a standard
> part: C2688.
>
> <http://search.digikey.com/us/en/cat/discrete-semiconductor-products/transistors-bjt-single/1376376?k=c2688>

Standard, yes. But not a stocked item. Nothing magical about it: easy to 
find a substitute. Key parameters are 300V, 10 Watt, 200 mA max current, 
hFE 40 to 150 and 50 MHz.

http://alltransistors.com/crsearch.php?mat=Si&struct=npn&pc=10W&ucb=300V&uce=300V&ueb=5V&ic=0.2&tj=150%C2%B0C&ft=50MHz&cc=3&hfe=40/160&mnf=&caps=

81 results found :)

-Alex.

[toc] | [prev] | [next] | [standalone]

#3162

Alex Freed <alex_news@alexfreed.com> wrote:
> On 10/18/2011 10:41 AM, D Finnigan wrote:
> 
>> I just researched the transistor number, and it appears to be a standard
>> part: C2688.
>> 
>> <http://search.digikey.com/us/en/cat/discrete-semiconductor-products/transistors-bjt-single/1376376?k=c2688>
> 
> Standard, yes. But not a stocked item. Nothing magical about it: easy to
> find a substitute. Key parameters are 300V, 10 Watt, 200 mA max current,
> hFE 40 to 150 and 50 MHz.
> 
> http://alltransistors.com/crsearch.php?mat=Si&struct=npn&pc=10W&ucb=300V&uce=300V&ueb=5V&ic=0.2&tj=150%C2%B0C&ft=50MHz&cc=3&hfe=40/160&mnf=&caps=
> 
> 81 results found :)
> 
> -Alex.

Wow!  That seems inordinately beefy for a video output transistor. ;-).
10W?  And 300v seems like a lot for a transistor stage that only needs to
drive a CRT grid/cathode...  

Sounds like a significant overdesign. 

-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon

[toc] | [prev] | [next] | [standalone]

#3165

On Oct 19, 4:08 pm, Michael J. Mahon <mjma...@aol.com> wrote:
> Alex Freed <alex_n...@alexfreed.com> wrote:
> > On 10/18/2011 10:41 AM, D Finnigan wrote:
>
> >> I just researched the transistor number, and it appears to be a standard
> >> part: C2688.
>
> >> <http://search.digikey.com/us/en/cat/discrete-semiconductor-products/t...>
>
> > Standard, yes. But not a stocked item. Nothing magical about it: easy to
> > find a substitute. Key parameters are 300V, 10 Watt, 200 mA max current,
> > hFE 40 to 150 and 50 MHz.
>
> >http://alltransistors.com/crsearch.php?mat=Si&struct=npn&pc=10W&ucb=3...
>
> > 81 results found :)
>
> > -Alex.
>
> Wow!  That seems inordinately beefy for a video output transistor. ;-).
> 10W?  And 300v seems like a lot for a transistor stage that only needs to
> drive a CRT grid/cathode...  
>
> Sounds like a significant overdesign.
>
> -michael - NadaNet 3.1 and AppleCrate II:http://home.comcast.net/~mjmahon

they probably got a deal in bulk

[toc] | [prev] | [next] | [standalone]

#3172

On 10/19/2011 2:08 PM, Michael J. Mahon wrote:
> Wow!  That seems inordinately beefy for a video output transistor. ;-).
> 10W?  And 300v seems like a lot for a transistor stage that only needs to
> drive a CRT grid/cathode...
>
> Sounds like a significant overdesign.

Not as much as one would think. A voltage of 200 V is quite typical for 
driving a CRT. In order to have good frequency response you need to use 
a relatively low load resistor - say 10K. So we have 20 mA of current 
and up to 4 W heating up the transistor. Add a little safety margin and 
you get a circuit that works for 25 years till someone makes a short :)

So it's a bit of an overkill, but reasonable. Medium power transistors 
are cheap.

-Alex.

[toc] | [prev] | [standalone]

Back to top | Article view | comp.sys.apple2

csiph-web