D Finnigan

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/

D Finnigan

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.

Grent

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

D Finnigan

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.

D Finnigan

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/

Michael J. Mahon

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

D Finnigan

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>

Grent

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

Alex Freed

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.p...160&mnf=&caps=

81 results found

-Alex.

Michael J. Mahon

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.p...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