Caple C871i Manual do Utilizador descarregar pdf (Página 363)

100

101

Apple II Computer Info

frequency response of video amplifiers. A transistor

amplifier has unavoidable capacitance that bypasses

high-frequency video components. It's these high-frequency

signals that enable the CRT to change brightness quickly as

it forms sharp edges. Inductance, such as that found in a

peaking coil, cancels capacitance. If you choose just the

right amount of inductance you can compensate for the

unwanted capacitance, which is exactly what happens when a

peaking coil is doing its thing.

The open inductor in my monitor's circuitry allowed the

green cathode on the CRT to drop about 100 volts lower than

normal, and this action biased the green gun into

saturation.

It was obvious that something was amiss once I'd

determined that the peaking coil had100 volts across it's

terminals. I had two good peaking peaking coils for

comparison in the red and blue circuitry. Each of these

good coils measured about 10-ohms resistance. The flakey

coil was open. Shorting the open coil for a test eliminated

the excess green on the screen but at the expense of poor

green bandwidth.

How do I get a replacement peaking coil? The local

electronics supply dealer was about as helpful as he'd been

with the Sams Photofact deadend. "Peaking coils almost

never fail. You have to go back to the manufacturer to get

a replacement! Forget it!" This time I stood up to him,

asked to see his stash of peaking coils. I needed a

20-microHenry inductor, and he did have a 22-microHenry

unit. $1.95.

The 10% excess made no noticeable difference and my

monitor seems as good as new.

Being green is not so bad if your monitor can also be

red and blue.

***

I wrote him and asked if the part was labeled L6G2, a small

blue component with a red and black dot, just above the

transistor Q6G2, on the board hanging on the back of the

CRT. This is the reply:

Subj: Re: RGBlues

Date: 92-08-26 11:25:01 EDT

From: AFA DonB

To: DynaSoar01

You have the right part... It's a small blue component with

one black dot on the side, a black and a red dot on top...

Measure the dc voltage across the component. If it's open

(defective), you'll measure somewhere around 100 volts...

If it's good, the voltage drop will be negligible.

Another way to check: Measure the voltage at each of

checkpoints on the CRT socket. All three should measure

Apple II Computer Technical Information : Apple II Family Hardware Info

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Caple C871i Manual do Utilizador Página 363

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