Apple 2 Plus Color Burst goes to h$%@ In Circuit

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Apple 2 Plus Color Burst goes to h$%@ In Circuit

Hello there. First time here, Noob.

I have an apple 2 that is driving me crazy. Problem is that i dont get color in HRG. I DO get color in GR.

I have studied the circuit and traced it out with a scope (best i can). Here is what i see.

The clock is spot on.

The divide by 4 at chip B1 is producing perfect 3.58mhz

Im getting 3.58mhz all the way through B13 and B12

the burst signal then goes to a 1k resistor before entering the 'color killer' portion of the circuit.

At the in side of this resistor i get the 3.58, on the out side it goes to hell jumping from khz to 5mhz.

It is NOT the color killer transistor, Q6 as i have removed this completely during testing so the path to ground is open.

The only thing along this path connecting to ground is a variable cap (have changed it and also tried a fixed cap of 47pf) a stand alone 47pf cap (have changed it) and a 27uh inductor (have lifted its leg at ground - got worse)

Then the signal goes up to a 2.7k resistor where it joins the NTSC stuff to drive Q3

So the section between the 1K resistor and the 2.7K resistor swings wildly. rock steady up to the 1K.

What am i missing? is this how it is supposed to work due to backfeed from the NTSC through the 2.7K resistor?

 

THANK YOU for any help you give.

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Process of elimination...?

In the multi-page schematic in Apple's Addendum to the Apple II Reference Manual your problematic signal is identified as "C14,6" [correction: R5] where it crosses from one page to another.  The connections to that signal are:

  1. Output from pin 12 from 74LS11 at B12 through a 1K resistor.  You've verified the signal is OK here.
  2. Input to inductor L1, 27µH to ground.  You tested this.
  3. Input to capacitor C2, 47pF to ground.  You tested this.
  4. Input to variable capacitor C3, 5-50pf.  You tested this.
  5. Pulldown transistor Q6 to ground.  You tested this.
  6. Input to pin 5 (D2) at 74LS194 at A10.  Not mentioned in your description.  Did you test this?

The shift register at A10 has a connection to 14MHz, so it's possible that an internal fault is driving interference back through its pin 15.  I would probably try swapping the 74LS1974 at A10 with one of the video shift registers at B4 or B9, just to rule out that last connection.  If the parts are okay, then the swap won't affect anything.  But if A10 is faulty, then swapping it into the video shift registers will probably fix the color killer while distorting pixels that go through pin 5.

[edit]

Oopsie.  So many wires packed together -- I think I followed the wrong trace back there!

Clearly it should have been signal R5, not "C14.6".  The connection I listed as #6 doesn't really make sense, does it?

Sorry about that!

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It looks like you are 1 line

It looks like you are 1 line off (?) the color burst comes across 4 lines down on line "R5". and goes to the 2.7k resistor.

 

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New info..... I seem to be

New info.....

 

I seem to be getting green and purple in HGR.

No orange (it is white).

 

Again, GR all colors are fine.

 

Hope someone can help!

CVT
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Can you run the Apex II color

Can you run the Apex II color tests for GR and HGR and post pictures? You can download it from here.

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Will do later today. THANKS.

Will do later today. THANKS.

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A11 & A12
kirkj wrote:

I seem to be getting green and purple in HGR.

No orange (it is white).

 

Again, GR all colors are fine.

The Rev. 0 Apple board only produces black, white, violet, and green in high-res. Blue and orange are only possible on Rev. 1 or later boards, by using a 74LS74 at A11 to delay the color phase.

If your board is missing A11, congratulations: Rev. 0 machines are rare and fetch high premiums (some say fewer than 6,000 were made).

If not, (and I'm guessing not since you mentioned the color killer at Q6), the lack of orange and blue can be caused by A11 or A12. Do you have a scope? Measuring frequencies with a DMM on video signals is frustrating and uncertain.

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robespierre wrote:kirkj wrote
robespierre wrote:
kirkj wrote:

I seem to be getting green and purple in HGR.

No orange (it is white).

 

Again, GR all colors are fine.

The Rev. 0 Apple board only produces black, white, violet, and green in high-res. Blue and orange are only possible on Rev. 1 or later boards, by using a 74LS74 at A11 to delay the color phase.

If your bo

IT WORKS!!!!!   THANK YOU!

 

I swapped the 74LS74 at A11 with the same chip at J13 (cassette / sound). Sound still works so the bad half of the chip must be on the casette section.

THANK YOU!

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