Colour output missing (Apple II+)

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Colour output missing (Apple II+)

For some reason I'm not getting any colour output from my II+ clone. The monitor (composite input) is attached to the video output on the back of the computer. Other than that there are no other issues with the video (it displays fine, but only in black/white), and everything else appears to work on the computer.

 

So I looked up the Applebox diagram (which illustrates what each IC does) and I've swapped the following ICs, alas without any difference:

 

 

74LS02 (B14)

74LS02 (B13) color ref

74LS11 (B12) color burst

74LS51 (C13) color decoding

74LS32 (C14) video blanking

 

I intended to swap F14 (74LS259) as well, but didn't have another one to try out. I also haven't replaced transistor Q3 (J14) close to the video level trimmer, but I suppose since video in general works it's fine.

Where else could the problem lie?

 

Before getting my II+ in fully working condition I had a problem with the keyboard not responding (see this thread). It turned out that a circuit trace had corroded away, but of course was very easy to fix once I tracked it down (thanks to people in this forum!). Could the reason be likewise here?

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Fuji wrote:

For some reason I'm not getting any colour output from my II+ clone. The monitor (composite input) is attached to the video output on the back of the computer. Other than that there are no other issues with the video (it displays fine, but only in black/white), and everything else appears to work on the computer.

Possibly silly question: You are displaying this on an NTSC screen? If not you're not going to get the colour without an additional card. Apple II colour comes from some nasty NTSC signal hacking and is black and white on PAL region systems without a PAL card.

 

John

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Not a silly question at all.

Not a silly question at all. I wasn't aware of the monitor adhering to PAL or NTSC standards, thinking that was only an issue with TVs. So there's Composite PAL and Composite NTSC?

In any case the colour monitor (a Japanese NEC import, so it's definitely NTSC then) is the same monitor I've always used with this II+, and has always displayed colour from the II+.

 

That probably also explains why I neither got a color display on the monitor while attaching a (PAL) video camera composite output to it, nor when I attached the II+ rear output to our (PAL) LCD TV's composite input and yet again got a black/white picture! I wanted to see if the malfunction was within the monitor or II+, but assume from this that I would only get a black/white picture regardless because of the PAL/NTSC mismatch, so not much to gain from those tests.

 

While searching for a more specific answer I came across a very similar issue in this forum: no color for Apple II Plus. I gathered that the II+ clock has something to do with this and suddenly realized that my II+ (a clone actually) has a capacitor-trimmer mounted near the A1 area. It's not present in the original II+ schematics so I don't know what it's for and why, but could it have to do with fine-adjusting the main clock frequency? I do have a digital multimeter with a frequency counter mode: where do I point the probes to check the clock frequency while trying to adjust the trimmer to see if it makes any difference?

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First try this monitor with

First try this monitor with another NTSC composite  source to verify it is still capable of displaying NTSC color picture.  It may have a manual switch somewhere to select the color standard...It may have  color saturation control too...

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Monitor works in colour!

Yes, it (the monitor) works in colour!

I checked it with a multi-region DVD-player, inserted a U.S. (NTSC) DVD and luckily it had a video output jack in addition to HDMI. Glorious colour!

 

OK, so that eliminates one source.

On the II+ I believe I've replaced most of the relevant ICs except the 74LS259 (F14). Could this chip be the cause of such issues? It appears hard to find stores who sell it, but I'll keep looking.

 

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If I were you I wouldn't have

If I were you I wouldn't have wasted time with changing ICs especially led by this very doubtful German amateur source with the applebox diagram. At first glance this sketch lies about the data buffers near the CPU. The bidirectional 8T28 buffers cannot be replaced by 74367 unidirectional buffers. I would have tried to adjust the frequency of the 14.318 MHz crystal and its generator by adding small capacitance to it or by replacing the crystal itself...Nowadays this is still the most widespread and cheap quartz crystal used in every PC...

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Some moderator has very

Some moderator has very unwisely switched off the editing of the messages in this forum but I needed to add this...Check also the color killer transistor operation with a multimeter,  note that it should allow color only in graphic modes, so use at least GR from BASIC to enable your  computer to output color subcarrier. 

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I wasn't aware of the diagram

I wasn't aware of the diagram/site being doubtful, but being an amateur myself it seemed as a helpful start.

 

I would have tried to adjust the frequency of the 14.318 MHz crystal and its generator by adding small capacitance to it or by replacing the crystal itself...Nowadays this is still the most widespread and cheap quartz crystal used in every PC...

 

So apparently the trimmer capacitor in my II+ clone is there for what you're suggesting above. I measured (freq. setting from a digital multimeter) from each pin of the crystal while adjusting, but have only reached around 14.292 MHz. It's very difficult to adjust, so maybe I should use a smaller value of the trimmer (mine is 220 pF). Is it very critical? Regardless, there's no hint of colour.

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The colour killer transistor,

The colour killer transistor, is that the 2SC945 close to the "color trim" trimmer?

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Fuji wrote:

I wasn't aware of the diagram/site being doubtful, but being an amateur myself it seemed as a helpful start.

 

I would have tried to adjust the frequency of the 14.318 MHz crystal and its generator by adding small capacitance to it or by replacing the crystal itself...Nowadays this is still the most widespread and cheap quartz crystal used in every PC...

 

So apparently the trimmer capacitor in my II+ clone is there for what you're suggesting above. I measured (freq. setting from a digital multimeter) from each pin of the crystal while adjusting, but have only reached around 14.292 MHz. It's very difficult to adjust, so maybe I should use a smaller value of the trimmer (mine is 220 pF). Is it very critical? Regardless, there's no hint of colour.

 

Apparently I would have written to use the color trimmer capacitor but I didn't. You can't adjust the 14.318 frequency with that trim capacitor and since it is lower that required your only oprion is to replace the quartz crystal.  The transistor is Q6 (2N3904).

 

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I haven't seen your clone so

I haven't seen your clone so I can't tell for sure which transistor it is but yes, usually this transistor is placed near the trimmer capacitor. The above reference was for original Apple ][+.

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Casablanca wrote:

Apparently I would have written to use the color trimmer capacitor but I didn't. You can't adjust the 14.318 frequency with that trim capacitor and since it is lower that required your only oprion is to replace the quartz crystal.  The transistor is Q6 (2N3904).

 

I was referring to a capacitor trimmer right next to the 14.318 MHz crystal (which isn't present in the original II+) but in my II+ clone. It was broken from storage and I don't know its value, but the motherboard is marked 50 pF. I'm currently using a 200 pF trimmer as I didn't have anything else, but plan to order a 50 pF trimmer.

Do you think I need to replace the crystal as well?

 

I also have a "color trim" trimmer, which is marked 50 pF as well. It's worked in the past to adjust the color hue as far as I can recall. Yes, there's a transistor to the left of it -it's marked 2SC945 on my motherboard and that's what's used as well. I'm guessing it's an equivalent of the 2N3904 originally used, but the pinout is different (my clone motherboard was made in Japan I've been told, and those are perhaps more common in the Asian world than in the U.S.).

 

Unfortunately I don't have a replacement 2SC945 in my part box (and ordering online takes a while), but I do have several 2N 3904 so I'll double check if they're equivalent to replace and see what happens.

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You don't have to buy all

You don't have to buy all parts to verify their operation! Use some thinking.  If you measure the voltage between the collector of Q6 anf GND it should be low (appr. 0.3V) in text mode  and high (appr.  1V) if any graphics/mixed mode is active.  Just type GR and TEXT at BASIC prompt to switch the modes. By adding capacitance to the oscillator you can only lower the frequency.  So if with unconnected trimmer the frequency of your oscillator is lower than 14.318 MHz (AND you are sure in your frequency measurement!) than you have to replace the crystal. They tend to age and  their frequency drifts with decades espcially in those ancient generation of quartz crystals that was used in your A2 clone.

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Thanks for the clear

Thanks for the clear explanation.

The computer won't boot without the trimmer capacitor (the one next to the 14.318 MHz crystal), but I tried to measure its frequency regardless. The trimmer capacitor (removed) must be crucial as the frequency drifts as I measure. It's around 2.7 KHz. So for now I'll put the trimmer capacitor back (it's marked 50 pF on the motherboard), but I'm still not reaching 14.318 MHz. I've read something about problems measuring some things like this correctly without a buffer circuit, but I'm not sure what that means and if it applies here.

In any case a new crystal doesn't cost much and is probably worth doing with such an old computer.

 

OK, next is measuring the voltage between C (collector) of Q6 and GND:

TEXT: 0.0108V

GR: 0.0104V

 

So it looks like Q6 needs to be replaced. In my clone it's a 2SC945, but according to online sources it's an equivalent to the 2N3904 as you point out and I assume used in the original II+ (but a different pinout). Luckily I have a spare 2N3904 in my parts box so I'll give that a go.

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OK, I've changed the

OK, I've changed the transistor (Q6) but it made no difference. The voltage readings are the same and I've also adjusted the "color trim" trimmer. No sign of colour. And of course the monitor is still adjusted to show colour (as tested with the DVD player).

Should I start looking for broken PCB tracks? In addition to the obvious (areas around Q6 and the "color trim" trimmer) do you have suggestions as to where else the problem might originate from?

 

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Check with a scope the

Check with a scope the presence/absence of 3.58 MHz at the collector of Q6 while in GR/TEXT.

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Keep in mind these are only

Keep in mind these are only color bursts and are not constantly present there. They probably can be seen better at pin 12 of 'LS11 (B12).

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You acted again illogically -

You acted again illogically -- you replaced Q6 without first checking the voltage at its base first during GR/TEXT! 

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I'm probably going to make

I'm probably going to make more illogical mistakes as I'm no engineer :-)

I just measured the Base of Q6:

 

TEXT: 0.67V

GR:   0.14V

 

Measuring pin 12 of B12 (74LS11) in both GR and TEXT mode I get around 15.7 KHz for both.

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I've done some further

I've done some further research and came across the Apple II red book 30th year anniversary edition which among other things explains where to take frequency readings from.

On page 143 there's a diagram that (if my understanding is correct) shows 14.318 MHz can be checked on pin 8 of B2 (74S86) and by adjusting the capacitor trimmer near the crystal I was able to get exactly 14.318 MHz, so that's good news!

 

There's no 3.56 MHz reading on pin 12 of 74LS11 (B12), but looking back at that same diagram (page 143 of the red book) I can read 3.58 MHz at pin 3 of B1 (74LS175) as labelled "color burst", so at least the signal is generated.

 

 

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I meant 3.58 MHz (not 3.56 as

I meant 3.58 MHz (not 3.56 as in my typo above).

Too bad it's not possible to edit postings here.

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I'm baffled!

I'm baffled!

I can't figure this one out, and just about all the "quick fix" documents (including Apple's own repair document) mention the following ICs for a "no color, everything else works fine" problem: 

 

B12 (74LS11)

B13 (74LS02)

C13 (74LS51)

 

I believe I swapped them all, but I'm not ruling out that my replacement chips could be bad. Unless I figure this out quickly I think I'm going to order a bunch of different ICs that I don't already have (or just one of) and try some swapping again.

 

So what I've done next is read through a bunch of repair/service documents and in "Sam's computer facts -Apple II, II+" there are handy schematics with voltage readings in various places. I checked all of the ones I could find related to this issue (page 36/37 from the above book) and many of them were off. Here's that page with my readings in red (different) and green (more or less the same):

 

Another thing I don't think I've mentioned before: measuring the frequency of C of Q6 I get the following when entering these modes:

 

TEXT: 15.7 KHz

GR: 31.4 KHz

 

I'm not sure how to use the oscilloscope and make sense of the measurements.

Can anyone make any sense of all these findings?

 

 

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Fuji wrote:

I'm baffled!

I can't figure this one out, and just about all the "quick fix" documents (including Apple's own repair document) mention the following ICs for a "no color, everything else works fine" problem: 

 

B12 (74LS11)

B13 (74LS02)

C13 (74LS51)

 

I believe I swapped them all, but I'm not ruling out that my replacement chips could be bad. Unless I figure this out quickly I think I'm going to order a bunch of different ICs that I don't already have (or just one of) and try some swapping again.

 

So what I've done next is read through a bunch of repair/service documents and in "Sam's computer facts -Apple II, II+" there are handy schematics with voltage readings in various places. I checked all of the ones I could find related to this issue (page 36/37 from the above book) and many of them were off. Here's that page with my readings in red (different) and green (more or less the same):

 

Another thing I don't think I've mentioned before: measuring the frequency of C of Q6 I get the following when entering these modes:

 

TEXT: 15.7 KHz

GR: 31.4 KHz

 

I'm not sure how to use the oscilloscope and make sense of the measurements.

Can anyone make any sense of all these findings?

 

 

 

I've had two boards do this and in both cases I replaced the crystal with a new one and it was fine. I guess they float out of spec with age...  You can find my thread on here from many years ago about it.

 

 

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Yes, I read that thread

Yes, I read that thread earlier, but confirming that I have a 14.318 MHz signal I thought I was OK and the problems must be elsewhere.

Did you measure fluctuations before replacing the crystal? It appears stable here.

It's worrying that you had to try several different crystals before it worked. I've just ordered three of these (14.31818 MHz) from Futurlec and hope it'll solve the issue. I've also ordered replacement ICs for what has to do with the colour signal. I've also started closely studying the motherboard itself for broken traces/vias. You never know (it was the cause of my previous keyboard malfunctions).

 

Regarding the voltage readings in my previous post -what can the cause of these be? The crystal can't be doing this, can it?

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Apparently, the crystal

Apparently, the crystal tolerance is extremely tight. I had some 14.318 crystals that did not work. The ones that did were "NYMPH 14.31818" in my case.

 

I originally tried to swap between 2 Apple II's and then color worked in neither, so it seems the soldering iron heat took the other one out of spec also.

 

 

 

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I suppose I can't go wrong to

I suppose I can't go wrong to replace the (almost 40 year old) crystal in any case. 

My II+ clone has a trimmer capacitor alongside the crystal which I found I could adjust to get the exact frequency, but for all I know the crystal may still not "be right" even though my meter readings don't pick up on it. From the documentation I've seen on the original Apple II+ there's no trimmer, so that probably means a tighter tolerance on the crystal itself is needed.

Do you remember the frequency reading when you replaced it? How much off the exact 14.31818 MHz would allow for it to still work properly?

 

Does the colour burst signal depend on the computer's main clock frequency at the time it's booted, or could I get colour to appear if I slightly adjust that frequency trimmer while the computer's powered on, then try to run a graphic test program (i.e. color bars or a game), exit back to text mode, then repeat with a slightly different frequency setting?

Or do I need to reboot every time I change the frequency for it to work?

 

If I'm not mistaken, the 3.58 MHz "colour burst" signal (which enables colour output) is as the name implies; a short signal "burst", then disappears again. My understanding might be totally wrong though as I'm just a hobbyist with a soldering iron and some practical skills, but that's my gathering from these postings and from the various books and service manuals I've read.  

 

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