Apple ][e PAL video circuit. Help please

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Apple ][e PAL video circuit. Help please

I'm not sure if anybody would be able to help, but I am trying to find some voltage readings from an Apple ][e PAL version. Specifically I am after voltage readings from the TCA 650 (UA13) that forms part of the video circuit, and specifically I need voltage readings from:

  1. Pin 14 - Power in. According to the PAL schematics I have, plus what the TCA 650 datasheet shows, this should be 12v. I'm reading 5v.
  2. Pin 4 - When the colour killer switch is off, it is open and pin 4 reads 2.5v-2.7v on mine. The datasheet states for PAL to be selected it should be 7-12v, and there is a contradiction here with the schematics that show 5v to pin 4 with the switch open. I suspect that the 5v signal may be enough to make it run fine, but since mine is half that, I can't help but think there is some sort of power issue, maybe a short somewhere or the PSU is falling short.

What I'd like please if you can help, are voltage readings from both of these pins with the machine booted and the colour killer switch turned off. Please note this MUST be a PAL machine as the NTSC machine has a very different video circuit.I have a suspicion that the reason that my colour display is turning off intermittently is because the voltage at pin 4 is somewhere in between the 0v-1v required for SECAM (Mono) and the 7v-12v required for PAL (Colour).The Colour mode will either go off or flicker between that and Mono, almost like the signal is floating. I will check my own machine with the TCA 650 removed to see what the voltages are like in case it's marginal and pullng them down. I would add that when the voltage reaches the higher end of the scale, so 2.7v, it is more stable, but once it drops to the lower end, that's where the mode switching occurs.

 

Any help would be gratefully received.

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On mine:

On mine:

 

Pin 14: 10.46 V

Pin 4 color on (SW1 set to COLOUR): 2.87 V

Pin 4 color off (SW1 set to MONO): 0 V

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Thanks so much for that.I

Thanks so much for that.

I checked pin 14 on mine again given your results and it's 5v with or without the TCA650 installed, which seems low and not at all in line with the schematics that state pin 14 is supplied with 12v. Yours seems to be correct.

However I also just left the machine running for an hour or so and the 12v Rail was 7.2v at the power connector. I powered off and on and it's now reading 12.2v again. I'm going to leave it on again and see if I can replicate.

Your pin 4 levels are very similar to mine, though mine are a bit lower. 0v is fine for Mono, and I get around 2.5v-2.7v set to colour. Currently, with the supply to pin 14 now reading  over 10v, I'm getting 2.448v on pin 4.

I'm now wondering if I have a more general PSU problem.

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Yes, I think you have a problem with the +12V rail of the PSU. 

Yes, I think you have a problem with the +12V rail of the PSU. According to the Apple IIe PAL circuit schematics pin 14 should be 10 volts, not 12. Check your +12V rail, R31, CR5 and C95:

 

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Thanks, I just went back and

Thanks, I just went back and checked and realised that it should be 10v (My copy of the schematic is a bit blurry), but it doesn't explain why mine was 5v, so I will check the other components are ok too.

Also has me wondering why 10v over the 12v in the datasheet for the TCA650, and also why my 12v rail hit 7.2v!!

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Most likely they wanted to

Most likely they wanted to protect the 12V rail by limiting the current using R31, which ends up dropping it to 10V.

My guess is that you also need to recap your PSU. This drop in voltage after some time is typical for bad electrolytics after they have temporarily reformed. After running for half an hour they get leaky again.

Do you have electrolytic caps on the motherboard or tantalum?

Also what is the voltage without a load? Just unplug the PSU from the motherboard and measure it on its connector.

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You're probably right about

You're probably right about protecting the 12v rail, but still, 10v seems a bit low for an IC expecting 12v. 

Funnily enough I had just tested the voltages on the PSU and get 5.28v and 12.55v, on load they drop a smidge, but not a massive amount, same with -5v and -12v.

Motherboard has no electrolytics at all, either ceramics, what looks like polyesters and some others I can't quite ID, other than 'not electrolytic'.

I'm going to leave it running this afternoon for a few hours, see what the voltage look like, but I had already been thinking the PSU needs the caps re-doing.

I also tested R31, CR5 and C95 and all report the expected values.

Given I've had some other odd behaviour, like the RAM/80 column card vanishing, it all kind of points to PSU issues.  

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Ronin47 wrote:You're probably
Ronin47 wrote:

You're probably right about protecting the 12v rail, but still, 10v seems a bit low for an IC expecting 12v. 

...

 

The supply voltage can be between 10 and 13 V according to the TCA650 datasheet.

It's pretty rare to have electrolytics on the motherboard, but I've seen it once or twice. Mine are tantalum STC:

 

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Mine are identical to yours
Mine are identical to yours above the power socket, the 4 to the right on mine are mixed types.
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I did look again at the

I did look again at the datasheet I have and you're right, the input voltage runs from 10.2v to 13.2v, annoying I missed it, though mine was at 5v anyway, until it suddently jumped back to 10.17v.

I also find it odd that the input to pin 4 sits between the PAL and SECAM switching ranges, but given your results and also those from somebody else on another forum, it seems right.

Perhaps it was a design choice because it worked stably at 2.5V and they didn't want to feed it any more to prolong the life of the IC.

So I'm going to give the PSU a recap for starters since it is 40 years old based on the age of the newest IC in the machine, it certainly can't hurt to ditch the old caps. 

Once I have that done I'll see if it stabilises the machine as a whole as I have been getting some other weirdness too, specifically when the machine is warm, like my RAM/80 column card vanishing, despite cleaning the contacts and tweaking them slightly.

I'll report back when it's done

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So the PSU is now recapped

So the PSU is now recapped and running fine, and I've dropped in a new TCA 650, which seems to have improved the video quality, leading me to believe the original is marginal.

I get 10.16v at pin 14 and pin 4 is 2.5-2.6v, so on a par with yours and others who've checked for me.

Appreciate the assistance, we seem to be back to relative normal now :)

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