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Post by kokovec on Jan 11, 2023 14:18:59 GMT -5
I was just curious to see if the current and voltage over temperature ratings were different for the new chips. At least, different from the datasheet. The question is why do the caps work fine with the original DACs? That tells me that the values might not be correct for the new chips. It could also be the new chip is not supplying enough current to bias the integrator amps. Lots of theories. I'm curious as to what the issue will turn out to be. It's usually the simplest explanation that wins out.
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Post by playvectrex on Jan 11, 2023 20:55:34 GMT -5
Hi Peer, Thanks for the updates! If you can characterize how the -13V line on pin 3 of the DAC behaves when everything is working, it might help you when you finally go to measure the non working DAC. Especially if you can get a time based reading with an oscilloscope and measure VCC (5V) at the same time. I'm suspecting some kind of latch-up issue where 5V comes up before -13V, and that keeps the DAC in a latched-up higher load state on -13V. As I said it could also be due to failing components in the -13V power supply... or it might be none of these things I think you are right though that it's likely not a temperature issue, based on how you described fixing things by simply booting the Vectrex without a DAC, and then immediately booting with the new DAC and it powered right up. But if the -13V investigation doesn't pan out, messing around with temperature of the DAC could be a good next step.
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Post by Peer on Jan 12, 2023 14:35:41 GMT -5
... If you can characterize how the -13V line on pin 3 of the DAC behaves when everything is working, it might help you when you finally go to measure the non working DAC. Especially if you can get a time based reading with an oscilloscope and measure VCC (5V) at the same time. Hi Brett, thanks for your help! I will try to get those measurements next time at the lab, as I do have not the necessary equipment here at home. That could actually be the case. Today I put a new DAC in a fully-depleted console (not switched on for two days). Before inserting the DAC, I checked by using a multi-meter, and there were only -78 millivolts on pin 3 of the empty dac socket. I guess that counts as 0 Volts. When I switched on the console, the new DAC did not work!
Then I removed the DAC again and did the switch-on-switch-off with the DAC socket being empty. Now, with the console being off and unplugged, there were still -12.98x Volts on pin 3 of the socket. I inserted the new DAC, switched the console on, and everything worked.
In then left the console switched off until the residue voltage on pin 3 had dropped to -12.010 Volts and then switched it on again. The DAC did not work.
So, from what you wrote, my layman's guess is that the new DAC seems to fire up correctly if there are already -13 Volts present on pin 3 when the 5 Volts come up. And if there aren't -13 Volts present == the 5 Volts come up before the -13 Volts, it does not work.
Assuming that this is the actual cause of the whole problem, is there possibly a remedy, or a workaround, to fix this and make the new DAC work?
Many Cheers, Peer
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Post by playvectrex on Jan 12, 2023 15:11:51 GMT -5
That could actually be the case. Today I put a new DAC in a fully-depleted console (not switched on for two days). Before inserting the DAC, I checked by using a multi-meter, and there were only -78 millivolts on pin 3 of the empty dac socket. I guess that counts as 0 Volts. When I switched on the console, the new DAC did not work! Then I removed the DAC again and did the switch-on-switch-off with the DAC socket being empty. Now, with the console being off and unplugged, there were still -12.98x Volts on pin 3 of the socket. I inserted the new DAC, switched the console on, and everything worked. In then left the console switched off until the residue voltage on pin 3 had dropped to -12.010 Volts and then switched it on again. The DAC did not work. That's good info, thanks. The one thing I think we are still missing is what does the -13V rail measure when you switch on the Vectrex with a new DAC (and it's not working)? Does it stay below -13V? Starting with -12V seems like it should have worked, that's odd. Still though, I'm curious what it measures when you actually turn on the Vectrex... is it stuck at -12V, does it drop further? You can probably speed up the discharge of -13V rail by just adding a 100 ohm resistor between -13V and GND. That should discharge it pretty quickly. As for how we might fix it, if it's consuming too so much current on the -13V (VEE pin 3 input) that it's folding back the -13V supply, it might just be your -13V supply caps need replacing... but it's also a voltage doubler circuit, so it won't really be able to drive a big load. But you can increase it's capability by doubling the last bulk capacitor on the -13V supply. It really depends on how much of a load the DAC is presenting to pin 3 though to know if this will help at all. If it's a race condition sort of a thing between 5V and -13V, you can also try slowing down 5V to the DAC VCC input. So if you added an RC filter in series with the VCC pin on the DAC, that could slow down 5V rising on VCC and allow -13V to rise(fall?) on VEE before it gets into the latch-up state. The RC filter could be something like a 100 ohm 1/2W resistor and 10uF to 100 uF capacitor. This is where looking at the rise times of VCC vs. VEE with a scope will be helpful, and then you can see what effect this filter has and tune it to help 5V rise before -13 on the DAC. 100 ohm 100uF 5V <----/\/\/\/\----(DAC VCC PIN)----|(----> GND To do this you might have to lift the DAC VCC pin... or make a custom socket for it that you can lift or re-route that VCC pin on.
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Post by Peer on Jan 12, 2023 15:32:31 GMT -5
…That's good info, thanks. The one thing I think we are still missing is what does the -13V rail measure when you switch on the Vectrex with a new DAC (and it's not working)? Does it stay below -13V? Starting with -12V seems like it should have worked, that's odd. Still though, I'm curious what it measures when you actually turn on the Vectrex... is it stuck at -12V, does drop further… Darn, I should have taken notes. If I remember correctly, pin 3 showed something like -1.5 Volts while the DAC was not working and the console switched on. And it was like this on both cases. When the console was cold, and also when I tried it with the -12 Volts residue. Does this help?
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Post by playvectrex on Jan 12, 2023 23:08:41 GMT -5
Yes, that helps. if the voltage drops like that it might be worth checking the 13V circuitry. I initially missed the 13V zener diode here which is probably fine along with R106, D106 and D107... but these are the caps I would look at replacing. C122 - 220uF 25V C121 - 47uF 25V C120 - 47uF 25V If that doesn't help, trying to slow down the 5V rise time to the DAC might be worth a shot... or lastly temperature stuff. Characterizing the rise times of 5V and -13V would be helpful. But you can go deeper into the investigation after these items if you want as well. It would involve trying to measure how much current is being seen on pin 3 of the DAC (-13V VEE pin) in all of the various situations and DAC types.
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Post by Peer on Jan 18, 2023 0:44:40 GMT -5
Yes, that helps. if the voltage drops like that it might be worth checking the 13V circuitry. I initially missed the 13V zener diode here which is probably fine along with R106, D106 and D107... but these are the caps I would look at replacing. C122 - 220uF 25V C121 - 47uF 25V C120 - 47uF 25V If that doesn't help, trying to slow down the 5V rise time to the DAC might be worth a shot... or lastly temperature stuff. Characterizing the rise times of 5V and -13V would be helpful. But you can go deeper into the investigation after these items if you want as well. It would involve trying to measure how much current is being seen on pin 3 of the DAC (-13V VEE pin) in all of the various situations and DAC types. Thanks a lot for this! Will keep that in mind. Just to keep you informed: Unfortunately I have not yet found the time to continue these investigations because the semester is coming to a close and the exams have started and will keep me busy for the next weeks. I will let you know as soon as I have any new insights. And I will likely ask you a lot more questions
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Post by VectorX on Jan 18, 2023 13:54:19 GMT -5
And I will likely ask you a lot more questions (paraphrased from Lost in Space) DANGER! DANGER! BRETT ROBINSON!
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Post by playvectrex on Jan 19, 2023 21:14:38 GMT -5
Questions are good, that is how we get to the answers!
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Post by D-Type on Jan 20, 2023 2:11:13 GMT -5
Is it possible to substitute in separate power lines from a standalone PSU?
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