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It is currently 27 Apr 2024, 08:53
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| bushytails |
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Posted: 06 May 2023, 04:15
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Well, I found another problem, but fixing it didn't change a thing. I decided to visually inspect the only board I hadn't looked at yet, the part of the FTSB that's under the mechanism. Found someone had put too long of a screw in the little access cover on the bottom, and shorted part of the "FTS SCAN" circuit to ground. It's the node with C21, R69, and R71 on the CLD-91 FTSB schematic. But, this is driven through a resistor and capacitor from the microprocessor, so it probably didn't burn out the output pin, and it looks like grounding it would cause tracking problems, not long-term damage, so I don't think it's likely to be the cause of the powering off issue... Or would it? How sensitive are these microprocessors to unusual loads on pins?
I'm about ready to put it back together and give up. I've been poking random things with my 'scope, and they look like random things doing random things, and without a lot better documentation that's about all I can tell... |
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| bushytails |
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Posted: 10 May 2023, 20:39
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Can't? :D
I'm not happy with my repair. I slipped with the iron and got a blob of solder on the outside of one of the tracking coils, probably shorted a couple windings. Really, really pisses me off that I did that. And, in the process of trying to solder on a patch wire, I ended up just making the new joint with solder blob, no new wire. But....
IMGP1418-small.JPG
And it even plays side B, too! I'm currently listening to Close Encounters of the 3rd Kind. I don't actually have a TV to connect it to to see how the video looks. (I mostly work on audio stuff; not a movie person)
And, just to make it clear for anyone searching, the coil repair fixed the powering-off-after-4-seconds issue. Here's a brief overview of how this is, from my understanding of it:
The FTS EMERG (focus and tracking servo emergency?) signal from the FTSB is two resistors connected to the focus drive and tracking drive signals, that then goes to the power supply board. The power supply board adds more resistors from the two switched +5V supplies and the -5V supply, to also allow it to detect power supply failures. If the servo outputs are zero, and all the power supply rails are good, this averages out to 0V. The power supply board then has a couple transistors to detect if this signal is significantly far from 0V, with a resistor and a capacitor to provide some filtering. If this signal goes away from 0V and stays there long enough, the power supply board pulls _EMERG to the control board low. And, approximately four seconds after this signal is pulled low, the control board shuts the unit down and tells the front panel to kill power to the unit.
If either the focus or tracking coils are open, the servo feedback loops try driving them to full power, due to lack of anything making it back through the respective return line and to the respective current sense resistor. These high voltages (15V rail minus one transistor drop) throw the FTS EMERG signal enough off zero to trip the threshold on the power supply board, set _EMERG, and cause the unit to power off. A shorted driver or other feedback loop problem could also cause full output, and if the coils were good, they'd be nice and crispy within seconds at full power - hence why cutting the power to the unit is the response to any continuous large signal from the servo drivers.
So, for anyone who finds this thread looking for why a unit shuts off, check the focus and tracking servos... would have saved me a lot of troubleshooting if I'd been able to find this information when I started! I was looking for power supply and other such issues.
Now to put some glue over the repair, and put the rest of the unit back together. Also should find a TV to test video output. I don't know how long this repair will last, but at least I've confirmed the FTSB and FTSD are good, and if it fails again, it just needs a replacement focus/tracking lens assembly. I'll be sure to advise the new owner of this unit's repair history, as this definitely isn't a repair I'd give any warranty on. |
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