"Bruce" <bruce@...> writes:
I agree that providing an uncurrent limited -10v was a mistake.
Some suggestions:
1) A loose part (screw, nut, washer) that found itself a place
shorting the -10v seems likely
2) A cap failure - how long was the 8566 off - long enough for the cap
to depolarize.
I hadn't considered that first possibility. Thanks for the hint! I did
at some point measure a short across the -10V rail, but I wasn't able to
reproduce the measurement. Maybe that could be a screw sometimes
shorting the rail. Will keep an eye out.
Do failing caps ever intermittently present shorts? I've read that most
8566 failures are from electrolytics, so I'm on the lookout for
that. But I'm a bit surprised that this rail is sometimes up and
sometimes not.
Thoughts on how to proceed:
Isolate sections of the power distribution on the PCB -pull cards,
pull connectors, etc.
If something smoked, it will be either a component on a PCB or the
motherboard (There are a few). Or it could be a PCB trace (bad news
if it is in a relatively inaccessible place.
Hardest but most through is to remove all cards, test for appropriate
voltages and then plug in a selection of cards. I usually start with
the control section (CPU, etc), then RF, then M/N, then 20/30. Don't
forget the front pannel, but it is easy to isolate (don't remember if
-10v is used on FP - think it is not).
For anyone else that sees this, the A8 rectifier assembly provides a
nice list of the assemblies fed by the various power rails. For the -10V
rail, I believe all the downstream modules are:
- A19 DAC
- A20 main coil driver
- A21 FM coil driver
- A11A4 YTO phase detector
- C3
- A11A5 sampler
- C3
- A6 RF module
- A7A5 reference
- A16 scan generator
- A10A5 PLL2 VCO
- A10A1 PLL1 VCO
- A10A6 PLL2 phase detector
- A10A3 PLL1 IF
- A10A7 PLL2 divider
- A10A4 PLL3 up converter
- A10A8 PLL2 discriminator
and for the -5.2V rail:
- A17 positive regulator
- A11 YTO loop
- A7A5 reference
- A15 controller
- A16 scan generator
Matt
