Dying VaxStation 4000 VLC - help?

From: Mark Tapley <mtapley_at_swri.edu>
Date: Tue Nov 19 07:49:00 2002

All,
        That does it. I'm turning off digest mode. Apologies if I miss an
important message, but this is just too annoying! ... well, that was easy.
Hopefully I'll hear back from myself in just a few minutes...

Toth asked:

>...Have you checked the solder joints and connections between
>components with an ohm meter? That might help turn up any problems.

More or less. I tend to probe both on the component and on the pit next to
it for SMD's or on the pad for devices with pins. But I have not been
rigorous about it.

Tony said:

>You will _not_ see the oscillations of a 393 comparator on a VOM. We are
>talking about hundreds of kHz / a few MHz...

Oh, right. But I'm assuming the duty cycle of the oscillations can vary,
and I might be able to see that on the (analog) VOM.

>RIght. On the other hand, if it _is _ a capacitor, then it might have
>gone leaky, in whiuch case it will have an effect on the DC levels.

Oh my. Well, I don't know of any way to test the capacitor in circuit. Once
the comparator is out, I could measure resistance on the lower leg of the
divider. If it changes while I'm watching it, that'd be a clue, but with
power off the system I don't see what'd make it change. Or if I knew what
the resistance was supposed to be, and it was in the leaky state when I
measured it, that'd be a clue, but at least the first condition isn't met
and maybe not the second.

>Are there any component refeneces (Rnnn, Cmmm, etc) on the PCB?

None that I've seen, other than the "+" markings on one end of the tantalum
caps.

>It could be a resistor, sure. And perhaps, then, there's a capacitor
>elsewhere to hunt for...

This comment, waiting for me in my digest as I found the component in
question, was what convinced me to off digest mode.

Tony and Toth agreed:

>Get some silver-loaded solder.

Ok. I'm sticking with my plan, to wit replace the comparator and see what
happens. I'll measure resistance on both legs of the divider while it's
off. If that doesn't solve the problem, I'll head back to the electronics
store and try to get replacements for the tantalum cap (and one more for
the backward one, while I'm there) and for the resistors in the voltage
divider - and some silver-loaded solder.

How will I know what values to shop for for the capacitor (maybe not so
critical?) and the resistor? Is there a catalog of 1992 or so surface-mount
devices?

Toth futilely wished:

>Erm, I just hope the iron is of fair quality, and is in the range of
>20-25W. Anything larger is likely to damage the board, especially those
>awful soldering irons Radio Shack sells (or used to?).

It's an awful Radio Shack pencil-type iron. It's about 20 years old. I
think it's about 20 W, don't remember for sure. I only have one tip for it.
Maybe I should think about a better iron while I'm at the electronics shop,
if it comes to that.
                                                - Mark
Received on Tue Nov 19 2002 - 07:49:00 GMT