12 Feb
2009
12 Feb
'09
7:18 p.m.
Philip Pemberton wrote:
Well, it's not the op-amps: I discovered last
night (of course, after
ordering the parts...) that the tape controller board has 3 MC1458's on
it, so I swapped a pair of them with the CPU board. No change, and the
tape drive still works fine, so I think we can rule those out.
I've ordered enough replacement caps to replace them in both the X/Y
channels so we'll see what happens once the parts arrive.
Thanks again,
Josh
Josh Dersch wrote:
Ah, whoops -- thought it was 5% like the others, but you're right.
Well, nothing blew up...
My multimeter doesn't go that high (I really
need to get a good one
one of these days.)
My Fluke 25 won't go that high either -- max range on that is 32
megohms. I've got a Solartron 7150plus bench DMM that can read down to
one milliohm in the 2K-ohm range (6.5 digits with averaging enabled),
but still limits at 20M-ohms max.
I replaced these with two 20M's in series
(figured it'd
be close enough to at least see if it changes anything, and I can't
find any place that actually stocks 39M resistors that aren't
SMD...) No change in display behavior.
The spec in the parts list was 1%. 1% of 39M is 390k-ohms either
way... 40M is well out of spec.
I'd have used a 20M, a 10M, an 2.2M and a 6.8M in series which gets
you 39M exactly, and if you use 1% resistors you should be within 1%
of the final value (assuming my math is accurate, which it probably
isn't). Note that all of those are E12 values, i.e. standard resistor
values. Found some MC1458P's on Mouser, ordered...
we'll see if this makes
any difference.
Well, based on what you've done, that leaves the capacitors and the
opamps (as you've already said).
There is a note on page 5-26 of volume 1 relating to the test fixture
and the vector display -- specifically:
Don't worry if both directions of vector drawing do not close for
the deflection magnitude may be well out of the linear range of
display deflection parameters.
I've just been (re-)reading Jed Margolin's "Secret Life of Vector
Generators" article (<http://jmargolin.com/vgens/vgens.htm>). There's
a bit in this article about capacitors in analog vector generators...
and how picky the integrators in AVGs tend to be.....
I'm starting to wonder if some normally-irrelevant parameter of those
poly caps has shifted and fragged the vector generator.
Catch is, because both channels are intended to be matched (there's a
bit about that in the theory of operation for the vector generator in
Vol 1, IIRC) you might have to replace the same parts in both channels
to get everything to match up... Thanks for the help!
No problem.