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Problem with Tektronix 475 "ADD" function
wrote in
ups.com:
This has got to be a DC offset in the inverter stage itself, that is
even though it night not be a seperate stage, whatever stage does it.
That's my opinion.It could be a invert balance adjustment,or a BNC that is
generating a DC offset(I've seen a few of these).
I doubt it's in the channel switch itself,that's just a fancy diode
gate,IIRC.
This offset is being compensated for by the front panel position
control, so after to get that straightened out you'll have to reset
it's range of course.
If the problem is indeed in a vertical amp IC, you probably can't get
it, but you may be able to swap it with the channel that doesn't
invert. You would actually be adjusting out a problem, but if it works
it works.
IIRC,the 475 vert preamp uses those TEK-made spider ICs,155-0078-xx,and
their gain and current supply resistors do go out of spec,they can cause
gain,offset,or compression problems.I'm working strictly from
memory,though,I have no access to schematics anymore.
There is a guy in this NG who can probably give you loads of more
specific info named Jim Yanik. Watch the thread, if you get a reply
from him it will probably be quite enlightening. You might also search
the group for his name and find something useful.
Thanks for the recognition! (He's emailed me already.)
Another piece of information that would be helpful is whether the 6
volt offset is at a certain scale, and reads 6 volts at different gain
settings, or is it simply at a certain position on the screen, i.e., if
you change the range does indicated offset or the trace position remain
constant ? This will indicate whether it's before or after the
attenuator. Also does the trace move when you use the variable control,
that is bringing it out of the calibrated stop ?
When working on DC offsets,I would not use any input signals.
If you can't solve this problem without parts and can't live with it
there are surplus places that might have one with serious screen burn
or a bad CRT or something. One such place is Electronic Surplus in
Cleveland Ohio. If they were open 24/7 guys like us could spend days
there looking at all their cool stuff. If you talk to the right person
there they will probably know if they have what you need. They also
used to have quite a selection of Tek plugins, but IIRC the 475 doesn't
use them. If it did it would simply be a matter of sliding in another
vertical amp. I don't have their number handy and at the moment I dunno
if they got a website. You may be able to get some NLA parts via mail
from them but it's nowhere near as much fun as being there. If you're
near Ohio it might be worth a visit if you have the time. If near
Florida you might want to check out a place called Skycraft in the
Orlando area.
You might run into me there! I love that place.
Their stock of TEK instruments has declined drastically lately,though.
I wonder if the local test instrument refurb shop bought them all up?
I would definitely call first, but if they have what you
need a visit would be interesting if it is feasible for you. In either
place I suggest you take your gold card, you will find some goodies
believe me.
JURB
--
Jim Yanik
jyanik
at
kua.net
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