Jim Yanik wrote in message ...
"bg" wrote in :
Jim Yanik wrote in message ...
"bg" wrote in :
Jim Yanik wrote in message ...
"bg" wrote in :
Jim Yanik wrote in message ...
"bg" wrote in :
Both channels on the verticle front end are damaged. One dual
FET follower is a definite (Q1297). It is tek part number
151-1090-00. I haven't seen one yet using google. Is there a
replacement or two discretes out there that will work?
They would have to be matched,else you get an offset.
check this out;
http://www.sphere.bc.ca/test/tek-parts/tekparts2.html
it appears they have some 151-1090's for sale.
they also cross ref to a "2N5911 family".
the 155-0078-xx is used in a lot of different scopes,several 7000
series plug-ins,too.
--
Jim Yanik
jyanik
at
kua.net
Thanks Jim. I had already been to that site and didn't see it the
first time, even with my glasses on!
Offset is the issue I am having.
bg
I've seen the input BNC connector cause DC offsets!
seems it absorbs contaminants and becomes a weak "battery".
--
Jim Yanik
jyanik
at
kua.net
I replaced the channel 1 connector, but plugged in one of the power
connectors backwards. So I blew one of the dual fets and the other
one seems to have a leakage problem. There is DC on the gate lead
right after the attenuator. The 1 meg resistor in the attenuator
output isn't low enough to pull the gate to 0 volts. Out of the
circuit I can't measure anything abnormal with the FET but in
circuit the DC is there. BTW , you are a great weath of info for
these tek products. bg
are there input protection diodes on that FET input? did you
disconnect the atten PCB and still have the DC?
(and then GND the gate and the offset goes away?)
--
Jim Yanik
jyanik
at
kua.net
I've disconnected the gate completely from all of the external
circuitry and then connected it to ground thru a 1.5 meg resistor ,
No,the atten grounds it almost directly.Maybe thru a ~47 ohm R.
I forget the exact circuit values.
Check your schematic.
1 megR is the basic input Z,it's no "ground".
You HAVE to see signals across that input Z.
which I assume is what it sees from the attenuator. Seeing as the gate
draws no current , there should be no voltage right, or perhaps just
microvolts. I get way too much. If I short the gate to ground or use
low resistance like about 50 ohms, I can drive it with a generator and
all is fine. But with High Z to ground, the offset is too high for the
verticle position control to bring the trace back on screen and none
of the balance or position pots have enough range. bg
Have you done your DC bal adjustments?
--
Jim Yanik
jyanik
at
kua.net
The output of the attenuator has a 1Meg resisitor shunted across it. The
gate connects to this resistor with a 470K. I think the short only exists
when the front pannel AC/OFF/DC switch is in the off position.
It's supposed to short the output of the attenuator. If I put that switch in
the off, center position, the trace does come back to center screen. Balance
adjustment works, gain adjustment works. The attenuator measures 1 meg in on
all ranges. I've ruled out DC in the attenuator by disconnecting the FET
from it.
I think I just need to get a new fet because everything points to the gate
having leakage of some sort.
bg