"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 :
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.
DC-GND-AC.
You just said that shorting the FET gate thru the 470K R removes your
offset;that tells me the offset is coming from the atten/BNC.
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.
Not relevant. You're dealing with mV offsets.
I've ruled out DC in the
attenuator by disconnecting the FET from it.
And grounding the gate,you still get an offset?
I think I just need to get a new fet because everything points to the
gate having leakage of some sort.
bg
The FET gate has an extremely high input Z(100's of Megs),the 1 MegR -sets-
the scope Z to the desired input Z.
--
Jim Yanik
jyanik
at
kua.net