--- In TekScopes@y..., Stan or Patricia Griffiths <w7ni@e...> wrote:
mtmpsmp@y... wrote:
--- In TekScopes@y..., Stan or Patricia Griffiths <w7ni@e...>
wrote:
mtmpsmp@y... wrote:
I have a 454 that has no vertical deflection on ch1 or ch2,
using
another scope I am able to trace signal from the preamps to
the
deflection plates. The sweep appears to be in good shape and
the
trace is centered on crt. ( position controls have no effect
on
trace) What I don't understand is that I have a voltage
imbalance
on
vertical plates and the trace is still centered!
Boy! I don't understand that either. I have to wonder if the
voltage
unbalance is actually getting to the deflection plates inside
the
CRT.
Perhaps there is a broken connection INSIDE the CRT. If the
unbalance is
actually on the CRT plates, I don't see how the trace can be
centered
vertically . . .
Stan
w7ni@e...
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..
Stan,
Upon further investigation I have found that when the scope is
turned on the collector voltage is the same (74.6 volts) on each
output device. The voltage will stay in that condition ( flat
centered trace) until I start making voltage measurements with my
voltmeter ( high impedance digital) then the top connecter drops
to
56.8 -62.0 volts and the bottom to 32.5 -29.8 volts, the voltage
of
course being determined by position control. The trace is at this
point off screen in all modes ( chop, add ect) I can seem only to
bring this condition on by taking voltage measurement on either
vert
plate.
Do you think maybe a bad output transistor? thanks for your help.
Mike
Maybe, Mike. Looking at my 454 manual, I see that the collector
voltage on
both Q394 and Q494 should be about 41-42 volts measured with a
voltmeter.
Also, I don't think the voltmeter should "load" the circuit such
that the
voltage changes very much because of attaching the voltmeter.
There are TWO
sets of vertical deflection plate pins. Are you sure all FOUR pins
are
connected?
A good troubleshooting method for balanced circuits like vertical
amplifiers
is to short together like points such as the upper and lower CRT
deflection
plates in order to force them to be at the same voltage. This
should force
the trace to be centered vertcially and should not damage the
circuitry. If
the trace is centered with the CRT vertical deflection plates
shorted
together (this is also the same as shorting the collectors of the
vertical
output stage transistors together), it is a very good indication
that the CRT
itself is good.
Next, you can remove the short and move it to the emitters of Q394
and Q494.
If the trace is once again centered vertically, most likely Q394
and Q494 are
good. Next, move the short to the base of Q374 and the base of
Q474. If the
trace is once again centered, most likely Q374 and Q474 are good.
You can
work your way a stage at a time through the vertical amplifier this
way until
you do not get a centered trace. Now you are very close to the
problem and
it is time to check for shorted or open transistors in this
immediate area.
Be careful not to brush ground with the loose end of the short wire
while the
instrument is powered up and the other end is connected because you
will have
a LOT of destroyed transistors if you do that . . . Also, be sure
BOTH ends
of the short wire are disconnected before connecting one end to
another part
of the circuit . . .
Let me know how this works for you.
Stan
w7ni@e................................................................
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Stan,
I got no farther than Q374 and Q474,
I replaced both transistors and the output transistors collector
voltage dropped to 40.5 and 35.2 volts. The trace is now in the upper
half of screen and I have a small deflection of a square wave signal
so gain is poor. Should Q374and Q474 be a matched duo?
This thing about having to touch the collectors to start the
transistors to conduct is really odd, you don't have to leave the
probe on just make a momentary contact with deflection pin or
collector.
I don't have a manual for this thing so I'm working mostly in the
dark. I would like to thank you very much for all the help you have
given me. mike
