mtmpsmp@... 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@...
