Do you think maybe a bad output transistor? thanks for your help.Maybe, Mike. Looking at my 454 manual, I see that the collector
both Q394 and Q494 should be about 41-42 volts measured with avoltmeter.
Also, I don't think the voltmeter should "load" the circuit suchthat the
voltage changes very much because of attaching the voltmeter.There are TWO
sets of vertical deflection plate pins. Are you sure all FOUR pinsare
A good troubleshooting method for balanced circuits like vertical
is to short together like points such as the upper and lower CRTdeflection
plates in order to force them to be at the same voltage. Thisshould force
the trace to be centered vertcially and should not damage thecircuitry. If
the trace is centered with the CRT vertical deflection platesshorted
together (this is also the same as shorting the collectors of thevertical
output stage transistors together), it is a very good indicationthat the CRT
itself is good.and Q494.
Next, you can remove the short and move it to the emitters of Q394
If the trace is once again centered vertically, most likely Q394and Q494 are
good. Next, move the short to the base of Q374 and the base ofQ474. 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 thisway until
you do not get a centered trace. Now you are very close to theproblem and
it is time to check for shorted or open transistors in thisimmediate area.
Be careful not to brush ground with the loose end of the short wirewhile the
instrument is powered up and the other end is connected because youwill have
a LOT of destroyed transistors if you do that . . . Also, be sureBOTH ends
of the short wire are disconnected before connecting one end toanother part
of the circuit . . .w7ni@e................................................................
Let me know how this works for you.
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
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
You really need a 454 manual. The one I was using to remind me of the Q
numbers was the one I now have up for sale on eBay right now. You should
look at auction #1226363944 and think about bidding on it.
You probably have found at least one bad transistor, or maybe two. You can
test those for most defects with an ohmmeter right on the bench.
The next thing I would do is short the emitters of Q394 and Q494 and see if
this forces the trace to the center of the screen. If it does, then I would
say Q394 and Q494 are good and you need to progress with the shorting process
to the previous stage, etc.
You really need the manual . . .