Re: Star Wars OVERVOLTAGE

Looking at the schematic you would have one of the following problems
(you don't have to do all of this, it's mostly different ways to find
the problem):

1) R17 not being properly grounded either due to a bad connection at
P102/J102 or the DAG not grounded. (The DAG is the aquadag coating on
the outside of the CRT, there is usually a spring or braid that lays
against that and goes to chassis ground, and in this case to R17.)

2) Bad R17 or R18. Simple check here, put a short across C10 and see
if the HV will run. If it does the problem is most likely the DAG
ground, R17, or R18, or an open C10. If it still trips the problem is
most likely Q5. Remove the short from C10.

3) With HV tripped you could also measure the voltages on Q5. In a
tripped state you should have 0 volts on both the base and emitter, if
not then Q5 is bad.

4) With CR1 disconnected and the supply running Q5 base should be under
8 volts, emitter would be .6 volts less than the base. CR4 should drop
6.8 volts, so if Q5 emitter is less than 6.8 volts there shouldn't be
anything left by the time you get to R19.

My bet is on Q5, an arcing flyback could have damaged it (especially if
it was the focus block doing the arcing).

Jonathan Stanley wrote:

>I've gone over the overvoltage section 3 times now and haven't found
>a problem. All my regulator voltages look good, every part I can
>test or swap has been tested or swapped.
>
>I am looking at the second revision of the overvoltage schematic
>which matches my board. This is simplistic, but after studying the
>schematic it appears to me that voltage is sent from Pin 4 of the
>focus block into transistor Q5 2N3904.
>
>Q5 looks like the key (sole?) determination of an overvoltage
>condition, am I right? If overvoltage, it 'switches' to the LED CR2,
>which continues on through Q4 MCR100-3 rectifier which then shuts
>down the HV. In-other-words, the very first path from Q5 is the LED
>itself.
>
>Since my LED is on, AND the voltage is getting cut, doesn't that mean
>that Q5 has indicated that there IS an overvoltage condition and so
>everything "downstream" from there such as Q4, etc. is working
>properly?
>
>So basically, if the LED is on, then either Q5 is bad, something
>related to the focus block Pin 4 line is bad (such as the R17 20K
>pot) causing it to "report" an incorrect voltage to Q5, or else there
>really IS an overvoltage condition.
>
>Is there a way I can measure the actual voltage at Pin 4 of the focus
>block and/or the voltage at Q5 to see what is actually happening?
>
>And if it really is overvoltage, what possible reasons could cause
>this to happen?
>
>Remember, this is a brand new Wintron HVT, that replaced my previous
>Wintron HVT that was working fine for over a month, then failed,
>accompanied by internal arcing noises. At the same time the previous
>Wintron failed, that is when the overvoltage LED came on. And now
>even with the new Wintron, the overvoltage LED won't go off.
>
>Thanks everybody, I hope we can solve this problem so this thread
>might help somebody else in the future!
>
>
>
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>
>
>

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Received on Sun Mar 28 03:48:58 2004