15 Jun
2014
15 Jun
'14
1:55 p.m.
During preparation of the machine for VCFE i had a failure in the CRT's high voltage
part.
Yep, flyback burnt..
The high voltage winding was potted as a separate unit, enabled my to
rewind the flyback's primary (was burnt).
Are there only 2 windigns? What about any taps on them? A single shorted
turn anywhere will cause problems.
Then after replacing 2 transistors, 2 resistors and
1 diode I got a
brief few seconds of live out of the display again.
When it worked, was the display clear, correct size, etc. Was it stable?
Often EHT problems have otehr effects. Too high an EHT will casue the
iamges to be smaller and brighter than normal, for example.
The hisses accompanying this would indicate the maybe the flyback HV
isolation is not good anymore, however i did not see sparks flying...
Hissing often means corona/brush discharge, nnot sparking. Did you
reoslder anything on the EHT side (connections to the diode, capacitor,
etc). If so, an 'icicle' fo solder cna be annough to cause this, you want
well-roudned sodler blobs
It could also be internal breakdown in the winding (in fact it sounds
like it is).
Current working assumption is that the secondary
potted winding is
also compromised.
I will also replace the HV voltage diode, is the HV capacitor
something that fails regularly ?
Is there a spearate capacitor? Often it's just the coatingso n the CRT.
Yes, the rectifier diodes do fail, often. But if it was shorted you would
get no display at all.
What fialed after the few seconds? The output transistors?
Is the service manual/scheamtic on-=line anywhere? Does the flyback
transofrmer/output stage do anyhting other htan supply the HET (is i nteh
orizontal defleciton stage too)?
It is a 17 inch white monochrome Ball HD17-type monitor.
What are my options : repair ag ain, looking for a generic flyback ( is
there such a thing ?) , or dumping it into lake Zurich, which is my >
current
mood....
Don't do the last, I am not particularly 'green' but don't approove of
polution. Quaiate apart from the fact that the machine is worth
restoroing.
I am not averse to putting some modern solution in
place ( to generate
the HV that is ) >
If this stage also generaes the horizontal defleeciton, you might be able
ot get it working to do that (leave off the EHT winding) and use a
eparate supply (any old flyback transoformer suitable driven, for
example) toi supply the EHT. Of course if it's a separate stage anywahy
(like i nthe Moniterm units I've worked on), you might be able to come up
with somethign that will work.
I wonder about makign a new EHT winding. I've never done it, but soem fo
the model IC engine enthusiasts make ignition coils and vauum impregnate
them with potting compound. I suspect makign this windign is similar.
Might be soemting to look into.
-tony