Sorry, the last post was a test. Friggin Google will sit there and let
me type for an hour and THEN ask for login, which loses it all.

Anyway, I'd like to know the mechanism by which the CRT blows the
chroma, or more aptly the -Y outputs of IC200. I can't see how this
cannot be protected against, and I'm still having trouble even
believing this is the cause.

I have had wierd stuff in the past, V height for greyscale, open
speaker causing no HV, I am not a babe in the woods. What I'm having a
hard time understanding is how any phenomenon connected with the CRT
could do this again and again without blowing any other components in
the path.

I have solved a similar problem in a CTC169, I just found a transistor
that was fairly linear in gain, but had a 400 volt Vce to replace that
ECG123 (or was it a 159 ?) equvalent. It never blew the chip, but maybe
RCA's chip of the day had better protection. What's more IIRC that
transistor is the last stop in the Y driver chain.

Is possibly the collector to base capacitance saving these transistors
by delivering the jolt to the chip ?

Seems to me a way could be figured out to protect it from that, that is
if I knew the failure mode. I have never witnessed it and as yall know,
even if I had I might still know nothing. I might know something but I
might not. I would have seen it, but I have not.

Ideas ?

JURB

TIA

" bravely wrote to "All" (03 Aug 05 22:31:11)
--- on the heady topic of "Dammit !, anywat, NAP 31X3 chassis"


A Ne2 bulb or a Zener diode at the CRT pin to ground, your pick?
Could be the glitch is indirectly injected through a large junction
into the IC's supply lines. In which case paralleling a 0.1uF bypass
disc cap across the IC's supply might catch it better than the electro
sitting there. Sneaky little problem there...

A*s*i*m*o*v


ZZ From:
ZZ Xref: aeinews sci.electronics.repair:54920

ZZ Sorry, the last post was a test. Friggin Google will sit there and let
ZZ me type for an hour and THEN ask for login, which loses it all.

ZZ Anyway, I'd like to know the mechanism by which the CRT blows the
ZZ chroma, or more aptly the -Y outputs of IC200. I can't see how this
ZZ cannot be protected against, and I'm still having trouble even
ZZ believing this is the cause.

ZZ I have had wierd stuff in the past, V height for greyscale, open
ZZ speaker causing no HV, I am not a babe in the woods. What I'm having a
ZZ hard time understanding is how any phenomenon connected with the CRT
ZZ could do this again and again without blowing any other components in
ZZ the path.

ZZ I have solved a similar problem in a CTC169, I just found a transistor
ZZ that was fairly linear in gain, but had a 400 volt Vce to replace that
ZZ ECG123 (or was it a 159 ?) equvalent. It never blew the chip, but
ZZ maybe RCA's chip of the day had better protection. What's more IIRC
ZZ that transistor is the last stop in the Y driver chain.

ZZ Is possibly the collector to base capacitance saving these transistors
ZZ by delivering the jolt to the chip ?

ZZ Seems to me a way could be figured out to protect it from that, that
ZZ is if I knew the failure mode. I have never witnessed it and as yall
ZZ know, even if I had I might still know nothing. I might know something
ZZ but I might not. I would have seen it, but I have not.

ZZ Ideas ?

ZZ JURB

ZZ TIA


.... Which sparks some mnemonic circuitry.

A Ne2 bulb or a Zener diode at the CRT pin to ground, your pick?
Could be the glitch is indirectly injected through a large junction
into the IC's supply lines. In which case paralleling a 0.1uF bypass
disc cap across the IC's supply might catch it better than the electro
sitting there. Sneaky little problem there...

I think the bulb may be a better solution. I think a Zener would tend
to short. There is one way to implement it though, instead of a Zener
at each cathode, one Zener pulled up to near the 220V source and
bypassed, with very high speed diodes with anodes connected to the CRT
cathodes. RCA 243636s should do. The problem is to bypass it with a
quick time constant otherwise it could cause problems. If the set got
shut off and turned back on while the CRT was still warmed up you might
have unwanted beam current. This might happen before the vertical sweep
starts, for example, which would not be a good thing. The only solution
is lower capacity bypass, which protects the Zener to a lesser degree,
or more pullup current which would require a higher wattage Zener.
Seperate Zeners at the cathodes would probably just wind up shorting
out, producing a recall nonetheless.

This means added complexity, handlable, but sort of violates the KISS
principle. There might be good reason though, the neon bulbs might not
provide adequate protection. I think the neons are the first step, we
can afford more ICs, but since I did not change them the PCB has
suffered a bit of collateral damage. Not a big problem, but reading
about this online it seems to not be uncommon. If a viable solution is
worked out we could probably save quite a few 31" sets from the
landfill. These CRTs still produce a good picture, they just seem to
have this quirk.

Thanks for the idea, I'll let you know if it works out. I might even do
it with the Zener and the HS diodes in the first place, mainly because
I have a bit more confidence in that method being effective. I
subscribe to the KISS theory but I have a Law that stands above all
others "I have no desire to do it right, but I damn sure don't want to
have to do it again !".

I'll run it by the boss, it's his money and while he's not a super tech
anymore because of other things, he knows enough to make an informed
decision. I'm not sure we can find any neon bulbs anyway, I know you
can buy them but I want it off my bench.

Thanks again buddy, if anything works I'll be sure to post it.

JURB