6 Nov
2014
6 Nov
'14
6:06 p.m.
** Update - I think it's nailed **
I let the unit cool down sufficiently such that things were "working" again
- but this time, I was ready. I had the base & collector of the HOT
instrumented, as well as the first cap in the 50V supply. Recall that both
15V & 50V tertiary supplies are otherwise disconnected from the horiz.
section where they are produced.
With the 50V supply running fine at 44V, the signals at the HOT base
measured 212mV P-P, and the collector 15V P-P. This ignores any HF spikes,
I'm just concerned with the significant power region of the signals.
After a few minutes, the 50V supply crashed.. and guess what? The HOT base
signal actually increased a bit, to around 250mV P-P while the collector
fell by at least an order of magnitude.
So please correct me if my conclusions are unwarranted, but I believe the
situation is quite clear, now. The HOT has a thermal intermittent in the
E-C circuit - I just can't see any other explanation for this behavior. The
base signal didn't flicker, it actually +rose+ as the collector signal went
away. It's as if the E-C circuit is going high-impedance while the B-C
circuit remains intact.
Also - if something (like C116) were shorting the C to ground, we wouldn't
see a +rise+ in the base waveform.. we'd only see the base signal increase
if the load on the collector were to go away - eh?
IOW, It's just gotta be the HOT. From the get-go.
On Thu, Nov 6, 2014 at 6:39 PM, drlegendre . <drlegendre at gmail.com> wrote:
Forgot to mention - this afternoon, when the 50V
supply crashed while I
was monitoring it, I actually +heard+ it happen.. there was a very faint
little buzzy-sizzle sound "zzzzzztt" as the supply failed. Wish I knew what
made the noise.. but it could have just been audible harmonics coming from
the yoke or some other effective transducer, rather than noise produced by
the faulty component itself.
On Thu, Nov 6, 2014 at 6:37 PM, drlegendre . <drlegendre at gmail.com> wrote:
Hi Simon, All - for an update (below)
On Thu, Nov 6, 2014 at 1:16 PM, Simon Claessen <simski at dds.nl> wrote:
why do you keep looking at the hot? a transistor
fails or works.
transformers and caps can show the problems you describe. did I mention the
small 1kv caps around the whole stage?
You know the saying.. when all you have is a hammer, everything looks
like a nail.. ;-) IOW, it's largely due to my lack of experience.. I'm
sticking to the stuff I know, and when it comes to analog video, the
horizontal output section is about the only area I have any understanding.
Also, I do know a horizontal issue when I see one (vertical line) and when
you add-in the low tertiary supplies, the HOT is at the core of it.
As for the rest of the caps around the board, I've checked them all with
my eyes (for liquid leakage / blow-ups), an ohmmeter (for static shorts /
electrical leakage) +and+ a quality in-circuit ESR meter (Capacitor Wizard)
and can't find a bogie.
Not to get twitchy, now, but I heartily challenge your assertion that
transistors "either work or don't".. transistors have more than the two
(hard short / hard open) common failure modes - it's in the literature.
** General update commences here **
But the thing is, I've eliminated the tertiary supplies.. by lifting the
diode / wires as I did, they are disconnected from the horiz. section. And
in this state, the 50V supply still crashed on me, did so this afternoon -
and yes, I have replaced all caps connected to the 50V line. It is clearly
a thermal intermittent issue - things are OK when cold, and go haywire
after a few minutes time.
One reason that I'm still suspecting the HOT, is that the entire time I
had the NTE 375 replacement installed, the 50V supply never crashed. No, I
didn't run it for that long, but based on how fast things go bad (when it
warms up) I really should have seen something when the 375 was in place.
Oh, and to eliminate questions about shorts in the horiz. yoke, I pulled
the connection while the 50V supply was in the crashed state - and the
supply rose by perhaps a volt or two - so the yoke is clearly not the
source of a short-circuit.
I still suspect the HOT or components to the left (aka base drive). I
need to let the system get good & cool before I hook up the scope to watch
what happens to the base drive when the 50V supply crashes. if it's stable,
then it's either the HOT or possibly the flyback diode or the (what's it
called?) safety cap? The "special" 400V cap in parallel with the flyback
diode? I'm running out of stuff it could be.. of course, there's always the
horiz drive transformer, ugh.