On Tue, 16 Feb 2016 19:13:09 -0600, "Mark Zacharias"
wrote:
"Cursitor Doom" wrote in message
...
Hi again,
I know I said I was going to mothball this scope for the time being, but
I keep thinking of new things to try and just can't leave the damn thing
alone. It struck me I ought to next check out the main transformer
because if that's toast, the whole psu might as well be binned. I think I
*may* be on to something.
Here's the circuit again:
https://www.flickr.com/photos/128859...in/dateposted-
public/
The transformer in question is T1801. I've pumped in a test signal into
its primary winding (2V p-p sine wave @20kHz Zs=600 ohms) and scoped all
15 remaining pins. Every single one of them is producing clean sine waves
at various amplitudes just as you would expect, except one, which is
giving a really thick, noisy trace.
As this terminal has no ground reference, with the loads removed, you
can only measure its winding voltage by measuring between other pins
that are connected to the winding, or by introducing the ground
connection on one of them.
The amplitudes measured will not be various - they will be scaled as
you expect them to be, as reflected by schematic output voltages or
labels. Measuring with a high impedance scope, it is possible to see
a waveform that is simply capacitive transfer, if you're not making
rational measurement observations.
On the diagram, this is the 5th one
down on the right hand side (immediately below the 1kV out pin.) It's not
marked on this diagram, but is supposed to be 0V from another schematic I
have. Someone has replaced C1826 & C1827 at some time and the
replacements don't look good for more than 50V max to me, and that's not
enough since the pd across them will be much more (the parts list
specifies a 500V rating!) They're not showing short-circuit, so I'm
wondering if they've failed open maybe that could account for the
symptoms I've been getting: terrible noise in the primary circuit and the
rapid heating of R1814 (the 20 ohm power resistor intended to dissipate
the switching energy).
Any thoughts?
C1827 is 'Select On Test' (SOT) to trim some operating parameter in
the functioning unit - possibly the HT tolerance under load or even
just display noise. You'd need a manual to figure this out. The more
often you mention the actual model number (PM3264 ?), the more likely
it is that you'll get someone to cough one up. It's $20 from jetecnet.
Ceramic disc caps of this size were typically 500VDC rated at that
time. Only smaller square plate types were less (and some offshore
stuff). Lower voltages will generally be marked as such, or colour
coded for identification in the mfrs spec sheet.
You still haven't identified the diode substitutions made in the
primary, or reported on secondary winding amplitudes loaded/unloaded.
You still need to report IC pin3 voltage when loaded. (it is reported
as normal, when unloaded, indicating a regulated condition exists.)
RL