OT: help... - test-drb@ccmp.vtda.org

29 Dec 2000

From: Tony Duell &lt;ard(a)p850ug1.demon.co.uk&gt;

...
   This one uses
only direct mains (160V).  That bothers me less as it
 can't reach out abnd get me. 
That's fairly unusual (most manufacturers these days make PSUs that can
be adapted for use on both 115V and 230V). 

Doen't even have an IEC cord set, strictly domestic.  mains come in
through
EMI line filters and gow through a bridge and cap hence the ~160V.

...
 I'd still be careful of 160V DC at considerable
current. It could be
fatal if you connect yourself across it. Personnally, I treat any voltage
...
 over 50V with great respect. 

Don't have to tell me.  I've been kicked mor than once by Oscope supplies
and once by RCA VHF low (35mhz) 500W.

...
 But insulation breakdown is relatively rare. There are
a lot of
unpleasant voltages on the PCB tracks of most monitors just waiting for
you to touch them though. Which is why I maintain that in general the EHT
...
 is the least of your worries. 

Some of them really spikey and feel bad too.

...
 I would regard most monitors as being a lot safer to
work on than valve
transmitters... 

No, most monitors these days can stand up without the cover and
are mains powered.  At least Valve Tx are transformer isolated and
commercial gear well caged abd often interlocked.

...
 Right. A total breakdown of the flyback can cause this
as well There's
one winding connected from the main output of the PSU (around 100-150V)
and the collector of the HOT. There's normally at least one winding with
a tap connected to chassis. A short between them cna shut down the PSU.

If it's not a short there, then a shorted vertical output IC will trip
the PSU in some monitors. 

I suspect the driving transistor to the HOT.

...
 Some monitors, particurlarly multiple-frequency types
have _separate_
output transistors for the horizontal scan and the EHT generation, the
latter driving the flyback. Sometimes there are other transistors (or
MOSFETs) switching capacitors in the various output stages when you
change scan rates so as to retune the flyback. Shorts in any of the
semiconductors will shut down the PSU. 

sounds like this one.

...
   the test was
to bridge the likely candidates.. no joy. 
I'd suspect _all_ aluminium electrolytics. I once had an Xterminal PSU 

It's not that old, maybe 6 years max.

I'll poke some more but, at 130$ for a new{and better} one I'm not sure
it's worth
a lot of effort.

Allison