From: Tony Duell <ard_at_p850ug1.demon.co.uk>
>> 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
Received on Fri Dec 29 2000 - 23:24:36 GMT