"David Bourgeois" wrote in message
newsp.ulft1uy4vcva2r@localhost...
Hi,

My TDS540 doesn't start anymore. When I switch power on and press the
ON/STBY button, the fan starts and panel light up. Then within 30 secs the
display should start but it doesn't do it anymore.
I bought it second hand last year and it had the problem already, but I
let it turned on for a couple of hours and then it finally started
normally, all tests passed. After that, it could take from 10 secs to a
few minutes to start. I noticed that sometimes I couldn't turn it off with
the ON/STBY button, I had to power it off from the mains switch.

I didn't power it on for a few months and now it doesn't start at all even
after 24hours powered on.

I found the service manual and followed the diagnostic procedures (low
voltage power supply is OK), and eventually ended up at the line saying
"replace the DRAM processor/display board". The display should be OK, when
I turn it on I have a flash (line that quickly expands on the screen). The
digit LCD on the DRAM board displays hex from '3' to 'E', not '1' to 'E'
as stated in the service manual.

It seems there is something like an initial check, can it be temperature?
I've read that electrolytic capacitors on these DSO leaks and should be
replaced. I checked mines and they didn't leak. Can this still be the
cause? Should I get a cooling spray or heat some parts of the board to
check if that makes any difference?

Thanks for any help.

David Bourgeois

Those are the symptoms for bad electrolytics on virtually any reasonably
modern piece of electronics which employ a switch mode power supply - as I
think this item does. It's extremely common for switch mode power supplies
to refuse to restart after they've been left 'off-power' for a couple of
weeks whilst an owner is away on holiday, for instance. The only realistic
way to check electrolytics for this kind of failure, is to run over them all
with an ESR meter. This will quickly reveal any that are out of spec,
without having to remove them. Suspect any that are located close to
components which run hot, or show any signs of their tops bulging or
splitting. Changed one just today in my dear old mum's VCR. That too was
refusing to start up for several minutes from a full power off, and then
when it finally did, the display was locked and corrupt, due to the noise on
the rail from the failed filter cap.

Arfa