[Test-Equipment] PM 3712 Scope - help needed and a magic cure found!

[Paul Galpin]

Can anyone help? I need the circuits and other diagrams for the PM 3712. The
service manual that  I have are has been scanned in at a DPI that is OK for
the text pages, but the circuits component details are so tiny that they are
just a few pixels!

The magic cure? Well, my scope suddenly stopped working. In the end it
turned out to be faulty insulation between the filament winding and the
other coils in the 18 kHz switching transformer. The filaments are tied to
the cathode at -1500V, and the overall leakage resistance between findings
was low on my 250V tester, so heaven only knows what it was like at 1.5 kV!
Philips stopped supporting this model many years ago, so getting a new one
is not likely..


The cure was to use a line filter double choke from a old computer switched
mode PSU. This forms a 1:1 transformer with good insulation between the
windings, so the power transformer feeds the 1:1 transformer, which feeds
the tube filament. It works, holds off the -1500 from the switching
transformer and doesn't even get warm!


Paul Galpin
ZS2PG