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expect the unexpected
- To: tesla@xxxxxxxxxx
- Subject: expect the unexpected
- From: "Tesla list" <tesla@xxxxxxxxxx>
- Date: Mon, 02 May 2005 10:59:41 -0600
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- Delivered-to: tesla@pupman.com
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- Resent-date: Mon, 2 May 2005 10:59:56 -0600 (MDT)
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Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>
Hi all,
Having not found a service manual I made one last effort to fix my double
beam long persistence Phillips scope. The trace is permanently on.
I had to remove the tube and power supply to track the wring. After an hour
or two I had traced all the tube base connections. It looked standard just
a resistor chain for the cathode focus geometry.
But no grid DC biased only cap coupled blanking/bright which looked right
AC signal wise. I was using the scope to check.
I felt I needed to add a connection to the grid via say 5 meg to the most
negative end of the bias resistor chain even though the cable form looked
original.
I worked out were the connection should go and there was an unused pin on
that connector.
The guy I purchased it from said it had a heater cathode short and he had
added an isolation transformer for the heater supply.
Actually its does not. But it did have a hidden resistor between the
cathode and heater on the base tube.
The original power supply was floating anyway LOL
It had behaved erratically. The trace would disappear some times. I assumed
it was something to do with the HV I was playing with. Then it would start
working again.
Still hard to believe it got out of the factory with a floating grid.
I was only convinced when I did my fix and it started working!!!!!
While I still have it in bits I will ruggerdize it against surges by
reconnecting the original filament supply (better isolation from the mains)
add a line filter and ground the case view some transorbs it has no earth
connection at present. I will also add timebase and y output.
I has anyone got a service manual for a Philips PM3218 (35MegHz bandwidth).
I still need to fix the trig input for the delayed time base.
Incidentally its has two very thin long coils in each signal path so the
signal is delayed sufficiently to see the trigger event.
Robert (R. A.) Jones
A1 Accounting, Inc., Fl
407 649 6400