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Telequipment scopes


F F <ferfons@...>
 

Hiya Don and all the others,

I've been checking a few things here and there and indeed re the D43, there is no +B reaching the vert amps. One of the sections in between caps (i.e. power supply filter section) of a certain tubular multitap resistor is open. Now my problem is finding that value.

The problem in the other scope, DM53A, has nothing to do with the trigger section after all. As it happens, there is a very suspicious and scary buzz that is severely interfering with the triggering (hence my initial doubts). The noise comes from two bulky, in series components within the crt's high voltage supply (voltage doublers perhaps?).

Any ideas? Anyone?

Thanks in advance

Fernando Fonseca
Portsmouth, UK.
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Hi Fernando,
Check the power supplies in the
D43. It sounds like
something common to both the deflection circuits and
beam voltages.
Hiya Dan,

Firstly thanks for you email.

Yep that is my guess as well. I suspect something like eg a burnt resistor between filter caps - the sort of think that would block +B onto both vert amps. Furthermore the almost lack of focus could well share the same culprit. God I do need the diagrams otherwise I'm stuck.

It seems
the horizontal circuits are working (assuming you're
seeing straight
horizontal lines).
This is true.

It could also be a vertical
amplifier problem, not passing
a signal but either having hum signal or oscillation
causing the line to
thicken. Try different sweep speeds to see if it's
some signal causing the
line to thicken.
Both beams remain thick regardless the sweep speed. And I just noticed even the vert offsets don't budge (both channels)! I'm sure there's something dead upstream the vert amps. Got to roll up my sleeves and start doing some measurements but, altough internally the scope is remarkably well conceived and very easy to get into it it�s an absolute nightmare to follow the circuit without any documentation at all.

The DM53A must have a problem in the trigger
circuit. Have you tried an
external trigger to see if it's lack of internal
trigger signal? It must be
getting lost somewhere there.
I suppose if it hadn't the int trig working chances are I wouldn't see any beams at all. One thing I forgot to tell you is it sometimes triggers on the opposite polarity! The ext trig is a bloody good idea! I will do that and also clean off 30 odd years of noisy dust and dirt.

Cheers

Fernando Fonseca
Portsmouth, UK.
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John Lennon's dead. Phil Collins alive?!
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