Old oscilloscope help: ideas sought

[Chuck Guzis]

On 7/3/2006 at 11:39 PM der Mouse wrote:

>Back in the late '70s and early '80s, I used a particular oscilloscope,
>a "TYPE 304 H" "CATHODE-RAY OSCILLOGRAPH" from "ALLEN B. DUMONT
>LABORATORIES, INC., PASSAIC, N.J., U.S.A." (all quotes come from the
>front panel markings).  It worked fine then.
>
>After sitting idle for years - probably at least a decade - I've got my
>hands back on this 'scope.  It doesn't quiet work, but appears to be
>close.
>
>It's an entirely vacuum-tube design; even the power-supply rectifiers
>are valves rather than semiconductors, and all the wiring is
>point-to-point, with terminal strips used as necessary and most
>components self-supported by their leads.  (I mention this to give some
>idea of its age.)
>
>When I turn it on, it appears to power up.  All the filaments light as
>far as I can see (the one I can't see is the CRT, and since I get some
>light on the screen under some circumstances, I infer its filament is
>working too).  But I can't find the beam.  I've set both X and Y
>selectors to "off", which (based on my past experience with this
>'scope) should give me a single stationary dot.  If I crank the
>intensity all the way up, I get vague shadowy patterns of light on the
>screen, but no matter how I play with the X and Y position controls, I
>can't get anything definite.  The Y position control does something;
>the X position control does not, as far as I can tell from watching the
>screen.
>
>What memory I retain (which may be wrong) indicates that this display
>syndrome is typical of a beam which is far off-screen in one direction
>or another, but doesn't give any hints for what to do if the position
>controls don't work.
>
>Any thoughts?  I googled, to no avail.  Since the wiring is
>point-to-point, I could trace out a schematic.  I will if I have to,
>but I'm hoping that the above symptoms is enough for someone to point
>me in a useful direction to look for the problem.
>
>Unfortunately I have only minimal test equipment available - a
>moderately-decent electromech voltmeter is about it.
>
>Since the X position knob does nothing, I speculate that that pot has
>gone bad and is, in terms of the circuit it controls, always hard over
>against one margin.  Does this sound plausible?  I may try to find the
>deflection electrodes and apply the voltmeter to them, if it has a
>suitably high range (I'd not try it unless it has a range of at least a
>kilovolt).  Yes, it'd load the driving circuit more than it's designed
>for, but glassfets are mostly pretty resistant to that sort of thing,
>and the loading alone may well bring the beam back on the screen if
>that's what's wrong.
>
>/~\ The ASCII				der Mouse
>\ / Ribbon Campaign
> X  Against HTML	       mouse at rodents.montreal.qc.ca
>/ \ Email!	     7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B