Re: Tektronix 465 No Trace, No Dot


Paul Amaranth
 

Get the service manual for your scope; there are free versions available online.
Google "tektronix 465 service manual".

The CRT filiment is at -2450V, so you don't want to measure that. On the other
hand, if you do have a HV probe, you do want to check that. If it says 0V,
that's your problem. Don't check it without an appropriate HV probe though.

Check the fuse for the HV inverter power supply. A reasonably common
fault is for Q1418 or C1419 to short and blow fuse F1419. You can find
those on diagram 10, CRT circuit. The transistor is heat sinked to the
rear cabinet and the cap is under the HV shield. It's also possible the
HV multiplier has failed; the output of that is 14-16KV. i

Crank the intensity up in a dark room. If the HV supplies are working and the
beam is off screen, you should see some greenish glow. That likely means
a fault in either the H or V amp and you won't have to mess with the HV
supplies.

On Mon, Apr 11, 2016 at 01:19:10PM -0700, w7qed@... [TekScopes] wrote:
Hi Folks,


My old and trusty 465 was turned on the other day and nothing is displaying on the screen. Not a dot, not a line -- completely blank.


The indicator lights seem to be working, and it seems to find a valid trigger on vertical inputs, so I believe some portion of the circuitry must be ok.


I took it apart last night and did some snooping:


1. All the non-lethal bias voltages seem very good that I tested -- the +55volt, -8 volt, +5 volt, +15 volt (I that's what I tested). No ripple (under 1mV), and dead-accurate.


2. The z-axis amplifier, which I initially suspected, seems very good. The intensity knob can adjust the test point (TP1486) from 15 volts to 41 volts, and 27 does seem about the middle.


Pressing "beam finder" does not find anything.


Here is what I think I should check, and I would appreciate your thoughts and comments:
Vertical and Horizontal deflection plate voltage: If this were too high or too low (i.e., "very negative"), the beam would deflect into the plate or off-screen, correct? What's a reasonable range, differential, between a set of plates? I seem to remember applying 20 volts in my undergraduate physics class to a CRT on a table. CRT Heater voltage: Is there an easy-access test point for this 6.3 volt AC point? Should I try and read the current too? Or maybe ohm it out? Barring the above, time to check the cathode and anode voltage, correct? Thanks very much. The last scope I took apart had tubes in it and was larger than a mini fridge, so I really appreciate any insight you folks might have.


Thanks again,


--Elliott












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Paul Amaranth, GCIH | Rochester MI, USA
Aurora Group, Inc. | Security, Systems & Software
paul@... | Unix & Windows

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