Re: Repairing my Tektronix 454 oscilloscope need some advice


Albert Otten
 

Victor,

Obviously there is not enough secondary HV, or a diode is bad. If you remove the plastic cover of the HV box you can access at least one diode which is connected to a hot winding end (I don't remember which one, I think it has been mentioned in another recent 454 thread). With 1 V pp over the primary you could view the wave form at that secondary winding, using a 10 M (and 10X) probe. Then a bad (open) diode plays no role.
I estimated the primary impedance (about 10 R) by comparison of amplitude (1 V pp) with the open-circuit output amplitude of the 50 R generator. Did you also do such? Maybe tomorrow I will (just for fun with my 7854) view both voltage across and current into the primary. With the 7854 keyboard calculator it's easy to determine reactive and real input power to primary. I guess it will mostly be real power because of the (cold!) CRT filament load. (Of course the phase shift between voltage and current can be viewed on an arbitrary scope, without storage or calculator).
You might also view the collector waveform during normal operation. The 0.22 A current seems to be small and probably indicates that Q1430 can not deliver enough power to increase the oscillation to normal level; either because a fault in the feedback circuit (resistor string to Q1414 etc.) or a too heavy secondary load (shorted HV cap?) or ...(?)

Albert

On Wed, Feb 19, 2020 at 07:55 PM, Victor wrote:


Albert,
With the 454 energized today I could make some of your recommended test. I
start with the "less risky test" and the results was far from my expectations.
After follow your process I applied to Q1430 collector 1V pp and the voltage
at the HV test point was only - 3 to - 4 V. I change the frequency from 23 kHz
until 27 kHz and no results and no voltage increase to 77 V DC.
Otherwise: The current at F1437 measured is 0.22A. 
Also adjustment on pot R1401 give any variation on the HV test point.
The voltage on HV test point actually (in normal operation) is  - 64V. 
I could'nt make other test mentioned on your message on 02/17.
My test equipment is : scope TEK 7603, wave generator Sound Technology 3100A,
meter Fluke 87III.
Any thought from your side ? 
Thank you 
Victor     

-----Original Message-----
From: Albert Otten <aodiversen@...>
To: TekScopes <TekScopes@groups.io>
Sent: Tue, Feb 18, 2020 5:44 pm
Subject: Re: [TekScopes] Repairing my Tektronix 454 oscilloscope need some
advice

(continued)
I did an easier and less risky test. F1437 tied to GND, wire to "P"
disconnected. Sine wave generator FG504, same GND , 50 ohm output applied to
Q1430 collector (no other termination). Viewed waveform at Q1430 collector at
a scope. Kept amplitude pp up to only  1 V so  Q1430 would never conduct.
Measurements:
Resonance at about 23 kHz, not very sharp but clear enough.
Amplitude 1 V pp at collector resulted in 77 V DC at the HV test point as
measured with a 10M DMM.
Impedance was about 10 ohm.

Albert


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