Re: Repairing my Tektronix 454 oscilloscope need some advice


Albert Otten
 

Victor,

Today I did more waveform measurements. I always viewed the primary voltage and primary current ( A6302 current probe) waveform and each time I tuned for resonance. Last time I made a frequency reading mistake (when you turn the FG504 knob cw the the frequency deceases...). Without other probes attached it was 27 MHz. Probes at the secondary side lower the resonance frequency somewhat.
From the visible transformer right side 3 very short blank wires go to ceramic supports. From front to rear these are connected to winding terminals 7 (tripler circuit), 9 (cathode, D1452) and 10 (grid, D1440 visible). Resistances to GND: 547R, 243R and about 20k respectively.
Findings:
Always nice resonance with simultaneously primary voltage maximal, current minimal and no phase shift between these two. In hindsight this could be expected when the primary side acts as a nice parallel RCL circuit with not too low Q factor. R was about 10 Ohm. Also each time the secondary voltage was in phase with the primary.
Amplifications, crude values:
from primary to 7: 375, to 9: 185, to 10: 195.
I did a more precise simultaneous measurement of negative peak at 9 and the DC voltage at HV TP (with DMM). The "loss" was about 10 V (of value about 80 V), looks realistically.
Anyway, it could be very informative to view those secondary waveforms.

Albert

On Wed, Feb 19, 2020 at 09:36 PM, Albert Otten wrote:


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

Join TekScopes@groups.io to automatically receive all group messages.