Bob Dodson WB5APD EM84ak wrote:
> Hi Gang;
>
> Here is the story. Turned on my MLA-2500 and plate V. was 1600
> volts.
> Found (3) shorted diodes in HV string. Replaced ALL diodes w/new ones.
>
> Turned it back on. Still 1600 Volts. Checked caps withe VOM and (1)
> looks open. New diodes held and still ck OK. Questions;
>
> 1. Will one open cap in HV string drop HV from normal 2600 to 1600.
With the caps okay the off-load voltage will be equal to the peak
transformer volltage (less a few volts for diode drops). With no
capacitance, the voltage will be a series of half-rectified sine waves.
A moving coil meter would respond to the *average* value, which is 0.637
times the peak A true RMS meter would indicate 0.707 times the peak. If
the cap peak voltage is 2600 V, the average without the cap would be
0.637*2600=1656 - not too far from what you measure. Hence it does just
seem possible this is the problem. If you have a scope, and can make a
suitable divider, you should be able to see the waveform. Some DVMs can
measure capacitance, so that might help too.
Adding more capacitance can be a bad thing. The extra stored energy can
destroy tubes in an arc. However, I dont want to get into a discussion
on arcs !!!!!
G8WRB
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