Hi Bob...
One OPEN cap in the HV cap string definitely WILL reduce no-load HV - from
~1.41 x rms AC voltage (the peak value which a capacitance filter will hold
to under no-load conditions) to roughly 0.9 x rms AC (the average value of
the rectified AC which is more or less what a typical voltmeter will read).
This reduction to 0.9/1.41 = 0.64 corresponds pretty closely to the
1600/2600 = 0.62 reduction that you note. After replacing the bad filter
cap
it would be an extremely good idea to check voltage balance among all the
caps in the stack. If it varies more than 5-10% from one to another, I'd
highly recommend replacing them all. MLA-2500 filters are all getting
pretty old and electrolytics don't live forever (although very good quality
ones generally last at least 20 years if applied conservatively in ham
applications)
73, Dick W0ID.
-----Original Message-----
From: Bob Dodson WB5APD EM84ak [SMTP:wb5apd@stc.net]
Sent: Wednesday, July 29, 1998 1:41 AM
To: AMPS
Subject: [AMPS] MLA-2500 Help
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.
2. Does anyone have an original twist lock cap for this amp 150mf at
450vdc they waqnt to sell.
3. If I can not repair filter bank as original any problem using (6)
470mf at 450 vdc instead of 150mf at 450vdc. (going from 25mf to 78mf).
I always thought more was better. Thinking abt using 470mf at 450vdc
Nichicon 1.375" X 2.0" with 2 hole cap clamps. Not sure if holes will
line up with original, but looks like maybe.
Any thoughts on all of this is appreciated. BTW the HV xfmr looks OK
best I can tell. Running it on 220vac and I ohmed it out. Amp tbl showed
up at pwr up no load.
Thanks in advance. Before I start spending money on new filter bank
would appreciate your input.
Bob
--
73's
Robert E. Dodson
mailto:wb5apd@stc.net
WB5APD/EM84AK
"ex-WA4WQC, ex-WV6SKV"
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