Ok, here's one that has myself and a friend really baffled.
We decided to try and hang TWO x FWB's and TWO x HV filter caps... both
from the SAME plate xfmr.
Supply #1 uses a FWB [ 10 x 6A10's per each of the 4 x legs] and a high C
filter.. [ 12 x lytics in series]
Supply #2 also uses a FWB [ but more 6A10's per leg] and also a high
C filter [ 20+ x ltyics]
Dahl plate xfmr has sec taps like 0-2300-2900-3900-4600-5000-5400.
Supply #1 hangs off the " 0 and 2900 taps"
Supply #2 hangs off the "0 and 5400 taps"
RVS connected 6A10's wired between B- and chassis.. on both supplies.
Standard Pri setup, with contactor #1 applying 240 vac. Contactor #2
shorts out the step start R, after XXX seconds.
Good so far. When testing the supply's ...ONE AT A TIME [ IE: fwb on
UN-used supply disconnected from sec] everything is fine.
## supply #1 = aprx 4180 vdc Supply #2 = aprx 7800 vdc
OK, now we tie down BOTH FWB's to the sec. B+ of power supply #1 is
temp disconnected from VHF RF deck.
B- of supply #1 IS connected to VHF RF deck. BOTH B+ AND B- of
supply #2 IS connected to HF RF deck.
Ok, both supplies work fine. B+ meter's everywhere read what they should.
Plus a Fluke 0-40 vdc probe available.
Now, here's the part when all hell breaks loose. With B+ connected to
VHF deck... [ at this point, B+ and B- from each supply, are connected to
their
respective VHF /HF rf decks] and B+ switched on... this is what happens.
Voltage as seen on VHF deck sails up PAST 5 kv ! This is during the 20 x
second step start delay
sequence. Before he could hit the off switch on the power supply, BOTH the
AC HV fuse for supply #1 AND DC HV fuse, also for supply #1... blow
open!
In both supplies, a HV AC fuse is located between sec and input to FWB [
one leg only] A HV B+ fuse is located just prior to the respective glitch
R.
Supply #2 isn't even scathed, nothing happens to supply #2 [ no fuses blown]
In addition, the big 240 vac breaker that feeds the input of the plate xfmr,
ALSO trips open.
To sum up, everything is fine, UNTIL the B+ gets plugged into VHF deck .
We traced this thing 6 x ways to Sunday, and can't find any back door path
anywhere.
Funny thing though. With the Fluke HV probe going... we noticed that supply
#1 showed 4200 vdc, when measuring directly between B- and B+ . However,
when probe was between
B+ and CHASSIS... then we measured 5 kvdc !!
So what are we doing wrong here ??? What am I missing ? The concept was to
obtain two simultaneous, but different B+ voltages, and not have to buy a 2nd
plate xfmr.
Only one RF deck was to be driven, at any time. Moot point, since the plate
xfmr can handle the load of both Simultaneously.
There is NO resistor's between B- and chassis.. on either supply. [Only
rvs connected 6A10 diodes] Rvs connected diodes are also located in both RF
decks, also between B- and each RF deck chassis.
The rvs connected diodes ensure the B- can never float more than +/- .8vdc
from chassis potential.
Individually, EACH RF deck works superb.. as long as ONLY the mating B+
supply is hooked up to the sec of plate xfmr.
Is the dual supply /one plate xfmr concept flawed ??? We really don't
want to have to install a 2nd plate xfmr for Supply #1. Plan B was to
install vac relays, and wired such that only one FWB is tied to the
plate xfmr sec... at one time.
The only thing common to both supplies is the chassis. Also, both RF deck
chassis's are well bonded together. Only the '0' tap of the plate xfmr is
common to each FWB.
baffled........ Jim VE7RF
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