hi all.
could someone give me some ideas on the problem i have encountered on one of
my heathkit SB220 amps please .
basically i swapped tubes last week from a pair of eimac to a pair of amperex
3-500z due to the soldered filament pin sleeve on one of the old eimacs coming
loose and causing intermittent filament operation in one tube .
all was ok and amp ran fine for the whole week, mostly in standby mode without
any rf drive into it and a couple of short periods of transmit ..
today i had the sb220 switched on and had just tested it in transmit mode and
gone back to standby mode when i heard pulse type arcing noise coming from amp .
there is no high voltage showing on the meter in standby mode but it came up
when the PTT was engaged .
firstly i looked inside the amp and noted arcing from the zener diode from its
body to its mounting position in the shield of the rf deck , the insulating
washers werent that insulating by looks of it ..
subsequently i found a previous installation for the zener diode consisting of
2 roughly cut square pieces of very thin mica film and an almost worn out
insulating bush on the fixing stud of diode , i made 2 new ptfe insulating
washers and a new bush .
after fitting these there is no contact to ground from the zener diode as i
believe the zener is to be insulated completely from chassis .
after refitting of newly bushed zener the amp still continues to arc if in the
2.9kv setting .
grid metering is reading negative whilst amp is on but not in transmit .
i noticed that when the arcing is occurring it is doing so across the ptt relay
contacts , but obviously part of the circuit including zener diode is linked
to the ptt relay and carries a wire connection to the ptt relay back to the
zener connection on metering/rectifier board .
i can switch amp on in the lower or higher voltage setting and the filaments
are lit but there is no high voltage on the anodes as would normally be and it
will arc if i switch up to 2.9kv .
probing around the rectifier/metering board i am seeing high voltage on various
components and the plate voltage meter will rise whilst my probe is resting on
some of the resistors on the board although i have not had the time to check
all this out yet up against the circuit diagram .
on the zener diode itself i am reading 2kv on both ends and of course this
then follows down a wire terminated at the ptt relay , ie where i am seeing the
arcing ..
do you think my zener diode has failed or would it possibly something else to
look into .
the diode is a 1N3996A 5.1V 10WATT
there seems to be voltage present from the transformer , 990v AC before
entering rectifier and i am reading voltages on most of the diodes but need
closer inspection to all of them yet ..
i have no high voltage out of the rectifier/metering board to the anodes at
present .
i am unsure if the zener diode is good or bad and how ive lost anode voltage
and why .
hopefully its just a case of replacing the zener diode but id like to know what
measurements i should be seeing from the zener in normal operation .
the trouble occurred whilst in standby mode and not when inh transmit , amp was
just sat with HT on and filaments running and then all of a sudden the high
voltage went and the arcing started, to which i switched the mains supply off
instantly .
any ideas please , appreciated .
thanks
ian .
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