[Amps] sb220 zener diode problems

[almacottage]

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 .