Ian,
On receive, the cathode will have +120 Vdc applied to the cathodes
through the C.T. of the heater transformer. On transmit, the C.T. of
the heater transformer is connected to the 5.1 Vdc zener for bias
throgh the relay, then on trough a .82 ohm 2 watt resistor to ground
on the HV board. The plate voltage reading of the meter is
acomplished by sampling some voltage off of a resistor string to
ground on the HV board. This same ground point is also the ground
point for the HV board, the chassis ground, and ground for the zener
diode. By the arcing your mentioning, and the HV only showing up
under load on the meter, I'd look for a bad ground connection around
the zener and where the HV board goes to chassis ground since it's
all common at one point.
You mentioned the tubes heater pins coming un-soldered. This can be
fixed by cleaning all the old solder off and re-soldering it with
silver solder. This is a common problem with these amps because
there's not enough cool air hitting the heater pins.
Best,
Will
----- Original Message -----
From: almacottage <almacottage@btopenworld.com>
To: amps@contesting.com
Subject: [Amps] sb220 zener diode problems
Date: Fri, 8 Jul 2005 21:55:12 +0100
>
> 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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