Begin forwarded message:
From: R; Measures <r@somis.org>
Date: July 9, 2005 1:35:51 AM PDT
To: "Will Matney" <craxd@engineer.com>
Cc: amps@contesting.com
Subject: Re: [Amps] sb220 zener diode problems
On Jul 8, 2005, at 3:16 PM, Will Matney wrote:
> 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
> .
The cooling fan in a SB-220 directs high-velocity air at the filament
pins. However, if the fan bearings are not periodically oiled, the
cooling fan will slow and the solder will go.
- notes - When silver-soldering filament pins, first remove the lead
solder with solder-wick. Resolder with 6%-Ag / 94%-Sn solder and
liquid rosin flux.
> 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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>
>
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Rich Measures, 805.386.3734, AG6K, www.somis.org
Rich Measures, 805.386.3734, AG6K, www.somis.org
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