[Amps] sb220 zener diode problems

[R@contesting.com]

On Jul 8, 2005, at 1:55 PM, almacottage wrote:

> 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 .

Bad zeners typically measure under 0.1-ohms in both directions.
>
> 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 .

My guess is that the arcing in the SB-220 is being caused by an 
intermittent parasite at c. 110MHz.  Reducing the Q of the VHF 
suppressors may help decrease the VHF gain enough to stop the arcing,  
If you would like to try a parasitic suppressor retrofit kit, send me 
your address and we will send one.  If it does not stop the arcing, you 
owe us nothing - nada - zip.  If it works, the total price is $15.50.  
This malady is covered in an article about the 220 on my Website.
>
> 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 .

A familiar story, Ian.
>
> any ideas please , appreciated .
>  thanks
> ian .
>
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Rich Measures, 805.386.3734, AG6K, www.somis.org