On Sat, 26 Dec 1998 09:39:32 -0800 Rich Measures <measures@vc.net>
writes:
>
>>
>>
>>
>>On Sat, 26 Dec 1998 19:50:02 +0800 Alek Petkovic <vk6apk@eon.net.au>
>>writes:
>>>
>>>Hi All,
>>> A friend of mine in Virginia is having the following
>>>nightmare
>>>with his SB220. Can anybody help?
>>>My SB220 contains a small transformer with two windings. One winding
>>>supplies the filament voltage for the 3-500zs and the other supplies
>>>bias and relay voltage at 120volts. One side of the relay coil is
>>>connected to this little half wave supply. The other side of the
>>>relay
>>>coil goes a jack on the back of the linear. When this jack is
>>>grounded
>>>it closes the relay and the linear goes into the transmit. From the
>>>banded end of the diode rectifier, a 20ufd, 160 volt electrolytic
>>>capacitor is connected from this point to ground. Without this
>>>capacitor the relay will not close. It just chatters from the
>>>pulsating dc voltage. Just what you'd expect.
>>>
>>>I have gone through at least 10 capacitors, and each one explodes
>>>after>a short time.
>>
>>Starting with the obvious, you have checked or replaced the diode?
>>
>>>Six of them were supposedly new stock.
>>
>>Brand and date code?? That is a 1/2 wave supply and the ripple may
>be
>>too much for some bargain brand caps.
>
>? The original capacitor was a cheapo and it worked ok, Carl.
Worked OK until it leaked or shorted, that cap was always on the edge,
particularly the early ones in the brown/tan case.
>>Have you actually MEASURED the voltage?
>>Are R4 and R5 OK and in the ckt? If not the voltage will rise quite
>a
>>bit and can exceed the cap rating.
>
>? The secondary winding is 80Vrms. How can the peak-V exceed 160V?
Easy since you are incorrect about the AC rms voltage and the peak DC in
a half wave ckt with no resistive load.
73 Carl KM1H
.
>- later
>
>
>
>Rich...
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