[Heathkit] SB-220

[km1h]

I didnt see the original post but from some comments, erroneous and good, I
offer the following.


25-26 Ohms is correct for those chokes. If one or both is open the DC grid
path is lost and the tube(s) will not conduct. When they blow they often
take R3 and ZD1 with it. If the meter is in the Grid position it will also
blow.
To protect the meter, place a pair of 1N400x series diodes back to back
across the multimeter since it is now the grid return path when the R3
opens, The diodes may short but it protects the meter which is becoing a
scarce commodity.

It is also a good idea to replace RFC-2 with a 15-20 Ohm 20W real wirewound
resistor. This will act as a surge limiter during a momentary internal tube
gas arc. RFC-2 is useless anyway as it is self reasonant right around 150
MHz which just happens to be the natural parasitic frequency of a 3-500Z.
IOW, it creates a BIG parasitic potential.

While you are at it check the cut-off bias/relay voltage; it should be 
~120VDC. If it is lower replace C4.

If you can repeat the original problem and what steps you have taken I 
should be able to walk you thru the troubleshooting. I usually have 3-4 
SB220's a month in the shop.

Carl
KM1H





----- Original Message ----- 
From: "Duane Fischer, W8DBF" <dfischer at usol.com>
To: <w4ych at earthlink.net>; <wb8jkr at juno.com>
Cc: <heathkit at mailman.qth.net>
Sent: Thursday, November 25, 2004 1:09 PM
Subject: Re: [Heathkit] SB-220


> Tnx Mark -
>
> ----------
> From: wb8jkr at juno.com
> To: w4ych at earthlink.net
> Cc: heathkit at mailman.qth.net
> Subject: Re: [Heathkit] SB-220
> Date: Thursday, November 25, 2004 12:16 PM
>
> 26 ohms sounds high, I would think those chokes
> should only have maybe 4-5 ohms max. Make
> sure you have continuity between the supply B+
> output and the actual plate caps of the 3-500's
> you might have an open plate choke, although
> I would seriously doubt it.
> Also, make sure the grid current measuring
> circuitry is ok, any open in the DC path of
> the grid circuit would bias the amp off.
>
> Mark  WB8JKR
>
>
> On Thu, 25 Nov 2004 12:08:29 -0500 "Don Flenner" <w4ych at earthlink.net>
> writes:
>> Mark,
>>
>> The voltages are switching  when the relay is energized measured at
>> pin 8 of
>> the relay. Also measured from grid to ground and I get about 26
>> ohms. The
>> original 1mh chokes are still installed on the 3-500z's.
>>
>> 73,
>> Don, W4YCH
>> http://www.home.earthlink.net/~w4ych
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> ----- Original Message ----- 
>> From: <wb8jkr at juno.com>
>> To: <w4ych at earthlink.net>
>> Cc: <heathkit at mailman.qth.net>
>> Sent: Thursday, November 25, 2004 11:20 AM
>> Subject: Re: [Heathkit] SB-220
>>
>>
>> >  Probably a dirty relay contact. The bias should
>> > switch between about -100 volts and -5 volts
>> > between rec & xmit.
>> >
>> > Mark  WB8JKR
>> >
>>
>>
>>
>> -- 
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>> 11/24/2004
>>
>>
>
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