sb200

[JF]

W1JHS wrote on the Heath List:

> I guess I'm losing it but maybe the group can help.
> I have been re-doing my sb200 and it has been going well
 > except when I plugged it in and turned it on. It tuned up
 > ok on all the bands and then I noticed that the antenna
 > relay was not dropping  out when I went back to receive.
> I proceeded to check the caps and yes I did find the 2mf 150v
 > cap was shorted. Replaced it and guess what, there is still dc
 > on the winding. Checked the other caps in the ground circuit
 > all seem to be ok. Suggestions...I'm overlooking something!
> Tnx 73  Jack 


Not quite sure what you mean by "there is still dc on the winding".
As long as the linear is turned on there should be 135-170 VDC on the
hot side of the antenna relay's coil all the time;  the relay is
activated when a ground appears on the other side of the coil via the
ANT RELAY input jack on the rear panel, a small RCA-type jack. This
135-170 VDC will appear on both sides of the coil whenever there's no
ground on the keying line.

If the antenna relay will not drop out when you are not transmitting,
then either:
- the relay is stuck in the xmit position for mechanical reasons;
- a ground is present on the ANT RELAY line from outside (faulty coax
   or faulty keying line from the xcvr driver);
- a ground is present on the ANT RELAY line from somewhere inside the
   SB200.

Aside from possible antenna relay mechanical problems (as I recall,
it's not in a can and you can see the relay workings and the 2K/7W
resistor across the coil, R18), the SB200's antenna relay can cause
problems with the PTT key line from the external transceiver driver
(especially solid-state xcvrs) because it has such high voltage and
a hefty initial inrush current of up to 3 amps. This can pit or even
weld a xcvr's small PTT relay contacts, if it has mechanical contacts;
if it has a transistor PTT switch this can short it or burn it out,
thus providing either a constant ground or a constant open on the
key line to the SB200. Try disconnecting the cable from the xcvr
to the ANT RELAY jack to find if the SB200's antenna relay drops out;
if it does you will need to repair the xcvr (or key line cable,
whichever is at fault) and then install an isolation relay on the
key line between the xcvr and the SB200. The Harbach SoftKey
    "Jeff Weinberg W8CQ" <info at harbachelectronics.com>
         (317) 892-2641
         http://www.harbachelectronics.com/
modification is highly recommended for the SB200 (it goes inside
under the chassis), or you can make your own.

A short inside the SB200 could also place a constant ground on the ANT
RELAY key line - you would have to isolate parts of it one by one
(including the ANT RELAY jack itself) to find where the short is coming
from. Are you sure you didn't leave a short when you replaced the
2 mf/150V cap, or maybe even put in a bad cap? Pull out the tubes and
check the components on the key line going up to the grid terminals of
both tubes. Try disconnecting the lead that goes from R15 to terminal
1 on the meter switch. RFC 2, R15, R23, C18, the 2 grid resistors R21
& R22 (these drift with age and may need replacing anyway), and C14 &
C15 could all be suspect, as could the tubes themselves. If you bought
this linear from someone else, there could be wiring errors.

Finally, check the length of the screws that hold the 4 rubber feet to
the bottom of the case - one of them is underneath the antenna relay,
and if the screw is too long it can jam into the relay.
(Thanks to K5OVW for that tip.)

73    John   KE5ZB

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