John -
The relay circuit in the L-4B is a very simple half-wave supply on a
dedicated winding of the filament transformer. The you should measure
~30 VDC from one pin to chassis, the 'other' pin is grounded. The
reference about 'reversing the pins' is because some transmitters have
one relay contact grounded and the two-pin connector is not polarized.
If you plug it in so that the grounded relay pin is in the 'hot' lead on
the amplifier, the VOX will be keyed.
Use your DMM to measure between the two pins, you should read ~30 VDC,
with the positive meter lead being the ungrounded pin. If you switch the
meter to read current, you should see approximately 65 mA and the VOX
will 'key'.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
John Hudson wrote:
A simple question (as if there really are any) about the Keying
circuit of an L4B. Because I use several radios to drive the L4B I
have a relay connected to the amplifier keying circuit that really
only grounds the leads, that has now failed to work. I looked at the
schematic and it appears there could be voltage on the VOX pins and
it’s actually referenced in the manual that if you have the pins
reversed the plate current will go up and to reverse the pins. There
is no voltage present!
· Saying all that, what voltage should I see if any on the keying
circuit when not keyed?
· Has anyone experienced this before and if so what did you find?
Yes, before anyone says it, I know ultimately I have pull the unit out
of line and take it apart to trouble shoot but I also know symptoms
can provide a path to repair and a place to start.
Thank you (in advance),
John N. Hudson III, WA6HYQ
“C” line, TR4C, RV4C, L4B, MN-2000, TR-6 Owner-Operator.
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