Hank -
The NC-300 has two different methods for putting the receiver in STANDBY or RECEIVE mode. The
terminal marked STDBY is connected to the cathodes of the AF OUTPUT, a couple of IF stages and the
RF Amplifier. Lifting those cathodes doesn't kill the receive immediately because of the bypass
caps on those lines, allowing RF into the front end while the receiver is still 'listening'.
There is also a MUTE terminal on that same strip. Easiest fix is to connect a 100k (or so) resistor
from the -60 VDC BIAS supply in the T-4X to the MUTE connector on the T-4X. Connect about a 33k
resistor from the MUTE connector pin to ground. You may have to adjust the value of this resistor
to get about -25 VDC on that pin when the T-4X is in receive. Connect the MUTE cable from the T-4X
Pin and Shield to the MUTE and GND terminals on the NC-300 (remove the shorting wire if present!).
Leave the XMIT / RECV switch on the NC-300 is RECV.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Henry Foglesong wrote:
I am using a National NC-300 with my Drake T-4XC transmitter (using the Drake muting and antenna
changeover system).
When I key the transmitter in CW I get a rather loud "thump" in the receiver along with a very
brief squeal if I the receiver is not perfectly zero beat on the transmitter. When I key the
transmitter in SSB, I just get the thump. Obviously I am getting a little RF into the receiver
before it fully mutes. I do NOT have this problem with a Drake R-4A receiver that I have tried.
The NC-300 was just recently fully restored (new caps and replacements for out of spec resistors)
and aligned. Voltages are all in spec and the receiver appears to be working extremely well.
Have any of you run into this issue and if so, did you find a fix for the problem? It does not
stop me from using the rig, but it is certainly an annoyance.
I know, some of you are going to tell me to dump the NC-300 and use a good Drake receiver, and in
the end, I may do that. But in the meantime, I would like to use the National.
Hank
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