301/401 cw problem

[Darwin D. Geiselbrecht]

In a message dated 4/16/02 5:53:14 AM Central Daylight Time,
ggammon at BCGROUP.NET writes:

<< Hello all,

 My 301/401 pair has developed an interesting problem. When
 operating cw, at the end of a transmission, the relay opens
 but the received signal is knocked way down and a steady
 tone is heard. This tone gradually fades away until finally
 the full receiver audio is heard. I believe a second relay
 contact opens as there is a slight click from the 401 and
 the audio is restored all at once. The longer the
 transmission, the louder the tone is initially and the
 longer it takes to subside and restore full receiver audio.
 Blocked audio can take as long as 10 seconds to be restored.
 On short transmissions, such as a cq, the pair act normally
 with full audio after key-up.

 The twins are set up to operate in the xceive  configuration
 with the 301 controlling the 401. The full crystal pack is
 installed in the 401. SSB operation is normal.

 Any ideas? It liveable, but annoying as the first part of
 the other station's transmission is lost. This is often just
 the call signs  but sometimes it's not.

 Thanks for the help.

 73
 Gareth VE7CGG >>

Gareth - you have probably guess right that the relay RL1 is not completely
dropping out.

There was an article in January 1969 QST that addressed this issue in
addition to several others.  It suggests that the 7.5PF vox sidetone coupling
cap C131 be reduced to 2.0 pf.  I have done that with some success.  However,
if I remember correctly, someone on the list recently described a circuit mod
that used a zener in the cathode of the vox relay tube to make the relay
operate crisper.  If they don't chime in with info in response to your post,
I'll look in the mail backlog and find it.

The tone you hear is the carrier osc in the SB-401.  It seems to always be
present to some extent when operating CW with the twins cabled in transceive.
 The article deals with it by grounding contact 7 on relay RL1.  The reason
for this very worthwhile mod is that it eliminates screen emission from the
6146s when in receive mode. This screen emission gets worse with long
transmissions and powers up the plate circuit of the carrier generator tube
in RX mode.

I have 3 sets of the twins and have one on the bench now, so I'll try the
zener mod also. 73 - Darwin K5DOA

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