Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Keith -
Did you replace the bias filter cap? 8 uF @ 150V
The "DC" you are hearing on the phones is probably the result of using
low-Z phones, resulting in very low AF Gain settings for listening. You
can add a resistor in series with the phones, selected such that the AF
Gain control needs to be at 11-12 o'clock for comfortable listening levels.
73, Garey - K4OAH
Atlanta
Drake B & C Line Service CDs
www.k4oah.com
Darwin, Keith wrote:
(drum roll please .....)
AND NOW, LADIES & GENTLEMEN, THE MOMENT YOU'VE ALL BEEN WAITING FOR ...
Ok, enough hype, on to the real post.
Last night I finished my cap replacement in the R4A. I replaced the
PS filter cap module and, while I was at it, the handful of paper caps
that "should oughta be replaced". I also put in new blue filter material.
Overall, the replacement process was fairly easy. My R4A is a 13 tube
model which only used 3 of the 4 caps on the new filter cap module
which was fine. The effort went smoothly, although it did take a few
hours.
Prior to the replacement I saw a few small issues with the receiver.
There was some AC hum in the audio. CW tones were a bit raspy (not
really noticeably on SSB). The RX background tended to modulate up
and down just a bit as did signals. RX audio had a bit of a narrow
sound to it with limited lows and highs, regardless of the PBT
setting. Also, there is significant loss in the .4 position but I
didn't expect some cap changes to affect that.
Once I changed the caps, I turned the rig on and was quite happy to
have no smoke! Like a teenager in the morning, the radio slowly rose
to life. I give it a few minutes to warm up and then started
listening using headphones.
I still have some DC on the audio but it is not as strong. I think
this is just the way these rigs are as my 2B also shows some DC on the
audio. The slightly raspy CW tone is still there. The narrowness to
the sound is still there as well. The slight pumping of the gain
seems to be gone.
I have some hope the narrowness to the sound and the loss in the .4
position are both related to PBT alignment. I may do some o-scope
signal tracing to see if I can find when the CW tone goes from clean
to raspy. Hopefully this is just an issue with a tube.
- Keith KD1E -
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