SB-301 Problems

[Stewart D Lyon]

Hi Denny,
1. I think your major problem is the use of power supply diodes for high
frequency detectors. The reverse recovery time of these diodes is making
them act like resistors. I recommend you replace the two AGC diodes (D3 &
D4) with something like the 1N34. I have used NTE(or ECG)110MP which I
think are matched pair FM detector diodes.

2. You may have a gassy tube that is tied to the AGC line. This can be
determined by measuring the AGC voltage with a HI-Impedance voltmeter. If
the voltage is at all positive, one of the AGC-controlled tubes is
drawing grid current. About the only to determine which one is to replace
them one-by-one.

3. With the tube changes outlined below and using the calibrator as a
signal source, I measured the following AGC volts and S-meter readings:
No Signal  -0.7 v
3.7 Mhz      -4.28 v, S9+40db
7.2 Mhz      -3.71 v ,S9+30 db
14.2 Mhz   - 2.76 v, S9+5 db
21.2 Mhz   -2.67 v,  S9
28.2 Mhz   -2.15 v, S7
The drop in signal band-to-band is due more to higher harmonics from the
calibrator than from lessening sensitivity.

4. I place a posting a couple months ago about changing the tube types to
increase the sensitivity. In my SB-301, I changed V1 to a 6GM6, V2 to a
6AH6 and V6 to a 6HR6.
The V1 change (from a 6BZ6) was the most effective and was all the buzz
last month on the Collins reflector. (but you heard it here 1st!)  If you
change any of these tubes, you will have to realign the RF & IF coils
because of differance in tube capacitance - a two minute job.

5. I made some changes in the AGC time constants to improve the attack
and decay  time and will discuss them if anyone is interested.

Hope this helps.
73, Stu - W6CUX
Winnetka, CA
ARRL Life Member

On Wed, 4 Mar 1998 17:03:04 -0800 Dennis Heikura <heikud at DIRECTCON.NET>
writes:
>Hi folks, just finishing bringing a SB-301 back to life. Have it
>working pretty
>good but it still has a couple of anoying tendencies.
>
>First off, the AGC acts like it's working in reverse, that is,
>receiver is
>very quiet
>on weak signals. You have to turn the audio gain way up to read them.
>Then, when
>a strong signal or noise pulse comes through the rx springs to full
>life and
>stays that way until the signal is gone, then it slowly fades away.
>Just as
>if the
>AGC is working backwards. The rig had no AGC when first received due
>to a
>shorted
>AGC diode. I replaced both with garden variety silicon rectifier
>diodes  of the
>proper voltage as dont have the correct Heath replacements. AGC now
>works
>but as above.
>
>Secondly, receiver gets more and more insensitive as you go up in
>bands.
>Real hot
>on 80 and 40, then weaker on 20, weaker still on 15, and very weak on
>10.
>This seems
>to be common with the HW and SB rigs. Any cures? Am useing this rx
>with a Heath
>6 meter convertor which converts to the 10 meter band so greater
>sensitivity
>would
>be welcome.
>
>Thats about it. Any help or insight would be appreciated.
>Thanks for reading this.
>
>73,
>Denny==WB7EGG
>
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