SB-401 won't tune up

[Stewart D. Lyon]

Hi Jack,
First off, check the cables between the -303 and -401. It's easy to get them
mixed up. Not sure about a -303 but on a -301, these cables must be RG62 and
2 ft long -- especially the het oscillator.

In your letter you made no mention of adjusting the DRIVER or MIC/CW LEVEL.
I assume you did this.

On a couple 401s I rebuilt, I found the resistor that unbalances the
modulator for tune-up to be way off. This resistor is R401, is located
nearest the front panel on the left side terminal board and is 47K. I
recommend you change it to 33K. The balance modulator will now put full
output with the LEVEL pot at ~12:00.

I tracked the instructions you were given for tune up and even though that's
the way the manual sez, I don't agree with them!! Try this way:
A) Meter switch on PLATE. Function on TRCV. LEVEL at 3:00 o'clock.
B) Mode to TUNE,  PLATE I should read about 50 mA (at the small marker).
C) Adjust DRIVER to get a slight upward deflection on the PLATE current.
Turn LEVEL down so that deflection stays low and continue to peak DRIVER.
D) Turn LEVEL wide open and quickly adjust FINAL for dip in PLATE I.
E) Increase LOAD for a PLATE current of 250 mA (mid scale).
F) Switch meter to GRID. The meter should be now pinned. Reduce LEVEL until
meter reads 1/2 scale. Again adjust DRIVER to peak GRID current.
G) Switch meter to OUTPUT and reduce LEVEL until output just starts to fall
off. Adjust FINAL and LOAD _slightly_ to peak output. Max output may occur
at a PLATE current of less than 250 mA. Never exceed 250 mA or you may blow
the 4 Amp fuse.

If this seems like a lot of adjusting, I have a Kenwood TS-440 I'll sell you
(just kidding).

A common problem is the 200 ohm NULL pot on the Carrier board. This sucker
opens up at the termination rivet, making it impossible to null out the
carrier. As a matter of course, I replace it with a 10-turn trimmer (cost ~
$1.59).

Did you find a mic??

Good Luck, Stu W6CUX.

----- Original Message -----
From: "Jack Cantrell" <jack_cantrell at YAHOO.COM>
To: <HEATH at LISTSERV.TEMPE.GOV>
Sent: Friday, December 01, 2000 6:54 PM
Subject: SB-401 won't tune up


> I have recently acquired an SB-401 (and SB-303) and am
> attempting to get myself on the air.  I'm having
> trouble tuning up the SB-401 according to the
> operating instructions and the checklist on page 114
> of the manual.
>
> Specifically, this occurs first thing.  I set the
> function switch to Transceive, the mode switch to
> Tune, the meter switch to Grid, and then nothing.
> Nothing at all.
>
> I get a meter reading from HV of 8.  Otherwise
> nothing.  All of the tubes light up except for the
> V7/OA7.  I can't tell if that one lights up or not.
>
> Does anybody recognize these symptoms?  Any suggestions?
>

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