[Heathkit] SB-401/HW 101 Final Questions

[Kenneth G. Gordon]

On 23 May 2008 at 12:41, Glen Zook wrote:

> Replacing the 10 ohm resistors is pretty much a "must
> do" on the HW-100, HW-101, SB-100, SB-101, SB-102,
> SB-110, SB-110A, SB-400, and SB-401.  In the Collins
> 32S-1, 32S-2, 32S-3, 32S-3A, KWM-2, and KWM-2A there
> are 12 ohm resistors used for the same purpose.  Those
> need changed as well.
> 
> Another thing is tighten all of the machine screws and
> nuts that go through the chassis (especially those
> that hold the circuit boards in place).

This is extremely important: doing this very often makes a huge 
difference in how well they work, and many weird problems simply 
and suddenly go-away, especially stability problems.

>  All of the
> grounds in the units are made through these and they
> definitely work loose over the years.

Or build up corrosion around and between them. In one case, I had 
to take all those screws out (at least those I could reach), lift each 
board slightly, and clean the corrosion from between the boards 
and chassis. I then used a SMALL amount of DeOxit around each 
screw hole. Usually, however, simply tightening each one does the 
trick.

> 
> The various elecytrolytic capacitors in the unit are
> suspect and replacements from places like Mouser are
> pretty cheap.  I would consider replacing those.
> 
> The potentiometers in the SSB balanced modulator often
> go bad.  Replace these with multi-turn potentiometers
> for ease in nulling the carrier.

Yes. See below.

> 
> Other than that I cannot think (at least "off hand")
> other things that might cause problems.

That last thing is to do a thorough alignment, following the correct 
ORDER of adjustments as shown in the manual as closely as 
possible...then do it again. Three times doesn't hurt.

Try to set ALL the heterodyne oscillator voltages, as measured at 
the HetOsc test point, to -2 VDC, and be certain you are on the 
correct side of the slope as mentioned in the manual.

I prefer to do my alignments in the transmit mode, rather than 
receive mode, but do it at lower power levels: 10 to 20 watts output 
is enough, with a dummy-load connected to the output. Resonate 
the final amp first on each band, though.

Set the VFO to the exact center of each band, while setting the 
Driver/Preselector knob where the manual tells you to.

40 meters is especially critical in this matter.

Neutralization works much better if you use Heathkit's "Other" 
method which was published in a service bulletin: i.e., remove 
plate and screen voltages from the final amp tubes, connect an 
oscilloscope or HF VTVM (HP-401C) to the antenna jack, and 
adjust for minimum output.

Glenn is most certainly correct about the carrier-null pots! They 
were even a big problem back in the 1960s. Heath published 
several service bulletins on them.

If you cannot get adequate carrier-null, you may even have to 
change the values of a resistor, or move a capacitor from one side 
to the other, as mentioned in one of Heath's service bulletins.

Replacing the 4 diodes in the balanced modulator with hot-carrier 
diodes will often make a big difference too.

Ken Gordon W7EKB