[Heath] SB-101...more problems....CW - some suggestions.

[Kenneth G. Gordon]

On 28 Mar 2016 at 18:45, James Owen via Heath wrote:

> 
> Hi Guy,
>    A few thoughts here as I just completed rebuilding an SB-101 and have a
> HW-100 on the bench as the present time.    How do you determine that the CW
> position doesn't work? If it didn't then you would have no output when in the
> tune position as the CW  crystal is switched in only in the tune position and
> CW position on transmit. It's not used when in receive.

Well, not quite: the LSB heterodyne oscillator crystal is ALSO the CW 
crystal. I.e., the same crystal is used for both LSB and for CW for both 
receive and transmit.

Remember that in CW the relays are not keyed physically by the 
key-contacts, but by means of the sidetone oscillator through the VOX 
circuitry. If the sidetone oscillator is not working, as I remember it, you will 
not be able to send CW. There is a small resistance-capacitance "integrated 
circuit" that in my experience sometimes goes out. The values of the 
resistors and capacitors in that "couplate" are listed on the schematic, so 
replacing it with discrete components is relatively easy.

Another problem with CW COULD BE the VOX circuitry itself. There is a 
relatively simple mod (changing the value of one resistor) which will both 
speed up the first "dit" and make keying a bit more "positive". 

You can find several what I consider to be valuable modifications to these 
transceivers here:

http://www.w7ekb.com/glowbugs/ModsEtc/modifications.htm

and specifically in this article by Mark WB8JKR.:

http://www.w7ekb.com/glowbugs/ModsEtc/Optimizing%20the%20HW101.pdf

>     If the carrier
> won't balance completely several things to check. One be sure the balance pot is
> good as I found one end in mine was broken. Check for resistance across the pot
> (one end disconnected) and look as how smooth the resistance is from the center
> to either end.

In my experience, those 200 ohm balance pots are of very poor quality and 
very prone to failure. In addition, when mounted, they really should have 
some sort of insulator placed between the pot and the circuit board. As I 
remember it, the manual tell the constructor do this. Otherwise it can be 
shorted out. I replace mine with 10 turn pots. This makes a HUGE difference 
in how easy it was to balance the carrier.

>  If still not balanced you may have a bad diode in the balanced
> modulator or even a defective T1 though it's unlikely it's T1.

I have never yet found a bad T-1, but I HAVE found bad diodes in the 
balanced modulator. I routinely replace all 4 of those with HP Schottky 
Diodes which I buy from Mouser. Those cost something like $0.10 each.

Also, according to Heathkit, if you cannot balance the modulator even when 
everything is OK, then you should move capacitor C-18 from the "normal" 
side of the pot to the other side, then try it again.

>  This is a
> pretty simple circuit and not many things to go bad.     As far as the wires
> disconnected it sounds like someone modified the audio output to drive something
> other than the speaker. The green wire from the transformer goes to the speaker
> through the phone jack and you would have no output if that disconnected.The
> cable on "B" goes to one end of the VOX anti-trip pot and circuit would work
> with this disconnected. I would put the circuit back to the original design.

I agree, except for one thing: there is a large capacitor, installed under the 
circuit board, which connects one side of the output transformer back to the 
output tube. This is to provide considerable inverse feedback in order to 
increase the fidelity of the audio output. One undesireable effect of this, 
however, is that in order to get enough "listening volume" from the rig, one 
has to run the volume control almost at max.

Removing that capacitor does not noticeably degrade the audio-quality from 
the loudspeaker but it very noticably increases the available audio power 
output.

Instead of setting the audio gain pot at close to max, usually at least at the 
10:00 oclock position, I can set mine at the 5:00 oclock position, leaving 
ample audio output if I need it.

Ken W7EKB