Heath Heathkit HW-101 Instruction

Optimizing the Heathkit HW-101,

SB100-102 Transceivers

he Heathkit HW/SB series of transceivers were very popular in the late 60's through 70's and are fairly plentiful on the used market even today, nearly 30 years later.

This series of modifications will increase the audio quality of both receive and transmit, improve operation on CW, enhance strong signal handling of the rigs, and make the units useable with the low-Z headphones so often used today. All of these changes require no hole drilling or mechanical changes at all, and the rig can be easily restored to its original configuration at anytime if desired. The text deals with the HW-101, but the changes are applicable to the above mentioned SB-series as well.

Conversion to Low-Z Headphones:

These rigs are designed to be used with Hi-Z headphones, but if low-Z phones are used the outboard speaker will not mute completely with the phones connected. To convert to low-Z phones, make the following wiring changes referring to (Figure 1) of this document, and pictorial 8-4 (foldout from page 53), and pictorial 8-5 (foldout from page

67) of the HW-101 manual:

1.) Remove the black wire from terminal strip BA lug 2 and reconnect it to lug 3 (ground).

2.) Remove the green wire and the 100Ω resistor from jack AB (speaker).

3.) Connect the green wire to terminal strip BA lug 2.

4.) Connect the 100Ω resistor removed in step 2 to lugs 2 and 3 of terminal strip BA.

5.) Remove the jumper wire from lugs 1 and 2 of headphone jack L.

6.) Run a NEW wire along the wiring harness from speaker jack AB lug 1 to the headphone jack lug 2.

Now the external speaker should mute completely when low-Z phones are used.

Figure 1

Improved CW Operation:

In the CW mode the rig’s relays are energized by the CW sidetone amplifier's sidetone output driving the vox relay amplifier. The audio (sidetone) drive to the vox amp is a bit on the low side for fast relay action, and at speeds approaching 20 wpm the first dot sent is heard in the sidetone but isn’t transmitted due to the slow relay response time. Correcting this is a very simple modification – simply replace the 470KΩ resistor R328 on the audio board with a 1KΩ resistor (Figure 2). This increases the drive to the vox amp enough to cause the relays to pull in quicker. Now the rig will key reliably at 20 wpm. The first dot tends to become shortened at 25 wpm but the rig is useable at this speed.

Another annoying problem with these rigs is the fact the sidetone can be heard when the rig is in the CW mode without the key actually being pressed. Although the sidetone amplifier is in deep cutoff, there is enough coupling through the inter-electrode capacitance of the tube to allow the output of the sidetone oscillator to be heard at a constant low level even when the key isn’t being pressed. Fortunately this is also fairly easy to correct:

1) Connect a .001 µfd disk capacitor (DO NOT use a higher value) from relay RL1

pin 1, to ground (Figure 1).

2) Connect a piece of wire about 8” long from pin 1 of RL1 and run this wire

through the same opening in the shield as the wiring harness.

3) On the audio circuit board, replace 1MΩ resistor R326 with a 2.2MΩ resistor

(Figure 2).

4) At the circuit board junction of R326 and C311, connect one side of a .005 µfd

500v disk capacitor, and connect the other side of this capacitor to the wire from RL1 pin 1.

Figure 2

Now during receive, the sidetone will be bypassed to ground by the normally closed contact on RL1. During transmit, RL1 opens and the sidetone works normally.

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