[Heath] HW-16 mods collection

Kenneth G. Gordon kgordon2006 at frontier.com
Sat Feb 11 22:38:55 EST 2012

Here is a collection of modifications to the HW-16 that users may find 
of value. I have collected these over the past several years from 
various sources on the web and from my extensive collection of e-
mails, and I have no idea who originated most of them. I am not 
claiming any of them as my own, but cannot now give proper 
attribution to whomever originated them either. I have used several of 
these mods, and have found those I have used to be very useful. 
To Increase Receiver Sensitivity
Replace the following resistors:
R42 was 5.6K, change to a 50K or 51K 2 watt resistor.
R21 was 22K, change to a 10K 2W resistor.
The mod will greatly increase the sensitivity of the receiver
>From a user: "With a 100 K pot in place of R42 I found I liked it best 
at about 58 K - so I settled on a 56 K fixed resistor." 
15 Meter Sensitivity and Power
The 15 meter final tank L to C ratio is incorrect so that when you 
have the transmitter tuning as peaked as you can get it, it will not 
really be resonant.
Thus you will find the tuning capacitor clear over against the left stop 
for max power output, which in my case was only 15 watts, whereas 
power output was 50 watts on 40 and 80. Receiver sensitivity is also 
not peaked here. 
Therefore, unless you reduce the 15 meter tank inductance by 
spreading the turns as much as possible, neither the transmitter nor 
the receiver input will be truly peaked. However, make sure you don't 
short the bottom turn against the coil mounting bracket. I did this 
mod by simply pushing downward on the bottom few turns of the 
plate tank coil. Power output on 15 is now 50 watts.
This is NOT my idea: I got it out of an old "Hints and Kinks" in an old 
QST magazine.
Since the TX tank tuning circuit is also the receiver RF amp grid 
tuning circuit, making this change also helped the RX.
Keep in mind that 15m will always sound "quieter" than 40 and 80, 
which can fool you into thinking there's a receiver problem when the 
band is not open. I noticed that after these resistor changes 15m 
signals seem to really "jump" out at me, but the background noise is 
not that much higher.
When I tested my HW-16 after making the mods listed above, 
sensitivity on ALL bands was 0.1 microvolt for a completely 
"copyable" signal. I have no idea what that translates into S+N/N 
Receiver Drift
Change R-24 at V-2A, heterodyne mixer, from 10K to 47K.
Change R-28 at V-2B, heterodyne oscillator, from 1 meg to 270 K.
This reduced the very noticeable drift in my HW-16's receiver to 
almost nothing.
Sidetone Volume Reduction
There is a 470K ohm resistor one inch away and to the right of the 
neon bulb when looking from the front of the rig. 
Change this to 4 megohm. This will bring it down to a nice subtle 
level. Changing to 2 or 3 megs may be enough for you. 
Another possible fix for sidetone volume is to increase resistance R63 
- try a 1K to start.
A third possible fix: Side tone level comes from Bias Switch Q1 which 
does not quite switch OFF the receiver - R49 10K - try a 10k trim pot 
and adjust level to your liking.

Sidetone Partial Operation 
September 26, 1968
>From HW-16 Heath Service Bulletin:
Novice Transmitter HW-16-1
Side Tone 
When using the HW-16 with a VFO sometime a partial operation of 
the sidetone oscillator takes place due to an increased loading on the 
bias voltage.
This can be corrected by changing R-69 from a 120K ohm resistor to 
a 220K ohm resistor [PN 1-29].
Schematic Correction
Also, the original schematic shows R-16 as a 4700 ohm resistor 
which is incorrect. R-16 should be shown as a 5600 ohm resistor.
Sidetone Oscillator Frequency 
For many of us, the sidetone oscillator frequency is much too high. 
To reduce it, change R64 1.5M to 5M. You might want to install a pot 
to adjust frequency then replace with a fixed resistor.
Spot Button
Mount a DPDT push button switch somewhere convenient. One NO 
pole would be used to key up the transmitter by using the switch to 
the parallel the key jack. Another NC pole would be connected 
between C7 and pin 9 on V8, the driver. Now when you push the 
SPOT switch - it keys up the transmitter and also disconnects the 
input into the DRIVER/PA so it doesn't go out over the air. Be aware 
of possible problems of one kind or another if the leads between C7 
and Pin 9 of V8 are too long, or not dressed properly.
RF Power Control 

The original way to adjust the RF power is by varying the B+ to the 
screen of V9 the PA. This does not give you very much range, and 
does not work well. 

If you wish to fix this problem, disconnect and solder the wires from 
points 2 and 3 of the R13 power level pot together and insulate the 
ends with shrink-tubing, then tuck out of the way somewhere safe. 
This connects full screen voltage to V9, the PA, and it will now 
operate at full power at all times.

Disconnect the wire from the B+ buss which connects to R6, a 22K 
2W resistor which is the screen voltage feed to V6 the 6CL6 driver 
tube. Connect this B+ buss wire you removed from R6 to point 3 of 
the Power Level Pot. Connect a wire from point 2 of the power level 
pot back to R6. 

This duplicates the power control, but it is now attached to the 
driver. Power output can now be adjusted from full power down to 
about 2 or 3 watts. 

If you increase the value of R-14 you can adjust power down into the 
milliwatt range and still go up to full power with a twist of a knob. 
You will have to experiment with the value of R-14 until you get what 
you want.

Chirp with Crystals

There are several different suggested methods of minimizing the chirp 
which appears when using crystals with some HW-16s. 

Several have suggested voltage-regulating the oscillator voltage.

I did this first, and it did absolutely nothing to correct the chirp in my 

Others have suggested reducing the oscillator voltage by changing the 
point from which the oscillator voltage is taken to 125 VDC. 

I have suggested adding an additional feedback capacitor directly 
from the screen of the oscillator, pin 3 (or 8) to ground. 

I got this information after reviewing several articles from the 
literature on crystal oscillators. It was shown that some oscillators 
needed additional feedback between the screen (which is the 
oscillator's plate in an ECO) and ground. In this type of modified-
Pierce crystal oscillator, ground is the mid-point between the plate 
(screen) and grid, since the cathode is grounded directly. 

I ended up using 220 pfd, but others have found lower values will 
work as well or better. 

In at least one case, adding a small-value (2 or 3 pfd) mica capacitor 
between the GRID (pins 2 or 9) and ground helped eliminate the 
remaining chirp after the screen capacitor had been added. From a 
careful examination of the circuit, it is clear that this provides a bit of 
additional feedback.

Prior to "messing with" the feedback, I also voltage-regulated all the 
voltages to the oscillator tube in my own HW-16: the result was no 
change whatever in the amount of chirp!! 

Adding the 220 pfd capacitor between screen and ground eliminated 
all but a very small amount of chirp for me, only noticeable on 15 

Another user reported that adding the screen capacitor made his 
crystals "sluggish". However, when he reduced the value of the added 
screen capacitor to a much lower value, activity returned, and chirp 
was reduced. 

Another user reported that when he connected an HG-10 VFO to the 
HW-16, and left it plugged into the VFO input jack, but powered off 
when he used crystals, all chirp, even on 15 meters was gone. The 
capacitance of the coax between the HG-10 and the grid of the HW-16 
oscillator tube was the added factor that eliminated the remaining 

It is all about proper levels and relationships of feedback.


The QSK method used in the HW-16 generally works quite well. 
However, in some cases, there is a very annoyingly loud CLICK on 
break that is very tiring to listen to. My HW-16 does this, and I have 
not yet found the source of this click. Apparently others have noticed 
it and have tried to do something about it. None of those methods 
have worked for me. If anyone can give me a real solution to this 
problem I would be very grateful.

Full-Transceiver Modification

I have an article from one of the ham magazines which contains a 
somewhat complex method of using the HW-16s own internal HFO as 
the VFO for the transmitter, making the HW-16 into a true 
transceiver. However, although this works quite well, there is no 
method of RIT included, and in my opinion, the rig should have some 
form of decent AGC added in order to make the full-transceiver mod 
most effective.

If anyone really wants this article, I will be most happy to mail a 
copy. Just e-mail me privately.

Although I have not yet worked on it, I am certain that a really good 
AGC can be added to the HW-16 using a minimum of parts.

I also intend to replace the QSK system in my HW-16 with a relay-
based version somewhat similar to one which appeared recently in 
QST magazine.

I have also replaced the meter in the my HW-16 with one taken from 
an SB-101.

I have used my HW-16 as my backup rig for my NTS duties on 80 
and 40 and it is quite effective. I have also used it to chase DX on the 
low end of 80 and 40 meters. It is an excellent rig.

Ken Gordon W7EKB
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