Heath: HW8 Keying Problem

[John Farrington]

At 07:08 AM 11/29/2000 you wrote on the Heath reflector:

>I have an HW 8 that works fine except when I try to run it at speeds
>faster than about 16 WPM. At faster speeds the dits are run together,
>for example three fast dits are transmitted as one long keyed element.
>73s es Thanks,
>Steve K1EL


First, you need to check the adjustment of the break-in delay control,
trim pot R68 (50K) which, in series with R69 (1K), connects between
the 13.4V bus and the collector of Q12, and, through R71 (10K), to the
base of Q13:  if this trim pot is not set for higher speeds, or is out
of specs (or, if any associated resistors or other parts have drifted
out of specs - a common enough occurrence with older Heathkits), then
the break-in delay may be wrong and the dots & dashes could run
together at higher speeds.

R68 is located close to the rear of the pcb near the antenna socket
on the back panel, between the 4 crystals and the relay. The manual
doesn't say much about setting this control, only (on p. 68, under
"Operating Hints") to adjust it to obtain the desired hold-time of
the T/R relay.

If that doesn't help enough, try the following fix, from a writeup in
the August '82 issue of Ham Radio Magazine, p. 60, "The Ham Notebook"
column, by Robert Lewis, W3HVK:
----------------------------------------------------------------

To paraphrase W3HVK:

When keyed at and above 25 wpm, the HW-8 break-in delay circuit had
a long rf output decay time, even though the sidetone sounded good.
The parts involved were Q12, R66 & R67 (4700 ohms each, C92 (10 uf),
and keying transistor Q11.

If you look on your schematic, after the key is released capacitor
C92 discharges through Q12, causing Q11 to remain in conduction (for
100 ms in the author's case). This keeps the T/R relay in the xmit
position for that long after key-up - the time required for C92 to
discharge is controlled by trim pot R68.

The solution was to reconfigure Q12 to function as an ordinary diode;
then when the key is released, Q12 is reversed biased, thus effectively
disconnecting C92 from the keying circuit.

Modification to accomplish this:
leaving C92 as is, remove resistors R66 & R67;  then solder a jumper
wire directly between the base and collector of Q12. There will no
longer be a connection from the base of Q12 to ground through a bias
resistor.

The author said this modification had no noticeable effect on the
break-in delay circuit or the setting of the delay control.
---------------------------------------------------------------

73

John Farrington  KE5ZB

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