Product Detector to a Hammarlund HQ-100A
Before attempting this conversion, know that there are
voltages involved. Unless
you are absolutely
certain that you know what you are doing and have taken the proper
not perform this conversion. I cannot be
- nor will be - responsible for any injuries or
incurred as a result
of your reading this website.
Hammarlund HQ-100 (and later the HQ-100A) was a very popular Novice /
SWL, general coverage radio from the 1960's. While it was
designed for both AM and CW / SSB use, tuning in either CW or SSB
signals can be frustrating. With the HQ-100 version, there
BFO per se in the radio and the user was instructed to use the
'oscillating' Q Multiplier as a surrogate. The improved
used a separate BFO, but some users complained about a 'weak BFO
Since the HQ-100A version was not equipped with a
product detector, to tune in either a CW or SSB it was necessary to
disable the AVC and then 'ride' the RF gain control while adjusting the BFO circuit to maximize intelligibility.
The same was true of comparable radios from this time frame,
including the Hallicrafters S-85, SX-99 and their predecessors, even
the venerable SX-28 and the SX-40.
This website describes how
one may modify the HQ-100A with an integral product detector that
enables very pleasant CW / SSB reception. This is not a
'cookbook' type of article wherein every step will be described or a
schematic offered. Rather, it should provide enough
for a competent builder to both install this circuit and then to
improve it to suit his / her tastes. If you're interested,
- Build up a 6BE6 product detector circuit as described on
pages 72 and 73 of the June, 1997 QST magazine. Wire
up the tube socket before installing
it. Use RG-174 mini coax cable for the connections
to the BFO, the IF and to the audio amplifier stage. Each
shield must be grounded at the product detector's socket and ultimately at the 'other end'.
- Using a Greenlee chassis punch, make a hole for the socket
just behind the main tuning capacitor.
- The AM / AVC detectors and noise clipper are in the same tube as
the BFO. This is how the Hammarlund engineers provided BFO
injection - by saving another tube like the 6AL5 - clever, but cheapo! Maybe this is why some folks complain about a
'weak' BFO in this radio. For the product detector to work
properly, the AM / AVC detectors and clipper need to be separated from the source
of the BFO energy.
- Place a
1N4148 AVC small signal diode on the wiring that was in place between
pin 6 and pin 8 of V7. If you study the schematic and the
pictures, you'll see that some careful rerouting of the wiring is
required. Instead of placing a terminal strip to hold the
leads removed from V7 (too much rework), you can rely upon the heavier
leads from network Z2. It seems rigid enough. Don't
forget to include a new C56 (350 pf).
- BFO injection to the 6BE6 product detector is through a 10
pf capacitor from the cathode of the BFO tube, V7.
- IF signal injection is from the secondary of T7 through a
30 pf capacitor.
- Mount a 4 terminal strip under
the Noise Limiter switch,
and break the connection of C41 to the black wire on the top of the
limiter switch. Connect the loose lead of C41 to one
this strip. This is the input to the radio's audio stage
will be switched between the new AM Detector (2 1N34's in series) or
the outpot of the 6BE6 product detector.
- Mount a miniature DPDT switch on the front panel
in the space where the clock adjustment would have been. Wire
one pole on it to switch the audio input (capacitor C41) to either the
AM Detector or to the product detector's output. The cables are run
through the existing grommeted hole just below the mode switch. Use cable
ties and / or clamps to ensure they don't
interfere with the HF variable tuning circuits. It they get too close, rest assured that they will interfere.
other pole of the DPDT switch is used to activate the BFO. The BFO position on the mode switch has been disconnected.
- Build a small filter
consisting of a 2.5 mh choke bypassed to ground at either end by .01 my
capacitors. Mount this assembly on the rear, underside of the
chassis in the existing hole that the serial number tag covers.
My picture shows 3 chokes in series as I didn't have a 2.5 mh
unit available. Gauche? Yes, but it does work, at least for the moment.
- Identify the B+ feed to pin 3 of the BFO tube, V7. It
runs across the chassis (in a tight form) and ultimately directly by
the antenna trimmer before it is connected to the mode switch topside.
On my radio, it was a fuscia colored with a white stripe.
Even with the filter installed, some residual BFO energy was
still 'riding' theis lead. It needs to be removed and replaced
with a length of RG-174 coax, with the shield grounded at both ends.
- Clip both ends of this lead and remove as much wire as you can.
a short piece of RG-174 from pin 3 of V7 to one side of the filter
mounted on the rear panel,. Connect another piece of coax to the
other side of the filter and run it up through the chassis to the DPDT
switch. It may be necessary to drill another small, grommeted
hole because of the other additional coax leads installed to switch the
audio amplifier between the AM Detector and the Product Detector.
Ground all the shields together.
the radio, paying particular attention to centering the BFO and the Q
- In the LSB mode, turn the BFO adjustment two
or three notches
to the left.
- For USB, turn it two or three notches to the right.
CW operation, keep it vertical.
- When using the radio in the CW mode, keep the switched off Q Multiplier turned to one extreme, or the other.
Even though the Q Multiplier is not directly coupled to the first
mixer when the selectivity switch is in the off position, the thing's a
real BFO trap and will induce a small amount of BFO energy into the RF
chain, possibly desensitizing the radio on very weak signals in low noise conditions.
- If you want to use the Q Multiplier on CW signals, you may have to switch the AVC into the Manual mode.
Unfortunately, I was unable to completely eliminate the BFO
signals coupled into the Q Multiplier - and thence into the IF chain -
when both the BFO and Q Multiplier have been activated. Unlike
the HQ-110 and other contemporary tube receivers, the HQ-100A's BFO
circuit is not shielded by any enclosure as is the Q Multipier.
So - at least for the moment - either the RF Gain can be turned
back a tad and / or the AVC deactivated to use the Q Multiplier on CW
signals. Time permitting, I'll see what I can do about this.
Any suggestions would be most welcome.
possibility might be decreasing the B+ to the BFO tube to reduce its
overall output while increasing the value of the coupling capacitor to
the product detector. This tube draws just 6 ma and a simple
series half watt dropping resistor would probably do the trick.. Again, this is only a minor issue when the Q Multiplier and the BFO are turned on together.
could also replace the front panel BFO assembly with a solid state
version in a small minibox, and not make any changes to the radio
besides adding the 6BE6 product detector. Although I could not
definitively prove it, those 2 relatively long runs of coax from the
front panel to the BFO tube running virtually parallel to the IF strip
just might have some residual BFO signal riding on their shields. Dunno - time will tell.
I had to repair one of the IF cans because it was intermittently
crackling when it was moved. I removed it from the radio fully
expecting to have the dreaded mica capacitor 'disease'. However,
as it turns out, this IF transformer (T5) has internally mounted silver
mica caos (100m pf and 330 pf). The crackling was caused by one of the
very fine wires shorting out to the metal side of the can. It had
apparently become kinked, perhaps when the transformer was assembled,
or maybe when another ham removed the old micas, but it didn't look as
though it had been disturbed. A piece of black tape fixed this.
- Enjoy your improved boat anchor. This is really a nice, sensitive, stable and well calibrated RX. Oscar
Hammarlund would have wanted it that way!
73's - Joe - K3JLS