HQ-100A

Adding a Product Detector to a Hammarlund HQ-100A

Important Note: Before attempting this conversion, know that there are dangerous and potentially lethal voltages involved. Unless you are absolutely certain that you know what you are doing and have taken the proper precautions, do not perform this conversion. I cannot be - nor will be - responsible for any injuries or damages incurred as a result of your reading this website.

Introduction

The 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 was no BFO per se in the radio and the user was instructed to use the 'oscillating' Q Multiplier as a surrogate. The improved HQ-100A used a separate BFO, but some users complained about a 'weak BFO signal'.

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 suggestions for a competent builder to both install this circuit and then to improve it to suit his / her tastes. If you're interested, read on..

General Steps

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.

Install the 7 pin socket, getting filament voltage from a nearby tube and plate voltage from the OB2 voltage regulator.

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.

Remove the connections on V7's pin 9, 6 and 8. For the AM / AVC Detector, place two 1N34A's in series between the top terminal of T7 and ground. With just one 1N34A, the audio distorts on very strong signals - like those found on the broadcast band.

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 terminal on this strip. This is the input to the radio's audio stage which 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.

The 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.

Run 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.

That's pretty much it.

Re-align the radio, paying particular attention to centering the BFO and the Q Multiplier.

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.
For 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.

One 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.

One 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.

Finally, 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