[Hammarlund] HQ-129x IF alignment

[Richard Knoppow]

----- Original Message ----- 
From: "William A Kulze" <wak9 at cornell.edu>
To: <Hammarlund at mailman.qth.net>
Sent: Friday, August 24, 2012 7:31 AM
Subject: [Hammarlund] HQ-129x IF alignment


>A while back I was talking about utilizing an sdr as a 
>visual aid for getting a balanced passband in the IF. My 
>original idea was to use the panadapter to see the shape of 
>the filter. Well, that didn't work out because where you 
>measure the output, it's the neg dc at the detector. (it 
>works like WOW feeding it with the IF out of a 390a!)
>
> Anyway, lacking a sweep generator, I sent the signal from 
> the URM-25F to both the SDR tuned at 455kc (basically 
> doubling as a freq counter) and the injection points for 
> each stage, as I went. I monitored pin 5 of the 6H6 with a 
> scope. I peaked everything at 455kc and then went above 
> and below center freq until I reduced the neg DC level on 
> the scope by half. When all I did was peak at 455, I 
> noticed the peak was not in the center of the passband, or 
> if it was, the +/- 3kc were not equal . And if all I did 
> was peak at 455 I wouldn't realize that without sweeping 
> the generator through the passband.
>
> So I played around with the cores until I had the peak at 
> 455kc and the +/- 3 kc points balanced.
>
> When I did the part where you figure out what the xtal 
> freq is, it seemed like it was resonant at 453kHz, if I 
> did it right. (does that raise a flag for anybody?)  So I 
> tuned the whole IF strip at that peak and what a 
> difference it made.
>
> When I did the LO adjustments I didn't even inject a 
> signal from the generator. I used a sniffer coil connected 
> to the sdr, figured out the dial freq plus 453kc (or minus 
> on the top two bands) and set the sdr to that and adjusted 
> the LO, using the panadapter on the winradio to see when I 
> was right on freq. I used the generator to then adjust the 
> RF sections.
>
> I still have linearity issues on the dial, though. The 
> ends are right on but it's off in the middle.
>
> Another way the sdr came in handy, though, was I could 
> highlight its bandpass, say at 6kc, and quickly see when I 
> was on freq at +/- 3kc and then adjusting for a balanced 
> shape to each stage of the IF on the o-scope.
>
> I must say, it's quite the Rube Goldberg setup, but I was 
> able to do this with what I had on hand.
>
> The audio has a much better sound, that off-center peak 
> was giving it a brassy sound and it was hard to feel the 
> station center when tuning. On ssb it was more stable than 
> I thought it would be, and the xtal filter seems to work 
> well. Nice and clear with low noise, but I think one big 
> reason some of the older rigs sound good on ssb is that 
> you have the agc off, so it doesn't bring up the 
> background noise in the quiet spots like with the agc on.
>
> I've recapped (paper and electrolytic) the radio (from 
> hayseed hamfest, btw) and replaced some out of tolerance 
> resistors, but not all resistors or the mica caps.
>
> Oh, BTW, the serial number on the back of the chassis is 
> like 2435, or there about, I'm not where the radio is at 
> the moment. I am told I am the 3rd owner. It is an early 
> model with red 'make and model' lettering and original 
> chickenheads, except one that I broke!   8(     It has the 
> hammarlund 100kc xtal marker installed with a switch in 
> the lower left corner of the front panel, an adhesive 
> metallic label attached above it. I also have a jpeg I got 
> online of a qsl card my father had sent out in May of 1946 
> and it says he was using an HQ-129x. Must have been right 
> when they came out!
>
> There's a picture on my QRZ page from last winter. The 
> shack pictures are due for an update, though. Maybe this 
> weekend. It's nice to have a workbench after a long period 
> without one. This was my first real re-cap job and it's 
> given me some confidence to tackle the other radios I 
> have. I might even clean up the gear train in my R-390a!
>
> Bill Kulze W2NVD

    Unless the IF is way out of adjustment I suggest finding 
the crystal resonance first and then using that to center 
the sweeper or wobbulator for the rest of the set up.  Two 
of the IF transformers in the RX are overcoupled and must be 
set up using a sweeper.  There may be an alternative method 
of detuning one side as in the Collins receivers but I've 
never tried to work it out.  Simply peaking the transformers 
will make the set work but the selectivity will not be up to 
its capability. The whole idea of using the overcoupled 
transformers is to improve skirt selectivity, and with the 
critically coupled transformers, obtain a nearly flat pass 
band. These RX have excellent selectivity.  There are two 
adjustments for the crystal: the first peaks the input at 
the IF frequency, the second is the loading inductance. Its 
set by feeding a modulated tone at fairly high frequency, 
say 2khz, into the receiver, then set the carrier at the 
peak with the crystal at its narrowest, then set the crystal 
for its widest bandwidth and adjust the loading coil for 
maximum output of audio.  The loading coil broadens out the 
crystal.  After adjusting check the phasing adjustment.  You 
should be able to get a deep null on a carrier off-set from 
the center frequency by the same amount on either side. 
When listening to noise with the crystal at minimum it 
should have the least high frequencies when the phasing 
control is centered.  You may have to open the crystal 
filter housing to observe the phasing capacitor to insure 
the knob is centered.  If the null is not about symmetrical 
or the phasing does not produce minimum bandwidth with the 
knob straight up there is an adjustment in the form of a 
wire lead near the phasing cap that can be bent around to 
compensate whatever change there has been. The Hammarlund 
filter was patented and is the best of the single-crystal 
filters.  It was subsequently used by Collins, TMC, and 
others.
     Dial calibration:  I have never been able to get the 
dial calibration on my HQ-129-X to be really accurate and 
the band-spread has a large error on all but 10 and 20 
meters. I can't find the cause.  I compromise the 
calibration of the main dial to minimize the overall error. 
I find the greatest error is at the bottom.  The 
multi-section capacitors seem to be in perfect condition and 
I have checked for correct centering. I've tried everything 
else I can think of to solve this problem but nothing works. 
I am told by others on this list that the RX should have 
quite good calibration so something is wrong.  The 
arrangement of the capacitors and band-switch is clever and 
attempts to insure good LC ratios and high Q.  The two 
sections of the main tuning cap are used in parallel on the 
lower three bands and the larger of the two is used alone on 
the others  the band spread cap sections are used alone.
     Make sure you adjust the local oscillator _below_ the 
signal frequency on the top two bands and _above_ it on the 
others.  This is NOT mentioned in either the HQ-129-X or 
HQ-120-X handbooks and is important.
     This seems to be a surprisingly good receiver, 
reasonably stable and with good selectivity. Even the image 
response is pretty good for a single RF stage.  Works fine 
on SSB with some warming up.  Unfortunately, the RF and 
mixer tubes used were noisy.  You might get some improvement 
by changing tube types but the bias and AVC will not be 
right. If you really need better performance on the higher 
bands you need another receiver.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com