Hammarlund crystal filter not oscillating?

[KC2TAU]

I've been working on restoring my Hammarlund HQ-129X and I've had trouble getting the crystal filter to actually do anything. I know there have been many questions asked on this forum with regards to its function and the supposedly convoluted way to align it but I think this is another issue entirely. I am unable to get any sort of activity out of the crystal whatsoever. I am wondering if there is a wiring mistake somewhere that is causing this problem. The reason that I suspect this is that when the selectivity control is at its narrowest setting the tuning of the set does not become any more sharp or critical.

One would expect if the crystal was working, even if it was out of alignment, that the tuning in of a station would become much more critical whereas on my set the tuning is still very broad. There is no "whoosh" either that one would expect when sweeping across a station with the filter engaged. In addition the phasing control doesn't really eliminate heterodynes. When rotated to about "6" it just severely reduces the sensitivity of the set. The sensitivity returns in a relatively linear fashion as it is rotated in either direction of "6".

Now the most peculiar bit of this was at first I thought that the crystal filter was at fault so I had a friend send me a working module off of an HQ-120X and when I installed it the receiver behaved in the exact same manner. I'm starting to think it is much less of an alignment issue and more of a wiring issue of some sort. I have already cleaned the crystal and that did not help.

Any ideas would be greatly appreciated.

[KM1H]

Hit it with a signal generator (not very much signal on the 6K8 grid cap) and find its peak and measure the frequency. Do it in position 5.

It could just be low in activity and need a good cleaning. Those that are way low from 455 are the ones most inactive since acumulated dirt detunes them. If the IF is not aligned to the crystal and is far enough off the same as your symptom will result.

Carl

[KC2TAU]

Hello Carl,

I gave the crystal another careful clean and that did not seem to help things. Even with the signal generator injecting into the grid cap of the 6X8 the set responds in a very broad manner to the input signal even with the crystal filter on "5".

Attached are some photos I took of the IF section of my Hammarlund. I've looked it over and can't really see anything wrong at first glance.

Images:

http://i.imgur.com/iCa6Krp.jpg

http://i.imgur.com/KqdH4t3.jpg

http://i.imgur.com/nAzrDCH.jpg

http://i.imgur.com/eqtYVGF.jpg

http://i.imgur.com/wnhFAvj.jpg

[KM1H]

That piece of teflon coax is not stock and the C may be detuning the filter; the old shielded cable had less C.

Carl

[W7TFO]

It has been mentioned before, but it may not be that obvious.

The crystal doesn't oscillate per se, rather it IS a filter.  Highly frequency dependent, and as such, is untunable.  The scewdriver adjustments are present merely to effect an impedance match on the I/O, and perhaps give a bit of upper- or lower-sideband preference

Taking just the crystal unit out of circuit, you have to find it's natural resonance point by injection of a close frequency, then sweeping the generator and looking for a return loss dip whilst set for 5 on the dial.  When you find it, it will be obvious.

That frequency is where you need to align the IF, and it probably won't be on the nose as listed in the receiver data.

Since the radio has to be tuned to it, the sound to the ear is adjusted by how much effect is present in the final audio.  

An analogy for conceptualizing it is like a fixed single-channel audio pass filter, but for low RF frequencies.

I hope this helps in your quest.

73DG

[KC2TAU]

I made some progress. I isolated just the crystal and found a very pronounced peak around 456kc. I then put the crystal in line with the filter again and found that the peak had now moved to about 360kc. Seeing this I decided to pull the filter module out of the radio and disconnect the Teflon coax. I then measured the peak to be about 455kc or so which is much better.

The issue I'm seeing, though, is that the peak that the crystal created when it was isolated was very sharp whereas when the crystal is combined with the rest of the filter (even when the filter is removed from the radio) the peak is much more broad and not as pronounced. In addition the peak becomes progressively less pronounced the higher the selectivity setting I use. I need to keep feeding the module more RF with each successively higher selectivity setting. I've already replaced the mica for the crystal filter inductor which resulted in little change.

[KC2TAU]

I spent some more time looking at the filter as well as comparing it to the one that originally came out of my HQ-129X. First off, two of the resistors are out of specification. One 300 ohms reads about 600 ohm and one 2.2k reads about 2.7k. Secondly, the mica I removed from one of the inductors had a code that read (from left to right) grey, green and black. That would make this an 82pf capacitor, correct?

I've also noticed three differences between the two units.

First off, you'll see the arrangement of resistors on the selectivity switch. On the stock unit from the HQ-129X you'll see the 2.2k resistor is connected in line with all of the other resistors whereas on the 120X unit you'll see it connects straight to the crystal filter inductor. Why is this so?

In the second photo you'll see on the 120X unit on the right there is a 10k resistor connecting from the wafer switch down and out to the radio whereas on the 129X unit on the left there is no resistor.

In the third photo you will see that the 120X unit on the right has a .02mfd capacitor from the phasing capacitor to where that aforementioned 10k resistor connects to whereas the 129X unit on the left has no such capacitor. The capacitor on the 120X unit is stock but I have replaced the original as it was a wax paper unit.

Resistor arrangement:

129X:

http://i.imgur.com/sAGdsEj.jpg

120X:

http://i.imgur.com/Z2HfqYc.jpg

10K resistor on wafer switch:

129X:

http://i.imgur.com/TDLt0qb.jpg

120X:

http://i.imgur.com/FApEsKN.jpg

.02 capacitor:

129X:

http://i.imgur.com/wg8iaUa.jpg

120X:

http://i.imgur.com/Ouym7Ot.jpg

[KM1H]

I spent some more time looking at the filter as well as comparing it to the one that originally came out of my HQ-129X. First off, two of the resistors are out of specification. One 300 ohms reads about 600 ohm and one 2.2k reads about 2.7k. Secondly, the mica I removed from one of the inductors had a code that read (from left to right) grey, green and black. That would make this an 82pf capacitor, correct?

85pf

I've also noticed three differences between the two units.

They are different filters and there is even differences between early and late run 120's. Hammy was still fine tuning their design in 39-41.

Take a look at the RBG manual, it is WW2 and will show the progression of a 120 to a 129.
Also early 129's have an alignment hole for a trimmer cap that isnt mentioned in the manual.....back to the RBG.

Carl

[w4bfs]

in my 129x there was a 75pF sm near the phasing control in the if can .... hi ho silver dendrites strike again .... things much better after replacement and much cleaner audio