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Author Topic: Hammarlund Noise Immunizer - Silencer  (Read 999 times)
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ns7h
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« on: December 29, 2022, 12:36:32 PM »

I have an HQ-170 that has the Lamb Noise Blanker option.   To be honest, I do not know if it ever was working, but the receiver has been very good and reliable until some issue developed with feedback on audio levels and high "signal" levels.  The issue was with the noise blanker - taken out of circuit the receiver works normally.   I have spent a long time trying to "fix" the circuit and may have introduced errors, but it sure looks like it is the original circuit after many physical circuit traces.   I have tested out of circuit the IF cans, they are 455 Khz IF transformers and peak well in both directions.   Resistance to ground at tube sockets appear appropriate to the circuit design.   I have replaced a number of bypass and blocking capacitors.   The result of this effort is that without the immunizer installed, the receiver works normally.  Now, instead of feedback with the immunizer installed, the receiver is dead as if the circuit (in line of the first IF train (455 Khz) is "blocking" 100% of the time instead of the instance of a noise pulse - out of circuit the receiver works (out of circuit means no electronic connection to the receiver 455 Khz IF circuit.  (Still has B+ and filaments for the three on board tubes (checked OK).

Does anyone have any experience on this circuit? ( I know K7PP has a you tube on its operation, but not troubleshooting)  It is difficult to signal trace in the HQ170 with it installed and I haven't done much to figure an acceptable jig to allow this while operating to measure voltages and trace the 455KHz input.  So I am hoping someone has experienced something similar.  I have not gone a to a total rebuild and reluctant to do so now since I want to get on to another project. 
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K9MB
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« Reply #1 on: December 31, 2022, 05:10:04 PM »

I found this installation manual for this option.
Frankly, I spent a number if years working on analog noise blankers for weak signal receivers I designed and the first thing you need to know is that noise blankers have to be early in an IF system where the bandwidth is wide. Mine were at 10.7mHz prior to crystal filters.
The more selective the IF, the more that impulse noise is stretched and the more difficult it becomes to blank it by turning off the noise gate stage.
It is possible that this circuit did a little on automobile impulse noise that was not energy dense, but by placing it at 455kHz, they assured that the impulse noise was wider than if it had been right after the firs mixer at a higher IF frequency (3035kHz). Unfortunately, that IF is not used on 160 and 80 meters.
Here is the manual and good luck getting it to work. 73, Mike

http://ww.w.hammarlund.info/info/accessories/immunizer/immunizer_install_11pg.pdf
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