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Author Topic: HQ-170A Nasty noise when Both selected  (Read 1414 times)
VE9UJ
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Posts: 2




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« on: February 25, 2008, 05:26:42 AM »

Hi All
I have an HQ-170A that has a weird issue.
When Sideband is selected to Both in any mode there is S9+ of noise. This is up from what would be normally quiet say s1 with U or L sideband selected.
After checking through the cct at different points I find that the noise is in the IF section after the 3rd mixer.
I pulled the 3rd mixer and the change in noise is still there.
It appears as tho this may be a regulation problem as the noise appears on the voltage lines and the oscillator looks like it becomes some what unstable.
Just wondering if anyone may have seen this issue before or have any ideas they might like to share.
Thanks & 73
VE9UJ
Hugh
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KL7AJ
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Posts: 340


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« Reply #1 on: February 25, 2008, 09:07:55 AM »

This may be a "weak BFO" symptom.  The noise level in DSB may be NORMAL, but the overall gain in CW/SSB is way down.  In pre-"true product detector" radios, they generally relied on random non-linearity in the first audio stage for BFO mixing action....a very inefficient process.  Many early radios suffered from weak bfo injection.

If you have a good modulated R.F. generator, do a complete I.F. alignment with that....your noise level may be a lot less than you think.

Just a thought.

eric
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W5RKL
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Posts: 1034




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« Reply #2 on: February 25, 2008, 10:44:50 AM »

What happens to the noise level when you remove the
antenna and short the antenna to chassis ground?

73
Mike
W5RKL
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VE9UJ
Member

Posts: 2




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« Reply #3 on: March 02, 2008, 06:27:27 AM »

And the winner is Eric
After a lot of messing around I found that the IF on this rig is very finicky and was out of adjustment enough to cause the issues I was having.
Now I have to tweek it to get the U, L, & Both lined up a little better.
I had originally done a "by the book" alignment which looks for max smoke on the agc line but does not account for noise.  I also wonder if there may be a component or 2 out of tolerance in the 60 khz IF/Filter section.  More checking I guess.
Thanks for the good advice and as I progress farther I'll check on the BFO injection but it seems to be OK at least.
 
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