HQ-170 loss of sensitivity

[WB4AQL]

Hello everyone.

My HQ-170 has lost sensitivity on 160 and 80 meters. The upper bands still cooking along. I had an NC-303 that did this one the upper bands years ago and it was the 6C4 oscillator tube. The HQ-170 uses the same tube, but I`m trying to understand how this could affect 80 and 160. The HQ-170 is dual conversion on 80 and 160, but I don`t remember if the 303 was.

Anyway, she`ll come out of the cabinet this weekend and was just hoping for some thoughts from the world of AM wisdom!

73,

Michael

[w3jn]

"Reduced" sensitivity, or "no" sensitivity?

If the latter, it's almost assuredly the 6C4.  You may need to try several to find one that works correctly.  Sensitivity is also proportional to the LO level into the mixer - if the LO is pissweak the sensitivity will be reduced.  Could be the Q of the LC oscillator components is lower at the lower frequencies hence an oscillation problem or lower osc injection at those frequencies.

Also look at the switches associated with switching the various IFs around.  Not sure off the top of my head where the 170 switches from double to triple conversion, but that might be a start - what's different between 160/80 and the other bands.

[WB4AQL]

Johnny,
Greatly decreased sensitivity on 160 and 80 only. These are the double conversion bands. A strong signal still copyable, just barely moves the S meter. Noise level is nil and S meter just lays on the peg.

I wondered about a bandswitch problem but behavior not typical i.e. intermittent.

[KG6UTS]

"Reduced" sensitivity, or "no" sensitivity?

If the latter, it's almost assuredly the 6C4.  You may need to try several to find one that works correctly.  .......

Also look at the switches associated with switching the various IFs around.  Not sure off the top of my head where the 170 switches from double to triple conversion, but that might be a start - what's different between 160/80 and the other bands.

I'd go to the switch contacts for the first two tubes, as well as the 1st mixer. My RX is a 170A VHF that had problems with noise on strong sigs...it was V12 the 6C4. At the time I had two sticks of NOS RCA 6C4s and 7 of 10 had grid-cathode problems.

This is from feeble memory.... ~8^)=

EdZ KG6UTS

[w3jn]

Could be the ant coils for those bands are fried also.  Because they have a lot more windings than the higher frequency coils they're wound with thinner wire and thus more subject to burnout of Hammy Hambone loaded his xmitter into the thing.

If you have a sign gen and a scope this will be simple to troubleshoot.  Could be hard otherwise.  ANyway try those suggestions and see how it goes.

[WB4AQL]

Well, I`m not finding any one thing, but have replaced several weak tubes (still working, but below minimum on the TV-7 D/U tester). Sensitivity is way up and I have yet to replace the 6BV8 AVC-detector (had to order one) that tested way weak.

In these situations I always wonder if the tubes or cleaning the pin contacts are making the difference.

Anyway, this is the first time I `ve had it out of the cabinet and it seems to have been mostly re-capped; always a good idea.

73,  Michael