HQ-170 120 Hz audio hum

[WB4AQL]

Hello everyone. Spring is coming!

I have been chasing a persistent 120 Hz audio hum that is present at constant volume and unaffected by the volume control or standby/rcv mode. Not deafening, but loud enough to be a bother. Cathode bypass caps in audio stage are new and tubes have been swapped. My first suspicion was heater to cathode leakage, but I think I`ve chased those possibilities. Multi-section can filter cap has been replaced. I thought sure that would stop the hum as the 60 mfd section is the cathode bypass for the 6AQ5!. Didn`t change a thing.

There does seem to have been a previous owner mod in the audio section involving the 1st audio 6AV6 and final audio 6AQ5. I know there were some mods to "improve" the HQ-170 audio, but I can`t seem to find a description. Hard to believe that any wiring changes for "improvement" would have intentionally induced 120 Hz hum.

I know Hammarlund put out some tech bulletins concerning hum. I`ve been through those. Just wondering what ya`ll think.

73, Michael  WB4AQL

[The Slab Bacon]

just a thought:

Most of the negative feedback mods for the gray faced Hammarlunds create some type of feedback loop from the plate of the 6AQ5 back to the plate of the first audio. Now, the audio output is fed B+ from the input side of the CLC filter. (before the choke) I have done these mods before and gotten some hum in the audio. I have usually cured it by moving the feed for the 6AQ5 plate and screen to the output side of the B+ filter. (after the choke) (I dont remenber if they had a small choke or just a resistor) I cant check the skizmatic cause I'm at work. A little careful investigation should tell the tale.

[WB4AQL]

Slab, I think you are in the neighborhood. One part of the mod has a 1 megohm 1/2 W resistor in series with a 0.01 mfd ceramic disc connected back to the cathode of the first audio 6AV6.

Michael

[WB4AQL]

Also, I can`t find the cathode bypass capacitor at all for the 1st audio 6AV6. It should be a 20 mfd 25V electrolytic. I think it was removed during the mod-can`t imagine why. Guess it`s absence could be contributing some hum.

Michael

[The Slab Bacon]

Slab, I think you are in the neighborhood. One part of the mod has a 1 megohm 1/2 W resistor in series with a 0.01 mfd ceramic disc connected back to the cathode of the first audio 6AV6.

Michael

That's it!! I believe it was Larry's  (W8ER or whatever his call is now) mod. Try snipping the joint between the resistor and cap and see what happens. If that cures it, it's your decision- improved audio, Hum, or relocate the B+ feed for the outpoot tube.

[The Slab Bacon]

Also, I can`t find the cathode bypass capacitor at all for the 1st audio 6AV6. It should be a 20 mfd 25V electrolytic. I think it was removed during the mod-can`t imagine why. Guess it`s absence could be contributing some hum.

Michael

Michael,
           You have to remove it for the feedback mod to work or it would bypass the inverted audio right to ground. It being missing, is not going to cause your hum.

[WB4AQL]

Slab! Hum is gone!! It was the feedback from the audio mod. I`ll have to find a decent AM signal to critique audio, but not  missing that hum.

Should the cathode bypass cap on the 6AV6 be re-installed? Seems to work FB without it.

Man thanks OM.

Michael  WB4AQL

[WB4AQL]

Regarding the bypass cap-that is IF I leave out the feedback circuit...

Michael

[Steve - K4HX]

Then don't replace the cathode bypass cap. Without it you get some negative feedback.

Regarding the bypass cap-that is IF I leave out the feedback circuit...

Michael

[The Slab Bacon]

Regarding the bypass cap-that is IF I leave out the feedback circuit...

Michael

Without it you will get a little degenerative feedback. It is your decision, try it both ways and see which way you like it better. Dont expect breathtaking audio from an
HQ-170, though. The IF bandwidth is far far too narrow for anything HI-FI. they are, however a good "battle conditions" receiver. (I have one) You may also want to disable Hammarlund's crappy "Auto Response" circuit. I have yet to find a good excuse for them.
But that is up to you. (if it has the feedback mod, it has prolly allready been done)

[WB4AQL]

Okey doke. Thanks to all.

Michael

[W2PFY]

I too have one of those receivers. It has quite a hum. It does not have a three way plug on it with a ground to the chassis. I need to do that as voltage appears on the chassis.  It is enough to give you a tickle.

Well anyhow the point I'm trying to make here is that I greatly reduced the hum just by reversing the ac plug at the wall socket.