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HQ-145A




 
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KU8L
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« on: January 14, 2020, 10:06:24 AM »

GM All:

Working on a HQ-145A.  WHen it arrived, it all worked but I noticed some audio hum.  THe can cap checked OK but I added a couple of 47MFD caps in parallel in the pi PS filter.  Also upped the value somewhat on the Audio stage bypass cap.  It helped but the Hum was still there. 

The 3 position can electrolytic had been replaced and had higher values than original to begin with.  (the radio came from the Defense Air Technical Center at Wright Pat. AFB in Dayton.  It had some interesting "features" like and N antenna connector and some interesting Meter set-up from the detector.)

So I figured perhaps the can cap was leaky and ordered and installed a new one from Hayseed.  It came with the original values.   Now the Hum is much worse than original. 

Tube swaps make no difference
I did the audio couplate removal mods from the technical mod section here but no difference either way.

Is there some common wire dressing/routing issue with the 145A that makes it prone to ground loops or other Hum source? 

Thanks for any suggestions

Curt  KU8L
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WA2SQQ
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« Reply #1 on: January 14, 2020, 10:31:52 AM »

I would pull out my old audio signal tracer and start looking where the hum is coming from.
Here's what I built - easy and it works great!

Part 1
https://www.youtube.com/watch?v=PbsnI0uM5BM
Part 2
https://www.youtube.com/watch?v=5x5op1V_LFc
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KU8L
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« Reply #2 on: January 15, 2020, 12:16:28 PM »

OK..so I figured out the issue but not the fix yet.

THis Rx has a pi  LC power supply filter.   Most of the rx is either powerd from the output of the pi filter or from the subsequent VR tube.  The 6AQ5 audio output plate is powered thru the primary of the output transformer form the INPUT to the pi filter.  THis point has an expected amount of ripple on it.  This is exactly like the diagrams for most of these HQ's.  Normally any noise from that source is not heard in the output.

I followed the Mod for HQ audio improvements from the mods section here and removed the couplate, etc.  Part of that is to add a feedback path from the plate of the output tube back to the cathode of the driver.  I did this.  as soon as this is installed, it couples the sawtooth PS ripple back to the driver and the hum appears regardless of volume setting.  It is very visible using a scope at each end and is there despite isolating the input from the detector.

I temporarily moved the 6AQ5 plate supply from the input of the pi filter to the output of pi filter and problem is gone.   

Doing this removes the small difference in screen-plate voltage that is normally the case since both would now be sourced from th same point--infact, the screen ends up higher becuse it does not go thru the output trans winding.

I vaqueley remember seeing a note somewhere about the inverse feedback connecting to different point than the cathode of the driver but cannot find it.

I can leave the output amp powered from the filtered end of the pi and use a screen resistor and fix this, but just wondering why this has not been seen by others?

For reference, there is 80mv p-p of 120hz ripple sawtooth at the input to the PS filter at 265V.
at the plate end of the output trnas 238V and similar ripple.  233 on the screen.  THis all when connected as per the original.

What am I missing?

THanks,

Curt
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WA2SQQ
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« Reply #3 on: January 15, 2020, 02:36:46 PM »

Can you provide a link to the mods - I have an HQ-180 that might benefit from some of them
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KU8L
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« Reply #4 on: January 15, 2020, 07:04:45 PM »

Likely the 180 won't benefit from them since the IF bandwidth is limiting.

http://www.amwindow.org/tech/htm/hqaudiomods.htm

Also AB2RA wireless girl site has extensive info on the 180 and 145

http://www.wirelessgirl.com/

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DMOD
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« Reply #5 on: January 17, 2020, 10:07:31 PM »

I usually replace the electrolytic filter can with individual caps of 68 uF@450V caps and the hum usually goes way down to almost inaudible levels.

Most of those transformers were 400Vac CT transformers. If your rectifier tube has been replaced with no change, then check the AC voltage at each plate to make sure you're getting the full plate voltage of 200 volts rms (560 volts peak-to-peak).

The AC ripple in the DC end of things should should show up on a scope as ripple with an 8.3 ms repetition rate. 

Phil
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KU8L
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« Reply #6 on: January 19, 2020, 10:03:51 AM »

Thanks Dmod.

Yep, brand new can.  It was one of Hayseeds'  65mf on output, 45 on input so I added another 60 on the input.   Only minor difference.  This is the A version with SS rectifier.  

I think I have a solution now.   I am going to put a 10W resistor between the rectifiers and the filter input.  This should knock it down some.  Also, without the feedback modification, the hum is now tolerable.  I would say it is somewhat better than stock.  THe resistor will reduce the plate and screen voltages on the 6AQ5 which is OK because the thing has pleanty of output.

ALL of the audio examples I can find take feedback from the out of phase side of the output trans secondary back to the cathode of the driver.  OR, as the Wireless Girl site mentions from plate back to plate.  THese are correct negative feedback--as is the original Hammarlund setup for auto-response.

But it really doesn't matter in this case because ANY small amount of ripple that shows up on the plate of the output tube is going to couple back to the input and show up amplified.

So I will be adding negative FB from the secondary of the output trans, back to the input of the driver or the cathode of the driver to see if it is any better, if not, I'll leave the autoresponse FB net in place but apply it so the FB is there all the time, not just at lower levels.

THe  129x got it right by using two PS chokes!

Curt
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Tom WA3KLR
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« Reply #7 on: January 19, 2020, 12:27:05 PM »

Something that irks me about the HQ-129 and HQ-120 designs is that the audio output transformer is next to the power transformer where magnetic coupling from the power transformer to the output transformer can result in hum.  The output transformer’s core in my HQ-120X is oriented 90 degrees from the power transformer. This may be deliberate.  It looks like the audio transformer in the -145 is underneath the chassis and is a ways from the power transformer.  Some receivers and hi-fi audio gear put the transformers in opposite corners of the chassis, not necessarily required, but was guaranteed to eliminate the magnetic coupling.

Another source of hum in the speaker is a ground loop in the ground side of the output transformer secondary to the speaker.  It looks like the output transformer in the -145 is close to the rear and the secondary wire to ground might be connected directly to the speaker jack ground; this is the best connection as it eliminates a ground loop with the filament currents.

If either of the two above hum coupling modes exist, they can be listened for by pulling out the HV rectifier and turning on the radio and listening in the speaker for hum.  Ideally there should be no hum in this test.
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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
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