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Author Topic: Hammarlund HQ 129 X MAN AVC BFO problems.  (Read 2356 times)

Posts: 93

« on: March 23, 2013, 06:47:19 PM »

Referbing a Hammarlund HQ120 and have changed all resistors and caps as well as all the other things that are needed and the OLD GIRL works very well on BCB, CW and SSB. The meter is sensitive and very responsive and if anything I think the rig has a bit to much drive. I need to turn down the sensitivity to zero on local AM stations and reduce it often on cw and SSB. When I switch from AVC to MAN, I get a loud burst of volume and then the audio decreases to 0. Waiting for a few minutes, it seem to try and come back. Voltage increases by 2 when switched from AVC to MAN at V1 P8. When I switch from MAN to AVC it takes about 25 seconds to get up to 1.9 volts. In a effort to explain the problem better, here is the routine I go through. I tune to a local BCB station that has a strong sig, with the AVC on and sensitivity set at MAX, I read 1.9 volts at V1 P 8. I then turn the sensitivity to MIN or 0 and I read 29.volts. When I turn the Bfo switch to MAN and the sensitivity at max, I read 4.7 volts at V1 P8. I then reduce the sensitivity to 0 and I read 27.8 volts. At that point I turn the BfO switch back to the AVC and it takes about 25seconds to get up to enough voltage to hear audio. Another thing, I removed the speaker leads from the radio and I still can hear audio coming from T6. You can't understand the spoken word, however you can recognize the tune being played.
- Hope someone will help.
« Last Edit: March 23, 2013, 06:50:09 PM by KD0ACY » Logged

Posts: 4162

« Reply #1 on: March 23, 2013, 06:59:33 PM » takes about 25seconds to get up to enough voltage to hear audio.

Take a close look at the squidmatic for a condenser across the AVC line intended to dampen the response time. There's usually something to reduce the chances of volume overshoot / pumping / or blasting on a station or mode change, and a leaky condenser can take a while to charge. Failing that, look for a resistor intended to pull up, pull down, or bias the AVC voltage that doesn't do that trick like it used to. The AVC line is a simple circuit if you can see the trees through the forest.

I removed the speaker leads from the radio and I still can hear audio coming from T6.

Very common with any audio amplifier that uses a transformer for interstage or output coupling. Also not unusual for the grid of an output tube to talk to you if you listen closely...........    Cool

Never put off 'till tomorrow what you can do the day after tomorrow    -  Samuel Clemens                

Posts: 93

« Reply #2 on: March 25, 2013, 05:34:46 PM »

Thanks so much! After looking at the AVC part I was able to see that there was a 50K resistor missing. I know I put it in there. and I checked it at least three times to make sure I had don it correctly.LOL I am sure a gremlin's removed it when I wasn't looking.
Knowing where to look was a big help and time saver for me, who has little knowledge. The rig work so well, I am doin a jig all round the shop. There is still on item I am concerned about, that is how much power the sensitivity has. Non of my other Hammarlunds have this much drive. I worked 20 40 80 today and they all are booming in. I am using a Slinky for antenna, however I also am wondering If I am looking a gift horse in the mouth.
Again    Thank you .
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