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 Post subject: SX-28A- Sensitivity & AF Question
PostPosted: Mar Mon 20, 2017 9:26 pm 
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Joined: Jun Fri 27, 2008 3:10 am
Posts: 91
Location: Killingworth Connecticut 06419
So I have spent hours on my SX-28 acquired 6 weeks ago and ALL caps and out of spec resistors have been replaced. It works very well on the first 3 bands, up to about 12 MHZ but I as I move past there not only do signals get weaker but the audio goes down. In fact there seems to be little- no background noise.

For example, on the BC band, stations are loud, and I can them across the band. If I go up to 20 meters, the speaker is pretty much dead. If I turn on the BFO I can locate several signals but even cranking AF gain up they are not very loud. But there is little, or no background noise. Almost like there is no IF gain at the higher frequencies.

I'm used to the older rcvrs being less sensitive on the higher bands, but still having background noise.
Was the internal design of this particular receiver so good that there was little background noise, or is something else going on?

Any thoughts, or is this the nature of this beast and current band conditions?

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Carl
WA1KPD


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 Post subject: Re: SX-28A- Sensitivity & AF Question
PostPosted: Mar Mon 20, 2017 10:22 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 4218
Carl,

Check the voltages on the HFO and if they are good try a substitute tube at this position. Tube performance often falls off at higher frequencies with a failing tube and a tube checker will NOT catch this fault. With an antenna connected to mine there is a definite noise peak when the antenna trimmer is adjusted on the higher bands so something is up with your receiver.

Rodger WQ9E


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 Post subject: Re: SX-28A- Sensitivity & AF Question
PostPosted: Mar Mon 20, 2017 11:41 pm 
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Joined: Oct Sun 15, 2006 12:57 pm
Posts: 5075
You make no mention of doing any sort of alignment. Have you done any alignment yet?

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 Post subject: Re: SX-28A- Sensitivity & AF Question
PostPosted: Mar Tue 21, 2017 1:40 am 
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Joined: Jun Fri 27, 2008 3:10 am
Posts: 91
Location: Killingworth Connecticut 06419
Mike,
All aligned

Rodger,
I have replaced the Osc tube, but I can try another.

I will check the OSC pin voltages, and let you know what I find.

Thanks for the suggestions

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 Post subject: Re: SX-28A- Sensitivity & AF Question
PostPosted: Mar Tue 21, 2017 2:00 am 
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Joined: Mar Wed 16, 2011 10:44 pm
Posts: 443
Location: Peekskill, NY
Oddly this is the reverse of my sx28 - which is a bit weak on the BC band but good at the higher
bands. Suggest you concentrate strongly on the first RF stage, as that's probably your issue.

The alignment on the higher bands is strongly dependent on the having the bottom plate in place
when it is being done, and having the set inside its case and up at temperature.


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 Post subject: Re: SX-28A- Sensitivity & AF Question
PostPosted: Mar Tue 21, 2017 2:04 am 
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Carl,

You might also try substituting at the mixer position also; I did run into the same issue with a SP-600 where the 6BE6 mixer ran out of steam above 14 Mhz. but this sudden loss of gain with increased frequency is usually a HFO fault.

If the tube substitution doesn't cure the problem try injecting a signal at the control grid of the two RF tubes and at grid pin 8 of the mixer tube to locate problems in the coils or band switch.

Rodger WQ9E


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 Post subject: Re: SX-28A- Sensitivity & AF Question
PostPosted: Mar Tue 21, 2017 3:57 am 
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Joined: Aug Fri 29, 2014 6:17 pm
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Some folks run a 6SB7Y mixer tube, and even a 6SB7Y osc, to hit the mixer a bit harder.


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 Post subject: Re: SX-28A- Sensitivity & AF Question
PostPosted: Mar Thu 23, 2017 9:52 pm 
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Joined: Jun Fri 27, 2008 3:10 am
Posts: 91
Location: Killingworth Connecticut 06419
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Rodger,

I checked the resistance on the osc and mixer and found that 3 pins (tied together) were off by a significant factor.
Unfortunately to get to the resistors I had to get under the osc trimming caps and coils. Luckily I did not have to pull everything apart this time but it was a job. A 6.8K resistor had drifted to 55K. Interesting the 10 ohm one was right on. With my big paws it was a chore but I got it replaced.

Interestingly, I found a wire in that compartment that was pinned between a bracket and the top of the chassis. See attached picture. It must have been that way since assembly I was going to loosen the bracket and move the wire, but the screw heads are hidden under a sub chassis. So I would have had to pull the whole osc section apart.
Considered replacing the wire but have been burnt enough times by just making one more little improvement and given that tiny working space I decided that it is not subject to streess or movement and has been that way for 75 years, so I would leave well enough alone. Surprised this got through QC.

Net effect, some improvement on the two higher bands, but not great. Not really an issue as I don't roam up there much. However 40 meters sounded much better so you sure nailed an issue.
I'm done, I've got about 5 weeks into this project and its working very well for my purposes. I'm going to pair it with the HT-9 on 160.

73
Carl

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 Post subject: Re: SX-28A- Sensitivity & AF Question
PostPosted: Mar Thu 23, 2017 10:37 pm 
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Carl,

Glad it is starting to work better and it will look great with the HT-9. I guess the builder really wanted that wire to be anchored in place. It will probably remain OK for another 80 years since that wire cover is pretty tough and the surfaces clamping it aren't sharp. It won't be like the kit built Eico signal generator I came across that ran the B+ to the oscillator compartment under a tightly attached shield instead of through the intended hole because that one created a dead short.

There was a Hallicrafters service note reprinted in CQ to improve 10 meter and noise limiter operation; I will send a PDF to your email for your files.

Rodger WQ9E


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