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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Wed 04, 2018 2:17 am 
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Hey Rodger -

So the voltages on the two RF amps are as follows:

Pin 2: 2.2v at full sensitivity, ~38v at zero sensitivity. (same for both 6AG5s)

Pin 6: 121/153 at full sensitivity, on 1st RF amp there's no change, but on second RF amp the voltage goes to 260v (on all bands.)

I notice on several of the schematics that there is a 100K resistor in that circuit off of the sensitivity control, but neither my radio nor the schematic that came with it has that component.


The sensitivity control also has an expected effect on the S-Meter. When I rotate it to min sensitivity, it deflects all the way to the right. Returning to max brings it back into the expected area.

There definitely is some change, but it's not cutting off the tubes completely.

baffling.


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Wed 04, 2018 2:49 am 
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The screen voltage is sourced separately for the two RF stages so that difference in screen voltage behavior is normal in this case. The first RF amp gets its screen voltage via dropping resistors off the regulated B+ bus and it will remain fairly constant while the second RF stage is using a much higher value screen resistor directly off the B+ bus and its screen voltage will vary widely based upon tube conduction.

The change in S meter deflection is normal with the change in the RF gain control and expected with this type of RF gain control system but what isn't normal is the sensitivity should be pretty much nil with the RF gain all the way down and the increase in screen current you saw with the second RF stage is indicative of a tube that has the current flow greatly reduced causing an increase in screen voltage since there is now little drop across the screen resistor.

I bought one of the SX-62 series that came with substitute tubes for the 6AG5 stages, maybe 6AH6 tubes if I recall correctly, and with these tubes the RF gain control had little effect until it was reduced to about the last 30% of its rotation but it still resulted in near cutoff even with those tubes.

Hopefully someone else has a good idea to explain this baffling behavior but with that much cathode bias being applied via the RF gain control sensitivity should be very low at minimum setting and certainly enough to prevent problems with CW/SSB demodulation. Check around C80 (the BFO coupling cap into the detector) to make sure nothing is amiss here. Perhaps the issue is lack of BFO injection level rather than too much pre-detection gain.

Rodger WQ9E

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Wed 04, 2018 6:02 am 
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I didn't bring this up earlier because I do not have my notebook that documented this anymore. However, I might as well mention that there was a factory error in the SX-42 production wiring that created increased distortion. There is an "unused" lug on a tube socket which the factory used as a tie point for components, common practice at the time. Except that in this case, for reasons I can't recall, the socket lug actually does make contact with some element of the tube. I believe this was in the IF section.

I ran across this in the files of an ancient Hallicrafters service center where I once worked, a decade or so after Hallicrafters went out of business. I don't know whether the mistake occurred in design or the tube manufacturer started tying that pin to some element in later production. I also don't know if the issue was corrected in later production runs. Manufacturers would never admit it and tell us that a mistake was made in design or production. Instead we would get bulletins saying something like "Engineering has found that performance can be increased by moving wire XXX to point yyy, and it is suggested that this be done whenever the equipment is otherwise in your facility for service." One of the regional shops discovered this and notified Hallicrafters, and as I recall I think they were ignored. It's bad publicity to admit you messed up. Part of the problem was the pressure to get the SX-42 to dealers. They were not de-bugged at the factory first, hence the scary level of RF section re-work in the many pages of service bulletins on the SX-42 which were sent out...

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Wed 04, 2018 1:01 pm 
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Have you checked the Noise Limiter switch contacts? I posted, above in this thread, that I had sticking contacts in the switch contacts on my SX-62, activating the noise limiter, and causing distortion. To make things even more interesting, the fault was intermittent, making it even more difficult to spot. Just throwing my $.02 worth into the mix. GL!

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Wed 04, 2018 1:55 pm 
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Geoff Fors wrote:
It's bad publicity to admit you messed up. Part of the problem was the pressure to get the SX-42 to dealers. They were not de-bugged at the factory first, hence the scary level of RF section re-work in the many pages of service bulletins on the SX-42 which were sent out...


George, I found Sevice Bulletin #21 over at BAMA: Is this what you're talking about, or are there others? I thought it was a simple document until I scrolled down and it went on and on... YIKES!

-Tom


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Wed 04, 2018 2:13 pm 
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One of the early issue of Electric Radio had a reader question asking if anyone remembered which tube it was that created the issue from a tie point that was internally connected in later revisions of that tube. He recalled going with his father to a Hallicrafters service center when he was a young child to have their 42 fixed but he couldn't remember which one.

I remember reading through SB 21 when I got my first SX-42 and thinking they really had some band 4 issues early in the run.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Thu 05, 2018 3:42 am 
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Success!

I went back through with the Rider manuals and voltage charts, and began to get frustrated because everything was reading like it should. Then I remembered something from repairing my Swan 400 -- I could never get the volume to go completely down.

As it turns out, it is the 10K pot that is the sensitivity control. Apparently, it was enough to kill both RF amps, but it was actually shorting out internally so that I wasn't getting the full resistance when it was under voltage and load of just the second RF amp. I put a 10K pot on the bench with some alligator clips, and boom: full sensitivity control restored on bands one and two.

I remember reading somewhere that this was a particular type of pot -- right now I just have a generic 1/2 watt linear on it. Does anyone have information on the appropriate replacement for it (power and taper?) I can get it from my parts supplier on my next mass order.

Thanks to all for the suggestions by the way. I did find some "odd" wiring in investigating this, and I have restored things to proper configuration according to the schematic.

--Jeff


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Thu 05, 2018 3:59 am 
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Tom, I think it was Service Bulletin 21. I don't have my file at hand. An amazing number of mods to the RF stage. I always thought "If I do all these, what are the chances this receiver is going to work right when I finish?" There was no customer who would have forked out for the labor to do the mods anyway, so I have the feeling that hardly any of the mods were done in the field, at least by authorized shops.

Yeah, I was going to mention checking the pot, although my thought was that someone might have put the wrong value into it. One of the most vexing troubleshooting sessions I had was on a multiband VHF radio where I could not get the squelch to work right. Eventually I figured out that the pot, which looked original and was not obviously messed, with, was actually a replacement with a far from correct value. After so many years it seems we now have to check the pots in equipment on the bench for the correct value, shorts to case (tin whiskers) and so forth. Just as mica capacitors are showing up bad more frequently, making a "trust but verify" situation.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Thu 05, 2018 4:22 am 
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Jeff,

Good troubleshooting and glad you found it!

I believe the pot should be a 2 watt wirewound type, 10K linear taper. This is one of those places where you need to use a wire wound pot because there is DC on the pot and a carbon type gets very noisy quickly with DC current flow through it. A 2 watt pot was commonly specified and although that much power rating generally wasn't needed I suspect wire wound pots weren't available with a lower power rating. You can of course use a higher power rating.

As Geoff stated you have to look carefully to catch problems, particularly when a previous owner made a bad substitution. The Johnson Ranger that came with my Desk KW was a prime example because it had a repetitive intermittent hum issue that wasn't bad but was noticeable with hum appearing on the carrier. The first time it showed up I found a clamper tube with a little heater to cathode leakage and replacing it seemingly cured the problem but it came back in a couple of months and this time it was the VFO tube instead of the clamper that solved the problem. A few months go by with a repeat and this time the keyer tube seemingly was at fault from heater to cathode leakage. I suspected high heater voltage was a problem but it was right where it should be. A few months go by and the problem returns and I finally found the real problem which was a substitute low voltage choke. It was exactly the same size, shape, and color of the Johnson original but a closer look indicated it was a replacement with a higher current rating but less inductance. It didn't provide quite as much B+ filtering as the original but as long as everything else was perfect the rig sounded OK. Once I replaced that choke I quit having to chase phantom hum in tubes. The choke looked right but it was the radio version of you can't judge a book by its cover.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Sat 28, 2018 11:00 pm 
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So I can't leave something alone... The OM says "I don't are if the SX-42 hears SSB or not...", but I'm a perfectionist.

Here's the latest trouble: I get pretty severe distortion on signals in the CW position still. I know how to tune SSB signals on these old radios; I am twisting all the knobs accordingly!

So here's what has been done:

Rebuilt entire BFO circuit, including can and new tube socket.
Replaced all resistors, coupling and interstage capacitors in IF circuit.
Rebuild 3rd IF can with new capacitors
Rebuilt discriminator can with new capacitors

I have the "old" BFO connection (pin 5 of the 6H6 - 89D210-D).

So far, no luck in removing the distortion on SSB and CW signals. It almost sounds like RF on the signal, except it's coming from inside the receiver.

Looking for any suggestions as to where to go next.

Jeff


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Sun 29, 2018 12:31 pm 
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As is the case with all of my receivers that don't use AGC when listening to SSB, I need to reduce the RF gain considerably in order to obtain clear, undistorted SSB audio on my SX-42.

Dave


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Sun 29, 2018 2:16 pm 
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Is it a raspy sound? This sometimes come from heater to cathode leakage in the BFO or HFO because oscillators are very sensitive to any hum modulation via either ripple from the B+ supply or leakage to the cathode via the AC heater. This usually isn't readily noticeable on AM but is very evident in CW/SSB.

Otherwise with vintage receivers of this design for CW/SSB I run with AGC off, audio at 65-80% of maximum rotation and ride the RF gain control to provide the desired audio level.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Sun 29, 2018 2:26 pm 
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And as Rodger will usually suggest, you can check the purity of the oscillator signals by monitoring them on another receiver.

Of course for the BFO you'll need to either have a receiver capable of hearing 455kc or monitor the harmonic at 910kc.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Sun 29, 2018 2:53 pm 
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Good call Chuck!

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Sun 29, 2018 2:53 pm 
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Rodger -

Definitely a raspy sound. It does almost sound like 60 Hz ripple (yes, all filters have been replaced) And yes, I've backed off on the sensitivity control to keep the distortion out in that regard. And also yes, AM sounds OK.

I did an experiment last night to see if I could start isolating it. I took my CE sideband slicer and connected it to the 3rd IF transformer. The CE has a headphone jack, so I connected the phones directly to it, bypassing everything after the 3rd IF. As expected, the rig is very sensitive and I'm able to tune SSB signals well, but I still have the raspy sound on the audio. I usually connect the slicer to the "phono" input on the other SX-42 I have, which kills the BFO anyways, so the 7A4 is out of the circuit completely. The snag is definitely between antenna and IF out. I'm going to have to see if I can come up with another 7F8 and I'll give that a try.


Jeff


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Sun 29, 2018 3:02 pm 
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And another 7F8 doesn't make any difference...


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Sun 29, 2018 4:50 pm 
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As Chuck noted listen critical to the HFO in another receiver to see whether it is clean. Use the other receiver's BFO and listen for any raspiness on the HFO.

If you have a choke take a look at the well filtered side of the B+ (after the choke) to see how the ripple appears. A weak rectifier can create an issue where one half of the AC cycle has significantly lower output than the other resulting in ripple that gets by the otherwise adequate filter. Make sure that the filter caps are well attached and grounded (good solder joints, particularly at the ground side).

A meter set to measure AC is somewhat useful in assessing ripple but some digital units will give very odd readings with a mixture of DC + an odd superimposed AC waveform so a scope is preferable.

If you have a scope available inject a clean unmodulated signal into the front end and trace from the detect forward to detect where the distortion begins. Using a scope in the front end requires a very low capacitance probe along with a highly sensitive scope so start tracing at the high level IF end and work forward as needed.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Sun 29, 2018 7:57 pm 
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Jeff,
In addition to Rodger's suggestions, and in case you didn't hang on every word of my saga above like everyone else :), I had a similar symptom with mine.

In my case, using the above HFO monitoring technique, I confirmed that the HFO was not producing a nice clean tone in the monitoring receiver.

Then, focusing my efforts on the 7F8 and preceding circuitry, isolated the problem to the 7F8 circuitry specifically.

Somewhat by accident I found that the 7F8 was not making good contact with the socket pins apparently, even though cleaning the pins and socket was one of the first things I'd done in restoring this beast, er, ah, beauty.

Since I'd swapped 7F8s in the troubleshooting process, I cleaned everything thoroughly again and cleared up the distortion - for now.

As you have probably concluded by now, the 7F8 area of the SX-42 is very fussy!

Happy hunting.

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If talk is cheap, it's because the supply usually exceeds the demand.


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Apr Mon 30, 2018 12:25 am 
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Advice taken.

I put a small loop on the SDR antenna input and went hunting. I placed the loop near the 7F8 and I can see the frequency move back and forth on the waterfall as I turn the dial. I am hearing the distortion on the HFO signal on the SDR...

Is the oscillator frequency the fundamental of the dial frequency, plus or minus 455?

So now, I need to figure out where it's coming from. Replacing the 7F8 with a known good one didn't help, nor did deoxit drops in the socket and a cleaning. I've replaced the bypass caps in the circuit, but as I made note of in a previous post, I may have had a bad bunch.

In looking at the oscillator side of the tube, it doesn't look like there's much to go wrong.


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