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 Post subject: Hallicrafters S-85 with no response half the broadcast band
PostPosted: Sep Tue 21, 2021 9:14 am 
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Joined: Mar Sun 07, 2021 1:51 pm
Posts: 151
Location: Wake County NC
I have a new, old radio that has some issues. One is half the broadcast band appears to be dead. I saw a thread in here a while back that addresses this issue, but I can't for the life of me find it. If anyone can redirect, drop a link, or add a quick comment about possible causes, I'd appreciate it. I haven't adjusted the tuning on this one yet. I was hoping to resolve any problems before attempting that. If you believe the tuning could be the problem I don't know what would have caused it unless a bad coil or cap, I suppose. Any help is appreciated.


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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Tue 21, 2021 2:10 pm 
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Location: Suburban Chicago
Like many old shortwave radios this one splits the AM broadcast band between two settings of the band selector knob. So, when you say "half the broadcast band" do you mean half of the tuning range on band 1 or do you mean you get reception on band 1 but not band 2? I restored an S-38 that had the latter problem, intermittently. It turned out to be a bad connection to the band 2 oscillator coil.

I have no experience with a radio that only works on half of one band. I would look first for the LO signal. You can use a lightly coupled scope probe (I usually clip the ground lead to the probe tip to form a little loop and set it near the LO tube or circuitry) to see if the LO is dropping out for part of the band. People who don't have scopes will often use another radio that can tune to the LO signal to sniff it out and verify if it is working or not. That might tell you generally where to look, the LO or the RF amp. I don't believe anything after the converter knows what band the radio is on or where the tuning cap is set so it seems most likely to be in one of those circuits. I would like to say that a bent plate on the tuning cap could short itself out for part of the tuning range but that would affect all the bands, not just the broadcast band.

I know I haven't given you much help, perhaps someone who has fixed a problem like this will chime in!


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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Tue 21, 2021 5:30 pm 
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Joined: Mar Sun 07, 2021 1:51 pm
Posts: 151
Location: Wake County NC
Thanks for the tip. I'll check the LO next and see how it's responding. I use my TinySA with antenna to check out LO's so I'll see what's going on with this one. What i meant is that half of the frequencies, or one side of the freq width of the band has dropped audio output to the point of hearing next to nothing. Just some faint static. Thanks again for your help.


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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Tue 21, 2021 10:06 pm 
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Joined: May Thu 17, 2018 12:16 am
Posts: 236
Location: Edmonds, WA USA
Only on the Broadcast band? If so that rules out the tubing variable cap bent plate theory. Does the bandswitch dplit the BC band or is it all on one position . We need more information.

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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Wed 22, 2021 2:12 am 
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Joined: Mar Sun 07, 2021 1:51 pm
Posts: 151
Location: Wake County NC
Broadcast band is all one setting on the band switch. I found the Oscillator padder was out a bit for that band and actually can get some faint station activity now, but compared to the high side of the band the signals are just north of nonexistent. I aligned the high side Osc, Mix, and antenna with little improvement. It's really fine. Just the Low side that's faint. I checked the oscillator and it seems like the oscillator continues to function throughout the band, so that's not it. I'm going to go through a complete alignment tomorrow and see if that helps. I'm wondering it it could be a marginal converter tube, bad cap, compromised coil or variable capacitor, poor coupling in the osc. circuit. Trying the alignment and I guess I'll move on from there.


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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Wed 22, 2021 2:54 am 
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Joined: May Thu 17, 2018 12:16 am
Posts: 236
Location: Edmonds, WA USA
Sounds like you are using broadcast stations as standards. What is your antenna? What are the frequencies of BC stations in your region as per the FCC documents available on the web.

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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Wed 22, 2021 3:27 am 
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I would suggest replacing the converter as a first step. When they get weak, the lower part of the BCB is the first to suffer.

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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Wed 22, 2021 6:34 pm 
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I mentioned using a scope or another radio to look for the LO signal rather than a spectrum analyzer since relatively few here have a SA. So of course you are one of the ones with a TinySA, which should work fine for that!

According to the manual for the S-85 the broadcast band is split between bands 1 and 2. It is mostly band 1, 538 to 1600 kHz, but band 2 covers 1550 to 4600 kHz. Since your issue is at the low end of the band this raises the possibility of finding a station in the 1550 to 1705 kHz range to use as a test signal. If you cannot hear anything above 1600 but you have some reasonably strong local stations then you may have an issue with the tuning cap since the problem exists on at least two bands. A station between 1550 and 1600 would be perfect since you should hear that on both bands if the problem were with the converter tube, for example, but only band 1 if it were a tuning cap shorting out when the plates are fully or mostly engaged. Now I did not check to see if you say where you are from before posting this. In the US and some other countries the BCB goes up to 1705, elsewhere it stops at 1606 which doesn't prevent you from performing this test but it does reduce your chances of having a station in the interesting frequency range. No problem if you have a signal generator you can use for the test.


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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Thu 23, 2021 5:47 am 
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Joined: May Wed 06, 2009 7:01 pm
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I ran into that identical problem which I finally tracked down to a series mica cap with a poor connection inside. Worked fine on the high side but not on the low side of the band. Mica caps do go bad but the cause isn't necessarily leakage as in paper/wax caps.

Also a similar issue when someone with a golden screwdriver had aligned the oscillator on the wrong side. Tracking was well off as you might expect. A check with a frequency counter revealed the tracking problem.

-- Rich


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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Thu 23, 2021 9:53 am 
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Joined: Mar Sun 07, 2021 1:51 pm
Posts: 151
Location: Wake County NC
Ok, all of this was really helpful. I was afraid I had a discrete component failure such as a cap, but I haven't found it yet. However, things have improved considerably since I did the complete alignment. It turns out someone who worked on the radio before me had aligned the .6Mhz slug to a harmonic/image instead of the primary freq from the sig gen. Once I realized this I went back through and re aligned the entire radio and sure enough for one of the alignment frequencies I had used the harmonic instead of the test freq. So, now I use my TinySA at every tuning step and check the output from the LO to make sure it's test freq plus IF or depending on the upper freq, minus the IF. It worked very well and the radio sounds much better. However, there is still a drop off on the low frequency end of the broadcast band on band 1 and no perceptible drop off on the rest of the broadcast band of radio band 2. Still looking for that component failure. Would it be unreasonable to just replace all of the mica caps in the tuning section? I don't see how I'm going to find it because I have doubts that the fault is detectable by my LCR meter. Thanks again.


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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Thu 23, 2021 10:00 am 
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Location: SoCal, 91387
Mica caps are generally reliable. Again, try replacing the converter tube. What you are experiencing is typical when they get weak.

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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Thu 23, 2021 11:20 am 
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Joined: Mar Sun 07, 2021 1:51 pm
Posts: 151
Location: Wake County NC
I've also heard that mica caps don't commonly go bad. It's the uncommon one I'm worried about. As far as replacing the converter, wasn't possible until just yesterday. I know have one that is proven to work properly. I'll give that a shot now. Thanks for the reminder.


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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Thu 23, 2021 10:06 pm 
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Location: Suburban Chicago
I tried to fix a problem with an S-38 on one band by changing out micas. They new ones sure look pretty, if inauthentic. They did not fix the problem. I even found that one of the ones I pulled out was a little leaky and changing it did not solve the problem. Fixing the dodgy solder job on one end of the LO coil winding for that band did the trick.

I saw a video of a guy fixing an S-40 of one generation or another. He had no operation and was drawing a lot of current in one circuit. That did turn out to be a shorted mica. It was visibly cracked and bulging. However one or two others here have reported fixing radios by changing micas. They can go bad and we may at long last be approaching the end of life on some styles of mica caps. I don't think we collectively have a way to track them down because we have never had to do that before. It is possible that we will need to learn that skill soon now....


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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Thu 23, 2021 11:05 pm 
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Location: SoCal, 91387
I would much rather see him swap the tube, before attempting to change the mica caps, ESP if there is no obvious physical damage to any of them.

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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Fri 24, 2021 2:04 am 
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Joined: Mar Sun 07, 2021 1:51 pm
Posts: 151
Location: Wake County NC
Switched the tube, no change. It's likely I'll go with it as it is. I get some reception on the low section of the broadcast band now after the alignment, so I may just set it aside for the time being. It's usable. I've got a HT37 and others waiting their turn. So, I'm moving on and maybe get back to it eventually, or just sell it for parts. I've got one S-85 working great now. So, I'm in good shape.

Thanks again for everyone's help. It went from working barely to working pretty good now, so it's a win. Learned a lot in the mean time, as well.


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 Post subject: Re: Hallicrafters S-85 with no response half the broadcast b
PostPosted: Sep Mon 27, 2021 5:47 am 
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Joined: Apr Sun 08, 2007 6:47 am
Posts: 5203
Location: British Columbia
If the S-85 uses a pentagrid convertor tube, look for a bad resistor in the oscillator tank circuit for the AM broadcast band, I've had one go out in at least two AC/DC radios, one would change value randomly and work some of the time, then other times I couldn't get the top end the band to work.
Regards
Arran


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