Zenith 9S232 sensitivity control - Printable Version
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+--- Thread: Zenith 9S232 sensitivity control (/showthread.php?tid=18557)
Zenith 9S232 sensitivity control - NostalgiaRadioTime - 06-17-2018
Having two issues with this chassis I just finished re-capping. First is the sensitivity control, the other is lack of volume.
The control that was in the radio works as far as the detents and stop, but the switch itself connected to the power leads is what is sticking and only making sporadic contact. I tried some deoxit which helped for a bit but as soon as I put it back together again, it still stays on in the off position. The whole thing is riveted with teeny tiny little brass rivets that are quite long.
Just wondering what that second "distant" position really does if you don't have the special Zenith antenna that was available back in the day? Is there a work around to where you can just eliminate that second position and use a traditional off/on pot? (I tried this, just connecting the power wires to the off/on switch of an off/on/volume pot, leaving that extra "distant" wire disconnected. The radio plays and receives stations with the antenna, but not very loud. If I connect the antenna to the grid cap of the 6L7 tube I get lots of volume.)
Am I missing something by not having that off/on-local/on-distant switch in the circuit that could be causing the lack of volume? Or where else could the culprit be if I can get lots of volume by just connecting the antenna to the grid cap of the 6L7? Connecting the antenna to the grid cap of the 6K7 RF has no effect. I did swap out tubes in that section to no avail. Tube shields are all in place.
Paging Dr. Terry
Here's a link to the schematic for reference: http://www.nostalgiaair.org/pagesbymodel/495/M0025495.pdf
RE: Zenith 9S232 sensitivity control - tbone - 06-18-2018
If the RF amp is running in Local, I would expect you might get more volume by connecting the antenna to the 6L7 cap. Most likely you might have a open coil in the RF section. A simple ohm meter check is warranted. In my 12S model, coil 1 had an open primary. This caused exactly what you are experiencing. A lightning strike or corrosion are likely causes of this coil to be open. This radio has excellent sensitivity when working right.
If you can't make the switch work, one thing that you can do is leave the switch in the distant position, or I should say wire it so that it is in Distance mode. I believe it is in the open position. You can temporarily use a gator wire between the two points for verification and to test Local/Distance mode so you are sure the RF Amp is working. Most cases It's fine in distance mode unless you are next door to a 50KW station.
Keep us posted.
RE: Zenith 9S232 sensitivity control - Radioroslyn - 06-18-2018
https://www.youtube.com/watch?v=f5e-2KmOnWA
So ah where is switch #8 it's seems to be invisible??
RE: Zenith 9S232 sensitivity control - Eliot Ness - 06-18-2018
I had a hard time finding it too Terry, but although it's not labeled SW 8, it is by the power transformer, but I can't see how that is controlling sensitivity (local/distance) unless there's another part of the switch I haven't found yet...... I just found it:
I'm thinking like tbone, an open coil before the 6L7.... also check the components in the area like R2, C2, C3, etc.
RE: Zenith 9S232 sensitivity control - NostalgiaRadioTime - 06-18-2018
LOL! Love Frank "Yeeesssss" Nelson! John is right, Terry. That's exactly where the invisible #8 is hiding.
And Tony, you were right on the money. Look what I found on the RF coil:
There are actually two separate broken wires on the coil. This one is just the most visible. It may be hard to find the other stubs where these wires came off of. Will have to take the coil out tonight and get out the magnifying glass. I suppose I could just use one of the universal RF coils from AES, too?
How did you repair yours, Tony?
RE: Zenith 9S232 sensitivity control - tbone - 06-18-2018
My chassis had the antenna coil inside a can on the chassis. I was lucky enough to find only one end broken on the primary. I scraped enough enamel off and soldered an extension on it. Then hot melt glue to hold in place.
I think your coil pictured is #14 the RF choke. These are notorious for being broken or eaten by mice. They will cause similar issues like you described. B+ is supplied to the RF amp plate through it. It’s very likely you have no plate voltage. Someone might have a better idea how critical it is, but with multiple taps I suspect it’s important.
I’ll look in my shed on my junker 1204 chassis.
RE: Zenith 9S232 sensitivity control - morzh - 06-18-2018
The switch is that power switch knob on the right saying Local/long distance. In LD position it shunts that resistor.
RE: Zenith 9S232 sensitivity control - NostalgiaRadioTime - 06-18-2018
Today's progress....I took the sensitivity control switch apart and did some cleaning, working it back and forth, and finally was able to get it to click into place in the off position as it should. It wasn't broken after all...just majorly dirty.
Before I did anything with the RF coil, all the set would do is squeal in the distant position. After removing the RF coil, I found out just how bad it was. Multiple strands of wire hanging off of the secondary winding. Not good
No matter what combination of wires I tried, I could not get continuity through that winding. As a result, I had nothing to go on as far as reconnecting the broken ends. It would've been a shot in the dark. So I went with one of the universal replacement RF coils from Antique Electronic Supply instead.
With that new RF coil wired in, I can now find a distinct change in signal strength from local to distant. It's quite a bit louder when I switch to the distant position. So I feel confident that at least that part is working. When I remove the antenna wire I have clipped on the A terminal, it gets much quieter, so it appears the antenna coil is working as it should too. The set isn't as sensitive to receiving distant stations yet, but I suspect I need a longer antenna wire than what I'm using. That, and perhaps a better alignment might help. I'd have to think if the RF and the ANT coils are now working, the DET coil would have to be as well?
Getting closer to the finish line on this one. It was a beast when I got it. Major filth and the chassis was pretty badly mangled....had to do some sheet metal straightening first. But it will be a nice upgrade to swap the 7S232 chassis in my Walton set with the 9S232
RE: Zenith 9S232 sensitivity control - tbone - 06-19-2018
I’m glad you are making progress. If you find that you need a true replacement RF choke let me know. I might have one.
One other thing I’ve found with the last couple of Zeniths I’ve worked on is bad mica caps. I’ve had intermittent coupling caps, maybe someone rotated them years ago, and the plate bypass cap in the audio section. Yours would be C12 the 500 PF unit. When these leak, the audio starts to lack highs. Just something to keep in mind.
I know you’ll really enjoy this radio. Even though it has a single ended output section, it will rock the house.
RE: Zenith 9S232 sensitivity control - NostalgiaRadioTime - 06-19-2018
Thanks for the offer, Tony. It couldn't hurt having the original RF coil. Let me know if you find one that tests good on your junker chassis.
Will check out that mica, too. I want to check a few of the non dogbone resistors too again that I didn't replace. Sometimes those little 1/4 watt jobbers can be even further off than the big 1 watts.
RE: Zenith 9S232 sensitivity control - tbone - 06-19-2018
The weekend looks good for me to check my chassis. To much going on during the week with work, PT, yard work, and trying to get some exercise. I was looking at the chassis last weekend and it’s easily accessible. I only keep it in case I need a power transformer or major part for my 12-S-265.
RE: Zenith 9S232 sensitivity control - NostalgiaRadioTime - 06-19-2018
No rush, Tony. I appreciate your kind offer to look.
I have to ask....when you're looking under your junker chassis, does it happen to have the off/on-local/on-distant sensitivity control? Mine is still acting finicky.
RE: Zenith 9S232 sensitivity control - Arran - 06-20-2018
I would try melting the wax off of the old RF coil to see if you can find where the end may have broken off, it may take some poking around with a exacto knife or a small screwdriver to find it. If the break is on the outside you can unravel about 1/2-1 turn of it to get enough wire to reconnect it, if it broke on the inside that will be much harder. It's kind of a vulnerable spot to place such a thing so I expect it was some sort of mechanical damage rather then corrosion, perhaps when the chassis was mishandled in the past.
Regards
Arran
RE: Zenith 9S232 sensitivity control - NostalgiaRadioTime - 06-21-2018
Thanks, Arran
I did exactly that but there were too many loose strands of wire to determine which were the correct pair. Plus the coil was wound cross ways not straight around making it even more complicated.
The AES replacement seems to be working just fine so far.