Hi All,
I am still beting my head on the radio which had the freqency agile 12AT7. The 12AT7 was of course OK.

I suspected that the capacitor across the LO coil may be the culprit, so I replaced it.
The radio did something I did not think was possible. The low end of the dial had the stations of the upper part of the dial, the upper part of the dial had the stations from the low end! Somewhere in the middle of the dial I could get the station at 107.9 tune a bit higher I get the station at 88.5MHz.

Since the capacitor I took out passed the freeze test and tested as good as the capacitor I took out, I put it back in.
that problem went away, but now I have the problem that I can't get a station above 106.7 Mhz because I hit the top end stop. Channels are spread too far apart. I can get all the low end.
I thought that IFs were out of alignment. I realigned them and it did no good. I still can't get above 106.7.

I have been doing radios quite a bit, but am weak on front ends. Anyone have an idea?

Best Regards,

Bill Grimm

:Hi All,
: I am still beting my head on the radio which had the freqency agile 12AT7. The 12AT7 was of course OK.
:
: I suspected that the capacitor across the LO coil may be the culprit, so I replaced it.
: The radio did something I did not think was possible. The low end of the dial had the stations of the upper part of the dial, the upper part of the dial had the stations from the low end! Somewhere in the middle of the dial I could get the station at 107.9 tune a bit higher I get the station at 88.5MHz.

:
: Since the capacitor I took out passed the freeze test and tested as good as the capacitor I took out, I put it back in.
: that problem went away, but now I have the problem that I can't get a station above 106.7 Mhz because I hit the top end stop. Channels are spread too far apart. I can get all the low end.
: I thought that IFs were out of alignment. I realigned them and it did no good. I still can't get above 106.7.
:
: I have been doing radios quite a bit, but am weak on front ends. Anyone have an idea?
:
:Best Regards,
:
:Bill Grimm
:
Check your local oscillator reaquency. If you have it 10.7 mHz below the station frequency, would it not reverse the hetrodyne? And if the local oscillator quits, you have a 10.7 mHz TRF radio. I would use the old "second radio local oscillator" check.
Lewis

:Hi All,
: I am still beting my head on the radio which had the freqency agile 12AT7. The 12AT7 was of course OK.
:
: I suspected that the capacitor across the LO coil may be the culprit, so I replaced it.
: The radio did something I did not think was possible. The low end of the dial had the stations of the upper part of the dial, the upper part of the dial had the stations from the low end! Somewhere in the middle of the dial I could get the station at 107.9 tune a bit higher I get the station at 88.5MHz.

:
: Since the capacitor I took out passed the freeze test and tested as good as the capacitor I took out, I put it back in.
: that problem went away, but now I have the problem that I can't get a station above 106.7 Mhz because I hit the top end stop. Channels are spread too far apart. I can get all the low end.
: I thought that IFs were out of alignment. I realigned them and it did no good. I still can't get above 106.7.
:
: I have been doing radios quite a bit, but am weak on front ends. Anyone have an idea?
:
:Best Regards,
:
:Bill Grimm
:
Check your local oscillator reaquency. If you have it 10.7 mHz below the station frequency, would it not reverse the hetrodyne? And if the local oscillator quits, you have a 10.7 mHz TRF radio. I would use the old "second radio local oscillator" check.
Lewis

:Hi All,
: I am still beting my head on the radio which had the freqency agile 12AT7. The 12AT7 was of course OK.
:
: I suspected that the capacitor across the LO coil may be the culprit, so I replaced it.
: The radio did something I did not think was possible. The low end of the dial had the stations of the upper part of the dial, the upper part of the dial had the stations from the low end! Somewhere in the middle of the dial I could get the station at 107.9 tune a bit higher I get the station at 88.5MHz.

:
: Since the capacitor I took out passed the freeze test and tested as good as the capacitor I took out, I put it back in.
: that problem went away, but now I have the problem that I can't get a station above 106.7 Mhz because I hit the top end stop. Channels are spread too far apart. I can get all the low end.
: I thought that IFs were out of alignment. I realigned them and it did no good. I still can't get above 106.7.
:
: I have been doing radios quite a bit, but am weak on front ends. Anyone have an idea?
:
:Best Regards,
:
:Bill Grimm
:
Check your local oscillator reaquency. If you have it 10.7 mHz below the station frequency, would it not reverse the hetrodyne? And if the local oscillator quits, you have a 10.7 mHz TRF radio. I would use the old "second radio local oscillator" check.
Lewis

::Hi All,
:: I am still beting my head on the radio which had the freqency agile 12AT7. The 12AT7 was of course OK.
::
:: I suspected that the capacitor across the LO coil may be the culprit, so I replaced it.
:: The radio did something I did not think was possible. The low end of the dial had the stations of the upper part of the dial, the upper part of the dial had the stations from the low end! Somewhere in the middle of the dial I could get the station at 107.9 tune a bit higher I get the station at 88.5MHz.
:
:
:
::
:: Since the capacitor I took out passed the freeze test and tested as good as the capacitor I took out, I put it back in.
:: that problem went away, but now I have the problem that I can't get a station above 106.7 Mhz because I hit the top end stop. Channels are spread too far apart. I can get all the low end.
:: I thought that IFs were out of alignment. I realigned them and it did no good. I still can't get above 106.7.
::
:: I have been doing radios quite a bit, but am weak on front ends. Anyone have an idea?
::
::Best Regards,
::
::Bill Grimm
::
:
:
:Check your local oscillator reaquency. If you have it 10.7 mHz below the station frequency, would it not reverse the hetrodyne? And if the local oscillator quits, you have a 10.7 mHz TRF radio. I would use the old "second radio local oscillator" check.
:Lewis
:
Hi Lewis,
Thank you for your reply. What is the "second radio local oscillator" check?

By the way this is a Zenith Y723. It is slug tuned.

Best Regards,

Bill Grimm

Being a slug tuned front end set. It could be possible the coils are lose on the forms and have moved a little. If the winding have spread out that would cause inaccurate tuning.

:Being a slug tuned front end set. It could be possible the coils are lose on the forms and have moved a little. If the winding have spread out that would cause inaccurate tuning.
:
Hi Warren,
That is an interesting Idea. I have been suspecing the coil, but three things have lead me away from it. First, it will be very hard to replace. Second, it seems to track well with the antenna coil. Sensitivity is good all the way. Thirdly, I have just checked it and the coils are secure against the form. Some other damage may be there, though.
Still. I think you are on to something.

Best Regards,

Bill Grimm

The second radio check means to use a second radio, preferably one with a digital readout, to verify the lo frequency. You tune the second radio to a frequency that should be 10.7 MHz above the frequency the first radio is tuned to. You should be able to detect the lo of the first set.

Be careful of modifying the coils. This can get you to a point that can't easily be fixed.

The lo in these sets is aligned by changing the fixed position of the slugs, thus changing their range by affecting how far they move in the coil.

"Flipping over' the tuning means you got the lo on the wrong side of the tuned frequency. The cap must have been the wrong value.

How are you attempting to align the set? If you don't have a sweep generator you can use a regular signal generator for the if's and stations of known frequency for the rf. A counter or a sw radio with a digital readout can be used to calibrate the sig gen.

:Hi Lewis,
: Thank you for your reply. What is the "second radio local oscillator" check?
:
: By the way this is a Zenith Y723. It is slug tuned.
:
:Best Regards,
:
:Bill Grimm
:

Sounds like a capacitor in the LO tank circuit. If the LO tunes below the incoming frequency, and the capacitor opens up, remaining circuit capacitance might be such that the LO now tunes above the incoming frequency.

Another suspect would be corroded silver mica capacitors in the IF section that make and break connection. However, unless they all did this at the same time, you'd more likely just have reception that'd improve and degrade. It'd take a retune of the entire IF circuitry to reverse or spread tuning.

:Sounds like a capacitor in the LO tank circuit. If the LO tunes below the incoming frequency, and the capacitor opens up, remaining circuit capacitance might be such that the LO now tunes above the incoming frequency.
:
:Another suspect would be corroded silver mica capacitors in the IF section that make and break connection. However, unless they all did this at the same time, you'd more likely just have reception that'd improve and degrade. It'd take a retune of the entire IF circuitry to reverse or spread tuning.
:
Hi Thomas,
I tried taking the IF down to constrict the tuning. I took it down to 10.5MHz. It seemed to have no effect. The IF should be 10.7 anyway.

The Capacitor in the tank is marked red-red-black. On the schematic it says it should be 22mmf. I replaced it with a modern 22pf. That is when the weird split band started. I put the old one back in and it went back to the band spread thing.
Both the old and new 22pf capacitors tested the same. Could it be solder? I am really groping at this point.

Best Regards,

Bill Grimm

::Sounds like a capacitor in the LO tank circuit. If the LO tunes below the incoming frequency, and the capacitor opens up, remaining circuit capacitance might be such that the LO now tunes above the incoming frequency.
::
::Another suspect would be corroded silver mica capacitors in the IF section that make and break connection. However, unless they all did this at the same time, you'd more likely just have reception that'd improve and degrade. It'd take a retune of the entire IF circuitry to reverse or spread tuning.
::
:Hi Thomas,
: I tried taking the IF down to constrict the tuning. I took it down to 10.5MHz. It seemed to have no effect. The IF should be 10.7 anyway.
:
: The Capacitor in the tank is marked red-red-black. On the schematic it says it should be 22mmf. I replaced it with a modern 22pf. That is when the weird split band started. I put the old one back in and it went back to the band spread thing.
: Both the old and new 22pf capacitors tested the same. Could it be solder? I am really groping at this point.
:
:Best Regards,
:
:Bill Grimm
:

You could start by refreshing all of the solder connections. I do know that I had a mass solder connection problem in this Magnavox receiver that I fixed for a friend. Touching up all of the connections made a huge difference in performance, but the IF circuitry is on a board, so that's why there were problems in the first place. Still, it can't hurt so long as you don't overheat anything.

Try to stick to mica capacitors. Don't use ceramic caps in critical areas like the FM LO. However, to contradict myself, I did build a superregenerative FM receiver using all ceramic caps, and it performed very well. Ceramic caps probably will cause no harm, but they are less stable than mica caps.

You could also have a coil that's opening up, which would insert series capacitance and increase frequency, and might also spread the band. However, FM LO coils are often so rugged and small that it's unlikely. Still, check the solder connections. Check tube socket connections (contacts), too. ...Rock the tube. Perhaps also check your 12AT7s for grid to cathode shorts or grid to plate shorts, while the tubes are hot. Some of these tests aren't ordinarily performed when checking a tube.

Seems very much that the problem is in the LO. This is the most influential circuit in the whole radio. Detuning the IF circuitry can cause band spread and can cause station relocation, but this would require a mass detune of the IF circuitry.

Also, be sure that all necessary shields are in place. Spurious oscillation can cause the LO to behave irratically. Be sure that if the front end is on a sub-chassis that it has proper contact to the main chassis, and that all wires are connected properly. ....Etc.

T.

...Curious. By what method are the slugs moved? I own a 1951 Zenith consol chassis from a television/radio consol that someone parted out (shame!). The FM uses a cam of sorts to move a rod that carries the slugs. If your set uses a cam, and the cam is secured to the shaft with a set-screw, perhaps the screw has come loose. At first your station relocation might have been due to the cam shifting position slightly, or whatever that rides on the cam wasn't riding securely. ...And then if the cam came loose completely, it'd cause bandspread and possibly reversal of tuning procedure, depending on its shape and how it now sits on the shaft.

...Just a wild guess. I don't know how your radio looks or how it is designed.

Whatever the case may be, the slugs, if ferrite, should be farther within the coils for the low end of the band, and farther out for the high end. If this isn't happening, then something's wrong with the mechanics.

T.

:...Curious. By what method are the slugs moved? I own a 1951 Zenith consol chassis from a television/radio consol that someone parted out (shame!). The FM uses a cam of sorts to move a rod that carries the slugs. If your set uses a cam, and the cam is secured to the shaft with a set-screw, perhaps the screw has come loose. At first your station relocation might have been due to the cam shifting position slightly, or whatever that rides on the cam wasn't riding securely. ...And then if the cam came loose completely, it'd cause bandspread and possibly reversal of tuning procedure, depending on its shape and how it now sits on the shaft.
:
:...Just a wild guess. I don't know how your radio looks or how it is designed.
:
:Whatever the case may be, the slugs, if ferrite, should be farther within the coils for the low end of the band, and farther out for the high end. If this isn't happening, then something's wrong with the mechanics.
:
:T.
:
Hi Thomas,

The radio is slug tuned. The 1951 version of this radio, H723, was capacitively tuned and worked better. The radio did not drift during warm up. It was their first to use the new 12AT7 tube. The G723 from the 1940's, its predecessor, used slug tuning and a 12BE6. I consider it too ugly to bring in the house.
Zenith cheapened the design of the H723 in 1953 to slug tuning. This allowed them to use the same tuning capacitor on both their AM radios and AM/FM radios.

Best Regards,

Bill Grimm

Terry,
Couldn't possibly be at Bill's shop. They all live here with me, but they do well hiding, awaiting their most opportune time to appear.

marv

:::Sounds like a capacitor in the LO tank circuit. If the LO tunes below the incoming frequency, and the capacitor opens up, remaining circuit capacitance might be such that the LO now tunes above the incoming frequency.
:::
:::Another suspect would be corroded silver mica capacitors in the IF section that make and break connection. However, unless they all did this at the same time, you'd more likely just have reception that'd improve and degrade. It'd take a retune of the entire IF circuitry to reverse or spread tuning.
:::
::Hi Thomas,
:: I tried taking the IF down to constrict the tuning. I took it down to 10.5MHz. It seemed to have no effect. The IF should be 10.7 anyway.
::
:: The Capacitor in the tank is marked red-red-black. On the schematic it says it should be 22mmf. I replaced it with a modern 22pf. That is when the weird split band started. I put the old one back in and it went back to the band spread thing.
:: Both the old and new 22pf capacitors tested the same. Could it be solder? I am really groping at this point.
::
::Best Regards,
::
::Bill Grimm
::
:
:The problem with these "kids" today is that they don't know what a Gremlin is. These strange creatures, discovered during WWII, are found mainly in radio circuits. They love to mess around until you're about to go crazy, then they disappear. Some people believe they live in the same universe where your remote, and odd socks, visit for awhile.
:

Hi All,
The Gremlin has been found. This morning I replaced the coil for the Local Oscillator. The problem is now totally gone!

I have been looking at the old coil under the magnifier and could only find two small strands of copper fiber stuck to the coil. I find it a little hard to believe that those small fibers could have caused this problem.

After an alignment sensitivity came right up.

Then the silver Mica started kicking up in an IF. Will it never end? At least that is routine.

Thank you for the help.

Best regards,

Bill Grimm

Test the IF transformers for shorts between the primaries and secondaries. If there aren't any, consider spraying contact cleaner into the mica capacitor area. Then, once all has dried, inject some dielectric grease. It is non-conductive, and may help repair the connections and prevent future corrosion. However, sometimes the spring contacts just wear right through the silver, and no repairs can be made. If you can fix the transformers in this way, it will save drilling rivets and rebuilding them. Using the dielectric grease on the non-malfunctioning transformers might prolong their operation, too. Don't use WD-40. That stuff tends to carry debris and cause leakage paths. I have also found it to de-tune coils when they have become soaked by it.

T.