Zenith 9S262 and many other models. RF choke in the plate circuit of RF amp. Zenith part 20-135. It seems to be nothing more than a tapped RF choke that does not participate in the resonant circuit of the RF amp. Has anyone ever measured the inductance of one of these? Thanks. Tom

It does participate in the resonance of the RF circuit, as it is tapped and is changed with the band switch. As it is tapped into shorter sections, it will "tighten up" the impedance of the primary circuit of the RF transformer. This will cause this circuit to favor higher frequencies. Adding size to the RF choke will lower the frequency preference of the circuit.

Otherwise only the secondary of the RF transformer is tuned.

Thomas

:It does participate in the resonance of the RF circuit, as it is tapped and is changed with the band switch. As it is tapped into shorter sections, it will "tighten up" the impedance of the primary circuit of the RF transformer. This will cause this circuit to favor higher frequencies. Adding size to the RF choke will lower the frequency preference of the circuit.
:
:Otherwise only the secondary of the RF transformer is tuned.
:
:Thomas

Thomas: I can see it participates in the reflected Z of the RF transformer but did not notice a resonating capacitor. I need to know the L of the coil as I have an open one and could use RF chokes for replacements.
Thanks, Tom.

I understand now what it is that you are looking for.

It would be a bad idea, though, to have the choke be part of a tank circuit (condenser and coil), as this would tend to isolate a specific frequency. By choosing different parts of the choke, which favor certain frequency areas, you narrow down the area which the primary of the RF transformer works best at. If you were to put a condenser on the choke, it could not be a fixed unit for the reason mentioned above. It would have to be an additional variable gang added to the tuning condenser, which would greatly increase the cost of the set, which is why the idea is abandoned. Instead, Zenith already uses quite a liberal supply of tuned circuits and adds this broad-tuned choke as a little extra. Zenith was a remarkable company in that it didn't always make the PERFECT set, but it threw in all sorts of little affordable extras which made its sets pretty darn good anyway. This is how they were able to produce sets of such quality without making them entirely too expensive for all but a very few.

Also, puting in excessive selectively tuned circuits isn't always desirable, as this ruins audio quality due to the limiting of the bandwidth of the station passing through.

Thomas

::It does participate in the resonance of the RF circuit, as it is tapped and is changed with the band switch. As it is tapped into shorter sections, it will "tighten up" the impedance of the primary circuit of the RF transformer. This will cause this circuit to favor higher frequencies. Adding size to the RF choke will lower the frequency preference of the circuit.
::
::Otherwise only the secondary of the RF transformer is tuned.
::
::Thomas
:
:Thomas: I can see it participates in the reflected Z of the RF transformer but did not notice a resonating capacitor. I need to know the L of the coil as I have an open one and could use RF chokes for replacements.
:Thanks, Tom.

::Hi. I've been working on the same problem. I measured the small coil at .5mH and guessed by looking at it that the bigger coil would be 3-5 times as much. I tried a 2.5mH coil and the AM band worked. I still need to aling and fully test, but I think this is probably in the ball park. Like you, I would love to get an actual specification or measurement on the big coil, but I hope this helps. Please let me know if you learn any more. thanks, John
::It does participate in the resonance of the RF circuit, as it is tapped and is changed with the band switch. As it is tapped into shorter sections, it will "tighten up" the impedance of the primary circuit of the RF transformer. This will cause this circuit to favor higher frequencies. Adding size to the RF choke will lower the frequency preference of the circuit.
::
::Otherwise only the secondary of the RF transformer is tuned.
::
::Thomas
:
:Thomas: I can see it participates in the reflected Z of the RF transformer but did not notice a resonating capacitor. I need to know the L of the coil as I have an open one and could use RF chokes for replacements.
:Thanks, Tom.

John: Thanks for your reply. I am only attempting to replace the choke and not add additional tuned circuits as others seem to think. I repair a large number of radios and had a call from a customer today on a Zenith chasssis that contains the same RF amp as the one we are questioning. When I get his on the bench I will measure the L of the coil with my HP bridge and let you know the exact values. Tom S.:::Hi. I've been working on the same problem. I measured the small coil at .5mH and guessed by looking at it that the bigger coil would be 3-5 times as much. I tried a 2.5mH coil and the AM band worked. I still need to aling and fully test, but I think this is probably in the ball park. Like you, I would love to get an actual specification or measurement on the big coil, but I hope this helps. Please let me know if you learn any more. thanks, John
:::It does participate in the resonance of the RF circuit, as it is tapped and is changed with the band switch. As it is tapped into shorter sections, it will "tighten up" the impedance of the primary circuit of the RF transformer. This will cause this circuit to favor higher frequencies. Adding size to the RF choke will lower the frequency preference of the circuit.
:::
:::Otherwise only the secondary of the RF transformer is tuned.
:::
:::Thomas
::
::Thomas: I can see it participates in the reflected Z of the RF transformer but did not notice a resonating capacitor. I need to know the L of the coil as I have an open one and could use RF chokes for replacements.
::Thanks, Tom.

Noone seems to think that you want to add extra tuned circuits. Perhaps if you read thoroughly what I had said you would see that I was simply explaining your coil to you--as to what it is there for--that it does have a purpose in the RF circuit. I then went on to explain what other items could have been inserted instead of the rather broad tuned choke, but that these parts would increase the expense and complication of the set.

Read thoroughly and don't ASS-U-ME.

Thanks,

Thomas

John: I had a Zenith chassis come in for repair as I previously mentioned. I had time today to measure the inductance of the RF choke in question:
Section 1 = 180 microhenries
Section 2 = 5.2 millihenries

These values render this an easy fix using simple RF chokes. The L's here exhibit a ratio of about 52:1 and I ran a phasor equation on the circuit and discovered that if section 2 was at a frequency of 550 KHz then section 1 would be at 25 Mhz which is a little more than the short wave maximum frequency of the radio. I think these values are very close. Good luck on yours. Tom S.

:::Hi. I've been working on the same problem. I measured the small coil at .5mH and guessed by looking at it that the bigger coil would be 3-5 times as much. I tried a 2.5mH coil and the AM band worked. I still need to aling and fully test, but I think this is probably in the ball park. Like you, I would love to get an actual specification or measurement on the big coil, but I hope this helps. Please let me know if you learn any more. thanks, John
:::It does participate in the resonance of the RF circuit, as it is tapped and is changed with the band switch. As it is tapped into shorter sections, it will "tighten up" the impedance of the primary circuit of the RF transformer. This will cause this circuit to favor higher frequencies. Adding size to the RF choke will lower the frequency preference of the circuit.
:::
:::Otherwise only the secondary of the RF transformer is tuned.
:::
:::Thomas
::
::Thomas: I can see it participates in the reflected Z of the RF transformer but did not notice a resonating capacitor. I need to know the L of the coil as I have an open one and could use RF chokes for replacements.
::Thanks, Tom.

::Thank you very much, I really appreciate you letting me know. all the best, John.
:John: I had a Zenith chassis come in for repair as I previously mentioned. I had time today to measure the inductance of the RF choke in question:
:Section 1 = 180 microhenries
:Section 2 = 5.2 millihenries
:
:These values render this an easy fix using simple RF chokes. The L's here exhibit a ratio of about 52:1 and I ran a phasor equation on the circuit and discovered that if section 2 was at a frequency of 550 KHz then section 1 would be at 25 Mhz which is a little more than the short wave maximum frequency of the radio. I think these values are very close. Good luck on yours. Tom S.
:
::::Hi. I've been working on the same problem. I measured the small coil at .5mH and guessed by looking at it that the bigger coil would be 3-5 times as much. I tried a 2.5mH coil and the AM band worked. I still need to aling and fully test, but I think this is probably in the ball park. Like you, I would love to get an actual specification or measurement on the big coil, but I hope this helps. Please let me know if you learn any more. thanks, John
::::It does participate in the resonance of the RF circuit, as it is tapped and is changed with the band switch. As it is tapped into shorter sections, it will "tighten up" the impedance of the primary circuit of the RF transformer. This will cause this circuit to favor higher frequencies. Adding size to the RF choke will lower the frequency preference of the circuit.
::::
::::Otherwise only the secondary of the RF transformer is tuned.
::::
::::Thomas
:::
:::Thomas: I can see it participates in the reflected Z of the RF transformer but did not notice a resonating capacitor. I need to know the L of the coil as I have an open one and could use RF chokes for replacements.
:::Thanks, Tom.