BC-624 / SCR522 I.F. transformers

[Opcom]

I found a box of five 3rd IF transformers for the BC-624 receiver. It's a 100-156MHz AM set for aircraft.

The transformers are 12MHz. They should be useful for something. I can't find the bandwidth of the receiver written in the manual.

These are the 3rd IF, designed to be driven by a 12SG7 pentode and to drive the diode of a tube detector. Would that make them usually wider or narrower than the other IF transformers in the set?  (those have different part numbers).  

I hope they can be used for an IF.

[W7TFO]

They get the increased bandwidth via the caps in parallel with the primary & secondary.

Wideband tuning was an advantage with mechanically tuned radios up "in the clouds" AKA VHF, as it makes the dial rather broad and increases the odds of getting a message thru.

73DG

[G3UUR]

Patrick,

The bandwidth at 12Mc/s will be around 100kc/s. The final IFT usually had tighter coupling and consequently will be broader than the other IFTs. You can always pad them down to 10.7Mc/s, or even 9Mc/s, if you want to build a tube receiver with a more conventional IF.

73,

Dave.

[Opcom]

Thanks!  If I had a better idea of what "Z" is supposed to be on each side, I could set that up via a resistor on each side and take them to work where we have a spectrum analyzer with a tracking generator and see what the response is.

Am I right to think that, like with tube type plate tanks, the bigger the C to L ratio, the higher the Q or sharper the tuning?

These are made of two 1/2" diameter coils about 15 turns each of what looks like #16 or 18, on plastic forms with movable ferrite slugs in them, and across each coil the capacitors are soldered as the diagram shows. The coils are about 1/2" apart. I don't remember the exact turns, I should get a picture.

If the caps in parallel add up to 75pF, then the L should be 2.35uH

A thought was to lower them in frequency and at the same time sharpen them up by using a cap 3x to 10x the original value. It may not be practical to go too far.

I'm not so stuck on 10.5MHz or 9MHz, but that seems to be where most of the common filters are.

The other things is that if I want to cover a frequency range for example 3 to 25 Mhz, the 12MHz (or 10.5 or 9) IF is right in the middle of that. I prefer general coverage rather than just ham bands.
The last time I built a superhet receiver, there was a big problem when the RX was tuned to the IF frequency, too much leakage and I never really did get rid of all of it.

[G3UUR]

Hello again Patrick,

Thanks!  If I had a better idea of what "Z" is supposed to be on each side, I could set that up via a resistor on each side and take them to work where we have a spectrum analyzer with a tracking generator and see what the response is.

You can see what the terminating impedance needs to be by looking at the plate resistance of the tube driving the IFT and the grid resistance of the tube following it.

Am I right to think that, like with tube type plate tanks, the bigger the C to L ratio, the higher the Q or sharper the tuning?

It's sort of similar, but the dynamic resistance of the IFT needs to be less than one tenth of the tube plate resistance - the dynamic resistance is Q times the reactance at resonance, so for a tube rp of 1Megohm and inductor Q of 120 you'd want a reactance of about 800 ohms for XL or XC.

These are made of two 1/2" diameter coils about 15 turns each of what looks like #16 or 18, on plastic forms with movable ferrite slugs in them, and across each coil the capacitors are soldered as the diagram shows. The coils are about 1/2" apart. I don't remember the exact turns, I should get a picture.

If the caps in parallel add up to 75pF, then the L should be 2.35uH

A thought was to lower them in frequency and at the same time sharpen them up by using a cap 3x to 10x the original value. It may not be practical to go too far.

At a lower frequency the Q would probably fall off as the reactance goes down in proportion to frequency but the loss only goes down as the square root of frequency. However, if you're using a crystal filter to provide the main selectivity this won't matter

I'm not so stuck on 10.5MHz or 9MHz, but that seems to be where most of the common filters are.

The other things is that if I want to cover a frequency range for example 3 to 25 Mhz, the 12MHz (or 10.5 or 9) IF is right in the middle of that. I prefer general coverage rather than just ham bands.
The last time I built a superhet receiver, there was a big problem when the RX was tuned to the IF frequency, too much leakage and I never really did get rid of all of it.

If you want 3 to 25MHz, it looks like an IF of 1.6 or 2MHz would be more appropriate. It's either that or a high IF above 25MHz!

Dave.