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Old 26th Nov 2023, 8:37 pm   #1
LA9CHA Thorvald
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Default 465kHz IF signal pass.

Hello, I am working with an if adjusment on a Tanberg SS3 radio receiver, using 465kHz as if. In order to adjust for max signal response by the if coils, I connect the 465 signal to the grid if the ECH3 (hexode) mixer valve (as I have done for many other radios), but to my surprice I have no responce of the if signal on the ECH3 anode, before goin to the first if anode coil! However, when disconnecting the local oscillator input to the ECH3 triode grid, the 465if signal turns up and is tunable via both if cans! To my knowledge both the if signal, lo +465 and lo-465 schould pass trough. Hence the tuned station frequency, mixed by the + and - lo freq. should provide the 465 kHz (in this case). Any hints is welcome.
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Old 26th Nov 2023, 8:57 pm   #2
Silicon
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Default Re: 465kHz IF signal pass.

The tuning circuit on the grid of the mixer valve will appear as a low impedance at 465kHz and will attenuate the signal.
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Old 26th Nov 2023, 9:28 pm   #3
LA9CHA Thorvald
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Default Re: 465kHz IF signal pass.

Ok Silicon and tks for answer. I shall try to connect the signal to the topcap alone, hence avoiding the affect of the if damping circuit. However, why is it ok when disconnecting the lo to triode grid (disabling the local oscillator)?
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Old 26th Nov 2023, 9:34 pm   #4
LA9CHA Thorvald
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Default Re: 465kHz IF signal pass.

Ok and tks for answer. I shall try to connect the if signal direct to the mix. However, why is the signal ok when disabling the lo signal by disconnecting the triode grid?
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Old 27th Nov 2023, 10:53 am   #5
John10b
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Default Re: 465kHz IF signal pass.

Interesting, as you say you have successfully done this many times before. Apart from this does the radio work correctly?
John
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Old 27th Nov 2023, 11:00 am   #6
Nuvistor
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Default Re: 465kHz IF signal pass.

Does the radio have an ‘IF’ trap in the input to the ECH3? Many better radios had one.
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Old 27th Nov 2023, 4:40 pm   #7
LA9CHA Thorvald
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Default Re: 465kHz IF signal pass.

Hello John and Frank and thaks for answer. John, the radio does not work correctly yet as the IF has to reach the demodulator diode first. As for now I apply the 465 IF unmodulated to the mixer tube top cap only, while keeping the antenna circuit and if trap top cap plug disconnected. Hence it is only the 465 IF signal. This signal is very well measured at the anode (pin 8) with an ONLY when the internal IF is disabling the local oscillator! Also now the rest of the IF cans can be adjusted for max signal. I find this strange! Normally it is ok to inject the IF direct at the grid top cap!
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Old 27th Nov 2023, 6:31 pm   #8
John10b
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Default Re: 465kHz IF signal pass.

Do you have the circuit diagram? It would be helpful to see the mixer stage if possible.
Also do you have a spare mixer valve?
John

Last edited by John10b; 27th Nov 2023 at 6:37 pm. Reason: Extra question
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Old 29th Nov 2023, 9:00 pm   #9
LA9CHA Thorvald
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Default Re: 465kHz IF signal pass.

Hi again John. Thanks for your assistance, but I have now found the trouble. As I irafo box, injected the 465khz to the grid for the last if can I did not have any responce at th output feeding the domod diodes. Opened up and found a defectiver 100pf cap that measured 4pf only. Hence no resonance! Now ok with all! Mry xmas and tks!
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Old 30th Nov 2023, 9:51 am   #10
John10b
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Default Re: 465kHz IF signal pass.

Glad you found the problem. Happy Christmas to you.
John
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