1Tube + TA7642 (or LMF501) superhet

[andreidaniel]

Hello everybody!!

I am interested in building this design for MW and LW.

1tube & 2IC Super Hetrodyne Radio
**broken link removed**
**broken link removed**

I got the IC and the valve.

I have all the coils on a PCB with FM-LW-MW from a radio.cass.

The problem is the coils. Which type of coils should be used? The yellow core is 1st AM IF, the white is 2nd stage AM IF and the black is 3rd AM IF stage. The two coils should be the same for example (yellow-Filter-yellow) or not? (for example yellow-Filter-black).. Another problem is how can I know the numbering of those coils and the hot end and cold end of each section?

Thank you!

 

[chuckey]

With these coils( Intermediate Frequency Transformers) the secondaries are always a lower resistance then the primaries. On the primaries, if there is a resistance between all three pins then the collector/anode tap and Vcc/HT will be the lowest resistance.
I think the different IFT characteristics are:-
first IFT, lower coupling, hence highest Q, matching 20 K to 2 K
second IFT, tight coupling low Q, matching 10K to 2K
Third IFT, tight coupling matching 20 K to 5 K. . . . .Other ratios are available .
Ceramic filters are normally designed for a circuit impedance of 200 ohms, so they will damp the available IFTs.
Frank

 

[andreidaniel]

ok... in my case.. if you see the guy's picture from the device... if it is respecting the standard (yellow, white, black) uses two black coils and between them a ceramic filter. Is it ok to use this type black (3rd IF stage..one time is normaly,and after the filter is reversed).

Is there any problem with the anode curent which passes throw the coil at 12,6V? The IF transformers from stationary tube radios had a higher diameter of wires, and also a higher ferrite, but were not shielded. But...but.. at the end both coils from modern transistored radios and from valves have the same frequency 455KHz..

And the last problem - If I want to make a superhet for AM how can I know how many IF to put? I saw superhet with 2 stages (yellow-black) or 3 stages (yellow-white-black) or..maybe more.. How can I know if it is enough 2 or 3?

 

[chuckey]

Its a gain v. selectivity problem. A cheapo set would have one IF stage (three transformers) a communication receiver would have 3 IF stages with 5 IFTs and selectable crystal filters.
Frank

 

[andreidaniel]

Hi! I'm back after some researches... I didn't find two black IF coils identically with primary and secondary. All that I found for AM reception were: Red coils, Yellow, White and Black. I got the coils from some radio-clocks. Interesting is that two of them were identically, the same diagram but one had a black coil, and the other a white coil instead..

I found that works this combination. And yes, works like a superheterodyne. The local station doesn't cover the other nearby stations but even now sounds a little bit distorted.. The others sound good. At day now I can tune: 603KHz, 873KHz, 1053 (local), 1179KHz. At night before 22 I can tune some new frequencies like: 567KHz, 756KHz, 810KHz, 999KHz, 1332KHz. With the simple diagram of TA7642 I could only tune 1053 distorted and 603KHz at day. At night I also could tune more station after closing the local station with a little more sensitivity. Interesting is that I cannot do something for having the same level of sensitivity for entire MW band.. either I adjust the trimmer to have best sensitivity for 1000 to 1332KHz or I adjust the trimmer to have the best sensitivity for 810-540KHz.. I adjusted now at a medium point. The strange part is that I cannot cover the entire band.. I tune under 540 to 1332KHz. I tried another red coil but with worse results.

Connected either at an unregulated power or at a regulated power supply of 9 or 12,4V I got humm... Only if I put a battery I can have a clear sound or if I put the ground.. Interesting is that the same thing is happening at Graetz.. I should put the ground antena for MW. Curiously is that one transistored radio (Si technology) with internal regulated power supply has humm and another transistored radio (Ge technology) doesn't have humm if I connect it to a regulated power supply on the same bands MW and SW. The big question is "why"? Why one has humm and the other not?