UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

adibrook · 17th Nov 2006, 1:23 pm

Thanks.

A quick dig in the junk box turned up a couple of ECH81's a CCH35 and an X63, either of which could work i suppose. But i'm more enclined towards the ECH81 since they have a seperate triode oscillator, and are 9-pin and therefore easier to work with. Usually i would prefer octal valves for 1st prototypes since they are physically bigger, but for this 9 pin is easier i think because the valve holder can be soldered directly to metal, so I can use a recycled can for screening.

Any idea roughly what value the ''padder'' shoudl be? I think both trimmers C1 and C2 can be about 50-100pf, but the padder i dont know. In schematics i'v seen of real radios the padder is 470pf and is a fixed cap. but since i will probably be using homebrew (and therefore not ajustable) coils, an adjustable padder is needed.

Another thing that i'm not too sure about is IF frequancy. if i recycle IF transformers from a scrap radio, they woudl peak at the donor radio's IF, which is usually 445KHz

Am i right in assuming that radios use thesame IF sections for both SW and LW? So when you change bands the only thing that changes is the ariel and oscillator tuning coils? So in a SW superhet the IF is ALOT lower frequancy than the main frequancy.

So, for SW reception, the ariel and oscillator tuned circuits can have pretty much identicle coils, with the difference being adjusted out by the trimmers?

say if the signal recieved is 10MHz (10,000KHz), the ariel tuning coil would need to be tuned to 10,000KHz and the oscillator coil would need to be tuned to 9,555KHz. The difference is very small compared to the overall frequancy and therefore a small trimmer should be able to do it allright.

Sound right so far?

Thanks.

17th Nov 2006, 1:23 pm	#3
adibrook Retired Dormant Member Join Date: Dec 2004 Location: Weymouth, Dorset, UK. Posts: 377	Re: superhet Thanks. A quick dig in the junk box turned up a couple of ECH81's a CCH35 and an X63, either of which could work i suppose. But i'm more enclined towards the ECH81 since they have a seperate triode oscillator, and are 9-pin and therefore easier to work with. Usually i would prefer octal valves for 1st prototypes since they are physically bigger, but for this 9 pin is easier i think because the valve holder can be soldered directly to metal, so I can use a recycled can for screening. Any idea roughly what value the ''padder'' shoudl be? I think both trimmers C1 and C2 can be about 50-100pf, but the padder i dont know. In schematics i'v seen of real radios the padder is 470pf and is a fixed cap. but since i will probably be using homebrew (and therefore not ajustable) coils, an adjustable padder is needed. Another thing that i'm not too sure about is IF frequancy. if i recycle IF transformers from a scrap radio, they woudl peak at the donor radio's IF, which is usually 445KHz Am i right in assuming that radios use thesame IF sections for both SW and LW? So when you change bands the only thing that changes is the ariel and oscillator tuning coils? So in a SW superhet the IF is ALOT lower frequancy than the main frequancy. So, for SW reception, the ariel and oscillator tuned circuits can have pretty much identicle coils, with the difference being adjusted out by the trimmers? say if the signal recieved is 10MHz (10,000KHz), the ariel tuning coil would need to be tuned to 10,000KHz and the oscillator coil would need to be tuned to 9,555KHz. The difference is very small compared to the overall frequancy and therefore a small trimmer should be able to do it allright. Sound right so far? Thanks.