Substituting Variable Capacitor with Varicap Diodes

get,

Would I remove V.C. and do this?:

Coil---------C 0.01uF-------------Resistor 100K------ +ve | Varicap 300 odd pF. | Coil-------------------------------------------------------------- -ve | \\\\\\\\\\\\\\\\\\\\\\\\ Chassis

For AE coil and Osc coil.

Probably, pretty much, yeah. It'll still be parallel-resonant, and the .01 will have neglible reactance - if I was feeling tight-assed, I might put a ferrite bead at the left lead of the 100K, but where in the world do you get a 300 pF varactor? (That's the word that one of my tech school teachers said was the "proper" term, instead of "varicap". Sorry. ;-) )

Thanks, Rich

The variable caps in the gang are most certainly different values, in order to track over the different frequency ranges of the local oscillator and the radio frequency; the local oscillator is the lesser of the two.

Don

Digikey carries a 360 pf 20 V KV1520 and a 467 pf 16 V KV1560 by Toko. Well, they don't actually stock them, but they do list them. Less than $2 each, if you are willing to wait while they order some for you.

----- Original Message ----- From: "Rich Grise" Newsgroups: sci.electronics.basics Sent: Wednesday, January 11, 2006 12:56 AM Subject: Re: Substituting Variable Capacitor with Varicap Diodes

;-) )

Hmm, I thought that some (modern) BC band RX's were/could be tuned by varacters. Maybe I got things wrong? :c)

.01

I'd have to use special varacters like Phiips BB1122, BB212 and similar. Made especially for AM radios.

.01

I'd have to use Philips BB112, BB212 varactors. Special AM radio varactors.

varactors.

Thanks for the info. I actually paid a small fortune tonight on Ebay for a pair of (fairly rare) Philips BB212. Three pin devices with two varactors in them. I'll take apart, then rebuild on my own PCB, a generic 6 transistor "pocket radio" that I have. Luckily it had a tiny circuit stuck on the back plate. It will be interesting making things work. I don't think tracking will be a problem with a dual potentiometer. Tracking I think is a matter of padding capitors. Not sure though if each gang of these 2 gang variable capacitors you see in these small pocket radios are identical or not or specially devised to achieve tracking without padding etc. Mmm, I wonder if TOKO publishes data sheets on these things. I'll take a look.

if

order

the

Noted. No data about either on those miniature variable capacitors that were (invariably) Japanese. I've searched using TOKO, TSURU, MITSUI (?), SANESU (from junkbox) - but no dice.

My circuit does not give the tuning capacitor values, but as you say I think if it did, each gang would have a different value. No doubt someone's shematic of a "pocket radio" or similar would confirm. Trying to acieve tracking is going to be ineresting. I cannot find a schematic though on the WWW of an AM radio tuned by varactors.

As to varactors, TOKO Europe has some data:

One looks at: "Variable capacitance diodes for AM tuning"

if

order

the

Noted. No data about either on those miniature variable capacitors that were (invariably) Japanese. I've searched using TOKO, TSURU, MITSUI (?), SANESU (from junkbox) - but no dice.

My circuit does not give the tuning capacitor values, but as you say I think if it did, each gang would have a different value. No doubt someone's shematic of a "pocket radio" or similar would confirm. Trying to acieve tracking is going to be ineresting. I cannot find a schematic though on the WWW of an AM radio tuned by varactors.

As to varactors, TOKO Europe has some data:

One looks at: "Variable capacitance diodes for AM tuning"

if

order

the

Yep, here the values are:

Antenna caacitance: 3.0 - 141.6pF

Oscillator capacitance: 4.0 - 59.2pF

Tolerance ± 1.0pF +1.5% (at full rotation)

Miniature 2 Gang Poly-film Variable Tuning Capacitor For Broadcast Band

Maximum capacity: Antenna section. 15 - 140PF

Oscillator section, 10 - 60PF.

Trimmer capacity: variable to over 12PF

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Electronics australia did one in the earlt 80s it used a zn414 IIRC and a simple tuned circuit.

mk484 seems to be a similar device in current manufacture you might try getting the data sheet.

Bye. Jasen