Substituting Variable Capacitor with Varicap Diodes

If I were to try to replace the 2-gang variable capacitor in the a.m. bcb
radio shown on the webpage below, roughly what varicap would I need to get,
and how would I connect them? I'm not really so bothered about trying to
employ a 2 gang potentiometer. TIA.


http://www.electronics-tutorials.com/receivers/receiver-basics.htm

 

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

 

----- 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)

 

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

 

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

 

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.

 

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.

 

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

 

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:

http://www.toko.co.jp/products/index_product_e.htm

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

 

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:

http://www.toko.co.jp/products/index_product_e.htm

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

 

http://www.6v6.co.uk/candypress/scripts/prodView.asp?idproduct=1217

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)

http://www.oselectronics.com/ose_p98.htm


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
----------------------------------------------------------------------------
----

Miniature 1 Gang Poly-film Variable Tuning Capacitor For Broadcast Band

Maximum capacity: Single section tunes from 10 - 280pf
---------------------------------------------------------------------------
-----

Miniature Poly-film Variable Tuning Capacitor For AM & FM Bands

Antenna section AM 21 - 152PF

Oscillator Section AM, 10 - 74PF

 

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