Inductor value needed for Match Box size radio based on Transistors BC 548 & 495 - Page 1
Forum section: electronics / repair

author: gaurav279
author_rank: Contributor
timestamp: October 24, 2015, 09:33:56 am
content: Hi Folks...I have an pretty antique Chinese Match Box sized Radio which is based on majorly 2 Transistors BC548 and 495 with 3 resistors,4 ceramic disc capacitors,1no 33uF Electrolytic Capacitor,2 diodes,all mounted on a 3.5 cms x 1.6 cms small PCB,with a mono headphone jackThis radio works on a 1.5 Volt AA size batteryI am looking for an IFT COIL for this radio which broke accidentallyIts a 2 legged Small Vertical Ferrite Bead with very thin copper wire wound on it with several turns and the same was mounted on this PCBI tried searching the same everywhere in my city,couldn't find itCan any body suggest me any substitute part/coil or suggest me where I could find a Circuit Diagram of this particular radio.For your kind reference I am enclosing some close snapshorts of the PCB Top and the Back view,the list of Parts used in this Radio and the overall Radio Body as it looks from Outside.I request you'll to please have a look at the same and kindly advise me if you can figure out the approximate value of this small MW Inductor coil used in this mini Match Box Radio.I can then probably replace the same with small resistor type inductor and make these 2 radios working.Any suggestion would be of a great help.The Parts List for your reference is as below -Resistors (All 1/4 watt)R1 = 10 KR2 = 680 ohmR3 = 33 KCapacitorsC1,C2,C3 = 0.01 uF ceramic discC4 = 470 pF ceramic discC5 = 33uH, 16V ElectrolyticTransistorsT1 = BC 548 BT2 = 495 CDiodesD1,D2 = 4148 BMiscellaneous PartsMW Gang Condensor - 1MW Antenna Coil - 1Small Ferrite Rod for Antenna - 1EP Socket mono - 11.5 V Battery Holder /Snapper - 1MW IFT Coil Inductor with 2 legs - 1 (this is the one of which we need to find the Value of ...)

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 24, 2015, 10:48:03 am
content: That is not an IFT (IF transformer), but probably just a choke, this radio has no IF stage.Can you post a picture of the part? Otherwise just try something that looks similar, good chance that it will just work.If not, draw a diagram, can't be that hard to do.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: October 24, 2015, 10:53:07 am
content: Being a 2 transistor radio it will be a TRF or regenerative - not a superhet.Therefore there'll be no intermediate frequency.The inductor will therefore resonate with the variable capacitor on the frequency of the desired station (between 0.5 - 1.6 MHz).You could measure the variable capacitor's minimum and maximum value and work it out from there (there are LC & frequency calculators online eghttp://www.deephaven.co.uk/lc.html).Very roughly it will be in the hundreds of microhenries region depending on the size of the variable capacitor (20 - 200pF is an approximate range).As a starting point I'd be buying 220, 330 and 470 uH RF chokes (they're cheap) and experimenting with those.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 24, 2015, 10:57:37 am
content: Quote from: vk3yedotcom on October 24, 2015, 10:53:07 amI disagree, the tuning part is the ferrite and the variable cap, I strongly suspect the coil to be a choke to block RF and/or the regenerative signal.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: October 24, 2015, 11:01:57 am
content: Quote from: PA0PBZ on October 24, 2015, 10:57:37 amYes, especially as the ferrite rod will serve this purpose. Then its value won't be as critical and something around 1 to 2.5 mH should be OK.

author: gaurav279
author_rank: Contributor
timestamp: October 24, 2015, 08:20:40 pm
content: Guys thanks a lot for your posts in support of my query.Can anyone figure out the Circuit Diagram of this Small Matchbox Radio so that we can then easily trace the missing RF Choke's Value from the Schematic Diagram...

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 24, 2015, 08:24:37 pm
content: Quote from: gaurav279 on October 24, 2015, 08:20:40 pmIt's not rocket science to draw the circuit diagram from the PCB, so why don't you give it a try?If you have problems doing that please tell us what problems you see and we will be glad to clarify things for you.

author: gaurav279
author_rank: Contributor
timestamp: October 24, 2015, 08:37:07 pm
content: Quote from: PA0PBZ on October 24, 2015, 10:48:03 amPlease find an image of the Inductor Choke which I am looking for this particular Match Box Radio...The one in the Top Left Corner is much close to the one...

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 24, 2015, 08:40:08 pm
content: Quote from: gaurav279 on October 24, 2015, 08:37:07 pmSo find one that looks like that and try?

author: gaurav279
author_rank: Contributor
timestamp: October 24, 2015, 08:46:51 pm
content: I have tried a lot but no such parts are available for this Vintage Types Radio in the place I live (India).Also I am not that technical Geek to trace the circuit diagram of this Radio...Any suggestions please

author: AF6LJ
author_rank: Unknown
timestamp: October 24, 2015, 08:48:16 pm
content: Are there any numbers printed on the top of the choke??

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 24, 2015, 08:52:04 pm
content: If the parts are not available what exactly do you want us to tell you, how you can make your own?What is left of the original choke? Maybe it can be fixed or it would at least give us a clue about what it looks like...So again, any pictures of the original one?And:"I have tried a lot", do you mean that you have tested the circuit replacing the original coil with others? What was the result?

author: gaurav279
author_rank: Contributor
timestamp: October 24, 2015, 08:59:35 pm
content: NO not at all ...it was just this simple choke...with thin copper winding around

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 24, 2015, 09:22:03 pm
content: Quote from: gaurav279 on October 24, 2015, 08:59:35 pmSorry, Maybe it is a language barrier but I fail to see what you want.

author: gaurav279
author_rank: Contributor
timestamp: October 25, 2015, 07:05:22 am
content: Quote from: PA0PBZ on October 24, 2015, 09:22:03 pmMy reply was to AF6LJ...I meant that there were no numbers printed on the Inductor that I broke accidently...

author: AF6LJ
author_rank: Unknown
timestamp: October 25, 2015, 12:17:56 pm
content: Quote from: gaurav279 on October 25, 2015, 07:05:22 amJust wondering....There isn't much of a radio there, What did it receive?

author: AF6LJ
author_rank: Unknown
timestamp: October 25, 2015, 12:19:42 pm
content: Quote from: gaurav279 on October 24, 2015, 08:59:35 pmWhat frequency was this radio intended for, there isn't much there in a way of telling what frequency band it was intended for.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 25, 2015, 12:44:40 pm
content: My guess it's AM/MW whatever you call it, 500-1600KHz.

author: AF6LJ
author_rank: Unknown
timestamp: October 25, 2015, 12:56:39 pm
content: Quote from: PA0PBZ on October 25, 2015, 12:44:40 pmThat was my initial guess If that is the case, you could get away with something in the 90 to 200 micro henry range. All that receiver appears to be is a crystal set with an audio amplifier attached.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 25, 2015, 01:22:45 pm
content: As discussed before it is a choke and not part of the tuning circuit, that is the coil on the ferrite.

author: gaurav279
author_rank: Contributor
timestamp: November 12, 2015, 04:15:32 pm
content: Dear Forum Members,Thanks for all your kind Support on this Topic.After putting some efforts, I have traced and drawn the Circuit Diagram (Schematic) of this Matchbox Size AM Radio.I am uploading a JPEG Image of the same.I would request if any of the community members help me trace the Value of theMissing L3(Vertical Choke Inductor Coil), it would put Life in 2 of my such dead radios which are dead without this Choke Inductor Coil.Looking forward for your kind support...Thanks Once again...GauravIndia

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 12, 2015, 04:55:36 pm
content: Your circuit diagram is far from correct, but that doesn't matter. We already discussed this before, so why don't you just try an inductor of about 2mH?You could even make one yourself by putting about 100 turns of thin wire around a nail...

author: gaurav279
author_rank: Contributor
timestamp: November 12, 2015, 05:18:28 pm
content: Quote from: PA0PBZ on November 12, 2015, 04:55:36 pmDear PA0PBZ,Thanks for the kind revert.Can you please advise me where I am wrong in making this circuit.What did you did not understand from this circuit...I have made this circuit keeping the original PCB in front of me with all components mounted.The GANG Condenser is a AM Crystal Radio common Gang Capacitor for receiving AM signals (540 Khz - 1600 khz).If you need any other missing data then please advise me...Thanks once again for your kind support...GauravIndia

author: JimRemington
author_rank: Regular Contributor
timestamp: November 12, 2015, 05:45:34 pm
content: The circuit diagram has several obvious errors.1) one of leads to the "495" transistor is not connected2) the C and B of the BC548 are switchedPlease check everything again.Also, try using google image search for "two transistor radio schematic" -- there are a couple of similar results.This seems very close and indeed is labeled "match box size radio":http://bestengineeringprojects.com/electronics-projects/smallest-radio-circuit-using-two-transistors/

author: Andy Watson
author_rank: Super Contributor
timestamp: November 12, 2015, 05:59:27 pm
content: Quote from: JimRemington on November 12, 2015, 05:45:34 pmVery close, and it also describes the coil as being an indeterminate number of turns wound on to an "IFT" former.Quote

author: JimRemington
author_rank: Regular Contributor
timestamp: November 12, 2015, 06:09:39 pm
content: A similar design is shown herehttp://www.techlib.com/electronics/reflex.htm(see section titled "Two Transistor Reflex Radio"), in which the first stage amplifies both RF and audio. I had forgotten about that design!

author: Ian.M
author_rank: Super Contributor
timestamp: November 12, 2015, 06:14:42 pm
content: The closest example schematic for a single transistor, two diode charge pump detector reflex regenerative HF/AF amp stage direct coupled a ferrite rod antennae that I have been able to find is the Ladybird TRF radio by George Dobbs, (G3RJV)http://www.mds975.co.uk/Content/george_dobbs_trf_radio.htmlIf you want to understand it enough to draw the actual schematic of your radio, read the book, but remember, it was written for Germanium PNP transistors, so the positive rail is at the bottom going to the transistor emitters. To rearrange for modern Silicon NPN transistors, the bottom becomes the negative rail, still going to the emitters, all diodes must be reversed, and usually some attention to the base bias resistor networks is required, due to their higher Vbe and HFE. The feedback capacitor C3 can actually be two insulated wires twisted together as a gimmick capacitor (http://sm0vpo.altervista.org/begin/gimmik-0.htm) or even just a carefully positioned wire rather than a discrete trimmer or fixed capacitor, so its presence may not be obvious.Edit: Jim's just found a NPN silicon version of a non-regenerative reflex stage with a two diode charge pump detector. You will have to see which is closer to your set's circuit.The first stage circuit in your schematic is a total mess, and the second stage has B and C swapped.

author: gaurav279
author_rank: Contributor
timestamp: November 12, 2015, 07:25:02 pm
content: Quote from: JimRemington on November 12, 2015, 05:45:34 pmDear JimRemington,Thanks for the reply.You indeed made my day.This was the circuit I have been searching since a long long time.The circuit on bestengineeringprojects.com that you have pointed is exactly the one that I have drawn from referring my Tiny non working Matchbox AM Radio.By observing the same I have corrected the circuit in which the Base lead of Transistor 495 is going to the junction of 0.01uF capacitor and 33K resistor.But the Value of L3 Inductor Coil is still not mentioned and the author has advised to make it using 56 SWG over IFT Former... but he hasn't mentioned the length or number of Turns of this 56 SWG Wire to be used or the definite Inductance Value here.Can you pls advise me now on what Value of a Fixed Inductor (a Resistor Type Inductor) should I use in this Circuit to make it Alive again.Thanks !!!Gaurav

author: JimRemington
author_rank: Regular Contributor
timestamp: November 12, 2015, 07:34:10 pm
content: Asmany peoplehave already suggested, including the author of the circuit I posted, the value is not critical. Try a 1 mH inductor.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 12, 2015, 08:36:51 pm
content: Quote from: gaurav279 on November 12, 2015, 07:25:02 pmIt is mentioned in the article: "Counting of turns is not necessary. Wind the wire completed over former pulley."Or in other words, just fill it with wire. We already told you a number of times that it isnotcritical.