Icom IC202-S BFO Inductors - UK Vintage Radio Repair and Restoration Discussion Forum

Jon_G4MDC · 7th Mar 2016, 6:50 pm

Hello to all. I wonder if anyone has any information about this please.

The IC202 was a popular SSB rig for 2m using a VXO to generate local oscillator from 14Mhz crystals.

It appeared in a few variants over the years
IC202 & IC202-E had Upper Sideband Only
IC202-S intended also for Satellite use included LSB to deal with inversion through frequency transponders.

I acquired a 202-S some time ago as a box of remains. It lacked the BFO PCB so I built one from scratch by copying the foil patterns in the handbook.

There is a single 10.7015 MHz crystal in a Colpitts circuit whose frequency is pulled by 1x trimmer capacitor and 2x coils switched by diodes to produce 10.7015, 10.699.2 and 10.6985MHz.

The problem I have is with the value of the coils.

The frequency set trimmer works perfectly but the frequencies to set with the coils cannot be reached. This is because they were wound by total guesswork.

The original Icom Inductor Part number was LS-111. Does anyone have any clue what inductance that would be please? I have other IC202s so I could pull one apart for measurements but for obvious reasons that is left as last resort.

It will be bad enough getting the homebrew BFO out again to work on.

Many thanks Jon

orbanp1 · 8th Mar 2016, 5:47 pm

Hi Jon,

Do you have the BFO schematics for the 202S?
I could only find schematics for the 202.
I could plug the circuit into the simulator and see what comes out.

Regards, Peter

Jon_G4MDC · 12th Mar 2016, 8:15 pm

Thanks orbanp1!
That is something I considered but the simulation of oscillators is an interesting subject.

I thought about modelling just the crystal and pulling circuit components instead.
ARRL radio designer could cope with that - all that is available here.
Maybe that was your plan?

I wonder what you have available for simulation? Maybe it is full blown ADS!! Then oscillators for you should be no problem!

orbanp1 · 13th Mar 2016, 5:24 pm

Hi Jon,

I have been using LTspice for simulation.

I am enclosing a picture of the BFO oscillator schematics of the IC-202 that I used, and the simulation.
This is the plain 202 BFO, no pulling the oscillator by coil, only by cap, switched in value by the first transistor. I did not model the switch, just the two different capacitance values.
I used a "generic" 10MHz crystal model that was around the net.
I put the resultant frequencies as comments in the schematics.
The .measure statements do "measure" the frequency by measuring the time for 1000 cycles.
This oscillator had no trouble starting up in the simulation, but it took a while for the turn-on transients to settle down, that is why the data collection only starts at 4msec.
The output voltage is about 2Vpp, and the second harmonic is down by 17dB (no LC tank circuit).
My guess is that there would not be any problems including a coil as well.

Peter

Jon_G4MDC · 13th Mar 2016, 8:48 pm

Hello Peter - thank you for your interest.
I have come across LTspice but so far I have not explored it much. Probably I should.

Today has seen much work on this BFO. I decided to attack it and the results have been good. It's back in the set and working.

The story is long so I will save writing it up for another time.
I remembered a few things from when I first built it many years ago (10+ possibly) which all make more sense now.

The schematic you found is probably the IC202 USB only. The transistor switches in series capacitance to shift 10.6985 USB BFO up to 10.9995 MHz for CW TX with offset.

The IC202-S BFO generates 10.6985 (USB), 10.9995(CW TX) and 10.7015 (LSB) and here's why it has a trimmer which sets 10.7015MHz first, first coil shifts it to 10.9995, and a second coil in series with the first shifts to 10.6985MHz.

The coils I put in at first were guesswork. Not now.

Jon_G4MDC · 14th Mar 2016, 10:53 am

Now there is a bit more time here is the story of the IC202S BFO.

Icom used 2x LS-111 inductors and these have 3 connections. Obviously they are a main winding with a tap. This is all shown on the diagram which was the crib for the PCB foil pattern.

I used two inductors which “looked about right” to get the thing going. Originally these were crystal pulling coils in a Pye mobile radio and they were chosen because the can and pin pattern were correct rather than anything else.
.
I took the BFO out and removed the coils. A Grid Dip Oscillator and a known parallel capacitor made a crude way to estimate the inductance. One had a range of 1-2uH the other 3-6uH approximately.

Years later I find the junk box now contains some Toko 7K series inductors which also match the PCB – we are in business! The Tokos were 0.5-1uH and had 11turns so a rewind is in order. First guess using N squared was to try 28 turns. It was a few too many.

It is very happy with 20T as L15 and 24T as L16. If Icom used the same ferrite as Toko (and who can say) these were probably 24T with a tap at 4T.

Something to do on a Sunday afternoon! Now looking for contacts.
70.200MHz anyone Herts/Essex area?

7th Mar 2016, 6:50 pm	#1
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Icom IC202-S BFO Inductors Hello to all. I wonder if anyone has any information about this please. The IC202 was a popular SSB rig for 2m using a VXO to generate local oscillator from 14Mhz crystals. It appeared in a few variants over the years IC202 & IC202-E had Upper Sideband Only IC202-S intended also for Satellite use included LSB to deal with inversion through frequency transponders. I acquired a 202-S some time ago as a box of remains. It lacked the BFO PCB so I built one from scratch by copying the foil patterns in the handbook. There is a single 10.7015 MHz crystal in a Colpitts circuit whose frequency is pulled by 1x trimmer capacitor and 2x coils switched by diodes to produce 10.7015, 10.699.2 and 10.6985MHz. The problem I have is with the value of the coils. The frequency set trimmer works perfectly but the frequencies to set with the coils cannot be reached. This is because they were wound by total guesswork. The original Icom Inductor Part number was LS-111. Does anyone have any clue what inductance that would be please? I have other IC202s so I could pull one apart for measurements but for obvious reasons that is left as last resort. It will be bad enough getting the homebrew BFO out again to work on. Many thanks Jon

8th Mar 2016, 5:47 pm	#2
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Icom IC202-S BFO Inductors Hi Jon, Do you have the BFO schematics for the 202S? I could only find schematics for the 202. I could plug the circuit into the simulator and see what comes out. Regards, Peter

12th Mar 2016, 8:15 pm	#3
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Icom IC202-S BFO Inductors Thanks orbanp1! That is something I considered but the simulation of oscillators is an interesting subject. I thought about modelling just the crystal and pulling circuit components instead. ARRL radio designer could cope with that - all that is available here. Maybe that was your plan? I wonder what you have available for simulation? Maybe it is full blown ADS!! Then oscillators for you should be no problem!

13th Mar 2016, 5:24 pm	#4
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Icom IC202-S BFO Inductors Hi Jon, I have been using LTspice for simulation. I am enclosing a picture of the BFO oscillator schematics of the IC-202 that I used, and the simulation. This is the plain 202 BFO, no pulling the oscillator by coil, only by cap, switched in value by the first transistor. I did not model the switch, just the two different capacitance values. I used a "generic" 10MHz crystal model that was around the net. I put the resultant frequencies as comments in the schematics. The .measure statements do "measure" the frequency by measuring the time for 1000 cycles. This oscillator had no trouble starting up in the simulation, but it took a while for the turn-on transients to settle down, that is why the data collection only starts at 4msec. The output voltage is about 2Vpp, and the second harmonic is down by 17dB (no LC tank circuit). My guess is that there would not be any problems including a coil as well. Peter Attached Thumbnails

13th Mar 2016, 8:48 pm	#5
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Icom IC202-S BFO Inductors Hello Peter - thank you for your interest. I have come across LTspice but so far I have not explored it much. Probably I should. Today has seen much work on this BFO. I decided to attack it and the results have been good. It's back in the set and working. The story is long so I will save writing it up for another time. I remembered a few things from when I first built it many years ago (10+ possibly) which all make more sense now. The schematic you found is probably the IC202 USB only. The transistor switches in series capacitance to shift 10.6985 USB BFO up to 10.9995 MHz for CW TX with offset. The IC202-S BFO generates 10.6985 (USB), 10.9995(CW TX) and 10.7015 (LSB) and here's why it has a trimmer which sets 10.7015MHz first, first coil shifts it to 10.9995, and a second coil in series with the first shifts to 10.6985MHz. The coils I put in at first were guesswork. Not now. Last edited by Jon_G4MDC; 13th Mar 2016 at 8:53 pm.

14th Mar 2016, 10:53 am	#6
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Icom IC202-S BFO Inductors Now there is a bit more time here is the story of the IC202S BFO. Icom used 2x LS-111 inductors and these have 3 connections. Obviously they are a main winding with a tap. This is all shown on the diagram which was the crib for the PCB foil pattern. I used two inductors which “looked about right” to get the thing going. Originally these were crystal pulling coils in a Pye mobile radio and they were chosen because the can and pin pattern were correct rather than anything else. . I took the BFO out and removed the coils. A Grid Dip Oscillator and a known parallel capacitor made a crude way to estimate the inductance. One had a range of 1-2uH the other 3-6uH approximately. Years later I find the junk box now contains some Toko 7K series inductors which also match the PCB – we are in business! The Tokos were 0.5-1uH and had 11turns so a rewind is in order. First guess using N squared was to try 28 turns. It was a few too many. It is very happy with 20T as L15 and 24T as L16. If Icom used the same ferrite as Toko (and who can say) these were probably 24T with a tap at 4T. Something to do on a Sunday afternoon! Now looking for contacts. 70.200MHz anyone Herts/Essex area? Last edited by Jon_G4MDC; 14th Mar 2016 at 11:01 am.