[NLRS] Poundshop FM radios at Dollar Tree

[Dr. Gerald N. Johnson]

Moving the FM BC receiver down to 30 MHz, I'd consider tripling the 
values of the bypass capacitors for the front end.

Back a couple sunspot cycles I took on designing the Ditto Radio. A 
crystal controlled pocket radio for Ditto heads to listen to WRNO on 
15.420. I found a nice Toshiba AM radio chip with good AGC and good 
audio that ran on 3 volts in a RS Flavor Radio. Finding data on the chip 
was difficult, Toshiba officially told me the part number was invalid, 
at least in this country. Turned out they just didn't import analog ICs 
to the US. A rep in Cedar Rapids found a data sheet which he copied for 
me. It had little frequency information. But I found by experiment that 
it worked decently on 15.420 with a separate oscillator. In 1 MHz 
operation they used a bipolar transistor in the chip as a self 
oscillating mixer. I didn't want to spend the time getting that to work 
at 15 MHz with a crystal, so I spent the extra 4 cents on a separate 
transistor. I used a 15 MHz microprocessor crystal available for 39 
cents in quantity and I set it up with three IF coils instead of two for 
better selectivity skirts. 455 kHz IFs tuned down to 420 easily enough. 
I used a 4 turn loop on the PC board, about 1/3 the total board area for 
the antenna. We went through a broker for the chips and they cost less 
that way than buying Flavor Radios. Several thousand were built, I don't 
know how many were sold, the seller's guarantee didn't specify it didn't 
cover loss of propagation from lack of sunspots so he got many working 
radios back. I warned him up front that propagation would be a problem 
in a few months, and I produced the design in about 40 billable hours 
over 1 month's time which was what I promised. The custom case took much 
longer to get the mold and he didn't follow my advice to build the first 
few thousand using a custom machined stock case. He wasted too much of 
the remaining 20 meter propagation and got burned. Every time I 
suggested a stock case, he'd respond, "But once the mold is made, each 
case costs only a couple pennies." Not getting to the market quickly 
cost the whole project but I wrote my proposal and contract so I got 
paid on design delivery not dependent on sales or returns.

I had to experiment with the chip to find out what it would really do. I 
figured after my HF experiments that I probably knew more about it than 
the factory, for sure more than the factory would admit. Though I didn't 
look for a Japanese language data sheet and applications note that might 
have had much more information.

My first prototype to check the concept of the loop and a portable SW 
receiver, I used a NE602 as converter. Looking for chips I found Plessey 
had some but they quoted 50 weeks delivery on samples, I decided they 
would figure out how to make them if they had an order. I didn't have 
the luxury of that much time. I did built a bread board with NE602 and a 
couple op amps. NE602 conventionally as front end, then a couple with DC 
on the oscillator base for gain control as amplifiers, another with 
signal on oscillator and signal input as a detector and op amp to offset 
the AGC voltage. I probably used a LM380 or something like it for audio. 
It did work. the NE602 would still work as a FM detector with the right 
circuit around it, today a NE612 would work but there are so many FM IF 
and detector chips or even whole FM receiver chips that using separate 
chips wouldn't be good use of bench time. Modifying the $1 receiver 
should work well though it probably will need a metal case to keep out 
potentially interfering signals from AM BC to light.

The Toshiba based radio was hard to align because of the good internal 
AGC. I had to show the contract assembly shop how to do it. A noisy 
environment in their factory didn't help.

Problem with using the FM radio chip for 50 MHz CW or even ham bandwidth 
FM is that with an IF 150 to 200 kHz wide, oscillator stability isn't 
nearly as critical as it is for CW or 5kHz deviation FM. CW as a concept 
though shouldn't be hard, just get an oscillator near the receiver at 
the IF frequency to get a beat note. I found my good Fischer FM receiver 
copied noise quite fine when tuned off to the side of a carrier (LO of 
another FM reciever) good enough to copy thunderstorm static from 
tornadic clouds half a state or more away. (long yagi sure helped.)

On 12/12/2012 4:27 PM, Doug Reed wrote:
>
>
> <http://www.hanssummers.com/poundshop.html>
> I remember seeing articles about this cheap FM radio early this year and
> being surprised that anyone managed to turn a $1 FM radio into a 40M CW
> receiver. At the time we were doing a little email about using the 24GHz
> Gunnplexers to make simple WBFM rigs for the 10GHz & Up contests.
>
> One of the standard problems with using the Gunnplexers is always the
> 30MHz WBFM IF receiver. If you go looking in the old info, you will see
> that designs using the TDA7000 chip were common but not many people ever
> got them working.
>
> I was at the Dollar Tree store in White Bear (Cty Rd E & Hwy 120)
> yesterday and found they had a supply of the little auto-tune FM radios
> back in stock again. I bought one and opened it up this morning. It has
> a SC1088 FM receiver chip, which is a clone of the TDA7088 chip, which
> is a newer relative of the TDA7000.
>
> So take a $1 radio and change a cap on the PCB to move it down to 30MHz
> as a Gunnplexor IF receiver. Or follow one of the other articles on the
> web to make it a 40M CW receiver or anything else you can figure out.
> The various data sheets I've found would also give enough info to modify
> the unit for fixed or mechanical tuning and AFC if you wanted to operate
> that way....
>
> The other interesting thing is that I didn't see a frequency spec on the
> data sheet. It will probably handle 50MHz wouldn't it..... 50MHz CW?
>
> If you go looking for one of these receivers, you should look for two or
> three buttons. One is RESET, one is SCAN, the other is a flashlight. If
> you are lucky it also has a volume control.
>
> Beats the heck out of trying to build the TDA7000 receiver from
> scratch.... May not be as good as the Ramsey 10M FM receiver modified
> for crystal control, but it is at least $40 cheaper. :-)
>
> The HansSummer web site has another page about the "SuperDRG" AM-SW
> portable radio that often sells in the $10 range and various mods you
> can make to it. What is interesting about this radio is the 4-digit
> frequency counter it uses for a display and with various mods for CW &
> SSB from 20M and down. I haven't read that article in depth and haven't
> checked the local stores to see if I could find anything like it....
>
> 73, Doug Reed, N0NAS.
>
>