[maxhifi]

Waiting for parts to arrive for all the projects I am currently working on, reading the thread about the 7 transistor radio kit and having nothing needing to be done around the house, I decided to give this little radio a little attention.

I picked this up on a trip to Russia (I can't help but seek out vintage electronics no matter where I go, and iron curtain stuff is appealing to me because it's just so different). It has a couple of features which make it a little different from the typical pocket radio. First, two of the two integrated circuits are a pretty unusual design, the third is a normal DIP package.

The other thing, is the frequency bands it covers. Note: none of them are active here (Canada), at all. When I got it home and turned it on I got nothing but noise, and some strange beeping sounds from poorly shielded power supplies and other equipment in my house.

Yesterday I decided to have a little fun with the radio, and make it work on the 88-108 MHz FM broadcast band. First step was tracking down a schematic. Then I looked up data on all the chips, and set out to figure out how the radio works. Once I knew enough to be dangerous, I had a plan. A lot of people modify the tuning range of FM radios by messing with the coils, but I wanted to leave mine alone and instead change some fixed capacitors. My reasoning is I can easily put the old capacitors back, but once the coil is modified it's a one way change.

Given the values of the capacitors marked on the schematic, and the frequency band covered, I figured out the inductance of the tuning coil, and calculated new values for the capacitors to shift it up to 88 - 108 MHz. I calculated 16 pf and 33pf, and installed 10 and 33. I didn't have anything closer to 16 than 10 in my capacitor drawer. If I increase the 10uF to 16, it will remove some usless band coverage above the FM broadcast, but it's not a big deal.

Anyway, the modification was effective, and the radio has been entertaining me on and off all day. It could be a bit more sensitive (some radios using this IC employed a single transistor RF preamp), but for what it is I can't complain. It does tend to drift a bit, and I think that is partly down to design, and partly down to my modifications using really super cheap ceramic caps I got in a Radio Shack surprise box 20+ years ago.