23 January 2009

21st Century PTO

I was quite puzzled when I bought a used 2005 car in 2007 and it came with a peculiar el-cheapo radio. It was an Audiola SCD-857N for which there's no mention on the Web, with the following specs:
  • FM broadcast band only, very insensitive
  • analog-like tuning (yes, a knob!) but digital frequency display
  • obviously no memories
  • CD player for original media only, not even a CD-R
At the time being, January 2009, Audiola/Majestic still sell devices with similar receiver specifications in the Italian market at 50 euro entry level price tag.

You cannot imagine my engineer's happiness when the front panel hook broke and I had to replace the whole device: I could finally peek into the tuning mechanism! The replacement is an old FM/MW/tape Daewoo car radio that I kept aside 9 years ago because of its good receiver performance (so at least 14 y.o. device). Tape vs original CDs doesn't make much difference to my listening comfort! The replacement process will be documented in another post.

The Audiola radio was built in 2004 according to its outer labels. The CD player module is stamped 12/2003. Inside I found two large pieces of thin cardboard as insulators between boards. 3 ICs are SMT, the rest are through hole R/C/L. But the tuning thing...

there you see it, at the rightmost side of the chassis. Here's a zoom view:

First thing, the black disk. it is a sort of clutch that transmits rotation from the knob to the PTO. It is needed because PTO axis and knob axis are not aligned. Cool.
Inside the metallic box there are 6 cylinders, of which only two are used, and a plastic plate pushes inside them a metallic rod. Behind them there is a shielded circuit that I haven't opened yet. It has 6 wires plus ground, a couple of holes for adjustment (there's an IF-can). One line comes from the antenna plug and another goes near the display driver chip (frequency counter?). Other 4 lines (grey wires) land nearby a couple of ceramic filters.

I have been able to remove the whole block, but not taken a picture.

What is behind the variable L? A passive circuit? Active? Could I reuse this input stage for 2m or 4m? Has anyone played with similar things?

In any case I was surprised to find such an arrangment in 21st century instead of a mechanically simpler varicap or a PLL.