These are mods I did with a GE solid state table radio, like their model T1290A. A better AM detector, an AM RFI filter for its power feed, and a better FM front end, using a JFET in place of a bipolar transistor. For good FM strong signal performance, ie, better FM receive dynamic range, ie, a better intercept point. avoiding intermod products.



This homebrew set above uses a GE circuit board almost identical to that in the GE T1290A. Right click "view image" to see the schematic, with my mods. full size. To improve the FM front end, I changed a grounded (with respect to RF) base bipolar transistor (BJT) to a grounded gate JFET, a 2N3822, in the FM RF amp stage. This is to improve the RF stage to have good strong signal performance, ie, better FM receive dynamic range, ie, a better intercept point. avoiding intermod products. The JFET, running at a similar amount of current, will likely be more linear than the old BJT and that gives a better intercept point. The old NPN BJT device likely had low enough noise, but running it at low current (for high gain), yielded possibly slightly better gain, but more seriously, definitely a poor intercept point. You'd want better intercept point if you live near New York City, with its crowded FM dial (see below). The JFET also has a low noise figure and decent enough gain. You could increase the current thru the BJT to improve its dynamic range, but the noise performance would be worse than the JFET. And the mod would also need substantially different circuits to get beyond that, much more mods than I wanted to do.

I also tried other JFETs, a 2N5458 with a 180Ω. or MPF102 with a 220Ω, source resistor, and got results pretty much the same. I kept the 2N3822, as its data sheet states that it's for VHF work, and it seems just noticeably better. And here is a method of checking your junk box JFETs.

NYC's crowded FM dial. Below the before and after of the FM front end:

FM fool's predicted NYC dial:


Additional mods I did include a separate AM detector using a base-collector strapped transistor for better weak signal demodulation, an RFI filter on the incoming powerline.

Inside this homebrew set is the RFI filter, and series resistor.
This RFI filter knocks down crud from the powerline. Note, right of the power resistor, the white wire with orange and blue, like Syracuse Orange and Blue.

If you are working on GE radios like this, check the low value resistors. I had to replace a 100 Ω, a few 150 Ω and a 220 Ω resistors. They all went flakey. The bad 100 Ω resistor made the FM local oscillator quit at the lower end of the band. In a few spots, where the circuit board nodes were close together, found it easier to use the larger sized surface mount resistors to replace the old resistors that were in crowded areas on the component side of the board.