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.

Further below, I added an AM RF stage to this radio.



This homebrew set, now an "18 transistor", 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:

One version of the new FM front end uses a VHF TV set balun. But the JFET will see everything in the VHF spectrum. Another version uses an LC circuit tuned to around 99MHz. One turn of insulated wire forms the input from the FM antenna terminals but isolates the antenna terminals from the hot chassis shock hazard. Only thing here is that the radio will be less sensitive at both ends of the FM dial. Using a pair of LC circuits (one around 91MHz, the other around 105MHz) coupled via induction and stray capacitance to give a reasonably flat band pass over most of the FM dial helps here. Using a grid dip meter helps a lot in getting these LC circuits on these frequencies. I used a single ferrite adjustment slug mostly for the 91MHz LC circuit, I had to fiddle with trim caps to get both resonances where I wanted them. The single ferrite slug helps couple RF antenna energy across the LC circuits and into the RF amp stage. You can check the grid dip meter's calibration by seeing what its dial says when you jam an FM station of known frequency.


Closeup of the FM front end's input and band pass filter. The LC circuit further from the circuit board is peaked around 105MHz, the closer one 91MHz. Antenna coupling is an almost single turn of wire fed from the antenna terminals. Maybe this makes it around 300 ohms? Probably too short to matter. Reception is slightly better than that I had with the input balun. The curve is just a guess, stations around 98MHz come in pretty well, so they are likely just "couple o' dB" down. Sure, it might be better if I had a 3rd FM tuning gang on the tuning cap, but I don 't... (but see further down this web page). I did try a tuned circuit on the FM RF amp's input (built it with a grid dip meter to get its frequency range correct) with an independent tuning cap, but it didn't seem effective, about a db at best. Not worth the trouble of mechanically rigging it to the tuning dial.

The modified radio, with replacement resistors and electrolytics. And note the white wires with orange and blue (Syracuse!) colors on the tuning cap.


Adding an FM input LC tuned circuit, And the mechanical construct to couple it to the rest of the tuning caps


The main difficulty in adding a tuned LC circuit on the input to the FM RF amp is to mechanically couple it to the rest of the radio's tuning mechanism. Obtain another AM FM tuning cap and use its FM RF section, and ignore the FM osc and the AMs. Initially, I mounted the new cap coaxially with the existing tuning cap. And I used a long screw to act as a pin attached to the new cap that in turn fits into a new hole in the tuning wheel mounted to the existing tuning cap. So when the radio is tuned, this pin also makes the new cap also get tuned. A kludge, but it worked reasonably well. I then decided on a more elegant method, mounting the new cap to the existing piece of circuit board I mounted the tuning knob to. And using a pair of same size tuning pulleys and dial string to couple it to the main existing tuning cap.

Looking at the bottom of the "new RF amp" schematic above, you can see the new FM front end RF amp input LC circuit. With the new variable tuning cap "C1K" (with its own trimmer C1M, and the coil LN are adjusted using a grid dip oscillator (GDO) to have it track the existing tuning cap. Tune in a station near the bottom of the band, tune the grid dip oscillator to "jam" that station, and adjust trimmer C1M to get the new LC circuit to resonate (which is when the GDO dips). Do that with another station midway in the band, and another near the top of the band. and iterate a couple of times. And adjust the inductance of coil LN a little, by spreading or squeezing the turns. The GDO's dial may be off, but jamming a station tells you you have the desired frequency dialed up on the GDO.
A grid dip oscillator meter.

FM station signal strengths increased on the FM band after I was done. I could adjust the trimmer C1M and see, with this signal strength indicator tweak a peak in signal (easier to see with the station just barely strong enough to dimly light the indicator) at a few different spots on the FM dial. It can be hard to hear any difference except on really weak signals, which would ask the question as to why bother?


Now that I have a second AMFM tuning cap in this radio, I figured I could build an AM RF amp stage. The existing AM ferrite rod antenna L5 LC circuit would stay as is, but its secondary winding would be disconnected from the AM converter transistor. And now it feeds the new AM RF amp stage. The AM RF transistor's collector feeds a new RF transformer, and its secondary now feeds the AM converter. I took an AM 455KHz IF transformer and removed its capacitor. The ferrite rod antenna's inductance measured to be 590uH, and I was able to adjust the full primary of the transformer to the same inductance. I also checked the AM antenna tuning cap C1A, it maxes at 150pF, and the new tuning cap C1R also is the same. So this new transfornmer and the tuning cap C1R should resonate on AM stations, once the trimmer C1T and transformer slug are tweaked.

AM RF amp stages can be hard to tame, I first ended up selecting a rather small emitter bypass cap of 100pF, but the impedance of this would vary from one end of the band to the other by a factor of 3. I did a frequency insensitive feedback loop using resistors in series with 0.1uF caps. These caps, very low impedance here, are mainly to block DC to avoid messing up voltage biasing. The resistors, 51 ohm for the emitter (not really part of the loop) and 10K from the collector back to the base. These resistors keep the gain of this RF stage reasonably constant across the AM band. I had this loop surrounding the tuned stage, but I realized "DUH!" that would reduce the selectivity of that tuned stage... I then made this loop encompass the transistor only. I selected these resistors by trial and error mainly to get amplification without the stage going into oscillation. The collector resistor had the bigger effect.

I used small coax cables to feed this AM RF amp from the ferrite rod antenna, and another coax cable to feed the AM converter the AM RF amp's output. And I used one shield to deliver the AM switched positive supply to this amp, and the other shield the negative ground return. Another short wire connects this stage's RF ground to the tuning cap's RF ground, via a coupling cap, to keep the LC resonant currents local.

This diagram also shows the
JFET AVC assist and received bandwidth circuits. These help keep the strong stations from distorting.

AM MW propagation was good just before sunrise today (Oct 1, 2019), but I didn't expect to hear a station from Syracuse (my college town!) WFBL 1390KHz, but I did! "The Dinosaur". Never heard any station from Syracuse in New Jersey before!

Though I get birdies worse on the 2nd and third harmonics of the IF frequency (from the detector). Not surprising as the radio is more sensitive to signals and noise and stray from the detector.


I changed the half wave rectifier in this set to a bridge. Lower ripple. But I had to double the resistance of the old 230Ω series power resistor R43 to a new one of 464Ω 10W. Else the B+ voltage will be too high. The reason for this resistor change is that the filter cap gets to top off its charge at 120Hz rate instead of the old 60Hz rate. And thus each topping off takes half the current as before. Simulation shows that the resistance of R43 should be doubled to compensates for this reduced current, thus yielding the same B+ voltage. I had some confusion here, not realizing that the battery in my DVM needed to be replaced, Before that replacement, the measured B+ read higher than it really was... DUH!

Note that the ripple on the B+ is about 1.7dB lower with the bridge then that of the half wave rectifier,
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 carbon comp 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. While you're at it, check the rest of the carbon comp resistors, most in my radio went high by 20 to 60%. I decided to change them all out. After replacing them, the radio got more sensitive, like I remember it when it was nearly new. The resistors in the green RC circuit modules (some examples below center) have held up very well, not drifting in value. No need to change those out. These in the picture came from other GE radio circuit boards. My father used to get these boards from some surplus shop on New York City's Canal Street back around 1970.


Above left: unusual resistors, essentially single ended. Likely to sub for regular resistors that would have one lead folded over to have them stand up vertically on a crowded circuit board. They look like the sort you find in Japanese transistor radios, except these have both leads at one end. Usually, to mount an ordinary resistor vertically, you have to fold over one of the leads to run parallel to the resistor body to get back to the circuit board. The pictured resistors look to be a reasonable answer to this, don't know why these didn't become an industry standard. Look to be designed for use in crowded circuit boards. That pesky lead on the top is not exposed. These resistors look to be made using hollow ceramic tubes with resistive material on the outside (under the paint), and one of the leads passing through the hollow tube center and terminating at the top, and the other lead terminated at the bottom dogbone resistor style. Then they were dipped to coat them with the paint and then markings. Used a few of these in the above radio.

Above right is the cabinet my radio circuit board was intended to live inside.

Speaking of resistors, they made 1/8 watt carbon comps. Go easy with the soldering iron or dissipation on these, else the heat will cause the resistance to go up, making them go out of tolerance.



I found a back from a LCD VGA monitor that fit perfectly (horizontally) the wood cabinet I built back in high school. To replace a rather crudely made back I had before. And used an Orange (for my Alma Mater Syracuse! BSEE 1978) power cord I had. Turns out I can occasionally receive (weakly) an AM station WFBL Syracuse here in northern New Jersey.

I did the AM RF amp mod to another radio, and it worked just as well.

Also added the FM input LC circuit mod, and that worked too, though I didn't change the bipolar transistor to an FET.