Repairing a Vintage GE Model P945B AM Radio

2/22/2016

The GE Model P945B Radio is a 6 Transistor AM Radio first offered in 1964. It covers the AM Broadcast Band.

It is of the Superheterodyne design featuring the following sections:

Mixer/Oscillator - Used to convert AM broadcast band into an IF (Intermediate Frequency) of 455KHz.

IF Amplifiers - Bandpass amplifiers that pass/amplify a narrow range of frequencies in the 455KHz range.

Diode Detector - Removes IF envelope leaving audio output.

Audio Preamplifier - Prevents loading of the IF Amplifier and provides an impedance match to the push-pull audio amplifier.

Audio Amplifier - Efficient two transistor push-pull amplifier used to boost the audio to make it suitable for listening without earphone.

One of the unique features of this radio is its use of a high impedance speaker with reed suspended between 2 pole pieces that drives the paper cone with a stylus. It is directly driven in push-pull by a pair of collectors, without an audio output transformer.

This radio falls under the "vertical shirt pocket" type, although you would need a rather big shirt pocket to contain this radio!

It also has a jack so a high impedance earphone can be used. Below are pictures of the GE radio prior to repair.

Step 1 Obtain Schematic

While you can do some troubleshooting without a schematic, as basically the majority of "shirtpocket" AM radios from the 1950s up until 1980 are built the same. Having a schematic diagram greatly reduces the amount of troubleshooting time as you can quickly identify areas of interest. I was able to find a schematic and component placement diagrams for my model P945B on the RadioMuseum site. Click on the button below to go to the URL.

GE AM Radio Model P945B Schematic

Step 2 Determine Symptoms

This GE radio requires two AA "Penlight" batteries for power. I installed two fresh batteries then turned the volume knob to the "ON" position. I could not pickup any AM Radio stations no matter how high I turned up the volume or adjusted the tuning dial. In addition, I could not hear any atmospheric noise, that you typically hear in between radio stations, when tuning.

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Step 3 Disassemble

The disassembly process of vintage portable AM radios is much easier than tabletop or console models. The back cover typically just snaps in place for easy replacement of the batteries. You just need to insert penny in the slot at the bottom of the case then twist to liberate the back cover and expose the inner workings.

Only one screw, circled in red below, holds the printed circuit board in place.

The speaker is held in place to the front cover with two tiny bolts. I had to use needle nose pliers to remove. Below is a picture of the printed circuit board and speaker liberated from the case.

Step 4 Troubleshooting

The first thing I determined, by taking a Voltage measurement between the positive and negative power rails, was that the GE radio was not getting power. I should be able to see 3 Volts! Power rails are circuit traces on the printed circuit board used to power all of the different sections of the radio. They are typically located at the outside edges on the foil side. I determined that the issue was that the batteries contacts had a layer of rust on them that prevented a good electrical contact. I used a Dremel wire brush on these contacts until they were shiny then reinstalled the batteries in the right polarity. Success!
The radio was getting power yet my original "no sound" symptom was still there.

Next, I figured I would inject an audio signal at the Cathode of the Detector Diode to determine if the issue is with the audio section. One connection of my homemade audio generator goes to the ground bus, which happens to be positive in this radio due to its extensive use of PNP transistors, and the other side to the Cathode of the Detector Diode, which is the input of the audio pre-amplifier.

Again there was no audio, even with the injection of an audio signal into the audio pre-amplifier. After some investigation, I discovered that the tarnish on the contacts of the earphone jack was preventing a signal from getting to the internal speaker. The contacts are normally closed to allow the audio signal to the internal speaker but open when and earphone is plugged in to divert the audio signal to it. Spraying contact cleaner on the contacts removed the tarnish and finally I could hear the injected audio signal through the internal speaker.

The next issue was that the I would hear static when changing the volume level. I determined that the potentiometer used for audio control was dirty and sprayed a little contact cleaner where the wiper meets the carbon resistive element. I then worked the volume control potentiometer through its full motion until the static was gone.

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OK, so I fixed all the issues in the audio section, but I still couldn't hear any atmospheric noise or radios stations. This lead me to conclude there was an issue with the mixer/oscillator or IF amplifier sections. One way to test the IF Amplifier sections is to inject a modulated 455Khz RF signal into the input of the IF amplifier sections using an RF generator. You should hear the audio tone that is modulating the RF carrier if these amplifier are working correctly. In my case there was no audio sound from the internal speaker of the radio so I knew there was an issue with one of the two IF Amplifiers, but which one?

You can use the process of elimination to determine which IF Amplifier stage is at fault. Start by injecting the 455Khz modulated RF signal in the IF Amplifier that feeds the Diode Detector. If no tone is heard from the internal speaker then you know you isolated the bad stage. If a tone is heard, then by process of elimination you know the remaining IF Amplifier stage is at fault. In my case I had determined that the IF Amplifier closest to the Detector Diode was at fault. Now I needed to determine the issue down to component level.

Upon closer examination I noticed that there was a lot of corrosion on the metal shield that enclosed the IF Transformer of the second IF Amplifier. See area circled in the picture below:

I surmised that maybe the corrosion had leaked into IF transformer causing one or more of the ultra tiny wires to break. I few resistance checks told me that I was right! But where do I get an IF Transformer for a vintage AM radio from 1964, surely it would be made of "Un-Ob-Tainiam" and was impossible to get. Upon further examination it looked just like the IF Transformer from a bread-boarded AM radio kit from the 1980s I was about to scrap. The IF Transformer from the 1980s radio was exactly the SAME! Nothing in IF Transformer construction changed in over 20 years. The soon to be replacement IF Transformer is circled in red below.

This GE radio sprang to life once the IF Transformer was replaced and batteries were install. AM Radio Stations now came in loud and clear!

Step 6 Electrolytic Capacitor Replacement

As electrolytic capacitors age, their electrolyte dries up causing their electrical capacity to drop and leakage current to increase. It is definitely a good idea to replace electrolytic capacitors that are over 50 years old! This GE radio employed three electrolytic capacitors. All electrolytic capacitors should be replaced with one of the same or slightly greater capacitance and working voltage rating. I replaced the electrolytic capacitors circled in red in the picture below:

Below is a picture of all of the parts replaced in this vintage GE radio. The IF Transformer was so corroded that it crumbled when removed from the printed circuit board!

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Step 7 Detailing

The front and back cover and knobs of the radio were dirty and the plastic had yellowed. In addition, the vinyl carry case with strap was embedded with years of grime and had yellowed as well. It was AMAZING how well all of the parts looked after being cleaned with "Mr Clean Magic Eraser", the sponge looking thing at the bottom of the picture.

A toothbrush works good for cleaning dirt in grooves and holes in the speaker grill section. Plus your vintage radio is guaranteed not to get cavities!

I used solder flux cleaner to remove the original flux from the foil side of the printed circuit board and the new flux caused by the replacement of the IF Transformer.