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Eddystone EC10 communication receiver restoration and PSU add-in

Recently I got vintage Eddystone EC10 MK1 communication receiver from friend of mine and it is completely dead when it comes to me. EC10 MK1 is one of Eddystone's very early solid state communications receivers and it use 10 PNP Germanium transistors. This receiver cover 550 kHz to 30 MHz and come up with BFO for CW and SSB receptions.
Restored EC10 HF communication receiver

Restored EC10 HF communication receiver

The main problems which I noticed in my EC10 is corroded wires, components and PCB. I did restoration step-by-step and most of the useful steps are noted down in below:
  1. I got schematic of the EC10 MK1 from Eddystone user group. MK2 schematic is also quiet similar to MK1 and major differences between those 2 models are additional fine tuning control and carrier level meter.
  2. Remove 75% of the capacitors in the receiver and most of them are located in the top PCB (green colored PCB). Capacitors located in the bottom PCB are quiet difficult to remove due to mechanical layout of the unit and channel selector switch need to be disassemble to remove some of the capacitors and resistors.
EC10 repairing process on IF and AF amplifier board

EC10 repairing process on IF and AF amplifier board

  3. I keep all the inductors and trimmer capacitors untouched because all of them are seems fine to me.
  4. I replace two OC171 transistors (TR3 and TR5) because of famous tin whiskers effect. In both the transistors I notice short-circuits between Collector and Shield pin. As a solution we can cut down the Shield pin from PCB but I have additional OC171s in my collection so I decided to replace defective transistors with new once. In some sites I notice another solution for this problem. What they suggest is apply some high voltage (via capacitor) to all ECB pins and Shield pin to burn the whiskers. More interesting details related to this problem is found in this forum.

(a) damaged OC71 transistor (b) working OC71 transistor (c) AC125 is direct replacement for OC71

  5. Then I replace OC71 transistor (TR7) because its Base lead is corroded and disconnected. Initially I replace OC71 with new transistor but later I replace it again with AC125 because I have only one OC71 left in my collection and I want to keep it for my other experiments. I didn’t notice any difference between OC71 and AC125 transistor, and both performing equally well.
  6. After replacing components in the PCB I start rewiring. I replace all the wires with new wires and organize wire harness as same as older layout.
EC10 receiver after rewiring and component replacements. Both the PCBs are covered with protective plastic lacquer

EC10 receiver after rewiring and component replacements. Both the PCBs are covered with protective plastic lacquer.

  7. Both the dial lamps in my EC10 are burned and it is really difficult to fine replacement lamps from our local market. So I decided to use two high bright white LEDs as dial lamps. I connect LED to lamp connections through 1.2K resistor.
Wiring diagram for LED base dial light

Wiring diagram for LED base dial light.

  8. I replace existing speaker of EC10 with new 4Ω 12W 5-inch full range speaker. The existing speaker of EC10 is 5inch 3.2Ω but it is little bit difficult to find 3.2Ω 5-inch speakers in local market so I decided to go for 4Ω speaker.

In this restoration I replace majority of passive components which large amount of capacitors and couple of resistors and all the inductors and trimmers are keep as it is. Because of that it may not requires any tuning or calibration. I align the dial indicator of the EC10 receiver using another short wave radio.

The problem with EC10 is powering it with batteries, so I decided to integrate small power supply unit into the receiver. During the testing I notice it draws very little amount of current (aprox. 0.08A – 0.02A) and I build small 8V 500mA power supply using 7808 positive voltage regulator. I fix this power supply into battery compartment as shown in the below picture.

  Schematic diagram of 8V power supply unit.  

8V 500mA DC power supply unit on EC10 battery compartment.

After above changes EC10 starts to work again and with some external antenna is performs quiet well. Most of the times I use this receiver to listen to 4S7RS net on 40m band (7060kHz).