Restoring a 1960 Austrian 7-transistor MW/LW radio.

From https://www.abetterpage.com/wt/euro/EumigOkay.html

I have several projects that I've not been following through. Here's another one.

It's a 1960 transistor radio with the MW and LW bands, which at the time would have had a good coverage of programming in Central Europe. A contemporary review noted the cheap price and the good value for the money. 

Now the LW, which is mainly a European band, is practically dead, with the MW dying. If I can't get any actual stations, I have the signal generators.

Some notes:

-the construction is weirdly complicated, as if the designers had not yet clearly understood that the PCB is for making things sit nice and straight. There's a large number of jumper wires at odd places. The small signal AF part is an absolute traffic jam. I used 1/8W resistors in the rebuild for that part. I could have used them everywhere for the small power rating there is in the set, but the looks would not be good to my eye.

-the switch pack was likewise oddly arranged, I was able to simplify it a lot. There's one sub-switch that could have been broken. I figured out after some head-scratching I can make the switch set work all the same, but I'll change the seemingly darkened main power switch/speaker connections to other sub-switches and v.v.

-The sound amplifier part has not the ordinary 2nd output transformer, instead the amp feeds into the midpoint of the 3V batteries, which are of a German type I've not seen anywhere. I ordered two 3.6V lithium batteries instead. I think I could also have modified the amp circuit to feed into two "stacked" large value capacitors, and use a single 6V battery, remains to be tested.

-I replaced almost all components. The transistors and the capacitors were somewhat surprisingly all within tolerances, the resistors were all above, as far as I tested. This is the default condition for the non-film resistors.

-I used the AF306, AC 188 and AC153 transistors as replacements, but the originals were fine too. I just like to change things. I kept the old parts for now at least.

-The AF306 comes from the twilight of the Germanium transistor era, and were used in TV tuners well into the 1970s. The part fully disguises itself as a modern Si transistor, the only one I know of such. 

-The rule of thumb is that the higher the intended frequency area for a transistor, the smaller the hFE. I tested and selected examples with the hFE in the 50-100 area to suit the originals. The range was typically 20-40 for the part as indicated in the data. One of the set of 20 showed 4, and another thought he was a diode. Not unexpected at all for old Ge parts, even if they are stock. One AF306 had an absurd hFE of over 300! 

The 3 AF306s all went into the "high" frequency section. I'm hoping I can reduce the "backwards" 180 degrees out-of-phase compensation that was required for the transistor circuits of the time. Their collector-base capacitance was large by today's standards, so there is the added 70pF capacitor here between the stages to prevent the entire circuit from oscillating. I'm hoping I can reduce or even remove the compensation capacitor, and thus make the reception stronger, as the AF 306 has a collector-base capacitance of only 1.5pF vs the 9.5pF of the original transistors. (The set actually has different but equivalent Hitachi transistors installed for the HF part than in the schematics.)

-I have a number of AC188 "power" transistors that have an hFE of 80-90. The value should be over 100, so I took one for the AF signal duty, and used the "power" AC153 for the rest of the positions.

-The replacement resistors were of the classic 0.5W type for the looks, but I used 1/8W parts for the "traffic jam" part. The replacement capacitors are now film types.

-there is a major error in the schematic diagram. In reality, both the LW and the MW coils on the ferrite rod antenna have their respective take-up coils, not just one. So there need to be two 1-0-1 switches for taking care of the 2 switching functions for the antenna circuit. Two 0-1 switches are required to turn on the extra capacitors both in the antenna and the oscillator circuits for the LW band respectively. In addition there need to be two switches for connecting the batteries and the speaker, as otherwise the battery would leak through the speaker. I moved the battery switch to the "lower" 3V battery for the simplified wiring. The caps for the LW now connect on the ground side, not on the "hot" side, for the same reason.

-there's an inconsistency regarding two audio path capacitors, the wiring diagram shows one in the reversed position to the scheme, and for the other the polarity seems wrong if we believe the test voltages on the schematic diagram. I solved the problem by installing two 22u parts back-to-back for the 10u parts, so now I have unipolar 11u parts, and any polarity problems are gone.

Scheme with revisions:

,

Before: 

Afterwards still in the process:. The switch set is taken off for a better examination.

More to come.