G'day folks,

I migrated to Australia from Germany in 1974. I used to own a Schaub Lorenz T60 Transistor Radio in the 60s. For nostalgic reasons I purchased the same radio on German eBay. Quite expensive because of the shipping. It was advertised as fully functional. But it is not. All I get is very weak and distorted volume. I traced it down to the two AD155 final stage Amp transistors. Short between emitter and collector, measured desoldered. I'm actually a retired carpenter tinkering in old analogue gear - especially organs. I know a "little" about electronics. I suspect that unsuitable or defective gear was connected to the audio out. There are a few expensive AD 155 transistors on eBay, but there are also some cheaper Russian substitutes offered (GT403E or 1T403E). There is also the issue of matched pairs! I think the Germans call it "Gegentakt Endstufe". But I'm only guessing here. Please enlighten me.

Thank You

You actually don't need the exact replacements for these germanium transistors. A well designed circuit will make the current gain, for example, almost irrelevant provided it's over a certain easily achieved minimum. Early transistor radios had to be designed for transistors that were much further from ideal than the almost perfect devices you can get today.

What this means is you can replace germanium transistors with common, easily-available silicon transistors. The output transistors in your radio are probably PNP (I didn't check), intended for lees than 30v operation and less than 100mA current capacity. That means an ordinary BC557 will work just fine. The same transistor will replace even the devices in the RF stages. I have done this and ended up with a radio that was a very hot performer with much better S/N figures than the original.

The only reason you wouldn't go this way is if you wished to retain exact original internal appearance. But you could always cut the metal can off the old transistor and glue it over the new one.

The issue of matched pairs becomes moot when both transistors in the output circuit will now have current gains well over 200. You can choose BC557B or BC557C if you want a closer match for current gain.

You may hear some crossover distortion (depending on if there is negative feedback around the amplifier). If so, a small adjustment to the bias will fix it.

Oh, I just looked up your output transistor and it's bigger than I thought. I was thinking of a portable transistor radio. Yours has much more output power than that.

Same applies, BD140 will do the job. Readily available, different case but leads can be bent to fit the TO66 footprint.

Silicon transistors are much more rugged than germanium anyway.