Beolit 600 Type 1501- Converting Output Amplifier from Germanium to Silicon Transistors

I know this is a hot topic for all techs - every now and then I get some older equipment in for repair, and I have to tell the customer that parts are no longer available (or may be available at only uneconomical prices).

This time I got in a BeoLit 600 (1970's modern style), with both AC117 and AC175 Ge transistors defective, and numerous other problems. I explained the problem to the customer, and he said, no problem, use this radio to experiment on.

So I did. I wanted to get my feet wet with this issue, and besides being able to make cheaper repairs in the future, Si has much less distortion than Ge, so I can improve the sound.

Credit must go to James Hawes here, who didn't complain when we bashed heads together to solve the problem, and was very envious of me working on this beautiful radio.

So the first thing I wanted to do was eliminate other problems, so I replaced all the Electrolytics and Tantalums with my standard fare of Wima MKS2 and Nichicon KT 105-degree capacitors.

That got me a loud hum when power was applied - but it could not be a capacitor at this stage, so it was clear that the Ge transistors were not fully operational. R80 NTC130 Thermistor had obviously not done its job in protecting the Ge transistors, so it was a suspect, and it proved defective, as you can see in the voltage readings below.

Next job was to replace the Ge transistors TR8 and TR9, which I used well-proven SS8050 and SS8550 output transistors - perfect for this job.

I now got sound without a hum, but with distortion and low volume. Progress, but not enough. But no going back now.

There were a couple of issues, and I monitored the B-E voltage drop on TR8 and TR9, to see how the changes would influence performance.

D8, (1N4148) was dropping only 0.7V, and we needed twice that. We decided that putting another 1N4148 in series with D8, was far simpler than redesigning the resistor circuit.

After adding the 1N4148, the voltages came up to something almost normal, but there was still distortion.

That R79 220ohm, between the Bases of TR8 and TR9, was the final key.  Multiplying 220 ohm x 2.3 (the B-E voltage drop ratio of a Ge (0.30V)  vs Si transistor (0.70V) ) = 506 ohms. Next higher resistor = 560 ohms, and that's what went into the R79 slot.

That gave me the correct voltages and resultant sound clarity that was required.

Project complete!

 

Summary of modifications:

1. Replace TR8 PNP with and TR9 NPN with suitable TO-92 Si transistors - I used SS8550 PNP and SS8050 NPN

2. Remove R80 NTC130 Thermistor

3. Add 1N4148 Diode in series with D8, giving a forward voltage drop of 1.4V

4. Replace R79 220 Ohm with 560 Ohm

 

The only remaining question I have is, even though the radio is working perfectly, is - whether the NTC130 is really necessary after the change to Si transistors, and if so, whether the 130 ohm rating should be changed to something else?

Menahem

I just got an answer on the NTC - the NTC curve is not suitable for Si transistors, so recommended to not use an NTC at all, and leave R80 empty.

Impressive job of redesign. You substituted components with modern versions, but struggled to maintain the original performance goals. Thanks.

i have one of these quiet as the grave and i would like to get it going agin but i dont understand. if youfit modern parts and make circuits modern wont it sound modern? or is that the point? germanium semis have a uniq sound to them that cannot be achieved with silicons imo. and why fit sils when germans can still be found? yes af117 is always bad but e.g. af190r is just as fine and sounds great.

Minus

Sometimes, one has to be practical, and instead of wishing for a dead machine to come to life again with unavailable parts, one can just create a modern solution, and have the machine working again.

I doubt if you could tell the difference in sound between a BL600 Ge and a Modified BL600 Si. This is not exactly a piece of exotic High Fidelity. 

Perhaps on a fancy expensive guitar amp, you may be able to tell the difference, but not here.

BTW, it's an AC117 (which does not have a great reputation) , not an AF117.

Menahem