Well…that was a bad idea. Just started working on the BG again, finally got the matching paint and repainted the baseplate. Problem: Turns out if you travel the world for two months and come back you might not remember what goes where. And I didn't take pictures. Ha.

Umm…Beaker? Or somebody else who owns a 42? Could you please take a picture of the innards? Just with the bottom plate off. I have no idea as to how to route the cables or where exactly the voltage selector was located before...😳

Here you go. Anything else you need, just let me know.

Thanks! I now reassembled everything the way it should be.

Well...so close, and yet something rather important is still missing.

Thanks to Martin, there finally is some progress:

For future reference: The bearing on the ST/L and the Beogram 1000 tonearm is identical, the arm itselft is not (the ST/L arm is longer due to the different counterweight). So...bearing replaced, adjusted - speed is perfect, tracking weight is perfect, arm perfectly adjusted...put a LP on there, take the cover off of the SP7, lower the cueing lever - I can hear the faint sound of Fehlfarben's "Hier und Jetzt" emitting from the stylus, turn up the receiver...

Guess what...I've found the reason why the preamp was disconnected.  😕 It produces nothing but a faint hum. Let's see what we can do about that...

Does anyone have a good quality scan of the preamp-schematics? I can't really read most of the things on beaker's scan.

What I've found so far: AC126...wasn't it those that grow crystals or something?...both test ok, but the breakdown voltages measure about 0,15V on both transistors, whereas on the other two (2Nsomethings) it's the usual Germanium 0,3V. Also, one of the two diodes is open circuit. Can I replace this with a standard 1N4001 or is a Ge-Diode required?

ablaumeise wrote:

the breakdown voltages measure about 0,15V on both transistors

I have some AC151r's that should be a replacement for both the AC126 and the AC107 (as far as I can read beaker's schematic)

They have 180mV measured on B-C and B-E so the 0.15V aren't too bad.

And looking at the supply it shouldn't be a big deal using 1N400x or even 1N4148's as there are 12.5Volts AC so .4V difference won't lower them too much. And the OA85 had 50mA only...  but change both diodes anyway.

As far as I can see this is a voltage doubler circuit - I would expect some 20 Volts supply after the different RC-stages.

Probably the bipolar electrolytics?

  hx (still lovin' his BL4500's 😎)

The diodes seem just rectifiers for the power supply, I'm sure 1N400x:s or similar silicon diodes will do.

I think it was the AF11x types that are most likely to grow internal whiskers... but all germanium transistors are rather fragile by today's standards. Anyway if they work, I'd leave them alone; the exact measurements can vary a lot.

Thanks for your input!

Replaced the diodes with 1N4001s - still nothing. Checked the capacitors - capacitance and ESR both measure out fine.

Checked the transistors out of circuit - they also measure 1,7V e->c...they shouldn't do that, should they?

ablaumeise wrote:

they also measure 1,7V e->c

Did you leave the base floating? 0.1V are easily catched when left open and then they trigger the transistor - even with the voltmeter only.

(Was learning by doing for myself too - my germanium days are way back now...  Anyone remembers the black OC something's?)

Ground the base while measuring & check again.

Are the 20 volts ok? I found the diagram for a BG1000 with a little different schematic: From 14VAC they create 33.5V DC after the 2 diodes, 28.5V at the first cap and 16V at the 2nd cap. But there are different transistors, both already silicium. EDIT: oops, I meant silicon 😳

   hx

P.S. did it work without the preamp, using the short circuit plate? (yet another EDIT: just read there never was a short circuit plate - should say "did it work while bypassed"...)