(Vintage) Nortronic 811 Sound Analyzer

[arno]

Dear folks,

it is vintage electronics time again. The Nortronic (now called 'Norsonic') 811 sound analyzer is dated around 1975 and can measure sound pressure levels and even room reverb time. See the attached picture. Luckily I have the complete schematic, which is around 30 pages. In there are 20 cards in a row.

My unit has an odd issue, which I could not repair: Card no. 20 (see attached schematic) is the so called sample generator, which generates pulses for the (analog!) band pass filters, depending on the frequency you choose on the panel.
At room temperature the unit generates almost an rectangle signal. When heating up the marked transistor (AE254) just a little bit, suddenly the card starts working as intended.

So my two questions are: Does anybody have a datasheet for the AE253/254 transistors? What could possibly cause this problem? I have already swapped caps, diodes and measured all resistors (they were all good).

Thanks in advance!

[arno]

Dear all,

seems like this will become a bigger project. Have not found the issue yet.
I will redraw the whole schematic of the unit with KiCAD, whenever I have some free time.

You will find it on github:

https://github.com/adaeuper/nortronic

Started with card no. 5 (a 40 dB amplifier) today.

Best wishes