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Old 14th Jul 2019, 11:12 pm   #24
Join Date: May 2008
Location: Derby, UK.
Posts: 7,612
Default Re: Philips N4450 Reel to Reel Tape Player

Originally Posted by Michael Maurice View Post
I successfully repaired the flip flops on these using BC547ís and BC327ís.

The BC327ís have a similar current rating to the AC128ís and are pin for pin compatible
I'd have expected BC327s to be a good substitute in this application. I looked at the circuit and it only depends on the PNP polarity of the transistor, not the base-emitter voltage. The base-in-the-middle pinout makes it a very easy substitution.

A flip-flop fault could certainly cause your spurious recording lights problem. AC128s were once thought immune to Tin Whisker Syndrome, but apparently it just takes longer to show. Better to whip the lot out now than store up trouble for later.

As for why Philips used a germanium transistor: They were still making them in the 1970s, and they probably were cheaper than a similarly-rated silicon PNP transistor (it's harder to dope silicon to P-type than N-type; germanium is the opposite).
If I have seen further than others, it is because I was standing on a pile of failed experiments.
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