The best transistor replacement

Hi,

The BC550 is likely fine for all of these NPN transistors, since the actual transistor gain doesn't matter usually (because the circuit will control the actual gain). The BC550 is a very decent substitute for most TO-92 NPN transistors I expect. It's my standard go-to transistor (well, the BC549 version) for non-RF stuff.

You could swap out one transistor at a time, replacing it back if it makes no difference. However you have voltages labelled on the schematic, so you can actually do the testing in-circuit with a multimeter.

Are you sure it's not just a faulty switch or noisy potentiometer? Silicon transistors are very reliable. I have a ~60 year old radio receiver on my desk and it is still functioning.

You could also use a test tool (or a crystal earpiece) to view or listen to the audio too; the crystal earpiece is non-intrusive to most of the circuit. Ideally you'd want to use an oscilloscope if you have access to it however.

b139b said:

The recorder sounds like a thunderstorm, so I think one transistor is bad

Have you already replaced the electrolytic capacitors?

They are far more likely to have gone bad.

That and potentiometers ...

*crackle crackle

- Gough

Just a little more info please.  What does "sounds like a thunderstorm" mean?  When you adjust the volume does the sound change?  Does the sound occur for both tapes you recorded and pre-recorded?  Gordon

BC550C  can be an option. You need to study its datasheet to find out if the operating voltage and package type matches with your previous transistors.

that maybe it - dirty heads and/or degraded tapes... I have once seen a (long unused) VHS recorder that was outputting an image distorted by "snow" noise for some time until - it seems - heads cleaned themselves running against the tape and the image improved...

Or maybe mains frequency: either in speakers (so maybe dried-up capacitors) or from the mains transformer (I have one DVD player that has transformer vibrating and generating noise - sometimes tightening screws helps, but not for long in my case - maybe the core got damaged/loose after drop and some resin is needed)

https://uk.farnell.com/multicomp/bc237b/transistor-npn-to-92/dp/1574377

Thank you all for your replies! Great community! 

The Thunderstorm is heat depended...came when I thought the player was fixed after the backlid was on. I have changed all electrolytes and C2. With a oscilloscope/RTA I can see that the signal is ok at the left side of C2 but the noise is present at the right. I have tried to find the faulty component by knocking on the components, potmeters too. Cool them with freezespray... heat with a heatingun but no component changed clearly.

Thanks for the link to BC237B

The worst case noise figure for the Multicomp BC237 is much worse than for the BC330C or BC550 (although the typical figure is similar). The OP's schematic doesn't give enough context to know if that matters much or not .

My source for BC330 data is not that traceable either !

MK

Thanks 

So the BC550C would be a better choice? Both BC237B and BC550C are low noise types