Little problem with amplifier of magnetic head.

[axrl]

Hello,

I'm trying to repair the RIGHT pre-amp of the lecture head of a cassette deck. (SABA CD 360).

I have written in red on the files in attchment the values I measured on the transistors which make problem.

The values on the LEFT channel are correct because this channel works fine.

The values on the RIGHT are anormalous.

With this I'm not a specialist and can't know witch component is bad. Indeed the first transistor of the Darlington gives a tiny collector courent that makes the tension of collector of 2.38 V and not the right value of 2.02 V like the left channel.

But maybe it is the second transistor that doesn't work right because its collector is quite in short circuit with its emetter (Vcollecteur = 1.8 V instead of 5.18 V).

Or eventually the two transistor are bad (the first not giving enough courent and the second giving too much) ?

Thanks for your ideas.

Axrl

Note : The original values on the plans are in RECORD mode and not in PLAYBACK mode. So several values are different. You only have to compare the red values from one channel to the red values of the other.

 

[user_88]

The second transistor .... 1.8-1.71 looks a little suspicious.
I would remove/desolder this transistor, and with it removed from the circuit, check the collector voltage, and also the base voltage to see if they corrected to more of an expected value.
If the collector voltage on the circuit board measures somewhere around 5.2, then it might be worthwhile to try a new replacement transistor.
Sometimes, these transistors are installed as matched pairs, so you might want to also replace the primary transistor .... just guessing about that.
Can you provide a photo of the two transistors in question?

 

[axrl]

Ok. The transistors are 2SC1344. I removed the two transistors from the circuit. Tested the leakage of all capacitors : nothing wrong. The two transistor also were OK. Eventually I replaced the transistors and the two channels now work fine.

So I guess it was moisture or a bad contact...

Axrl.

 

[user_88]

... glad to see that you got it working,

... just curious as to what actually caused the deviant voltage readings that you obtained.
Electrolytic capacitors that have aged are sometimes subject to anomalous behavior, and can actually change with temperature or current flux. The question of interest is whether there was an excess magnitude DC current going through capacitor C109, rather than through the NPN transistor. Prior to your repair and re-soldering, you might have measured the DC voltage at the node between R116 and C109, comparing it to the working channel board.

... any future problems, try replacing C109.
... It would more likely be the problem than the NPN transistor.