Repairning Yugoslavian EI HSR 48

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#2
> When I replace the output transistors 2n3055 or MJE 3055, they burn out.

It is all direct-coupled. Other parts have failed, which is killing your new 2N3055s.

Leave the '3055s out. What is the voltage *across* the 470 Ohm resisors that I have marked. Also the top rail voltage, and the center-point voltage.

RadioMuseum, HSR-48 Radio Elektronska Industrija Ei; Niš, build 1974 ?, 5
 

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#4
Sorry for the long break. I determined, that both driver transistors were bad, so I replace them with bd140, Bd139,but now i have another problem. When i try to set the midpoint voltage to half vcc, the quiescent current shoots up to 300 mA. This only happens with brandnew transistors. When i try some 30 years old, the amp works correctly.

PS: no its not oscillating

Best regards,
electronicsman
 
#5
There's no way to adjust the output bus voltage. As this is capacitor coupled, it isn't that necessary, too. The only trimpot P804 in the circuitry adjusts bias, i.e. idle current. Hence the behaviour you observed is quite normal.


Best regards!
 
#7
So I am here asking again about this amplifier. The previous one i had I sorted out. But now someone has brought me another to fix. Its Missing the zener diode, labled ZF 1,4 V. I tried replacing it with two silicon diodes in order to acheive the voltage drop required, but the circuit distorts very badly. I cant get my hands on a similar diode either. Any suggestions would be much appreciated.(schematic of the power amp is on my first post almost a year ago)



Best regards!
 
#8
electronicsman said:
...Its Missing the zener diode, labled ZF 1,4 V. I tried replacing it with two silicon diodes in order to acheive the voltage drop required, but the circuit distorts very badly. I cant get my hands on a similar diode either. ..
Click to expand...
Hi electronicsman,

I just switched on a search engine typed "bzx 1v4" and got a bunch of results. Issue is that they are so cheap that it's almost impossible to get one single piece ;o).

Substituting a Zener with normal Diodes is inappropriate for several reasons, including that a Zener effect uses the steep reverse voltage curve to achieve a stable Voltage, a normal diode in forwar mode much less steep. Read more here.

Cheers,
Winfried
 
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