Identifying diodes

[PickyBiker]

I need help identifying diodes in a 1980 Marantz Amplifier. There are two diodes in series. They are connected at the cathodes. One is marked A27 and the other simply has an H as the only marking. These were added to the amplifier after production because they are not on the schematic. They are shorted out so how can I identify what they are?

 

[AnalogKid]

There are over 40 diodes in that amplifier. Which two are you talking about?

Photos??????

Are they in a power supply section, power output stage section, signal section, bonded to a power transistor or a heatsink, black, clear glass, greater than 1/10" in diameter?

ak

 

[PickyBiker]

There are over 40 diodes in that amplifier. Which two are you talking about?
\
Photos??????

Are they in a power supply section, power output stage section, signal section, bonded to a power transistor or a heatsink, black, clear glass, greater than 1/10" in diameter?

ak

They are in the power output section. These diodes are not shown on the schematic (if they were I wouldn't have a problem IDing them), so they must be some kind of a revision. They are tiny and the markings I talked about in the original post are the only markings on them.

It is a stereo amp, so there are 2 channels and 2 power output transistors for each channel one npn and one pnp. There are 4 pairs of these diodes, one pair for each output transistor. The schematic I have is difficult to read and the component markings are black on a very dark brown PCB and are impossible to read. I will try to draw out the exact connection to the power transistors.

If I were to venture a guess, they are one zener and one standard diode. Arranged together in series with the cathodes connected, I would guess their purpose is to limit the signal into the power transistors to prevent overload. (that is a pure guess). The power supplied to these output transistors is + and - 54 VDC. Is it a coincidence that 27 happens to be 1/2 of 54V and one diode is marked A27?

 

[PickyBiker]

Okay, here is the schematic of the right channel with the diode pairs shown.

 

[AnalogKid]

From your labels it makes more sense for H to be the zener.

ak

 

[PickyBiker]

Okay, turning them around as you suggest, do you have any idea what the values might be? Might A27 be a general purpose diode and H be a 27v zener diode?

Just trying to understand if the circuit makes sense with those values.

 

[AnalogKid]

Each half of the output stage is a triple-darlington. As such, A never should be more than 2 to 2.5 V above B (the Vbe's increase with increasing collector current). One possibility is that the zener is 2.7 V, not 27 V, with the tolerance code (A = +/-10%) as the European-style decimal point. But if that is the case it still appears to be in there backwards.

ak