Purpose of Zener Diode

[jheatac]

I'm still learning here, but is the purpose of the Zener Diodes on the transistors just a protection for overvoltage protection? 27vac comes in at top and output at bottom.

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[ArthurDent]

The zener is used as a voltage reference but can't handle the current required by the circuit so a transistor is used so the supply can output more power than the zener by itself. Think of the circuit as a amplified or higher current zener. Your circuit has two of these circuits for about 24 and 12 VDC. Check example #1 in this explanation for details.

http://www.idc-online.com/technical_references/pdfs/electronic_engineering/Voltage_regulators.pdf

[coromonadalix]

You can use an Zenner to drop an X amount of voltage(s) if you put it in serial  ... if you stay within the Zenner tolerance
You can use an Zenner to protect inputs of circuits ... to clamp excess voltages.

and the list goes on

[james_s]

That's a classic linear voltage regulator circuit, the zener diode sets the output voltage of the regulator and the transistor passes the current.

[jheatac]

Thank you for the replies. It took about 2 days of thinking about it off and on and staring at the schematic, but it finally clicked and makes sense. Something so simple but for whatever reason, it wouldn't sink in.

This is an old receiver. There is a "Phase Modulation" board that I am changing the caps on. It has 1.5uf/25v Elna electrolytics in it. The service manual calls for 1.5uf/15v tantalums. It is on the audio line and they are circled at the top of the schematic. Would a poly cap work? Should I get the tantalums? the 1.5uf electrolytic seem a little odd...

Thanks

[coromonadalix]

Not sure i would put poly caps in there       but if the capacitance value is the same, i would try it, if its 1.5uf needed there, its been calculated this way,  maybe i would have tried electrolytic 2.2uf

my 2 cents

[floobydust]

For the quadraphonic decoder, those 1.5uF capacitors are kind of important for the low freq. response. I would keep the value and use film caps such as Epcos 1.5uF MKT film cap 63V or parallel a 1uF and 0.47uF part.

[tkamiya]

I would just put in what the manual calls for.  They are just a coupling capacitor so I don't think it's all that critical.  But you have 4 signals coming in and circuits are identical.  So I suggest keeping all 4 the same. 

Seeing transistors are named 2SC and 2SD.  This must be Japanese made receiver and seeing 2SD92, this must be an old one from say, around 70s?  "D" in 2SD means low frequency response.  "C" means high frequency response.  2 means it's a 3 wire device.  I forgot what "S" means.  Knowing that sometimes helps in finding suitable replacements.  If you need help reading any Japanese text, please let me know.  I'd be happy to help.

[coromonadalix]

I would just put in what the manual calls for.  They are just a coupling capacitor so I don't think it's all that critical.  But you have 4 signals coming in and circuits are identical.  So I suggest keeping all 4 the same. 

Seeing transistors are named 2SC and 2SD.  This must be Japanese made receiver and seeing 2SD92, this must be an old one from say, around 70s?  "D" in 2SD means low frequency response.  "C" means high frequency response.  2 means it's a 3 wire device.  I forgot what "S" means.  Knowing that sometimes helps in finding suitable replacements.  If you need help reading any Japanese text, please let me know.  I'd be happy to help.

here's your definitions
http://www.matidavid.com/electronic_components/04.htm
https://en.wikipedia.org/wiki/Transistor

[gbaddeley]

That's a classic linear voltage regulator circuit, the zener diode sets the output voltage of the regulator and the transistor passes the current.

Agree. The regulated outputs are the emitters of the pass transistors, which is about 0.6V lower than the Zener breakdown voltage.

It must be a reasonable low current power supply, as there is multiple R-C decoupling / filtering with fairly high value resistors, to multiple outputs (pins 4, 6, 7, 8, 9).

There is even a Zener diode on its own (ZD0003) to provide some extra regulation. I assume that the hand-written '27' is not the voltage at this point, I would expect it to be around 11 Volts.

I hope ping 3 goes off to a high value electro cap, probably about 1000uF 50V in a chassis mounted can.

[gbaddeley]

For the quadraphonic decoder, those 1.5uF capacitors are kind of important for the low freq. response. I would keep the value and use film caps such as Epcos 1.5uF MKT film cap 63V or parallel a 1uF and 0.47uF part.

They are important, but would not be determining the frequency response. Tolerance on electro caps is too broad to trust them for that role. I would not hesitate in replacing them with 10uF 16V electros, or higher voltage 25V 35V if you can't get 16V.  Tantalum or Bipolar is not really necessary, but could be pressed into service.

[floobydust]

I'm saying they aren't 2.2uF or 10uF for a reason. Those values are used elsewhere, but here it might be for economy. The input impedance of the SQ stages with 1.5uF -  I'd have to dig up the SQ standard and see if it matters.