I’m trying to better understand solid state circuits; it’s a journey.
I’ve attached a schematic from a vintage receiver - a Pioneer SX-880. I’m hoping someone can explain how the regulated voltages are set. Let’s take the positive regulator, Q28 and Q30. Q28 I think I might understand. 14 volts is needed elsewhere in the circuit so the 62k resistor and 14v diode sets the emitter voltage. The 15k/8.2k divider sets the base voltage. I’m not sure why we need the D25 diode, perhaps to keep the operating point stable.
Q30 I don’t understand. It’s intended to set the regulated voltage at 41.5 and so the base needs to be at 42.1v. So how is that base voltage set? Q24 sends 2ma of ripple to the Q30 base for noise rejection. That doesn’t set the voltage does it? What does? Is it the conductance though Q28? Is the operating point set by diode Q25? These are just guess on my part. Please, someone splane it to me.
I’ve attached a schematic from a vintage receiver - a Pioneer SX-880. I’m hoping someone can explain how the regulated voltages are set. Let’s take the positive regulator, Q28 and Q30. Q28 I think I might understand. 14 volts is needed elsewhere in the circuit so the 62k resistor and 14v diode sets the emitter voltage. The 15k/8.2k divider sets the base voltage. I’m not sure why we need the D25 diode, perhaps to keep the operating point stable.
Q30 I don’t understand. It’s intended to set the regulated voltage at 41.5 and so the base needs to be at 42.1v. So how is that base voltage set? Q24 sends 2ma of ripple to the Q30 base for noise rejection. That doesn’t set the voltage does it? What does? Is it the conductance though Q28? Is the operating point set by diode Q25? These are just guess on my part. Please, someone splane it to me.
The divider 15k:8k2 is a feedback network. The reference is the 14V zener (plus one diode drop across Q28's BE junction), multiply
14.7 * (15+8.2)/8.2 = 41.5V
Q28 provides the voltage gain, C411 is its compensation cap, and Q26 is the current buffer.
Q24 is a constant current diode providing an active load to Q28
14.7 * (15+8.2)/8.2 = 41.5V
Q28 provides the voltage gain, C411 is its compensation cap, and Q26 is the current buffer.
Q24 is a constant current diode providing an active load to Q28
Thanks guys. I think I understand: The divider network has one node at 0v, the center node at 14.7 and so the top node must be at 41.5v as determined by the resistor values. Is that about right?
So how is the negative supply voltage set? It has no divider other than the one that sets the base of Q23 at -0.6v.
So how is the negative supply voltage set? It has no divider other than the one that sets the base of Q23 at -0.6v.
It's essentially an inverting amplifier that inverts the positive regulated voltage.
The negative supply is set in a similar sort of way, but uses the positive supply as it's reference, rather than the zener...
<edit> Exactly Marcel! and maybe more clearly than I said<edit>
<edit> Exactly Marcel! and maybe more clearly than I said<edit>
The situation is that the Jfet at Q24 failed and was replaced with a CRD (E-202 2ma) and the regulation has failed full on. The positive and negative voltages are 55.3 and -53.9. Q30 is at saturation I believe. Ripple is .36 on the + and half that on the -. The CRD was tested and found to deliver 2ma and this is a pretty common substitution.
By some Experimentation I have replaced the jfet with a 33k resistor to force the Q30 base to the proper voltage and so the voltages are on target elsewhere in the circuit. Ripple is 2mv on the negative which has a good jfet and 22mv on the positive with the 33k resistor. The severe hum is greatly reduced but not gone. But this is experimentation, not a fix.
I did pull and measure the resistors in the voltage divider and they were correct at 15.24 and 8.25. The Zener is at 14.3. The circuit is not behaving in accordance with the dividers influence.
By your generous help, I understand the circuit better but I‘m absolutely out of ideas here. Any help would be greatly appreciated.
By some Experimentation I have replaced the jfet with a 33k resistor to force the Q30 base to the proper voltage and so the voltages are on target elsewhere in the circuit. Ripple is 2mv on the negative which has a good jfet and 22mv on the positive with the 33k resistor. The severe hum is greatly reduced but not gone. But this is experimentation, not a fix.
I did pull and measure the resistors in the voltage divider and they were correct at 15.24 and 8.25. The Zener is at 14.3. The circuit is not behaving in accordance with the dividers influence.
By your generous help, I understand the circuit better but I‘m absolutely out of ideas here. Any help would be greatly appreciated.
It sounds like Q28 doesn't work. Did Q28's collector get disconnected somehow? Is its base-emitter voltage normal?
Did Q30 survive? 33K would not provide much base current, let alone load current if Q30 failed. Sounds like you tweaked to get the nominal output voltage at Q30 emitter.
I reiterate, could have the CRD have been installed backwards? It would have behaved like a forward biased diode and delivered heavy drive to Q30 base. It might have damaged Q28 as well because of available current to Q28 collector--- feedback would have driven Q28 to conduct in an attempt to lower regulator output voltage from ~+55V to +41V.
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Well this is a surprise. The original part is a 2SC945A. The foil side drawing shows the pads as being ECB. The replacement part is a KSC945YBU. It’s a EBC transistor. No bueno. I vaguely remember something about this and I even made a note of the pin out on the mouser label.
I installed a KSC2383YTA with a ECB pin out for a win.
Voltages are 42.1 and -41.4. Ripple is 3.6mv top and bottom. The hum is gone!
Is there any consequence of the 0.7 imbalance?
The next rounds on me!
I installed a KSC2383YTA with a ECB pin out for a win.
Voltages are 42.1 and -41.4. Ripple is 3.6mv top and bottom. The hum is gone!
Is there any consequence of the 0.7 imbalance?
The next rounds on me!
No, I even breadboarded a little LED lamp circuit so that I could measure the current and verify my understanding of the direction of the installation. And yes, that 33k resistor was just for my education. I didn’t leave it on for any longer than I needed to get voltage readings.
I‘m reasonably competent with tube circuits but solid state circuits I struggle with. I bought about six fouled up receivers as a learning experience and it’s been good fun and indeed I’ve learned a lot. This is number 4 and it was a challenge.
Thanks for your comments.