Power Supply - Pass Transistor

I would like to get a better understanding  of how this power supply works. Does anyone know of a good app note(s) or other design material for this type of power supply?

A couple of notes on the schematic. R1 and R2 were removed from later versions. It was necessary  for a PSU protection circuit that was deemed unnecessary.  T3 is part of a protection circuit that turns off the power supplies and tripps the circuit breaker if there is a Drive Amplifier failure.

I am really looking for an explanation of how the regulation works.

My understanding  of regulation is that a small proportion of the ripple is fed back into the pass transistor out of phase ,and so cancels the existing ripple , its easy enough to visualize that in the circuit you showed , even modern three pin regs are based on the same principle . Morgan Jones books on amplifier design might be a good place to start , you might find a pdf copy online somewhere or be able to read extracts of the text , its all written in very plain,  easy to understand language . I did buy the third edition ,so didnt feel too bad about downloading the fourth ,its mainly concerned with tube designs ,but all the basics are covered in any case .

The Emitter of T4 sits at a voltage determined by zener GR3or4 when the bas of T1 exceeds the emitter V by about 0.6 V it turns on T1 which in turn pulls  down the drive to T2 base.Pot R15 determines when this conduction happens, which is determined by the hopefully regulated output. If the output is low T1 wont turn on, so T2 turns on more, when the volts from the pot reach the T1 emiiter Volts + 0.6v it adjusts T2 Drive suitably. Pretty standard circuit. Havent figuired out T3

Gold said:

I would like to get a better understanding  of how this power supply works. Does anyone know of a good app note(s) or other design material for this type of power supply?

A couple of notes on the schematic. R1 and R2 were removed from later versions. It was necessary  for a PSU protection circuit that was deemed unnecessary.  T3 is part of a protection circuit that turns off the power supplies and tripps the circuit breaker if there is a Drive Amplifier failure.

I am really looking for an explanation of how the regulation works.

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A handful of basic moving parts.

#1- T1 + T2 form a basic "darlington" emitter follower. So the low impedance emitter follows the high impedance base, albeit 2 diode drops lower voltage.

#2 T4 is performing the actual voltage regulation with a divided down version of the output voltage fed into it's base, and it's emitter compared to a zener reference voltage. If the output voltage rises too high it turns on T4 sucking current from the base of T1.  Easy peasy... ;D  Note, there is a diode drop between base and emitter but otherwise similar to op amp + and - inputs. Just like with op amps the output voltage is a function of the divider ratio relative to the reference.

#3  T3 is a little extra something something something to act as an external shut down... when t3 is turned on, it sucks the base of T1 down to near 0V shutting off the supply.

This is a very basic pass regulator...

JR

Thanks guys. That explains a lot.

There are  three of these for each channel. Two are bridged ( if that’s the right word)  to give a +70VDC rail for the Drive (power) Amps which are also bridged. The third one runs the line level stuff.

Gold said:

Thanks guys. That explains a lot.

There are  three of these for each channel. Two are bridged ( if that’s the right word)  to give a +70VDC rail for the Drive (power)

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stacked?

Amps which are also bridged.

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bridged amp outputs drive drive both ends of speakers in opposite directions.

The third one runs the line level stuff.

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JR

JohnRoberts said:

stacked? bridged amp outputs drive drive both ends of speakers in opposite directions. JR

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They power the drive coils in the cutter head.

In an above post I said T3 trips when the circuit breaker opens. That's not correct. T3 trips if there is a drive amp failure.

Getting the VG66 rack up and running can be like chasing your tail. A bad power supply blows the drive amp and a bad drive amp blows the power supply. There is a calibration point to set the current at which T3 trips but of course you can't set that until everything is working.

The technical term for that is a "trapeze act" design ....  always fun...

JR

JohnRoberts said:

The technical term for that is a "trapeze act" design ....  always fun...

JR

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And of course setting that trip point for T3 is tricky. If you take too long to turn the trimmer you blow the Drive Amps and/or the PSU. Time to start over...

The Drive Amps use a BDY24 power transistor. They are unobtainium. I have a very small stash left but I really need to find a sub for those.