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[Jonathan Johnson]

Why does it have the speaker output in bottom left with the power plug ?

I thought it was a PEAKER DUTPUT.

[Greg Amidon]

I thought it might be fun to post the circuit below, the power supply section for a very popular late 60's early 70's product manufactured by a company known to us all. Well at least the old timers.
 
Points for spotting what some may call an issue with this power supply, and 1 million wizard points if you can tell me who's circuit this is, and for what product.

I may have this turned around. But my guess is that S1 and S2 are thermal switches that open when over temp. This diverts the current to the Thermal Overload lamps and limits the power available to the transformer.

Just a guess.

Greg

[Mike Sokol]

Points for spotting what some may call an issue with this power supply, and 1 million wizard points if you can tell me who's circuit this is, and for what product.

I think that S2 and S3 are door interlock switches and the R23 component is the actual thermal overload switch. The neon light in parallel with the string will light up whenever there's a thermal overload, but also will light up when a door interlock switch is open. That's the problem, since an open door (or whatever) will make you think there's a thermal overload.

Since there's an EGC connection to the chassis, so there's no good reason I see to have a polarity flip switch on the AC power, but maybe they're trying to make it safe for when someone cuts off the ground pin in the power cord or plugs into a ground lifted outlet? But I don't see any stinger cap as a chassis-to-neutral shunt, so unless they're planning on the internal leakage of the transformer performing this function I don't have an explanation for the phase-flip.

[Bob Leonard]

Points to those who recognized the Vocal Master circuit. This circuit is specifically for the Shure Vocal Master Pro va300.

S2, and S3 are actually thermostats. R23 is a resistor. S1 is a toggle switch.

Mike's reason for the toggle switch is correct, although this switch can actually cause more problems than it can cure. But, back in that time period using a 2 prong adapter was not uncommon as most 2 prong outlets had yet to be completely replaced throughout the USA. I have to wonder if anyone ever used the little green wire to attach to the plate ground, or if they did was there really a ground? I never used it and equipment of this type would often require the spit and spark test while holding your guitar and touching the mics. Also note that ALL of the input and output jacks are tied directly to the chassis ground.
From the manual;
 

"Turn on the Power Switch (10) located on the front panel of the Console. Set the “Master Volume” Control to “5.” While talking or singing into one of the microphones connected to the console, turn up the individual volume control for that microphone to a normal level. If at this point a slight hum is heard in the speakers, switch the Power Switch (IO) to whichever “On” position results in the least amount of hum coming from the speakers."

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"The VA300 is equipped with thermal sensing switches on the heat-sinks of the output transistors. The thermal switches are set to shut off AC power to the amplifier when a temperature of 90°C (194°F) is attained on the heat-sinks; the switches will automatically recycle and return AC power when the heat-sink temperature reduces to 73°C (164°F). A thermal overload light (9), located on the front panel of the VA300, will indicate if thermal cycling has occurred. Thermal cycling may occur if air is not allowed to circulate through the grilles of the amplifier."

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