Reverse engineering a Mascot 7416 Power Supply

[LaryPant]

(See attachments for schematic and pictures.)
Hello there,

I found a dirt cheap - for Denmark - Mascot 7416 power supply in the local used market and I went ahead and bought it with the idea of reusing the chassis, heatsink and transformer to build myself a better power supply.
But before doing so, I wanted to reverse engineer the circuit for fun and academia and this is the story of doing that.

The Mascot 7416 features an output voltage of 8-16 Volts at a current of 1.6 Ampere, but after a quick test comparing it to a reference LM317 design it turned out to perform quite terrible - there was a lot of ripple on the output. The front panel of the power supply is also pretty useless. There are no banana plugs or similar, but simply two wires coming out of the front. 
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Why did they design it like this? This is obviously a lab supply, so why attach the wires directly with no way of turning off the power supply? This seems like a recipe for shorting out the power supply to me..... any thoughts?

Anyways, after some hassle getting the cover off - the screws were stripped - I finally got in to the good parts. The internal layout is quite simple: A transformer, fuse, bridge rectifier and the DC-DC voltage regulator with over current protection.
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The regulator is based on the Motorola MC1723CP which is a 14 pin voltage regulator that has a lot of different functions. Because this chip has so many different ways to use it, I decided to include an equivalent circuit model in my schematic, but note: This is an equivalent model which is only valid for this specific circuit.

I have attached the schematic to this post. Feel free to suggest any changes to make it more readable or if something is completely wrong.

The power supply seems to be a straight forward design. The desired output voltage is set with RP1, R1, R2 and the interval reference voltage of the IC. The output of the IC is then feed into a Sziklai pair which controls the output voltage and handles the high current.
The over current protection is achieved using a single NPN transistor to pull the output of the IC low if the current flow is to high. The NPN Q2 is turned on when the voltage drop across the 1 Ohm sense resistor R7 exceeds the base-emitter voltage of Q2. Actually the voltage across the base-emitter junction of Q2 is 95% of that across R7 due to the voltage divider formed by R3 and R4.

I think I managed to understand the circuit quite well, but one thing still puzzles me. What is the purpose of D5?
It could be a zener, but I just drew a standard symbol as I have no idea.

Based on the date codes of the IC and transistors this unit was manufactured somewhere in the late 80's. I imagine that they were mostly used in schools where price played the biggest factor, but I still do not get the lack of an ON/OFF switch, any thoughts on this?

Now the reverse engineering is done and I will start to design a replacement for the DC-DC regulator as this one is quite disappointing and I will definitely have to add some banana plugs and a ON/OFF switch.

Any comments and thoughts are appreciated!

Lary

PS: I can't seem to add the pictures as inline expandable thumbnails, I will try to fix this!

EDIT: Fixed a tiny error in the schematic.

[rust_collector]

Just for fun, here is the front of an older 7416.


Back then, it was called a battery eliminator, which I guess explain the wires.
Most of these I have seen, are running small transistor radios, in old peoples kitchens.

[LaryPant]

Just for fun, here is the front of an older 7416.
(Attachment Link)

Back then, it was called a battery eliminator, which I guess explain the wires.
Most of these I have seen, are running small transistor radios, in old peoples kitchens.

Ahh, that makes perfect sense!
This is not a lab power supply but simply a battery substitute for common appliances around the house, thank you for pointing this out!

It is interesting how they did not include the little label indicating which is the positive wire on your older front panel. This must have caused some amount of confusion, especially if these were to be used by non technical people.

For those of you who do not speak a Scandinavian language or did not notice this:
On the front panel of the newer version there is a little label with the text "BLÅ: PLUSS" which is Norwegian for "Blue: Plus". Mascot is from Norway.

Lary

[rust_collector]

mascot did make a "lab supply" which was called 719, I do not think they have the greatest reputation, but I do like how they look.