Manson EP-603 Power Supply

I bought a cheap "laboratory" power supply some time ago, one of these: https://www.manson.com.hk/product/ep-603/.

Manson EP-603 Power Supply

 

It is quite handy, but the ammeter has never worked and I decided to have a look. I can’t find a schematic for this particular model, though I did find some similar ones. The "BK Lab Power Supply Schematic 1670A_1671A_schem.pdf" seems identical. You should be able to find this easily online. All major semiconductors match, at least.

Interesting features include:

1. The +12V and +5V outputs are isolated from each other and the main output, which is handy and a surprise.
2. The regulators are all linear.
3. A relay is used to switch taps on the secondary winding for the main output, thereby allowing a modicum of efficiency over the output voltage range.
   
4. If you have to dissemble one of these, there are two hidden screws each side of the fixed output terminals. These may be accessed though holes in the PCB. It is wise to unsolder the meter terminals before removing the PCB.

The ammeter issue was due to bad soldering on the meter terminals. Now it is fine, and the meter calibration is about right. Despite this issue, the supply has not given me any other trouble and I do think it is reasonable value for money.

 

Internals

I took a few photographs while I had it apart. These may be useful if you come across a similar model. Click on the images for a more detailed view.

 

Front Panel PCB

Front Panel PCB (Reverse)

Heatsink Assembly

ELF Mini-Terminal

I was given this terminal many years ago, rescued from a skip. I don't think it has been used much in its life. It is is good condition and the CRT appears to have plenty of life in it.

It actually worked as soon as I turned it on, though I did clean it up a bit and adjust the internal monitor controls a little to improve the greyscale. Some switch cleaner is very useful for this sort of thing as it really improves potentiometer operation in old equipment.

I can't find any data on it online, though there is a similar (and I suspect somewhat earlier) model described on the Computing History web site http://www.computinghistory.org.uk/. It supports an impressive number of demonstrations with (presumably) recorded Prestel and other sessions to show off its capabilities. These include 80 column mode and several levels of greyscale. The display is monochrome.

Real Owls Don't Like Acorns!

I do wonder about the page shown above. The text on the bottom right says "Real owls don't like acorns!". Did someone have an issue with Acorn Computers?

Internals

The case is nicely designed, and just four screws allow the monitor section to be hinged out of the way. The power supply then simply lifts out after disconnecting the mains input.

Main PCB

Major devices are:

1. MC6803P microprocessor. This is a member of the then popular 6800 family of 8-bit processors.
2. UM6845EA CRT controller. This generates timings and addresses for the video display.
3. SAA5055 Teletext character generator (U.S. ASCII version).
   
4. SCN2681 dual asynchronous receiver/transmitter, for the line and printer RS-232 ports.
5. CDM6264E3 8K x 8-bit CMOS static RAM.
6. NMC9346N EEPROM, used to store configuration settings.
7. 27256 32K x 8-bit EPROM.
   

Power Supply

The power supply is a conventional switching unit. I imagine it provides +5 V, +12 V and -12 V. A negative supply is required for the RS-232 drivers on the main board.

Monitor Section

The monitor electronics is housed on a neat little PCB mounted underneath the CRT itself.

Terminal With Keyboard and Keypad

I have a numeric keypad as well. Strangely, I can't see a way to connect both. Perhaps for some applications, the keypad is all that is needed.

There is an idea lurking in my mind that could use this terminal, which was really the motivation for blowing the dust off it. More soon, perhaps!