Salvage: Newtronics Model 200SPC Sweep/Pulse/Function-Generator

It feels like we’re on a roll today – here’s another “salvage” thanks to the Electrical Engineering “dump pile” at UNSW. This unit will be familiar to most of the Electrical Engineers of UNSW – it’s a Newtronics Model 200SPC Sweep Pulse Function Generator 0.1Hz-2Mhz. It looks like it’s also capable of being used as a frequency counter (of dubious accuracy) with a switchable gating time between fast and slow. Commonly used in labs prior to their replacement by newer GWInstek units, these were one of the workhorses of our labs.

Interestingly, there’s really no mention of this model online – which seems rather interesting given that the internet knows everything. A few want to find the service manual – which no-one seems to have, not even me. There doesn’t seem to be a mention of the company anywhere either …

Sorry for the crooked image and the harsh lighting. The serial is barely readable – it says 8844. Option 01 is fitted – 5.5Mhz as it says on the back – but does that mean a 5.5Mhz output? Interestingly, this one says it’s configured for 230v (not 240v) which seems ahead of its time given that we only just recently “harmonized” our voltage from 240v to 230v.

The back also reveals a gem of history – at that time, Electrical Engineering and Computer Science were part of the same school! Judging from the plant number of 88xxxx and the serial of 88xx, this is probably a 1988 acquisition, making it a year older than I am! A stub of the security steel cable is still attached, cutting through that proved to be a job and a half with hand tools. I pulled the fuse from the IEC connector – it’s still good – so this is probably a workable unit (to some degree).

Taking a peek inside isn’t too hard, since there were pry marks from where previous attempts were made to pry off the plastic ring around the chassis which allows the two screws securing the (slightly rusted) metal lid to be taken off, and the lid to be slid off.

The internal layout seemed rather empty – in the rear is the transformer and IEC connector with suspiciously thin wires. Not that it’s much of an issue given the low currents involved, but it makes me wary of the insulation. The IEC connector’s primary goes to the front panel push-button as a “hardware” power button. Likewise, the front panel knobs are mechanically shaft-linked to potentiometers mounted on the main board, as well as the range switches. The rest of it is made in a rather modular fashion – with plug-in modules and socketed ICs. A few heatsinks are visible.

On the rear, left next to the transformer appears to be a positive linear 5v regulator – marked GL7805 using the rear metal plate as a heatsink. I believe GS in this case may stand for Goldstar (i.e. LG).

Next to that is a board with some 74 series logic from TI, Fujitsu, Motorola and Goldstar and an Intersil ICM7216 frequency counter IC.

On the heatsink below, we can see a Motorola MC7815CT, 7806CT and under the MC7906CT. These appear to be a 15v positive regulator, and a 6v positive regulator with its complementary 6v negative regulator to derive necessary rails for generating the signals perhaps.

The rest of that board has a large array of resistors, and plugins for the range switch and the display. Curved traces, a sign of “hand” drawn systems are clearly visible – elegant! There is a NE529N voltage comparator on the board as well as a RC4558P dual opamp. There’s a sprinkling of adjustment pots as well – although we have no real idea what they do …

On the other heatsink is a lone 15v positive regulator …

And the rest of that board houses more adjustment trimpots, as well as the potentiometers accessed by the front panel knobs. There’s also an interesting arrangement of Q12 – you can see a cable tie which secures two transistors face-to-face – this may be because they are used in a current-mirror configuration, and having the transistors at similar temperatures may improve their “balance”.

Interestingly, the front panel display is implemented with IC sockets – each 7-segment display unit plugs into the board’s sockets. That’s the first time I’ve seen that level of modularity.

On one of the plug-in boards responsible for the pulse functionality, there is a HA17458 Dual Opamp along with more trimpots. So many trimpots!

Not surprisingly, when plugged in, the unit functions to a degree …

(and yes, that’s a sink, toothbrush, hair products in the background. I do all my testing on inverter AC from a car battery, and usually close to a sink or bathroom as it means less risk if something catches fire (and I’m out of room in my room!))

It becomes obvious when playing around with the unit that the potentiometers controlling the output amplitude are well and truly on their way out, and the one controlling frequency is a little jumpy too. That’s what happens with mechanical parts … but it’s not entirely useless if one is patient.

Before was a square wave, well here’s a sine wave … tick!

And a triangle wave … tick!

And I can change the duty cycle too.

I even took the time to change frequencies and managed to get it to produce a wave with a period of two minutes! Sure, it’s probably going to drift like hell … but it’s still a valiant effort from something that’s likely to be … 25 years old, abused by students …