IPC Signal Generator and Amp

[Pamphonica]

This item was kindly given to me by a fellow forum member. Described as "has never worked", I wondered what I would find. Lots of charring and no output, that's what!

These devices (retail nearly £300!) are supposedly designed for teaching lab use. They comprise a 0.1Hz to 100KHz oscillator and a 4W amp stage. The amp stage (which can drive 4 ohm speakers or an attenuated 600 ohm output) can be used separately. Quite a neat unit for audio fault-finding. https://www.ipcel.co.uk/product/sign...tor-amplifier/.

With no circuit diagram, it was back to first principles as ever. All the power supply bits were working, as was the whole of the oscillator section plus display. That left the output stage, which had a number of badly charred resistors (with charred pcb underneath). The charring made working out values tricky but much schematic reverse-engineering later they had all been replaced, along with 5 of the 7 transistors in the amp. I suspect that the amp stage overheated when inadvertently short-circuited and the failing output PNP left an appropriate trail of destruction elesewhere.

The design finally emerged after a lot of tracing of tracks and looking up various ICs. The dc-coupled amp stage has no adjustment of standing current, just an adjustment for output zero offset. So both transistors dissipate about 3W each on standby. Poor design, I think.

Other "features" included the DVM display chip which measures the waveform generator control voltage to mimic the set frequency. There is also an unusual darlington pair arrangement at the bottom of the frequency pot, and no distortion set pots (which are tracked on the pcb but not installed).

So if you get one of these and it's dead, here are a couple of circuit diagrams (roughly drawn) and an annotated board layout to match.

-Jeremy