15 August 2021

Topward TFG-8104: the multimeter challenge

SPOILER ALERT. This post contains my take to the question asked in the previous post: "how to determine if it works with just a multimeter." If you want to play the same game please skip one post backwards and read my challenge.

Here we go.

The Topward TFG-8104 Function Generator powers up and generates enough RF that can be picked up by a nearby AM receiver. What else can be said with just a multimeter?

Since the output frequency can be reduced down to 0.1 Hz, all variations can be slowed down enough to be seen and appreciated even on a slow digital multimeter.

According to the manual it should produce a 20 Vpp signal. So I set:

  • lowest possible frequency, 0.1 Hz or so
  • square wave output
  • maximum amplitude
  • no DC offset
  • no modulation
  • no attenuation

The output alternates between -11.5V and +8V while it should be -10/+10V. When the amplitude is reduced readings collapse towards 0V and the asymmetry reduces, which is good. I can't think of further checks that are possible with a multimeter at the output connector. Well, there is output impedance.

So, even before checking the output at the oscilloscope, the asymmetry should mean something had happened to the output stage. A visual inspection of the circuit reveals signs of human intervention or fault on the output attenuator: some signal has tried to enter the function generator?!

While the box is open, the multimeter can check voltages. Both +15V and -15V are present. The circuit diagram mentions +/-20V but they are not marked on the board, nor there are accessible test points.