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.

10 August 2021

Topward TFG-8104: does it work?

Does the Topward TFG-8104 Function Generator (from the previous post) work? Well, considering the source, the correct question is: "What is broken in the Topward TFG-8104 that I now own?"

A Topward Function Generator TFG-8104 in mint aesthetic conditions.

While it looks good aesthetically - if you ignore the heavy uniform yellowing of the plastic case -, some tools are needed to determine how the inner electronics are doing.

First test: power-up. It is quite likely that someone else has plugged it in before selling the device, so a power-on harm has probably already been done. There's always a bit of adrenaline shock when pushing the ON button, and the red LED lights up. Nothing else can be tested using eyes and nose.

Next check: does it generate something? This step is easily done with the recently acquired Hantek oscilloscope, that is now resting in its box out of sight and immediate use.

First of all I tried tuning the generated signal on an AM radio. It is easier to slowly sweep the generator over the tuned frequency than doing it in reverse. Got it. With and without modulation. Good sign, it's worth going further. Oscilloscope? Not yet.

I opted for a more fun approach: understand as much as possible with a simple multimeter.

How would you do that? What can you check?

You can contribute in the comments, and read how it I did in the next post.

05 August 2021

Topward multimeter and function generator - for repair

I got two Topward instruments at a fundraising event: they give away truly untested and almost certainly broken electronic devices that might have a second chance of life in a different form. Most of the hardware available is composed of old computers and parts from 10-20 years ago, with the occasional electric/electronic lab novelty.

Topward TFG-8104 and 1302
Topward TFG-8104 and 1302.

I brought home a Topward Function Generator TFG-8104 and a Topward Digital Multimeter 1302. Built in mid-1980's in Taiwan by a company that is still around in 2021, and has released scans of manuals and diagrams.

Both boxes are very light, even though they contain a real transformer. I initially thought they were empty! I would say that even back in 1980's these guys were entry level devices and according to the front panel wear, they haven't been used much.

The multimeter seems OK. Voltage readings are all a bit low. Resistance is in the right ballpark and I haven't measured a current, which is the faulty part in other multimeters around my lab.

The function generator, while quite obsolete today, it provides uncalibrated 0.1 Hz to 2 MHz sine/square/triangle and AM/FM modulation, which makes it suitable for driving optical transmitters or "transmitting" a signal to a nearby AM radio. I had a look at the schematic diagram before buying it and its use of standard components makes it very suitable for repairs, which is the fun part I am looking for.