In another thread in the 'Items Wanted' section, there was mention at post #11 of the 'ESTON' signal injector:
https://www.vintage-radio.net/forum/...d.php?t=142793
I've had one of these injectors for many years - probably a cheap rally buy, and it works well, so I thought some notes might be of interest. I won't go into signal injecting or tracing techniques - it's been covered many times on the forum, particularly in reference to the excellent Vellman K7000 kit. Suffice to say that as far as I'm concered, a signal injector/tracer is second on the list of fault finding test gear after a multimeter.
My 'ESTON' t came with typewritten instructions on a strip of paper 3” x 8”which appears to have been cut from a page, and I’d guess it dates from the early 1960s. It has a low component count – 2 capacitors (plus an isolation cap), four resistors and two transistors, which I think will probably be PNP germanium types. The brand is ‘Impex’. They’re both marked ‘S30T’ and have a green spot on top. The circuit is built on a small plain drilled Paxolin board, (not a PCB) and is a simple multivibrator circuit, the frequency of oscillation determined by the choice of resistors and capacitors. The resistor values are as follows:
Marked
Value:
680R (625R) Measured Values.
680R (625R)
4k7 (4k4)
4k7 (2k2)
Unusually for carbon composition resistors, they've drifted low over the years, rather than high.
The two capacitors aren’t marked, but one measures 8nF - the other 20nF. They will originally have been the same value, (as with the two pairs of resistors) and I think it most likely that both 20nF. As with the resistors, the value is unimportant as it only has a bearing on the frequency of oscillation. The waveform as observed isn’t quite a square wave, but that’s unimportant and I guess that the lack of symmetry is due to the difference between the actual values of the 4K7 resistors and the two capacitors. There is a 500V .01uF isolation cap from the circuit board to the probe.
As can be seen from the picture of the underside of the circuit board, it’s simply cross-wired in ‘perf-board’ style, very much along the lines of a home-brew project, but nicely conceived and in a neat purpose made screen printed case.
Signal injectors widely differ in their frequency from as low as 400 Hz to as high as 2.5 kHz. This one measures 1.955 kHz.
The typewritten instructions that came with it (now very tatty) red thus:
THE ESTON SIGNAL INJECTOR
This signal injector is a two transistor square wave generator. The battery is a No 8 and as the drain is only 5mA it should last for many months. Signal injection is a technique used by most service engineers to trace faults in television and radio sets and all that is necessary is to touch the tip of the probe to the grid of the valve or base of the transistor and work back from the output stage towards the aerial. You just press the button on the injector to inject a signal into the circuit under test and if the signal is not heard in the speaker, this indicates a faulty stage.
(Verbatim copy of original instructions).
I've drawn what I think will most likely be the circuit - a fairly standard multivibrator, but I've not traced the wiring of the little circuit board, so the actual circuit may differ.
It used a 'Number 8' 3 Volt torch battery - long since obsolete so I used a 3V CR132 lithium battery in an adaptor I made, which I'll cover in another post in this thread.
There has been speculation about the Impex brand of transistors in a forum thread below, but no conclusive evidence as to who owned the brand and when:
https://www.vintage-radio.net/forum/...d.php?t=111971
The only other references I can find show that Impex brand transistor radios were made, such as these examples:
https://www.radiomuseum.org/r/unknow...ransistor.html
https://www.ebay.co.uk/sch/i.html?_nkw=impex+transistor
Some pics attached - more to follow.