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Old 25th Jul 2020, 6:39 pm   #5
Join Date: Mar 2015
Location: Milton Keynes, Buckinghamshire, UK.
Posts: 69
Default Re: Marconi TF2431 Counter Service Manual / Schematic

After some further investigation I think I may have located the problem...

The counter contains a wideband input amplifier using a number of cascaded emitter followers and common emitter stages. (See the attached schematic I created by reverse engineering the PCB). The transistors are 2N5179 which have an Ft around 1500 MHz.

The input amp is different to the older TF2430 which used an MC10216 as both an amplifier and a schmitt trigger. (I suspect the TF2431 uses a schmitt trigger made from discrete transistors to increase the bandwidth.)

Some probing with a 'scope revealed that one of the common emitter stages (TR7) is oscillating at around 120 MHz. Removing the 68pF peaking capacitor in parallel with the emitter resistor stops the oscillation whereupon the counter appears to work as it should.

However, this is not a good solution because removing the capacitor will reduce the bandwidth and affect the high frequency sensitivity.

Almost all the 2N5179 transistors on the beoard have 10 Ohm resistors in series with the base in a attempt to prevent parasitic oscillation.

Presumably the counter must have initially worked as it should with the capacitor fitted, so what could have changed that would cause TR7 to oscillate? As far as I'm aware, apart from electrolytic caps drying out and resistors drifting slightly, most electronic components don't change much with age.

I tried fitting a smaller 56pF capacitor and also swapping out the 2N5179 for another transistor of the same type. I've also tried adding some short lead decoupling capacitors to the 5V rail close to TR7. The oscillation is still there.
I may have to try a ferrite bead on TR7 base.

Any other suggestions?
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