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Old 24th Jul 2020, 8:09 pm   #1
brian_mk
Tetrode
 
Join Date: Mar 2015
Location: Milton Keynes, Buckinghamshire, UK.
Posts: 69
Default Marconi TF2431 Counter Service Manual / Schematic

I've owned the earlier TF2430 model for a few years.
It works fine - although the maximum frequency of 80 MHz is a bit low.
When I saw the later 200MHz TF2431 on eBay I decided to take a gamble.

As it turns out it has a fault - the displayed count is almost random and bears little relationship to the frequency of the input signal.

I have the service manual for the TF2430 but not the TF2431.

The front end is completely different. It uses ECL as you might expect.

I examined the PCB with a magnifier and found what looked like an unsoldered / bad joint on Vcc2 (pin 16) of a triple 2-3-2 input Or/Nor gate (MC10105).
This looked like the culprit so I resoldered the pin. Unfortunately the fault still exists.

I then connected the input to a 1kHz square wave and probed around with a 'scope. I'm not really familiar with ECL.
The output from the MC10105 appears to be the input signal gated by the main gate signal (The gate has a 1 second period on 'kHz' range). The output is a 1kHz pulse train held low for 1 second every second. That signal seems to be ok as far as I can tell.

It feeds into the CE2 (pin 11, Clock Enable) input of a MC10231 dual D type MS flip flop. The actual clock input (CC pin 9) is not used. The /Q2 output pin 14 is connected back into the D2 input (pin 10). Hence I would have expected the output of the flip flop to look the same as the CE2 signal divided by 2. Instead I get an almost random looking pulse train that the 'scope is unable to sync with.
I suspect the MC10231 may be faulty. I've checked Vcc1 and Vcc2 and the other input pins for noise / bad connections etc. but it appears ok.
I've ordered a replacement part - although I'm not 100% confident it will fix the fault.

It would be really helpful if I could get hold of the service manual or at least the schematic. Does anyone have a copy?

Cheers.
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