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-   -   Marconi TF2431 Counter Service Manual / Schematic (https://www.vintage-radio.net/forum/showthread.php?t=169327)

brian_mk 24th Jul 2020 8:09 pm

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.

Dickie 24th Jul 2020 8:37 pm

Re: Marconi TF2431 Counter Service Manual / Schematic
 
Alas I don't have a manual for a TF2431 but I used to work on them some 40 years ago and can remember a bit. After the main gate the counting chain is much the same as the 2430 you have, except it uses an MC10231 rather than an MC10131 to help it get to 200MHz. So you can use the 2430 schematic with a reasonable degree of confidence. Further on down the count chain the circuit is modified a bit to to allow an 8-digit display rather than seven.
Although the first MC10231 is configured as as a divide by 2 it is also wrapped up with other bits and feedback to give an overall divide by 10 function. So you might not be able to make much sense of it. It might be worth trying with a 10sec. gate time.
You may be suffering from dry joints. That generation of MI stuff used through-board rivets (AKA griplets) instead of plated through holes, that after many years of thermal cycling tend to develop cracks each side of the pcb. It would be worth running over all of them with an iron to reflow the solder joints.
Lets hope somebody has a manual!

brian_mk 24th Jul 2020 9:01 pm

Re: Marconi TF2431 Counter Service Manual / Schematic
 
Thanks for the reply Dickie.
Yes - I was aware of the unreliable griplet issue and have been over them with magnifier and applied a soldering iron where they looked suspect.

The decimal point was missing on the display but that was an easy fix - the edge connector just needed a clean.

So you must have worked for Greg Maton?
I also worked in R&D at M.I. St. Albans around that time.
I was there for 12 years so we probably know each other.
I spent most of my time a couple of labs along the corridoor in Telecomms (TF2356/57, TF2870/71 etc.). I also worked in TV Instruments for a while.
Happy days.

Dickie 24th Jul 2020 9:09 pm

Re: Marconi TF2431 Counter Service Manual / Schematic
 
Sorry - teaching you to suck eggs!

Yes, you've found me out. I worked at MI from '76 until they became IFR. Your counter should be fixable with or without a manual.

brian_mk 25th Jul 2020 6:39 pm

Re: Marconi TF2431 Counter Service Manual / Schematic
 
1 Attachment(s)
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?

brian_mk 25th Jul 2020 8:57 pm

Re: Marconi TF2431 Counter Service Manual / Schematic
 
Well the ferrite bead on Tr7 base didn't work. :(

I tried smaller values of peaking capacitor but TR7 still oscillates even with values as low as 4.7pF.

It makes me think there must be something else going on.
The copper ground area around TR7 is wide and looks like it's low immpedance.
The 5v rail decoupling looks ok.
The PCB tracks around the transistor are short.
The metal transistor can is grounded as it should be.
I'm baffled.

Dickie 26th Jul 2020 7:47 am

Re: Marconi TF2431 Counter Service Manual / Schematic
 
I don't want to labour the point (!), but go over all the griplets in the surrounding area, even if they look fine.

brian_mk 26th Jul 2020 3:13 pm

Re: Marconi TF2431 Counter Service Manual / Schematic
 
After more investigation today I think I've now found the true root cause of the problem:-

The counter contains an AGC loop.
This uses a diode detector which feeds a 741 summing amp.
The 741 adds in a variable dc offset voltage to set the gain using a trimmer.
I assume the output from the op amp eventually goes to the second gate of the dual gate MOSFET in the front end to control the gain (although I've not checked that).

The detector uses a pair of diodes in a voltage doubler configuration.
When I tried to measure the voltage at the detector output I got zero even with a 1V rms input connected!
At first I thought the problem might be the electrolytic capacitors in the detector. When I checked, the values and ESR looked ok.

Then I checked the two diodes using a trusty old analogue multimeter.
It turns out that one of the diodes was open circuit.
This is the first time in all my years as an engineer I have ever come across a failed signal diode.

The diode has no markings and I don't have a service manual, so I can't tell what type it is.
The older TF2430 used a single MBD102 schottky diode as the detector.
I assume the TF2431 also uses schottky diodes.
I don't have any MBD102 so I substituted an FH1100.

Low and behold - The counter now works as it should! :)

The detector fault explains the 120MHz oscillation I was seeing: The front end amplifier was operating at maximum gain all the time, even when a signal was connected. The oscillation only happened when a cable was connected to the input.

It's been a tough nut to crack and taken 3/4 days to trace and fix the fault.
It would have been so much easier with a schematic.

brian_mk 26th Jul 2020 3:45 pm

Re: Marconi TF2431 Counter Service Manual / Schematic
 
An MBD102 schottky diode has a maximum reverse voltage rating of only 4V. That could explain the failure.

brian_mk 28th Jul 2020 4:30 pm

Re: Marconi TF2431 Counter Service Manual / Schematic
 
I spoke too soon.

After fixing the AGC detector fault, I am still seeing HF parasitic oscillations / ringing in the wide band amp and the schmitt trigger.
Whatever the problem is, the result is false counter readings.
It appears to depend to some extent on the input signal and the AGC gain setting. Without a service manual I'm not sure how to set the AGC gain trimmer.
I haven't figured out if this is a design problem or a fault condition.
I'm still investigating.

Dickie 28th Jul 2020 5:15 pm

Re: Marconi TF2431 Counter Service Manual / Schematic
 
I can't remember the correct setup procedure from the manual, but the principle of the AGC is to provide a levelled input to the schmitt trigger that is just greater than its hysteresis. That way the effect of noise on the input is kept to a minimum.
So in the absence of anything better, I would put in say 100mV at 1MHz, (not at all critical, but it needs to be rock stable), then adjust the pot. to whatever end stops the counter counting. Then adjust the pot. slowly backwards until the counter displays a stable, accurate reading. That should not be far off the correct setting.
I doubt if its a design problem or it wouldn't have got out the door.
And just to clear up an earlier point. the detector diodes are configured as a peak detector, one to ground and one in series. Its not a voltage doubler.

brian_mk 30th Jul 2020 9:32 am

Re: Marconi TF2431 Counter Service Manual / Schematic
 
1 Attachment(s)
After further investigation I'm starting to believe the rogue 120MHz(ish) signal I am seeing is not caused by spurious oscillation but is more likely external interference.

The interfering signal is superimposed on any real input signal. This leads to false triggering of the schmitt trigger as the real signal causes the trigger threshold to be crossed. The effect is worse for small input signals (around 100mV). In this situation the wideband amplifier gain is increased by the AGC loop so the signal / noise ratio is worse.

It's difficult for me to accurately measure the frequency of the interfering signal as the timebase on my 100MHz scope only goes up to 0.05uS/division. I measure about 6 cycles per division which equates to roughly 120MHz (+/- 10% or so). I really need a spectrum analyser!

The frequency of the signal seems to remain solid - even if when I touch various parts of the input amp with a finger.

I'm not sure if the unwanted signal is mains born or radiated.
It's still there if I move the test setup to another part of the house.

I see no evidence of the unwanted signal if the input is left disconnected.

As soon as I connect a BNC lead to a signal generator the counter indicates random frequencies. This happens even if the generator is powered down or the output signal level is set very low.
I've tried a couple of different signal generators.

A BNC lead just connected to a 50 Ohm termination does not give the unwanted signal.

A BNC to croc clip lead with the two clips joined to form a small loop antenna does produce the unwanted signal.

Judging from the counter display, attaching a scope probe to various points in the wideband amp appears to makes things worse as does attaching the scope probe earth.

Unfortunately I don't have a screened room or a Faraday cage although putting the counter inside a grounded metal bread bin seems to help!

The UK frequency allocation data suggests the signal may originate from FM broadcast or Aeronautical Radionavigation:-
https://www.ofcom.org.uk/__data/asse...k-fat-2017.pdf

For now I have reduced the sensitivity of the counter by adjusting the 'Set DC' and 'Set AGC' trimmers.

Maybe I should cut a hole in the front panel next to the input connector and fit a LP filter switch as per the TF2431A model. This is a bit drastic and will only help when using the counter to measure LF signals.

I've attached a reverse engineered schematic of the input section.
It may not be completely accurate.

Dickie 30th Jul 2020 10:02 am

Re: Marconi TF2431 Counter Service Manual / Schematic
 
I wonder if the schmitt trigger is not working correctly? If you connect a 50R resistor across the input it should kill any pickup. then you can put an LF signal in and check the correct operation of it with a scope.

brian_mk 30th Jul 2020 11:25 am

Re: Marconi TF2431 Counter Service Manual / Schematic
 
From my earlier investigation, I think the schmitt trigger is working as it should.
If I've understood things correctly, the hysteresis is set by the diode forward voltage drop and is about 0.7V.

I'm pretty sure the problem is the interfering signal that I can't get rid of.
The counter makes a pretty good wideband receiver.
If it was FM broadcast, I would have expected to see a whole bunch of frequencies and general noise rather than a fairly clean looking signal that the scope can sync to.

brian_mk 30th Jul 2020 3:51 pm

Re: Marconi TF2431 Counter Service Manual / Schematic
 
Today I fabricated a half wave dipole cut to resonate at around 100MHz and hooked it up to my scope.
Guess what? I can see the same interference signal that's been bugging the counter.

The strange thing is that as I rotate the dipole the signal maximum occurs when it's oriented North/South. The closest VHF broadcast transmitter is Bow Brickhill which is to the South East. I have an FM antenna in my loft for my stereo system which points towards Bow Brickhill.
There are VHF broadcast transmitters at Northampton to the North or High Wycombe to the South but I would not expect a particularly high signal strength.

I suspect there is some other transmitter in the vicinity operating just above the normal FM broacast band.


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