Farnell DTC12 component tester board repair

[WaveyDipole]

I am trying to repair a component tester board on a Farnell DTC12 Oscilloscope. I have circuit diagrams for the oscilloscope, but the circuit for the component tester is missing. If anyone has the official circuit diagram and especially a manual for this, a copy would be appreciated. Instead I have had to reverse engineer the circuit and have attached my efforts for reference below.

The tester is an "octapus" style curve tracer that generates specific Lissajou patterns on the CRT depending on the component under test and its state. However, this tester is a little more complex in that is has three impedance modes: high, medium and low. High and medium impedance mode seem to work as expected, for example, a diode generates a right angle shape ( _|). However, low impedance mode shows a diagonal line i.e. like a slash (/) instead which does not make sense.

I have attached a circuit diagram and have eliminated the left hand side comprising IC1, IC2 and IC5. IC2 seems to operate in flip-flop fashion switching either R42 or R43+R44 into line with the inverted input of IC4 which controls the X axis and is driven by IC5+TR3. IC5 which also drives the LED via TR1 and TR2, the purpose of which I am not sure? In any case, I experimented with removing IC5 and IC2 and can test operation using wire links on IC2 pins either 8+9 or 10+11. This eliminates the left had side of the circuit.

The primary focus is on IC3 which provides the Y output. Each impedance mode has its own resistor chain arrangement:

High: R4-R8 + R11 + R13 + P1
Medium: R10-R17 + P2
Low: R19-R22 + R3

These are switched connected into circuit corresponding to the mode  switch that is depressed at the time.

I have checked the resistors R19-R22 and R4 and they all seem good and within tolerance. Since the circuit around IC3 works fine in medium and high impedance mode, it seemed unlikely that these resistors would be at fault, but just to be sure, I checked the 1M resistors R23-R26 as well as R29-R31 and all test good on a DMM. IC3 has been substituted as well but the problem still persists. I have also checked for dry joints and cracked tracks but can find no fault.

Perhaps someone can help me understand the purpose of R3? Having a 47Ω resistor across 1MΩ + 330KΩ in series seems to make little sense?

I lifted the ends of the two resistors R19+R20 at their junction, effectively disconnecting them from R25 at the inverted input of IC3A expecting a horizontal line but instead go a nearly vertical one!

Does anyone have any idea why the circuit is behaving in this strange manner? Is it meant to show a diagonal line in low impedance mode?

Please note that the slight ellipse in the low impedance mode image is on account of loose bypass wires. It is ordinarily a straight diagonal line.

[WaveyDipole]

Well this is bizarre! I modelled the circuit around IC3 and IC4 and the resistor dividers for the low and medium impedance modes in LT Spice. What I discovered was that the model behaved exactly the same as the actual circuit, so it would seem to be behaving exactly as it should, but I don't understand this behaviour. The model demonstrated that with those values of resistors there is no way that the horizontal line would be level. This begs the question why? Might the board have been a prototype? I have never seen an "octopus" tester that behaves that way.

With a bit of experimentation I was able to find resistor values that worked but R19 and R20 had to be substantially reduced in value for this to function as one might expect.

I guess the manual is a long-shot, but would be very useful is someone has one?