Need a schematic for Racal-Dana 9906

[wobbly]

I wonder if an expert in 1980s vintage Racal test gear can help me out here...

I've been hunting around the intertubes for a schematic for my Racal-Dana 9906 Universal Timer Counter.
So far I have only been able to find some very poor quality (almost completely unreadable) scans of a 9904 in the hope that this might be close enough, but I think the design is too different to help me.

My 9906 unit has a strange fault with the "Data output" port on the rear panel, but my attempts to diagnose it have stalled for now.
It spits out the display digits sequentially in BCD format - the problem is that whenever bit 2 is set then somehow bit 3 is also set as well.  I'm thinking there's a low impedance fault somewhere in the SN74LS75 bistable latch.
Oddly, the BCD output pins on the rear connector simply connect to the INPUTS (1D, 2D, 3D and 4D) on that chip - this has me a bit baffled, why did they do that?

This chip is TTL according to the datasheet but when I measure the two problematic pins (2D and 3D, which go to the rear-panel "Data Output" port) then pin 3D shows a square wave that seems to have it's logic low level at about 600 mV.
Worse still, pin 2D never falls below 2 Volts!!  But it's still a fairly decent square wave showing correct data.  3D seems to always get yanked high by a high level on 2D, as if the pins are somehow coupled together.

In-circuit resistance checks with a DMM seems to back this up (1D to 4D reads 9 Megohms, 2D to 3D reads 22 Kilohms!), can't tell if it is the SN74LS75 chip or something elsewhere.  I don't really want to start desoldering chips just yet.

Anyway, the rest of the unit works flawlessly, it measures beautifully and the front-panel LED display does show correct readings 100% of the time.

Does anyone have a good quality copy (digital or paper) of a Racal-Dana 9906 Universal Timer Counter main-board schematic (that exact model please)?

[eevcandies]

How about the 9905...the front panels look almost the same (at least a 2 second comparison)
https://elektrotanya.com/racal_9905_frequency-counter_sch.pdf/download.html

some misc info
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Racal/Racal-Dana_9902_9906_Counter_Operators_Handbook.pdf

[wobbly]

Hi there,

I'm sorry for the slow reply.  Thank you, the 9905 schematic did help a bit, luckily the part of the circuit that was causing some problems was pretty much identical to the 9905.

Long story short, the problem is 99.999% solved.  The remaining .001 is a nagging doubt.  I've been soak-testing this thing for many hours and I've not seen the problem again since I removed, socketed and replaced a pair of ICs.

Firstly the 74LS04 hex inverter that was driving the troublesome lines was removed and tested on a breadboard and found to be sort-of working (the output levels looked a bit iffy but the response time and noise levels were in spec).
So I binned it and put in a fresh one.  The problem immediately improved by 75%.

Secondly a 74LS75 bistable latch was swapped out in the same way and now the problem seems to be solved.

My home-brew data logger custom made for this instrument works nicely now that the BCD signals are not mysteriously sticky any more.

If anyone's interested, it's an Atmel ATTiny84 with an MCP23S17 SPI bus expander (which is super useful because it has built-in configurable inverters on it's inputs).
Data is output over a simple 5V, 9600 baud "8N1" serial connection that my PC can read easily.  Power for the logger is taken from the 5V output on the Racal's output port.

In the photo here it's the small black box on top of the Racal.  The serial port data output is shown on the right side.

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