Reviving the topic because I've hopefully finished repairing a Racal-Dana version of this DMM.
A couple of other topics relating to the model:
https://www.eevblog.com/forum/repair/thurlby-1905a-dmm-adc-repair/https://www.eevblog.com/forum/testgear/thurlby-1905-any-good/Problems:
Won't work, display shows SW version or checksum on power-up but won't respond to input.
Cracks in the case.
Someone previously had a try at it....
Checking power:
Transformer outputs marked 18-0-18 (AC) on service manual measure 22V, 7V winding measures 7.6V, all a little high.
18-0-18 voltages after diode bridge are +24V and -32V, +15V is good -15V is missing.
Replaced both 78L15 and 79L15 regulators for standard TO220 versions, the PCB was a little too heated under the regulators. (The 7815 fits as is, the holes are too small for the 7915.)
Fitted 470µF 35V for C44/C45 where 2.2mF 50V had previously been squeezed in between the mains socket and top shielding.
Replaced the four tantalum capacitors.
Re-soldered all PS diodes directly after transformer.
Replaced ammeter fuse.
This restored the ability to measure.
Next rewired the transformer for 240V, checked +/-15V power rails, readjusted -5V rail for -5.05V on CPU board.
Readjustment:
First noted all the positions of the adjustments on the lower shielding before re-fitting it. (As a side note: The grounding of the face plate is supposedly made through the upper shielding. In my case, due to plastic washers that may have previously been fitted in the wrong places, the front was not grounded.)
Service manual says to use a specific lower case with holes to for readjustment while allowing the electronics to run at closed-case temperature. I just bunged the sides and did the readjustment with DMM upside-down after 30 min warming.
I used a Fluke 5100B calibrator that needs some readjusting itself and Keithley 5100 DMM as actual reference readout. The Racal 4009 was set to hidden 6.5 digit mode with "filter" enabled.
I didn't fool around with any of the "SOT packs" resistor networks assuming the best possible had been done there in factory and that not being trimmers their drift should be minimal.
Only after going through the full SM's procedure once I started thinking that some of it didn't make much sense. (To me at least.)
Some trimmers are used twice in the sequence: A7 for 20k\$\Omega\$ then for 2kV adjustment, A8 for 200k\$\Omega\$ then for 200V this seems like a waste of time, you go to plenty of trouble getting the 20k and 200k Ohms adjustments spot-on then go and trow them off by making the voltage adjustments.
Going by the way the DMM is designed when adjusting the mentioned ranges one must choose between favoring either voltage or resistance accuracy. (I chose voltage.) either that or it's a headache and hard compromise between the two.
Rather than following the manual's sequence these were my steps:
1: Adjust 2VDC zero using A1
2: Adjust 20VDC zero using A2
3: Adjust 0.2VDC zero using A3 (I found there is a slight interaction with step 2, iteration may be needed)
4: Adjust 2VDC gain @1V input using A4 (ADC Vref. for V/A)
5: Adjust 0.2VDC gain @0.1V input using A5 (x10 buffer amp gain)**
6: Set to 2k Ohm range, adjust for 1mA current output on V/Ohm sockets using PR5 (Not crucial)
7: Set to 2k Ohm range, adjust to 1k Ohm reference using A6 (ADC Vref. for Ohms)
8: ?Best accuracy for V or Ohms measurements? For Ohms goto 13
9: Adjust 20VDC gain @10V input using A10
10: Adjust 200VDC gain @100V input using A8
11: Adjust 2kVDC gain @1kV input using A7
12: Adjust 2VAC gain @1V 50Hz input using A11
13: Set to 20k Ohm range, adjust to 10k Ohm reference using A10
14: Set to 200k Ohm range, adjust to 100k Ohm reference using A8
15: Set to 2M Ohm range, adjust to 1M Ohm reference using A7
13: Set to 20k Ohm range, adjust to 10k Ohm reference using A10