Sinclair DM350 calibration

[zerto]

Hello,

I have extracted my first multimeter from the attic and it is still working (I bought it in the early eighties !). A quick check on voltage measurement shows a 5% error over 2.5, 5, 7.5 and 10 V  reference voltage. I would like to try to calibrate it so I am looking for the Sinclair (Thurlby Thandar Instruments) DM 350 digital multimeter service manual, anyone who could share it with me ? that would be fantastic !

I found a web site with the manual but it is rather complicated to download it... even the registration to the web site is quite complicated, I am not sure what they are asking for.

Regards

[zerto]

adding some images...

[Andreas]

Hello,

at that time only few chips were used for DMMs. (ICL71xx)
I would look at the 28 pin Chip number and a data sheet for it.
Then adjust the reference voltage to the value of the data sheet (100.0 mV or 1000 mV)
This will help at least for the DC-voltages/currents.

But before I would clean/reseat the contacts of the chips in the socket.

with best regards

Andreas

[zerto]

Thanks for the tips, the chip is a MP7138A, but I could not find the datasheet yet.
I will cleanup the contacts as you are suggesting and check the reference voltage if I can get the service manual.

Best Regards

[iMo]

A brief info on the chip:

https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Poptronics-IDX/IDX/70s/78/Poptronics-1978-03-OCR-Page-0070.pdf

https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Poptronics-IDX/IDX/70s/78/Poptronics-1978-03-OCR-Page-0071.pdf

[zerto]

I wanted to share the great outcome of this topic:
After cleaning the main chip, the switches and the rest of the board using a contact cleaning spray, and with the help of the service manual I have been able to recalibrate the unit at better than 0.1% error on DC voltage ! I am really happy to be able to reuse my first multimeter, being confident on its accuracy
thank you all for the tips