Digital Test
Monsanto 110A

Manufacturer: Monsanto
Model: 110A
Year: 1968
Schematic: RE
Max. Rate: 85 MHz?
Display: NIXIE
Digits: 7
Logic: RTL, ECL

The maximum count rate on this unit needs to be measured. The first decade uses ECL flip-flops (MC1013) which are specified to 85 MHz. However, the second decade is labeled as "12.5 MHz", which would suggest the first decade is good to 125 MHz. Whether or not the rest of the input circuitry keeps up with either of these possibilities is unknown.


- Unit Log -

Serial Number: 916-A-561
Year of Manufacture: 1969 (Components stamped with 6745 - 6927, 1968 in foil of PCBs).
Date of Receipt: 1998 Dec
State upon Receipt: Good physical condition. Functional. Missing power cord with oddball connector. Fan bearings are bad, noisy. Unit had seen previous repairs:
- Some ICs on one decade counter module had been socketed and replaced (7647 dates on replacements). This provided cross references for some IC types.
- Transistor pairs on A and B input boards appear to have been replaced.

Date: 1998 Dec
Procedure: Cleaned.

Date: 1998 Dec
Procedure: Oddball circular power connector replaced with IEC standard.

Date: 2006 May 11
Procedure: AB Interval mode observed to be not working. A will trigger the count to start but B will not stop it. Traced to IC MT-4. Output MT-4.2 is not pulling high enough to send MT-4.5 low. Isolating MT-4.11 (V+) shows it to be open to all other pins. IC removed for replacement, but is OK after heat application during removal. Socket installed and old IC re-used.

Date: 2006 May 13
Procedure: Fan cleaned and oiled.

Date: 2009 Feb 16
Procedure: AB Interval mode problem (see 2006 May 11) is back. MT-4 (SC2217) replaced with a MC789.

Date: 2009 Feb 16
Procedure: Fan disconnected.

Date: 2011 Oct 13
Procedure: 4th digit not working properly, count sequence is 0-1-4-5-0, no carry to next digit. IC-4 (M0191) on appropriate board removed, socket installed, MC790 replacement installed. Functional.

Subsequently, numeral 6 temporarily fails to light, BC junction measures open, but then self-repairs(!)

Supply voltage on counter boards noticed to be quite low, down around 2.8V due to 4.7R dropping resistor.

Date: 2011 Oct 13
Procedure: AB Interval mode problem again, will start & stop but seems to be measuring simple period. MT board checked while on extender, problem seems to have gone away.

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Early Digital Test Equipment