| Electronics > Repair |
| Solartron 7061 7.5 Digits Multimeter Teardown and Repair |
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| Kosmic:
Some time ago, I had the opportunity of buying 2 Solartron 7061 7.5 Digit Multimeter. They were sold for parts and only tested for power-up only. The Devices: The Solartron 7061 was sold in late 80's as a fast and accurate (7.5 digits) multimeter. It can capture 1500 readings per seconds, got an interesting set of math functions and has some scanning capabilities (with the second input or an extension card). Both of the units I bought were built in 1988 and are equipped with the revised PCB 13 & 15. The Problems: Unit #1: - Failing 0V Test. - A little bit unstable. At 6.5 digits the 2 last digits are jumping quite a bit. - VDC is off (the only thing tested). Unit #2: - Pass selftest. - Readings are erratic. Ex, if 10V is applied at the input, readings will somehow increase but never reach 10V. If the input is shorted, readings will go down but never reach 0V. |
| Kosmic:
Unit #1: I decided to investigate the test failure first. The manual of the 7061 do not mention anything specific about the "0V Test Fail" error message. Nevertheless, a quick search on the web revealed some information hidden in the manual of the Solartron 7081 (big brother of the 7061). They recommend investigating IC201 in the integrator. Since the integrator of the 7081 and 7061 are similar this was a really good lead. The Integrator: The role of the integrator is to merge the forcing wave and input signal together and present a ramp up/down to the comparator. The comparator then generate 2 pulse trains, called +Glug and -Glug. Those pulse trains are then simply counted and digitalized. The technique used is similar to the Delta Sigma modulation used in many modern analog to digital converter IC. After I quick inspection I realized that someone already tried to repair the instrument. IC202 has been socketed (normally only IC201 and 203 are on socket) and the LM301 that was supposed to be there has been replaced by a RAYJM38510. I quickly replaced the RAYJM38510 by a new and working LM301 but the 0V test was still failing. A quick scan of the manual revealed the different test points of interest in the integrator. Testing the + and - Glug revealed that something terribly wrong with the negative Glug. Expected traces: Observed traces: |
| Kosmic:
Based on the fact that the -Glug was struggling to go back to 0V, I decided to inspect and verify TR202 and then the -Glug Drive and associated transistors. To my surprise I realised that TR202 was just not there (completely missed it when quickly inspecting the board :palm:). The pads were perfectly clean and it was almost like TR202 was never soldered to the board. Might have been a case of cold joint. Unfortunately the mosfet was nowhere to be found. So TR202 is actually a WM222 (Solartron part # 300555940). Now I couldn't find any information on this part but the 7081 has the same Solartron part # for the same part in the integrator. But the manufacturer par number on the 7081 is VN67AK and specsheet is available on the web for that part. Since the Solartron part # are the same I speculated that the parts were interchangeable (WM222 / VN67AK). After a bit of search I found the Microchip VN2210 with specs equivalent to the VN67AK. |
| coromonadalix:
Surely someone had a go in it and never finished the job ??? It would had never pass Solartron quality control ??? |
| Kosmic:
--- Quote from: coromonadalix on January 25, 2019, 07:52:55 am ---Surely someone had a go in it and never finished the job ??? It would had never pass Solartron quality control ??? --- End quote --- If it's the case, that person is really good at de-soldering! :) |
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