Author Topic: Solartron 7061 7.5 Digits Multimeter Teardown and Repair (Read 8578 times)

Kosmic · « **on:** January 13, 2019, 02:53:20 am »

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 · « **Reply #1 on:** January 25, 2019, 02:19:36 am »

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 IC202 has been socketed (normally only IC201 and 203 are on socket) and the LM301 that was supposed to be there has been replaced by a RAYJM38510.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 · « **Reply #2 on:** January 25, 2019, 02:41:33 am »

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

). 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 · « **Reply #3 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

Kosmic · « **Reply #4 on:** January 25, 2019, 02:53:46 pm »

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

If it's the case, that person is really good at de-soldering!

coromonadalix · « **Reply #5 on:** January 25, 2019, 09:19:55 pm »

With solder wick, you can obtain theses results ?? or a desoldering pump too ??

Unless maybe unless a part was not put on the pcb, i had a problem like this in the past with an sony cinema center, a missing resistor made the speaker output drift in dc and trigger the speaker protection, but the drift was really slow in time.

Macbeth · « **Reply #6 on:** January 25, 2019, 10:09:25 pm »

That part has been de-soldered and PCB cleaned up. You can still see some flux residue and also the solder mask show signs of heating around the pins.

kony · « **Reply #7 on:** January 25, 2019, 10:32:05 pm »

Quote from: coromonadalix on January 25, 2019, 09:19:55 pm

With solder wick, you can obtain theses results ??

Yes, with decent flux and bit of practise, no problem at all.

Macbeth · « **Reply #8 on:** January 25, 2019, 11:05:13 pm »

You can see that every one of those 8 transistors including the missing one have been played with, though the bottom ones are out of focus.

Just look a bit north of the board at the diodes and resistors and note no shenanigans with solder mask, it's all clean and even.

Kosmic · « **Reply #9 on:** January 29, 2019, 04:05:13 am »

You're probably right. Unit#2 is a lot cleaner and there is no flux residue around the transistors.

This is a picture of the integrator of Unit#2

Kosmic · « **Reply #10 on:** January 31, 2019, 04:14:19 am »

So replacing the WM222 by a VN2210 fixed the -Glug. Nevertheless I decided check the rest of the integrator before calling it a day.

For some reason (VN2210 being not exactly the same as WM222 ?) the -Glug Drive squarewave was curving a bit.

Trace 1 = +Glug Drive
Trace 2 = -Glug Drive

Increasing the capacity of C808 fixed the issue. I played with different value and ended up using a 1uF tantalum capacitor.

Trace 1 = +Glug Drive
Trace 2 = -Glug Drive

The new transistor and cap installed on the board.

Kosmic · « **Reply #11 on:** January 31, 2019, 04:23:13 am »

Unit #1 is now passing Self-Test and seem to be fixed

Kosmic · « **Reply #12 on:** January 31, 2019, 04:29:54 am »

Next steps:
- Teardown
- Testing all electrolytic capacitors
- Replacing the Schaffner filter
- Replacing the memory backup battery
- Replacing the input connectors

coromonadalix · « **Reply #13 on:** January 31, 2019, 04:33:45 am »

Congrats for your repair

Kosmic · « **Reply #14 on:** January 31, 2019, 04:44:16 am »

Teardown:

All the 7061 PCBs

PCB 13 - Digital Board

Kosmic · « **Reply #15 on:** January 31, 2019, 04:55:14 am »

PCB 15 - Analog Board

PCB 8 - Oscillator Board

Kosmic · « **Reply #16 on:** January 31, 2019, 04:56:01 am »

Front Panel

Kosmic · « **Reply #17 on:** January 31, 2019, 05:19:03 am »

New NI-MH battery in place

I spent some time testing electrolytic caps but they were all fine. Now since I'm pretty much done with soldering on the PCBs, I started cleaning everything with IPA.

branadic · « **Reply #18 on:** January 31, 2019, 04:52:02 pm »

What is the reference used in this unit?
Congrats to your progress in refurbishing the meter.

-branadic-

Kosmic · « **Reply #19 on:** February 01, 2019, 03:15:37 am »

Quote from: branadic on January 31, 2019, 04:52:02 pm

What is the reference used in this unit?

I think the 7081, 7071 and 7061 all use 1N829 Zener diodes.

Kosmic · « **Reply #20 on:** February 01, 2019, 04:15:21 am »

D301A is the voltage reference

Kosmic · « **Reply #21 on:** February 01, 2019, 04:42:20 am »

The original Fischer connectors being way too expensive, I decided to replace them by cheap XLR 12mm 5 pins connectors I found on ebay. I know they are probably not that good but I wanted the opportunity to create a lot of different test leads for the 2 meters I have.

At least they are fitting on the existing front panel without any modifications.

Kosmic · « **Reply #22 on:** February 01, 2019, 04:57:14 am »

Back connectors for rear input and secondary input channel were also replaced.

Kosmic · « **Reply #23 on:** February 20, 2019, 02:29:24 am »

Unit #1 fully reassembled.

eevcandies · « **Reply #24 on:** May 23, 2019, 02:06:53 am »

I have one of these 7061 models, been sitting around at least 12 years, since I plugged in (It came up and ran fine last time). I didn't have any way of make any connections to try it out (where are the banana jacks?), so I tossed it aside & reverted to my fluke meter. I seem to remember not liking the readout, it put commas in the readings (like 12,437 V)? Or perhaps I didn't fiddle enough with it.

I have a precision R6871E, which is nice, but I still use my Fluke!


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Author Topic: Solartron 7061 7.5 Digits Multimeter Teardown and Repair (Read 8578 times)

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