Electronics > Metrology

Yokogawa 7651 Programmable DC Source

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Hi group,

I am going to share some teardown pictures and the repair of a Yokogawa 7651 Programmable DC Source.

Link to the Operators manual


I bought this unit in broken condition.

It was first advertised on this forum:


and later sold on eBay:

Front Panel

Specifications (from the Operators manual)


I was curious about this configuration. The idea of using two DACs, in coarse and fine configuration normally doesn't work very well. The coarse DAC has to be extremely linear to achieve good accuracy. In Linear Tech application Note AN86, an LTC2400 high resolution ADC is used to correct two 16-bit DACs in coarse/fine configuration to obtain 20 bits of accuracy.

Link: https://www.analog.com/media/en/technical-documentation/application-notes/an86f.pdf


With the outer cover removed you can see a shield.

With shield removed you can see the main board.

Here is the main board removed from the case.

To be continued …




Here are some of the sections of the main board

Output Amplifier

The Output stage is capable of +/-32V output and +/-100mA.
It is made from discrete bipolar transistors, the kind that you would find in an audio amplifier.

Main Analog Section

There are two AD7541A 12-bit DACs

Interestingly Yokogawa did not use the best grade.

The colored paint dots indicate, selection and or burn-in or both

I suspect that the coarse DAC was selected for linearity.

Reference section

Under all paint dots, it is an LT1021C-10




It was a gamble buying this instrument in a broken condition. The fault was described as:

This is a PARTS UNIT.  Sold AS-IS.  When you turn on the Output (Voltage or Current) it immediately goes to -36V.

I couldn't find a service manual or schematics on the internet for this or similar Yokogawa instruments. So I knew I was going to have fix this without a schematic.

When I confirmed that the output was at -36V on all the ranges.

I traced out the power amplifier section, expecting to find a shorted transistor. All the transistors turned out to be good.

I then found the op-amp that was the driver for the output stage. If I shorted it output to ground the output also went to 0V. Indicating that the output amplifier was not the problem.

I determined that the op-amp, an LT1012, was damaged. Initially I replace the op-amp with an OP07. This is a similar op-amp but not as good.
This didn't fix the problem.

I looked for more op-amps that weren't working properly. I found a damaged NE5532 op-amp that was also not working. When I change this the unit started working properly.

These are the two amplifiers that I changed.



There was also a 90 \$\Omega\$ precision resistor that burnt. It was consequential damage from the output being stuck at -36V.

I believe that the resistor is in a divider for the mV ranges.

I decided to make my own replacement resistor rather than try and find the original Vishay precision resistor.

I built the resistor on a tiny PCB
There are two 182 Ohm 0.1% resistors in parallel and a 7.87k \$\Omega\$ in parallel to trim the value.

The resistor measures 89.99358 \$\Omega\$

Here is a picture of the resistor installed, it is R111.



Here are some pictures of the unit working.
This is after repair, but before any calibration.




I would be interested to hear from anybody who schematics for this unit.



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