Yokogawa 7651 Programmable DC Source

[Jay_Diddy_B]

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

https://cdn.tmi.yokogawa.com/IM7651-01E.is.pdf

I bought this unit in broken condition.

It was first advertised on this forum:

https://www.eevblog.com/forum/metrology/put-my-fluke-8505a-and-a-broken-845ab-on-ebay/msg2983972/#msg2983972

and later sold on eBay:


Front Panel



Specifications (from the Operators manual)





Outline




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

Teardown



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 …

Regards,

Jay_Diddy_B

[Jay_Diddy_B]

Hi,

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





Regards,

Jay_Diddy_B

[Jay_Diddy_B]

Hi,

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.

Regards,
Jay_Diddy_B

[Jay_Diddy_B]

Hi,

There was also a 90 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 in parallel to trim the value.



The resistor measures 89.99358



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

Regards,
Jay_Diddy_B

[Jay_Diddy_B]

Hi,

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


100mV



10V




10mA




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

Regards,
Jay_Diddy_B

[Jay_Diddy_B]

Hi,

This is an approximate schematic of the output stage:



The actual transistor numbers are marked in purple.



This is how it behaves with a normal load and in current limit:



It might help somebody in the future.

Regards,
Jay_Diddy_B

[jbb]

Nice repair.  Are you going to get a fancy replacement resistor or rock what you got?  (If so, I suggest a coating of some kind to keep crud out of it.)

On the output stage: interesting that they are effectively using 2 separate drivers for the main transistors.  I guess R1 and R2 are big enough that a few hundred mV of Vbe mismatch doesn't result in much current flow.  Alternatively, if the Vbe mismatch goes the other way the stage will go Class B, and that's fine because it's a DC output and crossover distortion isn't relevant.  It does make current limiting easy!

Are you sure about the connection of R4? Perhaps it should connect to Q3 emitter for current limiting?

[Jay_Diddy_B]

Are you sure about the connection of R4? Perhaps it should connect to Q3 emitter for current limiting?

You are right there is a mistake in my drawing.

Here is the correct schematic



And the modelling results




Regards,

Jay_Diddy_B

[Jay_Diddy_B]

Nice repair.  Are you going to get a fancy replacement resistor or rock what you got?  (If so, I suggest a coating of some kind to keep crud out of it.)

I am going to leave it alone for now.

It is tough to find the a better solution. The nearest I can find in stock is to use a 100 in parallel with a 1k . That is 90.9 and hope that the calibration can fix it. Those resistors are about $15 USD each.

I thought about dipping it in Epoxy or some form of conformal coating, but I am not sure.

Regards,
Jay_Diddy_B

[2N3055]

Nice repair.  Are you going to get a fancy replacement resistor or rock what you got?  (If so, I suggest a coating of some kind to keep crud out of it.)

I am going to leave it alone for now.

It is tough to find the a better solution. The nearest I can find in stock is to use a 100 in parallel with a 1k . That is 90.9 and hope that the calibration can fix it. Those resistors are about $15 USD each.

I thought about dipping it in Epoxy or some form of conformal coating, but I am not sure.

Regards,
Jay_Diddy_B

Hello,
here you have good ones for decent price. They are real.

https://www.ebay.com/itm/0-1-5ppm-0-5W-Very-High-Precision-Vishay-SFERNICE-Foil-resistor-values-2R-3K3/132092526341

They have 90 OHm..

Regards,

Sinisa

[Jay_Diddy_B]

Hello,
here you have good ones for decent price. They are real.

https://www.ebay.com/itm/0-1-5ppm-0-5W-Very-High-Precision-Vishay-SFERNICE-Foil-resistor-values-2R-3K3/132092526341

They have 90 OHm..

Regards,

Sinisa

Hi,

That is a good find, they have 90R9 0.1% which would probably work. In normal circumstances I would have bought the resistor from them. Unfortunately they are unable to ship using the Polish Post Office at the moment. This means (relatively) huge shipping costs for one resistor (to Canada).

I went through the 7651 calibration procedure, described in the manual, using an HP34401A DMM this morning.

I will see how stable it is with the DIY resistor.

I used 2 pieces of this resistor

RQ73C2A182RBTD

Digikey number: A139685CT-ND



I am not sure how, or what the resistor is used for in the Yokogawa 7651.

Regards,

Jay_Diddy_B

[2N3055]

Hi,
Sorry that you cannot get those resistors.. They would be even an upgrade to those VSRJ resistors in there...
VSRJ has specified slightly better tempco and stability than what you made, but  it seems Ok enough , at least for now...
Those are quite nice resistors, vith TaN passivation layer, they should be stable enough. One thing I'm missing from spec is voltage coefficient for them
For VSRJ  Voltage coefficient is specified at < 0.1 ppm/V..

Regards,

Siniša

[ch_scr]

For completeness here are bottom & top pictures of GPIB Option board.
On mine it seems like PCB marking for capacitor was wrong way around - also capacitor may have been replaced by former owner, but wrong way around again
I had to put replacement cap on bottom, vias are corroded away. (In picture it is still installed wrong! Top fill is GND, bottom fill is VCC, I also got confused at first.)
Someone want to reverse memory card pinout?

[ch_scr]

Ok, I could not resist, here is preliminary pinout & thoughts. This device does 8kByte card only, but proposed pinout would support up to 64kByte - which seems maximum offered by Yokogawa in this style.

[maxwell3e10]

I recently got a 378904 memory card (256kB) to use it with 7563 temperature Yokogawa meter. The manual for 7563 (also for 7561/7562 and 7551/7552) specifies only up to 378903 card (64kB) as an option, but the bigger card works OK. The battery it uses is CR2325. The only strange limitation I found is that it can save only 3500 points in memory, whereas the smaller 378903 card is supposed to have a capacity of 8000 points. 3500 points is not even an option among other cards specified. So there is something funny going on with capacity detection, it assumes a capacity of 32kB.

[maxwell3e10]

Grounding pin 32 of the card connector, which is left open in the 7563 meter, fixes the problem so maximum possible capacity is recognized. Apparently the size of the card is determined not by a voltage but by scanning of the memory. For reference here is the picture of the insides of the card. I can post the pinout of the card if there is interest in replicating it.

[ch_scr]

That would be very kind of you, I would be interested to see how "far off" my guess on the pinout was. But in replicating the cards - the tiny, non-standard connector would be the real roadblock anyway? Modifying the instrument for a different one might be of limited practicality.

[aronake]

Anyone tried to pimp one of these with a better voltage reference, say LM399?

Any views on linearity?

[DavidKo]

Grounding pin 32 of the card connector, which is left open in the 7563 meter, fixes the problem so maximum possible capacity is recognized. Apparently the size of the card is determined not by a voltage but by scanning of the memory. For reference here is the picture of the insides of the card. I can post the pinout of the card if there is interest in replicating it.

How do you verify, that the card is working? I have some flash type memory card and seems to me that the device does not detect it (it does not have metal contact on the top of the card).

[maxwell3e10]

You have to issue a command to initialize the card first, after that one can save data and save setup to it on the 7563.

[Ordinaryman1971]

I have one of those 7651 DC Sources with bad transformer so if anybody out there has one for parts I would be very much interested.
But my question is about memory card for those Yokogawa meters.
How does the connector look like.
I’ve got a card with one of the meters I’ve bought long time ago but it doesn’t look right

[DavidKo]

It should be yokogawa 3789 card. It has 1 row female connector.

[maxwell3e10]

Yes, that card does not look right. Here is the picture of the connector.

[Ordinaryman1971]

So, yes, the card the pictures of I've posted is definitely not the correct one. They are very hard to come about. I've managed to acquiree two of them since. The batteries were dead though... still need to replace them.
I would be interested in some pictures with modification to the pinout so the cards with larger capacity could be used.

[maxwell3e10]

To use higher capacity card it is not necessary to modify the card, one can just ground pins on the meter connector that are connected to extra address lines. When left floating, the memory capacity detection is unreliable. In principle one could also modify the card by adding a tiny switch to enable use of different memory segments.