EDC MV-100 repair - success

[rigrunner]

I recently acquired an EDC MV-100 voltage reference from ebay. It was no bargain, but the price was ok and i've kind of gotten used to the idea that most stuff for sale on ebay these days is broken in some way or other so didn't expect perfection.

The reference worked in that if you select 10V or 100mV on the first decade the output will be close enough. The problem with this particular MV-100 was that when you stepped down through the voltage range the steps were not linear.

I tried to locate a service manual and schematic for the MV-100 that matches the main board but no luck so far.
After comparing the board and the schematic from Dave's MV-106 it seems that the only difference between the two boards is how the power is regulated and the MV-106 has a 10mV range.

I went through the calibration steps in the MV-106 service manual and found that up to selecting and adjusting 1V on the second decade everything was fine. The next step is to adjust for 2V on the first decade and this is where it failed and i was unable to adjust to any voltage above 1.999. TiN confirmed that his calibration at this point had plenty margin for the adjustment. 

I disconnected the decade chain from the board and checked each selector for values from 0 through 10. The second decade should switch n X 1K Ohm but from switch position 3 upwards the values were high and sometimes erratic.

[rigrunner]

The 2nd decade switch had lots of tiny particles stuck to the posts and inner surface and the outer surface of the switch was covered in some kind of grease. I stripped the switch and cleaned everything in IPA and reassembled.
The values were now 1K Ohm per step but when slight pressure was applied to the resistors the values moved significantly. I retouched all solder joints an the switch and it now works.

Recalibrated again and the MV-100 seems to work perfectly

Before the repair the only way to get 10V and 100mV on full range was to bodge the calibration and over adjust the 2nd decade 1V stage which makes me think that one of its previous owners knew about the problem but bodged the settings at some point.

[Vgkid]

Nice job. It is funny what sort of problems to much grease+age can cause. I had similiar issues with my cs-152.

[rigrunner]

At least with the wafer switches you can pull them apart to clean and refit 

[TiN]

Yikes. My own calibration of MV106 was documented, while ago. Just in case somebody needs.

[SeanB]

I see most of the capacitors have been replaced with Sprague already. You probably want to replace the ROE ones as well on the left, along with checking all those carbon composition resistors for drift, even those high stability ones.

[rigrunner]

All parts on the board are original. Looks like they fitted Sprague as standard at that time.

Edit: Date code on the 741 and the RC4194 is 8th week 1978.

Checking the electrolytics was next on my list. If everything is within limits is it worth taking the time to check the resistors?

[rigrunner]

The 3 x 470uf smoothing caps read 700+uf 
The Two blue 10uf Sprague read 9uf and 11uf
The long silver 75uf Sprague reads 85uf

The 10uf are close enough, but given the age i might as well replace them too 

[Performa01]

The 10uf are close enough, but given the age i might as well replace them too 

I would not replace anything as long as it is not broken.
If you still think you have to do it, you should make sure to get parts of comparable quality, which - surprisingly - might not be as easy as most folks think.

Use a proper LCR/ESR meter and check the original caps, and then see if you can get something equally good to replace them. You might find out that even a brand new 105°C LL A-brand cap cannot touch a 40 years old electrolytic from Sprague or Roederstein.

It's the same for the resistors. If they have a color ring for the tempco, I very much doubt it'll be carbon composition, as this technology rules out stability in the first place. So it might be wire wound resistors after all. From what I see, many of them should have 25ppm/K, which is not too bad and impossible to get with carbon film, let alone composition resistors. So here again only touch them if you really know you have something better to replace them with.

[rigrunner]

The three ROE 470uf 50V are the same and two of them perform the same role as reservoir caps before the RC4194.
I would expect them to read very similar values. One reads 806uf ESR 1r3 and the other 700uf ESR 1r0
The third ROE 470uf provides smoothing for the main output supply and reads 867uf ESR 1r1

Most of the caps i have had to replace aren't this old but don't the high values mean that they need changing? 

I'm using a cheap component tester so the readings can't be assumed to be accurate, but as comparison a new Vishay 470uf 63V reads 485uf ESR 1r1.

[Performa01]

Okay, so what you've found is that these old caps have a higher capacitance, yet an ESR as low as your new Vishay part.
The high capacitance values just mean that in the good old days, A-brand manufacturers like Roederstein liked to have their parts on the high end of the tolerance field (which for such caps might be up to +70/-20%). ESR is still fine and also not so important in this particular application - we just want to check it to make sure we detect a faulty component.

As this caps are just for reservoir and supply voltage smoothing, this is all you need to check.
If an electrolytic is used to stabilize a more sensitive point in the circuit, such as the reference voltage, you would want to check the leakage current also. But here again, other than popular believe, good electrolytics can have leakage currents almost as low as mylar film caps...

Your unit is working fine, nothing suggests one of the components is near the end of its life or even defective, so I strongly suggest to not touch it. never change a running system ...