Electronics > Repair

Fluke 332D DC Voltage Standard Restoration

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Roman oh:
David,
Thanks for the link. First thing I notice is "Doesn't post to Australia". Then TerraOperative's experience...Hmmm...might just keep looking.
Thanks again
Roman

Roman oh:
Hi  again Terra Operative,
Fyi, I have made it a project over several years to "improve" this beast mainly with a view to reducing its power consumption, so I wouldn't have to wait hours for it to stabilise. (No load power consumption after ref heater warmup is now around 11W, instead of the 30-40 that is was). As part of that, some years back, I "improved" the series pass by making the series string of 2N3739s into a darlington configuration using some 2N6517s in a TO92 package. This enabled me to increase those hot 22K resistors(R28-R34) feeding the bases to 220K and save a few of those watts. So the dependence on hFE of the 2N3739s was reduced. Worked reliably for a number of years, so probably not a factor in my recent mishap.

Out of curiosity (since you have yours opened up) I notice that my board has, for for R6,8,10...18 and R21-26, uniformly 150K 1/2W CC resistors fitted whereas the schematic shows a series of stepped values from 56K -100K for the former, and 100K for R21-26. The board does not appear to have been modded, but hard to tell. Spice model of my configuration (both with and without the darlington mod) suggests a pretty good distribution of voltage up to 1200V in across the string, nowhere approaching the 300Vceo limit of the transistors, so I'm not too concerned. But I would be interested to know whether your boards are similarly configured.
Roman

TERRA Operative:
I am definitely interested in your mods!
I have some PCB's made way back when I started looking at this thing to replace the chopper amp with modern silicon which will be installed once I get it all working in it's original factory configuration, and I was thinking to replace the zener regulators on the right-most board (Current Limit board) with good quality linear regulators too for another easy upgrade.

I have a couple versions of the series pass board, so I'll check tonight and let you know. Will take a few photos roo.

kawal:

--- Quote from: TERRA Operative on January 05, 2023, 11:07:54 am ---Ok, so after poking away and testing this and that, I'm down to testing the pass element transistors (seven of 2N3739) and all eight 200V zener diodes.

The Peak Zener tester only goes to 50V, so I'll have to dig out a power supply capable of 200V and some resistors for current limiting to test the zeners.
My only power supply is a huge heavy rack-mount HP thing... yaaaay...

As for the transistors, not sure. I'll have to figure out an easy way to test them. I have no space to pull out the Tek 577 curve tracer to fix it. :(

--- End quote ---

I had a lot of issues with the same unit. The old transistors would test good in diod mode but had gain close to 1.  Check the gain of the transistors!

TERRA Operative:
Ok, updates.

I mixed and matched the pass transistors with what I had on hand to select the best ones.

Now the fault is still there! It will start and work fine in the 100V and 1000V modes, but it doesn't work in the 10V mode....  |O
When switching from standby to operate in the 10V mode, it trips out, and if starting in 100V or 1000V mode then switching to 10V mode, it doesn't trip but it gives no output.....

I really need to find a modern equivalent for these 2N3739 pass transistors so I can buy brand new parts that I know and can trust are 100% good...


Ugh, I'm getting tired of this thing. I'll poke it some more before I throw it on the 'for sale as-is' pile.....

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