Fluke 335A DC Voltage Standard

[Inverted18650]

I recently purchased this unit on eBay and it arrived today. It was sold as "powers up" but needs work. Unfortunately for me, I am not getting anything at all on the output. So I get to open it up straight away and begin the exploration, troubleshooting, and repair of this unit. I have already printed the entire manual from the Fluke website and starting working through some of the layouts. I have also read all the other posts from fellow 335A/D (and similar) owners and hope you all will come to my aid when I am ready with intelligent questions. ( It seems that Dr. Frank is the resident go-to on these units and hope he will have some free time to help me as well).

Here's a look at the unit now. I am also filming this project and will be uploading it to 'The Combat Engineer' on YT. Notice someone removed the current limiting vernier and labeled the unit, 25mA max. Finding and removing that botch and/or repairing the current limit board was going to be my first undertaking, now it takes a back seat to getting it to power up.

[Inverted18650]

Update #1:

It seems at least one of the cards wiggled itself a bit loose during shipping because after I applied a little downward pressure to them all to ensure they were seated, the unit powered up and now has a voltage on the output. You can see on the 100V & 1000V ranges, the full potential is sitting on the output with all the dials set to zero. On the 10V range, the voltage immediately spiked up to 5.780V and started slowly but steadily declining. At the time of this report, it is sitting at 4.994V and falling.

On the 100V range, I turned dials to 10 X X X X X X (all the way up) and the max output voltage is 111.9. Once the output settles on the 10V, I will take the unit up to the shop and repeat the test with the Keithley 2000 and record the results. Right now I think dials 2 and 3 are the only functional dials, because the voltage moves up when I adjust them, the others do nothing at all. Once I am able to record these initial results, I will try to undo the current limit hack that was installed and run another test.

[Inverted18650]

Update #2:

I started going through the "Troubleshooting," Section 4-65 of the manual today. All went well and the reading were well within spec until I started the resistance measurements on the current limit card (4-68,c). Pin's 1 & 3' started at 25k ohms and kept climbing, so I noted the issue and went back to the manual. I went through the schematic and used the page DW6. 335A_1002 (SHT 5 of 6) to test and record all the resistor values on the card. R26 was 2.29K and R8 was just a shorting link.

Then I moved on to the next section and removed the Diff Amp card and plugged the meter in between (- Sense) and Pin 5 of the Diff Amp card slot. Section 4-68, d states the initial measurement should be less than 0.5 ohm, but I had no such luck. I got around 1M initially with little-to-no-change in value when I switched the 1st-4th dial.

My next step will be to tear down the front end of the meter to gain better access to the "sample string" so I can test and record each and every resistor in the string. Based on the units behavior, I have a feeling that once I find and replace the open resistors, this unit may be ready to go back into service (after calibration, etc).   

Note: I did get some cool photos and will update this post with the pics shortly.

[Dr. Frank]

Update #2:


My next step will be to tear down the front end of the meter to gain better access to the "sample string" so I can test and record each and every resistor in the string. Based on the units behavior, I have a feeling that once I find and replace the open resistors, this unit may be ready to go back into service (after calibration, etc).   

Note: I did get some cool photos and will update this post with the pics shortly.

Don't disassemble the front. you might damage or pollute the string assembly.

To check the resistors, simply measure the resistance from beginning to end of the string, and dial through the different resistor settings, either in 1-2-2-2-2-2 or 1-2-4-4 sequence for each decade, depending on the revision of your instrument, see manuals. You need to pull A4 & A5 first.

Pictures and further hints are already available, search for '.. 332B in the slaughterhouse..'.

I doubt that the string is the problem, because if one resistor is defect , other settings will give a precise reading, anyhow.
If you have the old mechanical chopper amplifier, there's probably your problem, also the HV generation card may also fail.

Frank

[Inverted18650]

Glad I took a little nap, my brain was fried and I needed a break. It does not make sense that the sample string is the issue, since the full potential of the 100 and 1kv are on the output even with the dials at zero. Would the HV supply be a problem since it is generating 1kV? The chopper seems a good place to look. 

Here are a few pics. I have already noted a handful of other issues, namely resistors that are bad/missing.

I will reread the 332B slaughterhouse post.

[Inverted18650]

more pics

I am going to get the scope (Tek 335) and see what the base of Q3 on the Chopper Amp looks like. I will probably set up the camera and do a live feed on YT.

[texaspyro]

First things first... replace ALL the small electrolytic caps on the board.   These machines are notorious for their crapassitors.   They can also have problems with the power supply caps.

[Inverted18650]

For sure, on the caps. Looks like most of them were removed and at least tested (based on the soldering patterns/old work). All looks well on the Chopper board and given the symptoms I think the Series Pass Element is next, then the Diff Amp board (or vice-versa). I did replace R17 on the Chopper board and now have a very stable 10V,100V and 1000V, but I can only adjust them using knobs 2 & 3 (by 1V or 100mV respectively). The other knobs do nothing and I cannot get a zero reading, its always 10/100/1000. I'll sleep on it and try again tomorrow.

[Inverted18650]

[Dr. Frank]

Very probably the shaft of the first decade is loose and does not rotate the switch.
The shaft consists of two parts, a plastic one mounted in the knob, the other Aluminium one stuck in the rotator of the switch.
In the middle, there's a bearing with two stud screws, connecting both parts of this axle, and these screws are probably to be fixed.
You see these bearings in your 2nd picture.
Or maybe there's something broken..

And again, if you want to check the decades, simply do it decade by decade, dialing 1-2-4-6-8, that will be a systematic check.

Frank

[Inverted18650]

I found the issue with the first range dial, the contact is not rotating. Looks like I will be removing the front panel.

The shaft from the indicator dial seats into a plastic cup which is broken away, so when you rotate the dial, the shaft just spins inside the cup. Hopefully I will not have to remove too much from the front end, because everything else appears to be in good working order.

[Inverted18650]

Very probably the shaft of the first decade is loose and does not rotate the switch.
The shaft consists of two parts, a plastic one mounted in the knob, the other Aluminium one stuck in the rotator of the switch.
In the middle, there's a bearing with two stud screws, connecting both parts of this axle, and these screws are probably to be fixed.
You see these bearings in your 2nd picture.

And again, if you want to check the decades, simply do it decade by decade, dialing 1-2-4-6-8, that will be a systematic check.

Frank

I posted another video about it a few minutes ago.

[Dr. Frank]

If the plastic axle is broken, I think that you may remove it by simply disassembling the first knob only.
Otherwise, you'd have a lot of fun, as you'd have to remove all knobs.. and the front plate

Frank

[Dr. Frank]

Very probably the shaft of the first decade is loose and does not rotate the switch.
The shaft consists of two parts, a plastic one mounted in the knob, the other Aluminium one stuck in the rotator of the switch.
In the middle, there's a bearing with two stud screws, connecting both parts of this axle, and these screws are probably to be fixed.
You see these bearings in your 2nd picture.

And again, if you want to check the decades, simply do it decade by decade, dialing 1-2-4-6-8, that will be a systematic check.

Frank

I posted another video about it a few minutes ago.

Yes, that's because I wrote to do it systematically, instead! (see schematic)

[Inverted18650]

Thank you Dr. Frank. I am grateful you are willing to help. The video I posted cut out when my phone rang so it’s pretty terrible. I shoot the footage with an iPhone 7S which has a good camera and normally turn it to “do not disturb” while filming. I know some people hate “phone footage” but I have to use the tools I have. Anyways....

 I am considering removing the pop rivets in the support plate and going in from the back because taking this front end apart could, like you say, damage something else. But I’m confident in my ability to do it correctly and with care. I also need to replace the input sockets on the front end. I am to ponder this one a bit. If removing just that dial will work, that’s certainly the way to go.

Prost.

[Dr. Frank]

Thank you Dr. Frank. I am grateful you are willing to help. The video I posted cut out when my phone rang so it’s pretty terrible. I shoot the footage with an iPhone 7S which has a good camera and normally turn it to “do not disturb” while filming. I know some people hate “phone footage” but I have to use the tools I have. Anyways....

 I am considering removing the pop rivets in the support plate and going in from the back because taking this front end apart could, like you say, damage something else. But I’m confident in my ability to do it correctly and with care. I also need to replace the input sockets on the front end. I am to ponder this one a bit. If removing just that dial will work, that’s certainly the way to go.

Prost.

Just inspected my 332B remainder front plate and switches.. you can replace the plastic axle w/o completely disassembling, just undo the knob, the indicator scale and the bearing from the top.
The jacks van be replaced from the bottom side and a bit of skill.

The rest of the instrument seems to work fine.. you have the modern MOSFET chopper, but the old reference oven only.. hopefully the null voltmeter chopper is ok (neons).
Later on, you may want to replace this Z-diode reference by an LM399H, should be possible to fit that into the existing circuit.. should have better stability and less noise..

I'm always happy, when such beautiful instruments were brought back to life.

Good luck with the repair, .. I'm having the Hesse Äppelwoi (apple wine) meanwhile.. to your health!

Frank

[lowimpedance]

 While browsing through your pictures I noted that R3 on the current limit PCB looks like its seen better days. Might be worth checking (replacing).
And those horrible old carbon composition resistors like drifting high in value too so a quick sanity check with your meter might be a good idea.
Best of luck with the repair.

[Inverted18650]

Most recent progress:

Swapped the 7th digit "decade dial extension" up to the 1st, and now the 1st-6th decades are fully functional. Also rotated the input terminals from the rear recorder input up to the front panel, and replaced the burned out front panel range lights with LED's (63V to the lights). I have to file the edges of the standard led to make them fit into the frame but so far all is well. Its back together and better than before, amazingly, I didn't break anything or have any leftover parts. 

The voltmeter is also spot on while in Voltage Standard mode, hopefully it works just as well in Diff Mode.