Fluke 332D repair..still sorting issues

[Fred_47]

Hi all,

I allowed the unit to operate for about 4 hours, and is supplying a voltage in the "correct" range more or less to 3-4 places after the decimal.  Ive found myself in the position of not having a volt meter with sufficient resolution to view the output voltage accurately.  I think a voltmeter upgrade is in my future.

This morning I discovered a cap on the current limiter has leaked electrolyte during the 4 hours it was operating.  This is disturbing to me because it tested fine on my component tester.  I think I may replace more electrolytic caps to avoid future issues.  For starters I plan to replace C1-5 on the HV mother board, C1, C4, C6, and C10 on the current limiter, C2 on the series pass, and C2 on the Aux Power board. 

Any thoughts on this plan?   I would not want to disturb the accuracy or linearity potential, I dont think any of these caps would effect this correct?

Good luck finding a high resolution VM at a reasonable price.

Replacing caps on any of the removable boards is pretty straightforward once you get the cap. I found that axial caps are a dying breed.

Replacing caps on the HV MB is very difficult. Fortunately I only had one (C7 which parallels the interlock K1) which measure bad. Getting to the big 3, C1,2,3 is very difficult. I found that the 'paper' tube was not the actual capacitor but a cover. I my unit the actual caps filled about 2/3rds the height of the tube.

My first round of replacements allowed setting 0 volts  output on all ranges and dropped the power input from 40 to about 30 watts. Prior to replacement I couldn't get below a few mV at 0 setting.

As near as I can tell, linearization doesn't depend on any caps.

My 335D has been on or in standby continuously for almost a year with no problems except the decimal LEDs and null meter 0 setting on the 1mV and under ranges. (Optical chopper aging I think)

I originally set the 335D reference with a new DM3068 that was still in cal. The three meters on the top (2 HP3478As and an HP3456A) are as received from eBay sellers with far out of date cal stickers from well known labs associated with either the gov't or big manufacturers. I fiddled with the HP3456A on the bottom.

I've had the 335D up to 1100V a couple times to check a couple VTVM meters with 1500V ranges. Most of the 335D use is at 10V or less. I did use the 100V range a couple times to match neon bulbs for the optical chopper in the null meter and to check a couple zeners.

PS: I have modified the bench for better thermal management, i.e., I added some space above the 335D for better cooling.

[valley001]

Hello all,

Today I tried using the 1000v range on the 332D.  No dice.  When switched to 1000v range it could not make it to 900v.  Where should I look on this problem? 

[Fred_47]

Hello all,

Today I tried using the 1000v range on the 332D.  No dice.  When switched to 1000v range it could not make it to 900v.  Where should I look on this problem?

How did it not make it?

Voltage trip? Voltage fell to 0 when you dialed above a certain value.

Current limit? Voltage rose to a value and no higher as you dialed in more voltage.

1000V range calibration? The 3 ranges have separate calibration pots.

Was it approximately correct at voltages below 900?

[valley001]

Hello Fred,

Yes, it seems reasonably close at voltages lower that 900v.  When I switch for say, 100v to 1000v, it rises slowly and peaks out around 850-900v. 

[Fred_47]

Hello Fred,

Yes, it seems reasonably close at voltages lower that 900v.  When I switch for say, 100v to 1000v, it rises slowly and peaks out around 850-900v.

I'm not sure if the 'slow' rise is normal. I've never switched my 335D from range to range at full scale setting of a range. Too much potential for excess excitment ;-)

What happens when you make 10V steps on the 100V range? (This checks the sample string. You should get ≈10V/step)

On the 1000V range start at 100V and raise in 100V steps. How close is the output to the dialed in value. Assuming problem with range calibration setting (A4 Range Calibration Assy), you would get something like 90V per step.

Could also be preregulator or series pass element. (I have no experience with problems there.)

Have you replaced any caps yet?

What manual are you using? Do the schematics match your unit?

Edit:
The first diagram was supposed to be the original 'simplified' diagram from the 1978 rev of the 335D manual. What is posted here is a combination of the 2 pages from the manual. It is slightly modified based on my observations of the interlock circuit and isn't exactly the same as the diagram in the manual.

[Fred_47]

Edit:
The first diagram was supposed to be the original 'simplified' diagram from the 1978 rev of the 335D manual.

Here is the diagram that I intended to post. It differs in the A8 interlock connections.

[valley001]

Hello Fred,

Perhaps it is not a good idea to switch full range to range, ill refrain from that.  I will test 10v increments in the 100v range and see how it behaves.  I know on the 10v range it tracks fairly well one volt at a time however I only have 5.5 digit meters that I have repaired, nothing that has been properly calibrated. 

On the 100v range, stepping in 100 volt increments it tops out 850ish volts and will not go higher.

I have not replaced caps outside the one that looked bad.  I am learning but do not yet know enough to read the schematic and know which circuit/s could be causing this problem. 

[Fred_47]

Hello Fred,

Perhaps it is not a good idea to switch full range to range, ill refrain from that.  I will test 10v increments in the 100v range and see how it behaves.  I know on the 10v range it tracks fairly well one volt at a time however I only have 5.5 digit meters that I have repaired, nothing that has been properly calibrated. 

On the 100v range, stepping in 100 volt increments it tops out 850ish volts and will not go higher.

I have not replaced caps outside the one that looked bad.  I am learning but do not yet know enough to read the schematic and know which circuit/s could be causing this problem.

This is a dangerous piece of equipment to learn on. Plus, I have found some mistakes and confusing (maybe mistaken) circuitry on the schematics. Exercise care when probing this beast. Even with the unit in STDBY there are very high (>500V) voltages present. The only way to have no voltage is to unplug it and let the caps discharge.

The reason for checking the 100V range in 10 volt steps is that it uses exactly the same resistors in the sample string as the 1000V range in 100V steps. (As does 1V step on the 10 volt range so maybe the sample string is OK.) Actually, you're looking for all the steps to be the same size on any given range.

Am I right that on the 1000V range you are getting close to 100V steps until you get to above 800V?

Edit

I always seem to think of one more thing after I hit post.

Does the voltmeter on the unit work?

Does it agree (approximately) with your bench meter?

Does the meter on the unit show any current when you try the 332 on high voltage?

[valley001]

I take heed of the high voltage warnings, and have done all tests with the extender installed and diagnostic connections made without power applied.   I am particularly nervous with powering it on case open but I take precautions for safety. 

Yes, when stepping 100v steps in the 1000v range it will make 800 but then when switched to 900 it labors (seemingly) and stalls.

[Fred_47]

See the edit to my post above your last post.

We're trying to eliminate all the stuff on the right hand side of the simplified schematic.

Operation of the meter should be confirmed as it will give you an easier way to monitor the output. Also, operation of the current limit should be checked. If it is somehow becoming active at around 850V, it will hold the voltage down to keep a constant current. It's been a while since I operated my 335D in current limit, but IIRC it hums rather noticeably.

The meter and protective ckt procedure begins at Para 4-32 in the 332D manual Rev 1 3/75. For now I would skip doing the current tests at full voltage on the 1000V range. Maybe try them at a lower voltage like 4-500V after you get a feel for how it works.

[valley001]

I had some time to experiment with the 332D this evening.

-I verified 100v range steps.  I see 10v change per step: 10v, 20v, 30v, etc to..100v.  The analog meter tracks with each range along with the digital meter hooked up to the output. 

-Going through the current limit setting procedure per 2-18.  When switching to operate and attempting to set the current limit via the current limit knob the meter will rise to 10ma and then stall, sometimes simply failing to rise higher or will fall to 0. 

I am guessing the root of my issues lie in this discovery. 

[valley001]

Investigating this morning, so far:

C7 on the HV mother board reads 3500pf, so I guess that one is toast.  Will test more caps I guess. 

Uh oh... I tested three more of the small sprague electrolytics on the current limiter board, C2, C3, and C5, all test in the 1000pf range out of circuit.  Unless I am missing something these are all toast right? 

[Fred_47]

I had some time to experiment with the 332D this evening.

-I verified 100v range steps.  I see 10v change per step: 10v, 20v, 30v, etc to..100v.  The analog meter tracks with each range along with the digital meter hooked up to the output. 

-Going through the current limit setting procedure per 2-18.  When switching to operate and attempting to set the current limit via the current limit knob the meter will rise to 10ma and then stall, sometimes simply failing to rise higher or will fall to 0. 

I am guessing the root of my issues lie in this discovery.

Could be. It's certainly not the expected behavior.

The meter circuit is on DWG 332B-1001-2 Series Pass Driver. R22 is the shunt. It's the only resistor on the schematic that doesn't have a value listed. The BOM says it's 10 1%, 1/2W.

That same shunt drives the current limiter but I've never figured out exactly how. The end result is to reduce the drive to Q8 on the series pass element.

What does the current do when you repeat your original test that stalls at 850V? (Not shorted)

Does the current limit LED come on? (Do you know that it works? I think it comes on with the unit in STDBY.)

I've added PS voltages to the ~ where they were missing. (Hasn't been double checked). Note that the ±35V supply common is connected at flag '10' which is the center tap of the T1 winding that feeds the ±35V supply.

Your 'Still sorting issues' post came thru while I was drafting this. I'll post this then look into the cap issues.

[Fred_47]

Investigating this morning, so far:

C7 on the HV mother board reads 3500pf, so I guess that one is toast.  Will test more caps I guess. 

Uh oh... I tested three more of the small sprague electrolytics on the current limiter board, C2, C3, and C5, all test in the 1000pf range out of circuit.  Unless I am missing something these are all toast right?

Somewhere up thread, I posted a PDF of the caps I measured (mostly in ckt) with an Extech LCR200 using the 120Hz setting. I confirmed the choice to replace by measuring them when I made the replacement.

Of your list I replaced C7 on the HV MB & C3 on the Current Limiter/±35V supply along with 11 others scattered across the other boards about a month after I got my 335D. That dropped the STDBY power draw from 40 to 30W and allowed setting the zero outputs on all 3 ranges. Except for the null meter (probably the neons in the optical chopper) my 335D seems to function properly.

I replaced 5 more Ta caps last Nov along with one of the neon lamps in the chopper trying to improve the null detector. It worked for a while.

C7 on the HV MB had an insainly high ESR along with low capacitance. It parallels one of the K1 relays (shown on the drawing I attached to my post about 2 up in this thread). This circuit is confusing because the diode seems backwards on some drawings. I actually have a 335D dwg that shows 2 diodes in opposition. Originally I thought the cap was to slow the pickup of K1 but it could also slow the dropout. Figuring out the diode is one of the projects when I open the beast to work on the null detector. Right now a Wavetek 114 is in the way.

I also replaced C3 in the ±35V supply which had about 20X the esr of either C2 or C5. All 3 had acceptable capacitance.

I think you're on the right track.

[valley001]

OK great.  At frist so many were testing bad I thought it might be my methods but my component tester and my DMM both show very low capacitance on the ones I have taken out of circuit.  The component tester would not give an ESR reading.  My DMM says something crazy like 10 megs resistance.

There are a ton on of these small sprague electrolytics on the chopper amp and aux power board, guess ill be busy. 

[valley001]

I investigated a number of small electrolytics on the chopper amp and aux power board, none that I removed from circuit tested particularly bad.

I am letting it warm up and will see if replacing C7 on the HV motherboard and C2, C3, and C5 on the current regulator have an impact. 

[valley001]

With these 4 caps replaced the unit seems more stable, but there are still problems with reaching higher voltage and setting the current limit.

There does, however,  seem to be improvement with the current limit setting.  Now when switching to operate per 2-18 the current needle will rise slowly to 60ma.  If I operate the current limit knob the current limit light will flash once and the needle will go to zero.  I have to cycle the operate knob to get it to show current on the meter.

[valley001]

I went back in and replaced C5 on the series pass (high ESR and low capacitance) as well as C1 on the current limiter, just because it looked bad (tested fine, but seemed to be bulging a bit and possibly leaking electrolyte). 

Results are more stable operation and now it is reaching up to near 1000v, so I am seeing improvement. 

I think im going to order the other caps for the current limiter, C4 and C6. I have some 680uf 100v on hand, the originals are specd at 350uf +75% would I be pushing it installing the 680uf caps?   I did test the three big caps on the HV board and they are all showing about 160uf. I may order those and replace them as well. 

It is frustrating that a cap will test good on the component tester but it is obviously bad in practice. 

[valley001]

I all,

Sorry for all the posts but I encountered yet another issue.  What I thought may be problems with the output voltage turned out to be a problem in the range selector switch itself.  With the output set to 0.000000 I will see a reading of approximately 0.000040 (around 36-44uv flickering).  If I apply clockwise pressure to the range knob it will jump to approx 0.000140.  I can get it to go back anf forth between the two (0.000040 and 0.000140).  I assume this is a problem. 

The switch is several layers of sealed contact assemblies (see pic).  I know from experience these do not readily disassemble.

Would it be a good idea to apply a small amount of deoxit D5 (the only type I have) to the contact through the hole in the plastic for each layer?   Applied with a toothpick or something like that.  I have also read to clean with Isopropyl alcohol but I dont know how I could accomplish this short of soaking each contact assembly. 

[Fred_47]

I all,

Sorry for all the posts but I encountered yet another issue.  What I thought may be problems with the output voltage turned out to be a problem in the range selector switch itself.  With the output set to 0.000000 I will see a reading of approximately 0.000040 (around 36-44uv flickering).  If I apply clockwise pressure to the range knob it will jump to approx 0.000140.  I can get it to go back anf forth between the two (0.000040 and 0.000140).  I assume this is a problem. 

The switch is several layers of sealed contact assemblies (see pic).  I know from experience these do not readily disassemble.

Would it be a good idea to apply a small amount of deoxit D5 (the only type I have) to the contact through the hole in the plastic for each layer?   Applied with a toothpick or something like that.  I have also read to clean with Isopropyl alcohol but I dont know how I could accomplish this short of soaking each contact assembly.

Don't know about contact cleaner but my 0 problem went away after I replaced some caps. I was then able to adjust the 0 setting for each range and they have been good for about a year with small  adjustment for drift after about 6 months.

[valley001]

Don't know about contact cleaner but my 0 problem went away after I replaced some caps. I was then able to adjust the 0 setting for each range and they have been good for about a year with small  adjustment for drift after about 6 months.

I discovered yet another bad cap, this time on the aux power board (C7).  I also replaced C2 on the series pass board just because I had a 10uf 450v cap on hand. 

I also checked all of the electrolytic caps on the chopper board, many have ESR in the 3-6 ohm range. I think ill replace all of those eventually. 

With C7 replaced on the aux power board the drift on 0 setting has settled, flickering only 6-7uv so far but its only been operating for 30min or so.  Maybe what I was seeing was not related to the range switch at all.

Another thing, when the unit is first turned on I can get some life out of the current limit function, but when its been on for a while and trying to set current limit it will fail to operate.  When switched to operate the needle (on current setting) will flicker and peg past 0.   I plan to assess the transistors listed in the current limit description section 3-41, is this sound?.   I discovered this now that I have been allowing the unit to operate for longer periods.

[Fred_47]

OK great.  At frist so many were testing bad I thought it might be my methods but my component tester and my DMM both show very low capacitance on the ones I have taken out of circuit.  The component tester would not give an ESR reading.  My DMM says something crazy like 10 megs resistance.

There are a ton on of these small sprague electrolytics on the chopper amp and aux power board, guess ill be busy.

A number of the aux power supply filter caps had been replaced before I got the 335D in Mar 2018. As I mentioned up-thread, the HV filters C1, 2, 3 only fill about 2/3rds of the paper tube so they may have been replaced. The (1978) 335 manual lists Fluke as the mfg for them but both the(1971) 335 and (1975) 332 manuals lists Sprague. I think the ones in my 335D are Mallory.

This is what I replaced in April 2018 (The silver Sprague were mostly original circa 1980 the blue ? were circa 1990)

Edit... I'll have to look at the current limit but checking the transistors is a good idea. It sounds like there is some intermittent or thermal problem. The current limit and voltage trip have been dependable on my 335.

[valley001]

A number of the aux power supply filter caps had been replaced before I got the 335D in Mar 2018. As I mentioned up-thread, the HV filters C1, 2, 3 only fill about 2/3rds of the paper tube so they may have been replaced. The (1978) 335 manual lists Fluke as the mfg for them but both the(1971) 335 and (1975) 332 manuals lists Sprague. I think the ones in my 335D are Mallory.

This is what I replaced in April 2018 (The silver Sprague were mostly original circa 1980 the blue ? were circa 1990)

Edit... I'll have to look at the current limit but checking the transistors is a good idea. It sounds like there is some intermittent or thermal problem. The current limit and voltage trip have been dependable on my 335.

Hello Fred,

Are there date codes on the blue caps?  Mine has a mix of the Sprague and the blue ones and I think they are all original based on the intact coating on the rear of the PCB.   So far only one of the blue caps on mine has tested bad with high ESR.  The majority of the bad Sprague caps were testing in the pf range, some with high ESR.  Pic of my dead caps attached.

I think I have largely solved the stability problem, the 332D has been operating for a few hours now and it is flickering around 4uv.  It has settled at -104uv +/- 4uv on my Dana 5004 which was last calibrated in 1998, with the readout set to 0.000000 

[Fred_47]

Hello Fred,

Are there date codes on the blue caps?  Mine has a mix of the Sprague and the blue ones and I think they are all original based on the intact coating on the rear of the PCB.   So far only one of the blue caps on mine has tested bad with high ESR.  The majority of the bad Sprague caps were testing in the pf range, some with high ESR.  Pic of my dead caps attached.

I think I have largely solved the stability problem, the 332D has been operating for a few hours now and it is flickering around 4uv.  It has settled at -104uv +/- 4uv on my Dana 5004 which was last calibrated in 1998, with the readout set to 0.000000

Pack rat that I am, I still have all the caps that I removed. They now include a few that I replaced in Nov 2018. The count is up to 16 + a couple Ta that I couldn't find (but they're here ...somewhere). Most of the first batch were from various boards and most of the second batch were from the Null Detector boards.

The "blue" caps were either MEXICO or PHILLIPS, none of the caps were leaking or obviously failed by visual inspection.
 
I've included a txt file with the data but in summary:

All of the MEXICO caps were either low capacitance, high ESR or both. (one turned out to be in spec)
The closest approximation to a date code is in the line with the name (MEXICO) and is either 030, 031 or 032. Based on the cal sticker from Fluke (Apr-14-92 by #39) and date codes on some other parts, my assumption is that this is 1990, 30 thru 32nd week. I don't think this instrument was worked on after that.
(3 were marked 47uT not 47uF)

The remaining blue caps are PHILLIPS and have a similar codes 031 & 032. One of these appears to be close to spec but the others wouldn't give a steady reading on my LCR meter. One actually read out as an inductor.

Of the 4 Sprague cap, 3 have date codes from 1980, '81 and one is 1990. All are type 30D and have very low capacitance wrt their marked rating. So, I didn't bother to calculate the ESR.

[valley001]

I went through Q3-Q7 on the current limiter.  None of them test broken but some are showing hfe in the 50-100 range, these are silicon transistors. 

I did replace C1-5 on the HV board, along with C1, C4, and C6 on the current limiter.  This did not have any effect on the current limit setting, though the output is closer to 0 with the readout set to O.  It still does not set current per the manual.  When I switch to operate the needle pegs to the floor.   I am not sure where to look next.