Fluke 5440b guarded transformer hum and resistor cooking

[rigrunner]

I've recently acquired a 5440b that the courier has smashed up by dropped it on its front. Thanks UK DX Freight!.

I've spent a while striping everything down, straightening out the metal work and glueing up the front panel.
It's back in one piece and only took a handful of parts to get it seemingly working 

I have a couple of questions for those who either already own one of these or perhaps have repaired a few.

How loud is the hum from your guarded transformer?
How warm should R1 on the outside guard regulator run?

The guarded transformer in this has a hum that's about as loud as the fan and R1 on the outside guard regulator has obviously been hot for a while as it has cracked the resistor, cooked the board.and lifted tracks. Attached are front and back pictures of the cooked PCB with the new R1 in place.

[e61_phil]

The guarded transformer in this has a hum that's about as loud as the fan

That's normal. All the 5440B I used (three) have this loud hum.

[lukier]

I've recently acquired a 5440b that the courier has smashed up by dropped it on its front. Thanks UK DX Freight!.

Blimey, that must have ended badly. 5440B is huge and very heavy.

If your VFDs are bust don't loose hope - I bought mine 5440B very cheaply as it was parts & repair and oh boy there was a lot to repair, but even worse, it was missing the Display Board PCA, so so far I've only used it via GPIB.

I'm working now on 2 PCBs to replace the displays - one with 12 x 16 segment green displays and pretty much the same logic as the original (two shift registers, but one being a LED driver). The other one will be a 20 x 2 LCD negative display with a small STM32F1 to read two serial lines, decode them and send commands to the HD44780.

How loud is the hum from your guarded transformer?
How warm should R1 on the outside guard regulator run?

The guarded transformer in this has a hum that's about as loud as the fan and R1 on the outside guard regulator has obviously been hot for a while as it has cracked the resistor, cooked the board.and lifted tracks. Attached are front and back pictures of the cooked PCB with the new R1 in place.

AFAIR that's a common problem, I've seen one of these power resistors mentioned here on the forum before. I've replaced them on A10 and A17 boards.

Regarding the transformer it is hard to say, there is some hum for sure. Recently, I've replaced the fan so and I thought it comes from the fan - huge full metal heavy 110 VAC fan without any rubber mounts is going to make noise.

My tip is not very original - measure all the voltages. Fortunately, most important ones are accessible on the top on the test pins, often labelled. This way I've discovered problems with multiple power supplies on mine (AFAIR 5V digital and 5V and +17V analog) and the dead switches in the DAC that were loading the reference from 13V to 2V 

Also, via GPIB you can issue self-test commands and observe the output on the serial port at the back. 

The service manual is quite helpful in both tasks.

[Dr. Frank]

This loud hum of the transformer is normal, especially directly after turning on the cold instrument.
The different ovens draw a lot of current, I suppose, so this humming gets less after a quarter of an hour, or so.
Maybe, ones gets accustomed to that sound, also.

Frank

[rigrunner]

Thanks for the replies.

I'm relieved to hear that the hum is normal. That transformer had sheared the 4 screws that secured it to the chassis side panel. I was worried that it had been damaged and that was the cause of the hum.

Blimey, that must have ended badly. 5440B is huge and very heavy.

I think the 5440b put up a good fight but the ground won. I've been preoccupied with stripping it down so I didn't take as many pictures of the carnage as i should have.

I think the only boards that hadn't dislodged were the IO, the outside guard term pca and the front panel controller. The boards had mostly left their plastic guides and become jammed. The aluminium internal chassis had bent to give way but that was simple enough to push back into shape.
The left side bottom corner was about an inch higher than the right and the front panel was almost in two. It's very fortunate that the display and keyboard are separate boards as they separated with the panel split.. The ribbon that connects the keyboard to the control panel snapped one of its headers though.

The only thing left to do is decide how to reattach the display cover.

I've attached a couple of pictures of the damage and how it currently looks.

[rigrunner]

Couldn't post all in one reply

(mods if you see this can you ditch the duplicate images?)

[rigrunner]

I'm working now on 2 PCBs to replace the displays - one with 12 x 16 segment green displays and pretty much the same logic as the original (two shift registers, but one being a LED driver). The other one will be a 20 x 2 LCD negative display with a small STM32F1 to read two serial lines, decode them and send commands to the HD44780.

The VFDs will eventually die so I can see this being VERY useful.   

My tip is not very original - measure all the voltages. Fortunately, most important ones are accessible on the top on the test pins, often labelled.

There were a couple of dead electrolytics that needed replacing along with R1. All power supply checks are in agreement with the service manual with one exception. TP4 on the outside guard regulator has a silk screen of 29V. The service manual suggests it should measure 28V whereas the schematic suggests it should be 24V. Mine measures 24V.

[lowimpedance]

R1 does eventually BBQ itself !. See my exercise in VFD replacement fun and two pics of R1 (one from a 5440b and the other a 5442)

https://www.eevblog.com/forum/metrology/so-you-have-a-fluke-5440b-with-worn-out-vfds-!/

for example :

[rigrunner]

That's exactly how my R1 looked.
Interestingly yours is a  140 mOhm too. The manual shows a 150 mOhm so that's what i've replaced it with.
The similar resistor on my in guard supply is a 150 mOhm and is in perfect condition. 

There are some nice display mods in your thread.

[jfphp]

The problem of the display is solved since a lot of time. Two solutions are proposed :

https://www.eevblog.com/forum/metrology/so-you-have-a-fluke-5440b-with-worn-out-vfds

[lukier]

The problem of the display is solved since a lot of time. Two solutions are proposed :

https://www.eevblog.com/forum/metrology/so-you-have-a-fluke-5440b-with-worn-out-vfds-!/msg1165119/#msg1165119

I saw this post, but I didn't see the price (it is quite big PCB after all) and I didn't like the LCD for the left display so in my design I've picked 6 x PDA54-11GWA 16 segment displays to resemble the original displays

What is the second solution? The rest of the thread is about the VFD replacement, but as I don't have the entire PCB it is of limited use as sourcing the VFD drivers would be very difficult (and VFDs themselves can be rare too).

[jfphp]

First contribution in the topic.

[LaserEng]

What revision A17 and A10 boards do you have?

The A17 board in mine has a 0.44 ohm 2W resistor as R1 and while the resistor doesn't look too bad there is evidence of cooking on the solder resist on the backside, my board is a rev H. While not knowing what the actual current drawn through this resistor is I know it has a 4 amp fuse in line and at maximum of 4 amps that poor resistor would be dissipating 7W  Was considering replacing it with a new 5W resistor mounted further off the board to help prevent the PCB getting into a worse condition.

I'm guessing my A10 is a lower Rev than the ones mentioned earlier in this thread and that Fluke changed the value to 0.15 ohms on later revisions.

On my A10 board which is rev R the R23 resistor is 0.15 ohm and doesn't looked cooked but then 4 amps  through 0.15 is a slightly more respectable 2.4W.

BTW, does anybody know of a link to a service manual that doesn't have the schematics split across pages? I have been keeping an eye out for a hardcopy manual on ebay at a decent price but no luck yet.

[wn1fju]

You asked, "BTW, does anybody know of a link to a service manual that doesn't have the schematics split across pages?"

I believe   https://doc.xdevs.com/article/fluke/   has a link to the 5440 service manual with single page schematics.  At least, I think that's where I got mine.

[LaserEng]

You asked, "BTW, does anybody know of a link to a service manual that doesn't have the schematics split across pages?"

I believe   https://doc.xdevs.com/article/fluke/   has a link to the 5440 service manual with single page schematics.  At least, I think that's where I got mine.

Thanks, I had assumed it was the same as all the other ones as the file name looked the same as the manuals on Fluke's website but when I downloaded it did indeed have the schematics on one page.


After looking into the schematics I'm starting to think that the 0.44Ω resistor for R1 on A17 is a later revision upgrade to the 0.15Ω resistor
as the resistor is used to set the current limit of the MC1723 then with a 0.44Ω resistor the current is limited to 0.66/0.44 = 1.5A so limiting the power dissipation to 1W.

Even though it is limited to 1W and the resistor is a 2W part, it is still cooking the PCB so will put in a new resistor mounted further of the board to help with heat dissipation.

[leighcorrigall]

...

Attached are front and back pictures of the cooked PCB with the new R1 in place.

Hi Rigrunner,

Out of curiosity, did you add thermal paste to the outside guard regulator PCB (A17) RCA 40251 TO-3 package? My unit has similar thermal damage and I have received a 'POWER SUPPLY FAULT TEST UNGUARDED POWER' (see Figure 2-7 and Table 4-1 which refer to A17 and A19) message at the instance that the calibrator enters into standby mode. My guess is that the calibrator is suffering from an overheating issue because it will return to normal operation after being shutoff. Perhaps I need to apply thermal past like you have done.

Regards.

[rigrunner]

Out of curiosity, did you add thermal paste to the outside guard regulator PCB (A17) RCA 40251 TO-3 package?

I refreshed or added thermal paste to anything with a heat sink on the A17. Some of the old stuff was dry and crumbly so probably wasn't doing much any more.

[coppercone2]

just get a silicone tube, stick it in your ear (I strongly recommend you boil a pot of water and sterilize it before you do this, ear infection is not fun and insurance will fuck you hard on medicine for that) and sweep the PCB for hum. On a older instrument (hp amplifier) I found a capacitor right next to the transformer that was making a hum that I mistook for the transformer. I only found it because I replaced the transformer, vacuum varnished it, and while the hum changed a tiny bit I could not believe it was still there... so I got pissed and just wanted to find exactly where it was humming, and it turned out to be a disk cap. IIRC I could follow the vibration on a trace.

A hp current source was doing the same thing, that transformer was already potted in epoxy, but it had a small PCB next to it for the cable that I think had some caps on it, and I might have replaced those to quiet it down. I could hear it from across the room and now I have to stick my ear into the chassis to hear the hum.

 I always do this now before going after transformers.

[leighcorrigall]

Out of curiosity, did you add thermal paste to the outside guard regulator PCB (A17) RCA 40251 TO-3 package?

I refreshed or added thermal paste to anything with a heat sink on the A17. Some of the old stuff was dry and crumbly so probably wasn't doing much anymore.

I did not know it was that easy! The TO-3 packages are simply held in place with two adjacent skews and the pins are socketed. The thermal paste was very simple to do. I should have done this for all of the regulators when I had the chance. Before the next calibration, I will definitely reinspect all the boards and add touch up the remaining thermal paste. For now, I think the 5440B is a very happy machine and seems to be behaving itself.

Thank you, again rigrunner.