Fluke 850X power supply overvoltage on +15V rail

[mdubinko]

Hey all,
Wondering if more experienced hands have noticed this before..

Of the 2 Fluke 8505A/8506A power supplies I've seen, both have checked out as significantly high on the +15V rail (and the +30V rail, which is referenced from +15).

The linear power supply uses 78XX and 79XX regulators to produce all the voltages *except* the +15V rail, which uses an adjustable LM317. There's a trimmer to adjust the voltage, but the best it can do is get the rail down to about +20.5V. Between pins 1&2 on the LM317, there is not the expected 1.25V but something a bit higher.

This small part of the circuit looks like textbook LM317, like this: https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator

R1=240 and R2=2400 + up to 500 from the trimmer. Under the standard calculations this shouldn't have a problem hitting +15V on the nose, with the trimmer around mid-range.. SO the first thing I did was replace the LM317 (with a part that honestly is probably of a similar vintage) but no improvement.

There's some threads here on the forum about the minimum 10ma current the LM317 needs to regulate properly, and that 240 ohm resistor doesn't quite cut it (despite it showing up in reference designs, etc.). I replaced the resistors with R1=120 and R2=1000 + up to 500 from the trimmer, and presto, now I can easily dial in +15.

Is this something others have seen? Do LM317s get more particular with age about minimum loop current? (And are there a bunch of vintage Fluke multimeters out there slowly accumulating damage from overvoltage rails?)

[SQ9MT_PL]

Can you (if you have) send a schematic or a fragment of the schematic of this circuit ?

[mdubinko]

We're talking about U3 here.

[SQ9MT_PL]

Have you checked U1? If U1 is damaged, the voltage increases to 30V (it's also good to verify CR3)

[mdubinko]

I managed to get my hands on a third 8505a power supply, and it has the same problem on the +15V rail (and accordingly on the +30V rail, which is referenced from +15). The lowest I can get it down to is 17.7V. Time to swap out the resistors again...

[med6753]

Gee, I suppose I should check my 8505A. I did a re-cap on the PSU last year but I assumed the voltages were OK since it was functional before and after the re-cap and didn't check them. I know, my bad. 

[mdubinko]

I'd love to see some more data points on this. If you've got an 8505a or 8506a let me know what your VA1 and VA3 rails read at.

You can access VA1 (+15) at TP3 on the power supply board itself, and VA3 (+30) at TP1, both relative to the analog return at TP6. It is tricky, but possible, to do this without removing the extra guard shield over this half of the board.

And the tiny little trimpot next to TP6 is R9, which adjusts both VA1 and VA3 up/down in unison. The official spec from Table 4-4 in the service manual gives the tolerances as VA1: +14.25 to +15.75 and VA3: +29.7 to +31.7.
-m

[med6753]

I opened up my 8505A and the +15V measures +15.45V. I was able to adjust it to +15V on the nose with no issues. The +30V after adjusting the +15V reads +30.69V. So both voltages were in spec.

I think you have a bad batch of LM317T. Yes, typically the "load" resistor is more like 120 ohms but 240 ohms should work just fine. And I think you should also consider a complete re-cap of the PSU. I did mine last year. It's super easy. The PSU pulls out the back.

[bdunham7]

I managed to get my hands on a third 8505a power supply, and it has the same problem on the +15V rail (and accordingly on the +30V rail, which is referenced from +15). The lowest I can get it down to is 17.7V. Time to swap out the resistors again...

Are you testing these in or out of the meter? 

[mdubinko]

All of these tests have been out of meter. It would take quite a bit of dexterity to reach those test points in-situ.

-m

[bdunham7]

All of these tests have been out of meter. It would take quite a bit of dexterity to reach those test points in-situ.

-m

Perhaps the LM317 circuit as designed anticipated a minimum load for proper operation, one that the meter would always provide?  Out of curiosity, I popped the top on my 8505A and was able to reach TP3 and TP6 to measure the 15 volts.  Mine was 14.85V.

[coromonadalix]

Why not use  7815 and 7915 fixed regulators   they have the same currents than a lm317 ?

[floobydust]

Also note, in Fluke's circuit the +30V regulator (lifted 7815) sources current into the LM317's output, an additional ~5mA or so. So the LM7815 could be aggravating things, and I don't see any output capacitors for the regulators, which is good practice.

I find in general the LM317 has lower noise than LM7815's. Some LM317's today are garbage, tons of LF noise.

[med6753]

All of these tests have been out of meter. It would take quite a bit of dexterity to reach those test points in-situ.

-m

My readings were with the PSU in place and the DMM powered up. It was tough but I was able to get on those test points with the Pomona hook leads. Your readings are invalid with no load on the PSU.

[med6753]

Why not use  7815 and 7915 fixed regulators   they have the same currents than a lm317 ?

A 7915 is used for the -15V supply. The +30V supply is derived from a +15V supply regulated by a 7815 plus the +15V supply derived from the LM317T. The LM317T was used to allow it to be precisely set.

[med6753]

Also note, in Fluke's circuit the +30V regulator (lifted 7815) sources current into the LM317's output, an additional ~5mA or so. So the LM7815 could be aggravating things, and I don't see any output capacitors for the regulators, which is good practice.

I find in general the LM317 has lower noise than LM7815's. Some LM317's today are garbage, tons of LF noise.

Agree on the capacitors. It's good practice to hang at least a 2.2uf - 10uf tantalum on the outputs. 

[cat87]

I've also had some issues with the PSU rails on one of my 8505A.
Apparently the caps on these units are really crappy. Especially the blue ones.  So I;d recommend changing as many voltage rail caps as possible on every board.

I found on my PSU board that a cap was dodgy and under load the PSU rail would basically sag too much. Also changed out the regulators and managed to fix it.
Also, on these boards there's never just 1 issue, so as soon as one gets fixed, something else pops up