Fluke 335A (another one) - Page 1
Forum section: electronics / repair

author: Cubdriver
author_rank: Unknown
timestamp: May 13, 2017, 06:10:55 am
content: Hi All -I recently snagged a 335A and have begun going through it. It has not yet been powered up. The only glaring thing found so far is an overcooked, burned out resistor (R3) on the A5A6 current limiter PCB. The schematics I've found thus far (and the board shown in the other recent 335A thread) are somewhat different than the one I have - it appears that mine is a newer iteration (marked 335A-3060 Rev H), as it is missing some of the resistors shown in the earlier schematics.The older schematics show a series of 2W resistors feeding the range indicator lights - R3, R8 and R25; 150, 120 and 120 ohms respectively. My board has only R3, and it is 24? ohms (red-yel-something unreadable as the multiplier band is cooked off). This does not appear to be a field mod, as the reference designators for the missing resistors are not present on the board (though the layout appears unchanged from the earlier version; the holes and traces are present but are jumpered out). Based on a quick look at the paste-together schematics I've found thus far, it appears that all this does is to feed the range indicators, which on the older schematics appear to be incandescent lamps. On my 335A, they are LEDs, so as they are fed from what looks like a 45-50V line, I'd expect the resistor to be in the 2.4k range (~20mA @45V). Does anyone have info on the newer(?) iterations of the board and know what the value of that resistorshouldbe? At 20mA, it seems it should dissipate roughly 1W, so a 2W resistor might be a bit marginal there (I was taught to go 3X anticipated power dissipation), but plan to dig into the circuit further to see if anything else is amiss there - it definitely got VERY hot.Board:Cooked resistor:Additional photos of the instrument are athttps://pmanning.smugmug.com/Electronics/Fluke-335A-DC-Voltage-Standard-Differential-Voltmeter-NullThanks for any info!-Pat

author: coromonadalix
author_rank: Super Contributor
timestamp: May 13, 2017, 10:48:48 am
content: Try this manual, it's fir the 335D, some schematics inside, the a5a6 board is at page 173http://assets.fluke.com/manuals/335d____imeng0100.pdf

author: t_ryner
author_rank: Regular Contributor
timestamp: May 13, 2017, 11:59:17 am
content: First off- nice piece of kit! There's no other obvious problems in the photo, but I found a tip that helps- go around on the other side of the board and look for discoloured solred- indicating heat damage. Also- test the capacitors and check for shorts. The schematic provided by @cormonadalix should help too.

author: Cubdriver
author_rank: Unknown
timestamp: May 13, 2017, 02:06:43 pm
content: Quote from: coromonadalix on May 13, 2017, 10:48:48 amExcellent! Thank you very much for the tip. I'd downloaded the 335A/D manual from the Fluke site, but never thought to look at any other models. This will be a big help, as it apparently has some newer boards in it.-Pat

author: Cubdriver
author_rank: Unknown
timestamp: May 13, 2017, 02:17:03 pm
content: Quote from: t_ryner on May 13, 2017, 11:59:17 amYes, I certainly lucked out on it! I did some quick in circuit probing of that board (just before starting this thread in the wee hours this morning) with a 4262A LCR meter and found nothing that seemed horribly amiss, but might try re-forming them with a small power supply before firing it up, as it has been sitting unpowered for at least five years, and quite possibly longer. I plan to check the other electrolytics in the unit as well. I'll add to this thread when I do so, and report new progress or further issues.-Pat

author: cncjerry
author_rank: Unknown
timestamp: May 13, 2017, 05:03:30 pm
content: Maybe send a note to Dr. Frank. He was very helpful on my 332D.

author: enut11
author_rank: Super Contributor
timestamp: May 14, 2017, 06:40:30 am
content: Yes. Dr Frank helped me with a 335A manual. It has the bulb indicators on the schematics. Sounds like you have a later model hybrid with LEDs. I have just finished converting mine to LED indicators and used 6.8K 1/2W resistor in series to give around 8 mA current for the LEDs - more than enough brightness for me.enut11

author: Cubdriver
author_rank: Unknown
timestamp: May 14, 2017, 06:58:11 am
content: Thanks for the input, folks. I'm leaning towards putting something higher value in there (perhaps on the order of 3.3k), but will also bump the power level, too, and go with a 3W part to give it more margin. The lower current combined with a greater power dissipation rating should make for a happier resistor.-Pat