Electronics > Repair

Fluke 5700a repair

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Heaters measure ok (HR5 and HR6). I don't have a plot of the drift yet, if there's some code for doing this with the Datron 1281 that would be great, if not I'll have to learn how to do it. Or manually take some readings.

I've also made a start checking the relay paths. K1,2,3,4,8 all seem ok on the A11 DAC and function as per the table in the schematic. One thing I have noticed though, when pressing "operate" I hear the relays make a positive click, but I also hear a strange fluttering sound, almost sounds like a moth fluttering kind of sound. It's only brief, and I'm not sure if it's normal or not. It could be the sound of a failing relay.

I can hear it definitely isn't coming from the A11 DAC board. I removed the A10, A9, and A7 boards and still hear the sound. So it must be coming from the A8 board or possibly motherboard.

Another update on this, long term project and I believe I've found a faulty relay.

So with A20+A19+A18+A17+A7+A8+A9+A10+A11 cards installed, running the self diagnostics revealed no faults (besides the usual complaints about missing cards) However I still seemed to have a problem with my DC voltages tens of thousands of % out of spec and drifiting. Trying to nail the culprit here is tricky. Even trying to identify the faulty card was hard. So I added in the A12 card so that I could learn a bit more and see if I could identify any functions not working at all that may help.

Well as soon as I added the A12 card then I got 4 A8 relay faults flag up. SO what has changed here. Well the A12 card adds some ACV functions, 2.2v and 22v. A8 then divides these down to make 2.2mV, 22mV, and 220mV ranges.

Going through the  troubleshooting revealed no problems until the last step
"Check the mV Dividers. Set the 5700A to 200 mV at 1 kHz, operate. Using a DMM
measure the ac voltage at the cathode of CR6"
Setting the calibrator to 200mV 1khz caused it to trip out to standby mode. In fact, setting any of the AC mV ranges caused it to trip out. 2.2v and 22v worked ok. So there is a problem here!

I investigated the divider circuits, and used the tables in the A8 schematic to trace through the relay paths. And I found the A8 relay K15 is stuck in the "SET" position. this should be in the "RESET" position for all the AC mV ranges. It could still be a faulty driver IC, so I removed the A8 card and manually applied 5v to the K15 relay reset coil... nothing, nada, no click. So it appears I have a faulty relay, probably the contacts have stuck in the "SET" position.

This is bad because K15 and K33 both connect to the out/sense low line, but not at the same time. With this fault it allows both relays to connect to out/sense low at the same time.  Maybe this is contributing to the DCV volt problems, maybe not, but it certainly looks like a faulty relay here and it needs replacing.

Two questions
1. Whats the best way to remove the relay without damaging the pcb ? I don't think solder wick is going to work here. A hot air gun may work... or it could just lift the pads. I'm aware this is through plated and theres a risk of pulling pads from the opposite side of the board.

2. where is a good source of replacement relays ? they are obsolete and out of stock now from most mainstream suppliers. There are quite a few sellers on ebay, but there's always a risk of buying fake or factory reject parts.

1. I used solder wick, worked well with good care. Best would be desoldering iron with pump. Worst is hot air gun, as 99.99% of them would ruin the board.

2. No good source. DS4E's are ebay-sourced, I did buy number of them and after a sniff test used most promising ones. DS2E's are available from Digikey DS2E's also obsolete  :rant:..

For DS2E series relays you can find something here.

I had a quick go with wick, but difficult to get all the solder out especially from component side. I've got a Hakko FR803, but I agree the hot air gun risks ruining the PCB. The foil on this PCB already looks crinkled and fragile in places. I guess this is a good excuse to invest in a vacuum desoldering station  ;D

Thanks for the link to relays, might buy a couple of those, works out £15.70 each with shipping though. I've not found the DS4 ones anywhere except ebay at £3 + free shipping to £17 + £19 shipping each 


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