Fluke 5700a repair

[veedub565]

I don't think I left it long enough last time before interrupting CAL_CHK. Here is the completed CAL_RPT_CHECK. Looks like DCV section works, but long way out of specification as expected.

[TiN]

How I would start checking relay paths for DCV output from A11. Also good idea to look at DC output on a scope, if there is any oscillation or noise there. Main boards to test are A8, A11 and mainboard. Without DCV working properly all other ranges and functions will be out of whack.

[veedub565]

So there is a fault here then and not just out of calibration?

Reading the manual, almost everything is derived from the DAC in some way. Makes sense if that is out of whack then everything else will be.

[Kleinstein]

It looks like there is some change in the offset to the DAC. Depending on what was changes in the repairs so far, this may be normal and just need a new calibration. However the change is quite large (like 0.5 mV) and thus more than I would expect even from a rather drastic change of parts (e.g. new OPs).
It could be still a faulty old calibration / zero adjustment.

I may be worth to check the DAC a bit more in detail. e.g. compart the DAC output for some -10 V to 10 V in 0.5 V steps to a good DMM. Another point would be to check the ouput stability at a low voltage (e.g. some 10 mV, but still in the 10 / 20 V range) for some 1 hour or so. Also check for ripply / osciallation on the ouput (the more critical setting here would be some 5-7 V).

[veedub565]

No major change in parts. Only capacitors and carbon composite resistors. And I don't think the DAC card has many of those.

I've got a Datron 1281 which is pretty accurate for testing output voltages.

I will try as suggested, and also have a run through the A11 troubleshooting guide to see if that flags anything up.

 The A11 card generates 22v and 11v, then A8 card divides it for 220mV and 2.2mV I think. The fact all of these ranges are a long way out would suggest  A11 is best starting point.

[TiN]

The A11 card generates 22v and 11v, then A8 card divides it for 220mV and 2.2mV I think. The fact all of these ranges are a long way out would suggest  A11 is best starting point.

Yes, but bad relay on A8 could also load/offset A11 output as well. So main focus would be A11 and A8.

[veedub565]

Well it's been a little while, work and other project have got in the way of progress on this. Anyway I've worked through the troubleshooting for the A11 DAC (attached)

I was able to work through all the troubleshooting steps with no issues, until the last one (step 16) Set the output to 10V, measure the voltage at TP8, then at the output high binding post. And the two results should be within 10ppm of each other

Problem is the voltage at TP8 is constantly drifting down. In the time it take me to switch the probes from TP8 to the binding post the output has drifted by 100ppm anyway. For example:

After 2hrs warmup = 9.9980882v
After 24hrs = 9.9973883v

I did make the measurement anyway after 24hrs
TP8 = 9.9973883v
Binding post = 9.9974139v

So this doesn't look correct at all. Not only is it not within 10ppm. The output voltage should be stable not drifitng. 

I went back and rechecked step 2 but with higher resolution this time. I measured 13.0805634 and the last 3 digits were also drifitng around a little ending up at 13.0805714. I mean it's still within the specifications of the test, but still.

[Kleinstein]

For the 13.x V  raw refrence is can be difficult to measure. Meters that use a divider at the input may not be stable enough for this.  If the meter needs to use the 100 V the meters drift may be main part.
It may still be good enough to compare the 13.x V and the nominally 10 V output. This would be mainly measuring the DAC gain.

[veedub565]

I'm using a Datron 1281 on 8.5 digit resolution for the long range measurements. If it's only a couple of decimal places required then I use a Fluke handheld DMM

I did notice the HR5 can was knocked loose allowing air in and heat to escape. I reseated that and it seemed to improve the 13v ref measurement. Didn't make any difference to the output slowly drifting down though.

[TiN]

You have the plot of the drift or CSV? Did you check that all heaters on DAC board hybrids are good and not open (should read ~42 ohm or so between heater pins).

[veedub565]

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.

[veedub565]

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.
 

[TiN]

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  ..

[picburner]

For DS2E series relays you can find something here.

[veedub565]

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 

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 

[veedub565]

This could be an interesting source of replacement relays https://cephosrelays.com/

**UPDATE**
These are £17.83 each, minimum order qty is 10, and £15 shipping (to UK)

So £193 for 10 replacement DS4 relays 

[veedub565]

Well bought a vacuum desoldering gun which made short work of removing the K15 relay. I was able to test the relay out of circuit and found that it sets and resets just fine. So I refitted that, and tried replacing the driver IC just in case there was a problem there, but still no joy.

I've made a spreadsheet and started logging all the relay test results from A8 Switch Matrix (attached) and made a couple of interesting discoverys.

ACV - K15 is always SET when it should be RESET. I know the relay physically works because I tested it, and the relay driver was replaced.

DCV - K23 is always RESET when it should be SET. In ACV it is in the correct SET position. So this proves the relay is physically able to switch and that the relay driver is working.


This is very strange because there's nothing wrong with these relays, they aren't sticking, or the driver IC's. For some reason the 5700 is intentionally setting the relays to these "wrong" positions. Either the relay matrix diagram is wrong, or the 5700 logic has gone wrong in it's old age. there's no reason at all K23 should be in the RESET position for DCV except for 1100V mode with external amplifier. 

[Kleinstein]

Control signals often use some shift registers like HC595 to ouput the large number of bits. These are somewhat error prone from ringing on the clock signal and in case of the xx595 also from delays between chips. So exchanging one of the shift registers could also effect the signal integrety. At least the clock is worth checking.

[veedub565]

The 5700 uses a software controlled 82C55 with 4028 decoder. This strobes the data into several 5801's, which then set/reset the relays. A8 K15 and K23 are consistently set to the wrong state, I'd have thought a dodgy clock would be a bit more random. It's definitely worth checking though.

For a 5801 with 4 relays, each with a set & reset coil I imagine it would be strobing in 10101010 to set them all to "set" state. To get 3"set" and 1 "reset" it would have to look something like 10011010. K15 and K23 are set by 2 separate 5801's (one of which I already replaced) so I'm discounting relay or 5801 fault..... it's not a random error, more like a deliberate act,  They've gotta be receiving incorrect data from the 82C55, which in turn gets it from software. Or the manual is wrong.   

[Kleinstein]

A wrong manual is a very real possiblity. One may be able to check at least 1 connection to see if the direction is correct.

A broken chip that gives out the wrong possiblity is rare - though I have heard of such an example (some old Comodore ISA GPIB cards that could be fixed with an external inverter when they were broken in the usual way).

[picburner]

I have had a completely dead ucn5801 driver in the F5700 but, as Kleinstein rightly said, it is unlikely that a faulty IC will cause the relays to operate in the exact opposite way.
On the other hand, I found an interrupted track in an internal layer of the A13 pcb that drove me crazy before being discovered!
If you find it useful and your eprom programmer allows it, I made an adapter to test the 5801's by vector test.
Since I bought the spare parts in China, through this adapter I checked the devices before inserting them in the pcb!

[veedub565]

That must have been a nightmare fault finding, and it's an interrupted track somewhere.

At this point I'm not even sure which card has the fault on it. Only A8, A11, A17, A18, A19, A20, A21 fitted at the moment (and of course motherboard). Main suspects are A8 and A20 board. I can't see anything else affecting the relay switching.

Slim possibility it may be A21 causing a problem. This is the card which I blew the UCN5801 on using a 9v battery to test the relays (before I knew better). I did replace the IC, but maybe there is some other problem still. I say this because K23 relates to the B SENS LO line which is only used with the external amplifier. Maybe the 5700 thinks it has an amplifier or or something attached, and incorrectly setting the relays. That theory doesn't really fit though because even if 5725 amplifier was fitted K23 is still only reset for 1100v range. It may be worth double checking that card though.

Also way back when before I knew better. I did try swapping over the A8 card from a working 5700 (sadly no longer available to me) and still got the same A8 relay errors. So May not even be a problem with the A8 card.

[veedub565]

Well it's been a while since the last update. I've done some pretty exhaustive testing on the A8 card. And coupled with the fact I did previously test this card in a working unit with no errors. All leads me to believe the A8 card is fine, nothing wrong with it.

K15 and K23 in the A8 card are indeed set wrong, but there's nothing wrong with the relays or drivers. And the card works correctly in another unit anyway.

So it looks likely the problem is with the A11 DAC card. I followed the troubleshooting guide for A11 and there were no problems. I checked out all the A11 relays switch correctly, and they do. So it's a bit of a mystery now as to what the problem is with this A11 DAC. Quite how or why the A11 DAC card causes 4x A8 relay errors is a mystery.

There can't  be that much wrong with it else it wouldn't pass all the troubleshooting steps. Gotta be something simple causing  all these problems. 

[TiN]

I'd check motherboard connections ;-) I had dodgy A3 (motherboard backplane) with open connection between cards, had similar shenanigans too, "good boards" were no good with this A3

[veedub565]

Thanks Tin, I thought I'd fixed it by replacing A11 U31 but I was a bit premature in my celebration.

I don't think A3 is the problem, I was able to try my A11 card in a working 5700a and got the same fault. I know this isn't good practise, but it 100% proves it is the A11 DAC card that is the problem here somehow 

Troubleshooting procedure reveals no fails, so core functions seem to be ok. Something very strange and probably very simple has gone wrong. It might be worth me checking the motherboard connections on the A11 DAC card though. Maybe same problem you had on A3 has happened to my A11.