ASL AC Resistance bridges F26 F300 etc.

[doktor pyta]

I'd like to start a new topic about AC resistance bridges.
They are used mainly for platinum RTD, SPRT measurement.
They operate at 75Hz excitation frequency for 50Hz mains.
The F300 seems to be upgraded F26. GPIB commands are in most cases the same.
There is a user's manual for F300 (attached file).
There seem to be a common problem with non balancing around some values.
I got an info from trusted source that the F300 device went to ASL for repair and after some time the problem occurred once again.
It may be related to procedure of adjusting all of these trim pots on analog boards.
There is no service manual available to download so any info, tips regarding adjustment are highly appreciated.
Any experience with operating or servicing these bridges?

Some theory from a pioneer of this technique: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwiF6bqLyf7uAhVLpYsKHdb4B7YQFjAAegQIARAD&url=https%3A%2F%2Fnvlpubs.nist.gov%2Fnistpubs%2Fjres%2F74c%2Fjresv74cn1-2p15_a1b.pdf&usg=AOvVaw28uc86mIODcVGXSlBwXVa-

Below some photos.

First is F26.













Now F300.

[Sengcid]

Hello doktor pyta. I may be able to help. I don't have schematics but I know quite a bit about the F26. It is not the pots - mainly offset trims for the opamps (LF356). If you can connect to a variable resistor you should be able to identify the manual balance settings that don't work. I suspect a dry joint on a (ratio) transformer tap.
Chris
If you pm me we can go deeper.

[doktor pyta]

Sengcid,

thank You for Your reply.
Do You have experience in servicing ASL gear ?

Could You please clarify one more thing:
In F26 I noticed that decades ACBxxxx are obtained by switching taps of the transformer (or inductive voltage divider) in 000 ... 399 range.
I know this because I hear reed relays clicking.
Decades xxxDEFG are obtained by monolitic multiplying DAC AD7525. However this DAC is 1999 counts only, so 0000... 1999 range.
Do You know How are rest of the xxxDEFG decades made?

Oh, I will try to look closer on soldering joints on transformer, thanks for a hint!

Additional info: output of current/voltage converter connected to the MDAC is at TP39.
TP48 seems to be output of the phase sensitive demodulator.

[Sengcid]

Hello doktor Pyta. I have limited access to the internet (a whiff of free wi-fi through the window on a tablet) so would prefer to keep messages short and attach .pdf documents/pictures. If you would like anything in msword format please let me know.
I tried to pm but no luck.
Chris

[Sengcid]

Just seen your blog. I used to be in charge of r&d at asl, many years ago. The extra digit is specmanship (sales thought it was a good idea), otherwise lose a bit of resolution (e.g. display in calculated kelvin). I vaguely recall that its manual only version (F16) was much better than expected than the ad7525 (bcd input) lsb would suggest.
N.B. the trimpot that scales the mdac to the xformer is a critical tweak.
Chris
P.S. please attach hi res pics of the amplifier/synchronous detector so that I can download/see chip numbers. I should be able to recontsruct the cct diagram fairly quickly.

[TiN]

Sengcid
Thank you for sharing!
I tried to get your collection of monographs from link, but it leads to 404 not found  .

[doktor pyta]

https://drive.google.com/folderview?id=1TbomXeoBbIe-IVADaOOmyyLH9le-3wAt

These are great lectures !!!
Thank You Chris.

[BoseEinstein]

Wow, very interesting. I acutally did my master thesis on those AC bridges. Those monographs would have saved me many hours of studying the schematics/windig diagrams and trying to understand what Wolfendale/Yewen (and Daykin of course) did there (I got about 95% of it, but it took some serious amount of time... ) It's fascinating how easy it is to build such a good voltage divider with the IVD principle. And it's really cool to watch a bridge balancing to last digit   

[doktor pyta]

'Test 1.
Two 100Ohm resistors connected as RS and RT as fig.1.1.1 Set current to 1mA; ext RS; sensitivity (detector gain) high; ratio mode; cal off.
Does the bridge balance in auto? Try again in low and med gain – auto balance should slow down.
You should also see 100mV RMS on RS and inverted on RT (input and output of active guard circuit (see fig. 1.2.1, basically an inverter). Do not connect scope 0V to virtual earth – use the earth terminal on rear panel.'
100mV RMS +/-1% at 100R. Other current ranges also correct within 1%.
Result: OK. See photo below.
25Hz ripple visible on the scope.



'Test 2
In manual mode set the ratio to 1.00000; low gain and observe detector output (BNC rear panel). Vary the manual ratio setting up and down (top 3 decades from 0.00 to 3.99). The detector DC output should be proportional with no obvious gaps.
Note: On max gain you should get approx ±1V for ±1 digit in the 5th decimal place (±0.00001) and pro-rata for med and low gain settings. Similarly for different current settings: more current = more DC change per digit.
If there are jumps suspect a dry joint or one of the relays (transformer tap selection) not working.'

Result: OK. No gaps.

'Test 3
The last three digits is implemented with a multiplying DAC and added in series with the main xformer output.
Connect both RT volt sensing leads to same point on RT (virtual earth side) equivalent to a RT=0.
Manually set ratio to zero and check for balance. Increase one digit at a time on max gain. The DC output should change accordingly, with no gaps.'

Result: OK. No gaps. It was not mentioned, but if I start to adjust 6th decimal place there is some weird behavior. from 0 to 4 nothing changes, then from 5 to 14 is some constant value.
I suspect this 6th decimal place can be obtained from residual voltage measurement (?).



Below two more detailed photos of the analog part. The size is limited to 500KB on my website.





P.S. power supplies seem to be working correctly with reasonable voltages and relatively low ripple.

[Sengcid]

The low noise pre-amp is a bit unusual (see attached). Look for some 50 or 100Hz interference at the output (zero ratio test) – it could explain the 25Hz ripple. I suspect 100Hz due to current pulses that charge the PSU smoothing caps (every half cycle) which, with poor wiring layout, transmit magnetic flux.
Chris
P.S. Look after those Vishay ref. resistors – like vintage wine. Also, the ratio transformer is much better than required: probably <1ppm, comfortably and even better as an IVD.

[doktor pyta]

answer 1):
metal can IC30 is LF356H. I guess it is a part of current to voltage converter for MDAC.

answer 2):
CD4046 IC9 PIN 14- 50.0Hz ; PIN 4- 75.0Hz
CD4046 IC19 PIN 14- 75.0Hz; PIN 4-19.2kHz

3) probably, + AD7545 has also 1 bit higher resolution

P.S. Will the photos of the bottom side of the PCB be helpful?

[doktor pyta]

Maybe this video will push the diagnosis further.
It shows the problem with blinking CHECK indicator.
I couldn't get the bridge to a point when it cannot balance itself.

[ArnsteinO]

Hi Chris and many thanks for the very useful info on the ASL F bridges. My company used to be ASL representative in Spain from 1988 until 2014 and now there is a problem with technical support and repairs. It is a shame because the briges are so good and popular. I have two F17's with unstable reading (null detector) and will read through your documents to see if I can trace the fault on those. Would you have any info on the F250? I have one which reads open circuit on both channels. I have changed the multiplexor chip at the inputs (IC1)  and several of the OP amps and all the voltage test points are good and stable (capacitors changed). Many thanks and best regards Arnstein