ASL F17 and F700 circuits - Page 1
Forum section: electronics / general

author: Sengcid
author_rank: Regular Contributor
timestamp: April 09, 2023, 06:27:21 pm
content: The F17 and F700 (a later version) are 1ppm resistance bridges for thermometry: low operating current and SPRT up to 1.5kOhm.

author: Sengcid
author_rank: Regular Contributor
timestamp: April 10, 2023, 06:27:03 pm
content: With apologies, some errors in the intro. Herewith an update and a nice picture.The latest version can be found via the link: -https://drive.google.com/file/d/17pjnY0KpezlSFMIzelvaAceONad_kv5X/view?usp=drivesdk

author: doktor pyta
author_rank: Frequent Contributor
timestamp: April 11, 2023, 06:07:13 am
content: Are there any secret voo-doo techniques used while servicing such ASL bridges (core degaussing etc) ?Or known typical issues ?

author: Sengcid
author_rank: Regular Contributor
timestamp: April 12, 2023, 06:40:39 pm
content: Greetings Doktor Pyta,I don’t know much about Voodoo but I have always found electronics quite magical, especially magnetics and Faraday’s Law – the theory that underpins ratio transformer accuracy.The level of knowledge and skills (and equipment/tools etc) required to fix an instrument like the F17 is about the same as for an audio frequency pre-amp with filters. You don’t need quantum theory to diagnose and replace a dodgy op-amp.There are a few tricks of the trade, however. I do recommend the use of a differential amplifier when looking for mV level signals. Connecting a ‘scope to a circuit inevitably creates an earth loop that can result in spurious signals on the screen.With regard to degaussing: I have never come across a case. It is highly unlikely but not impossible. If, for example, both voltage followers fail, connecting the primary windings to DC power supply, the ampere-turns may be high enough for long enough, before the regulator fails, to permanently magnetise one or more of the cores. Fortunately it is easily diagnosed. In the F17, for example, the current through the windings passes through R40 (100Ω, see section 4.2). One side of R40 is connected to main PSU 0V (earth) so that you can easily monitor it with channel 2 of a ‘scope. In a recent case we eliminated magnetisation as the source of a few ppm error by gradually increasing the reference voltage (across the external reference resistor) beyond the recommended maximum (700mV RMS). The current waveform (see attached) shows the expected peaking as the core is driven into saturation. The flat top is the follower output stage current limiting (an LF356 op-amp). The symmetry indicates no permanent magnetisation. Reducing the reference voltage gradually restores the state of the core.The only failure mechanism I worry about (apart from ageing) is electro-static discharge. I have never had a problem with electrolytic caps, preferring the old adage: “if it isn’t broke, don’t fix it”.

author: mzzj
author_rank: Super Contributor
timestamp: April 12, 2023, 09:37:11 pm
content: Big thanks!I have "inherited" both F17 and F700 that are still in production use in our lab so these documents probably come handy one day.Both bridges need calibration/verification soon with the RBC100 and from what I have heard I'm expecting some problems.Reed relays ever got stuck on these? I heard some rumors like that

author: Echo88
author_rank: Frequent Contributor
timestamp: April 14, 2023, 02:51:49 pm
content: Is it possible to get some teardown photos of the bridges? I think ive never seen the innards of such high resolution bridges.

author: Sengcid
author_rank: Regular Contributor
timestamp: April 15, 2023, 06:41:10 pm
content: Quote from: mzzj on April 12, 2023, 09:37:11 pmI am investigating. The most likely mechanism is years of stress on the diodes (1N4148) in parallel with the relay coils resulting in failure of the driver (7407). It is easily diagnosed using the zero ratio check mode, setting the gain low and stepping through each digit (manual mode) and observing the analogue output (rear panel BNC). Best to stock some spares before they become unobtainium.Edit: The relay coil drivers for F17 are 7407. The 7445 are for F18.

author: Sengcid
author_rank: Regular Contributor
timestamp: April 15, 2023, 06:45:49 pm
content: Quote from: Echo88 on April 14, 2023, 02:51:49 pmHerewith the main analogue PCB. More anon with thanks to other eevbloggers.

author: Sengcid
author_rank: Regular Contributor
timestamp: April 15, 2023, 06:50:50 pm
content: Herewith part of the microcontroller PCB with the window comparator circuits. This was the add-on to the original (manual only) bridge to automate the balance.

author: Sengcid
author_rank: Regular Contributor
timestamp: April 15, 2023, 06:56:11 pm
content: Relay PCB.

author: Sengcid
author_rank: Regular Contributor
timestamp: April 16, 2023, 06:28:12 pm
content: Quote from: mzzj on April 12, 2023, 09:37:11 pmThe RBC is an excellent idea but one could also consider a DIY two-stage IVD. Wika sent theirs to PTB and the 11ppb uncertainty (at 75Hz) was all theirs (PTB's). I could supply practical tips on construction and testing if anyone fancies having a go. Suitable cores can be recovered from RCDs currently being replaced by electrical installers for the new types - not "blinded" by DC.Herewith a first installment.

author: Sengcid
author_rank: Regular Contributor
timestamp: April 16, 2023, 06:36:33 pm
content: I do not have a pic of the F17 ratio transformer so here is the nearest equivalent. It is a bit smaller but probably good for 1ppm at 75Hz and 0.1ppm used as an IVD at 375Hz.See separate thread for ASL F26 and F300 circuits. Thanks again to other eevbloggers. You know who you are.

author: mzzj
author_rank: Super Contributor
timestamp: April 16, 2023, 08:12:35 pm
content: Quote from: Sengcid on April 16, 2023, 06:28:12 pmI’m all ears!Recycled RCD core sounds like good idea.

author: Sengcid
author_rank: Regular Contributor
timestamp: May 08, 2023, 06:46:13 pm
content: Quote from: mzzj on April 12, 2023, 09:37:11 pmSadly there is substance to the "rumour". The failure is not due to the diode but the relay coil - a short between the winding and the (earthed) screen. If it happens the faulty coil and driver chip need to be replaced.Also, have you seen the thread DIY IVD RBC. I am hoping someone will start making them in small batches.

author: mzzj
author_rank: Super Contributor
timestamp: May 09, 2023, 05:00:49 pm
content: Quote from: Sengcid on May 08, 2023, 06:46:13 pmSure, was just foraging for RCCB cores.