Racal-Dana 9300 vs. 9300B - Page 1
Forum section: electronics / testgear

author: Electrole
author_rank: Contributor
timestamp: September 17, 2019, 03:11:45 pm
content: Last week I got a Racal-Dana 9300B RMS Voltmeter in a decent shape and at a reasonable price. As the first step when I received the unit I took it apart, of courseI found that the resistors that determine the gain in the chain of attenuators and amplifiers in the (older?) 9300 is a mix of 1 % and 0.1 % resistors. However, these resistors are all 0.1 % in the 9300B. I did not spot any other differences, so I wonder if the changes from the 9300 to the 9300B were just about these resistors (in addition to the different casing)... Anyone who knows more about the differences between the two, and perhaps about the history of the two?I have uploaded photos of the 9300B, with the shielding over the input section removed. The power inlet is not the original, but nothing else seems to have been replaced. For those of you with X-ray vision you may notice, however, that R14 is not the original. The original R14 is a carbon film resistor, and I replaced it with a metal film resistor (or in this case two metal film resistors in parallel) which I found increased the stability of the DC voltage on the back panel.

author: JohanH
author_rank: Frequent Contributor
timestamp: September 19, 2021, 05:34:03 pm
content: I know this post is a bit old, but I thought it better to keep knowledge in one place as there are not many posts about this meter, even though it is such a nice instrument. It is all linear and has mostly discrete components and should be almost 100% serviceable.Anyway, I bought the Racal Dana 9300B meter on ebay and tested and adjusted it a bit. It is in very good shape. Calibrated in 1995 according to a sticker, so probably not opened since then. It is not completely linear over the ranges (can't test all, though) and I did have to adjust every trim pot and trim cap to get anywhere nice readings.I replaced the same resistor, R14. It was within tolerance, but far from spot on. I traced the schematic and measured what I thought most critical in the signal path. Most resistors are really good, way better than tolerance specs. I found only two that were out of tolerance. One was R3 1M, so shouldn't make much difference, but I replaced it anyway.Another resistor that was way off was R28, 2.26k 0.25% and measured 2.4004k, but as this was in series with one of the trim potentiometers (R29 500R), it doesn't matter too much unless you end up with it bottomed.R28 is actually 2.4k in this model and not 2.26k as it says in the schematic (according to value on the resistor). The trim potentiometers are annoying because the smallest touch makes the readings jump. I'm thinking on replacing them with cermet 25 turns for easier adjustment.The user and maintenance manuals are available on archive.org:https://archive.org/details/racal_Racal_9300_User_Manual_4-11-89/https://archive.org/details/racal_Racal_9300_Maintenance_Manual_4-5-85/There is one error in the parts list. R7 is listed as 1k in the parts list, but in the schematic it is 56k. The latter is correct (verified in my meter).I haven't done anything to capacitors yet, but I'm thinking on replacing at least the electrolytics (only four in the whole meter!). There are many tantalums in the meter and I thought about replacing some of them that are in the signal path (coupling capacitors). Problem is to find modern equivalents. Not all could be replaced by tantalums, but I figured that polymer caps could work just fine. And while I'm at it, some of the coupling ceramics could be replaced with modern C0G MLCC ceramics or possibly polypropylene and micas for the smallest ones. Anyone any thoughts on this?

author: JohanH
author_rank: Frequent Contributor
timestamp: September 19, 2021, 05:40:38 pm
content: The only other info on the internet that I could found about this meter was a post from another forum explaining issues with a reed relay. That's the older 9300 variant, but the interior looks pretty much the same.https://www.vintage-radio.net/forum/showthread.php?t=92360

author: JohanH
author_rank: Frequent Contributor
timestamp: September 19, 2021, 06:10:47 pm
content: All this info is in the maintenance manual, but as I figured it out I thought I could as well post it. It is about which trim potentiometer affects which part of the instrument.R29 sets total signal path gain (for all ranges).R73 sets scale linearity (for all ranges).R82 calibrates the analog meter.R19 adjusts separately gain for the 40 dB attenuator. This affects following ranges: 10mV, 30mV, 100mV, 300mV, 10V, 30V, 100V and 300V (note, not 1V and 3V range!). This is not said in the manual, but you can figure it out from the schematic and the description of the attenuators and ranges.So basically you need to set total gain and linearity first with R29 and R73 using the lower ranges (manual says 3mV range). This is also the only adjustment for 1V and 3V range. Then adjust gain in higher ranges (that use the 40 dB attenuator) with R19.High frequencies are adjusted with C34 and C10 (sensitive ranges) and C3 (only 60 dB attenuator; i.e. 1V - 300V ranges). All of this is explained step by step in the manual.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2021, 07:34:14 pm
content: The ceramic an film caps are very stable. There is absolutely no need to replace them. The moden MLCC ones are usually no better, at least not relevant in most cases.With an old discrete buld RMS converter there is not really need for very high precision resistors. The accuracy is limited anyway.

author: JohanH
author_rank: Frequent Contributor
timestamp: September 20, 2021, 07:10:41 am
content: Quote from: Kleinstein on September 19, 2021, 07:34:14 pmI still like it anyway. For a non-professional in electronics like me (even though with an electrical engineer degree, I have forgotten a lot), it is nice to discover how the circuit works and see how good it works even though it is simple. There are hand selected transistors arrays and optocouplers, so care have been taken to produce the instrument. It is light and with maintenance it will last forever. Analogue display is a bit out of fashion, but you can use the DC voltage output. The range switch on the front is unlike older meters (other brands) not used for signals, but used to switch an array of relays. From googling I have seen people mention that they still use it for noise measurements for power supplies and amplifiers. It's wide bandwidth is good enough for this (-3 dB 57 MHz according tohttps://dabbledoo.weebly.com/rms-measurements.html).

author: theHWcave
author_rank: Regular Contributor
timestamp: November 16, 2021, 10:31:17 pm
content: I just got a 9300 (not the B-version), actually I got 1.5, one working the other for parts. I really like the meter and the circuit is so clever with automatic zero and noise compensation.I am interested if you decided to replace any caps, and if so which ones?Secondly, in the section on removal of covers, the maintenance manual mentioned "to not displace or damage any RF gasket material fitted". There isn't any trace of gasket material in either of my two (1.5) meters. Does your meter have any?

author: JohanH
author_rank: Frequent Contributor
timestamp: December 23, 2021, 07:23:11 pm
content: I have a bag of parts ready, been waiting for some spare time.I wanted to replace the inlet filter (Schaffner part no. 370-2), so I managed to get hold of a Schurter IEC filter with the same input voltage selector. This part was not stocked, so hard to come by. Note! I haven't fitted it yet, so can't be 100% sure. It's not exactly the same as the original. I will report how it fits. Part numbers are Schurter CE10.6100.151, Voltage Selector Insert no.1 4305.0048.01, double fuse 4305.0001.There is one other with voltage selector that almost fits, Schaffner FN379B-2-22, but you would have to make a bigger hole. Alternatively use any without voltage selector that fits and hard wire the transformer for one input voltage.I haven't seen any trace of "RF gasket material", so I can't imagine what it could refer to.

author: JohanH
author_rank: Frequent Contributor
timestamp: December 27, 2021, 11:44:30 am
content: Quote from: jukk on December 23, 2021, 07:23:11 pmThe new IEC inlet fits nicely. It is a bit deeper than the old, but there is plenty of room inside. The old rubber cover on the inside fits too. The transformer wiring (pinout) for the voltage selector is exactly the same, only the two rows are swapped, so watch out. Two 3 mm holes have to be drilled. I'm using 100mAT fuse as specified, only there are now two fuses in the new inlet. But seems to be no issue using the 240V setting with my 230V mains (settings are 100, 120, 220 and 240V). DC voltage on the secondary before the +-15V regulators is +-24V, that should be enough.Attached also a drawing of the difference between the original Schaffner voltage selector pinout and the new Schurter.

author: G0HZU
author_rank: Super Contributor
timestamp: December 27, 2021, 01:59:48 pm
content: Yes, the Racal 9300(B) is a very nice meter and I've used one all my career. There used to be one at work but I also have one here. Mine is the 9300 model and it has been a reliable workhorse for me.They can often be purchased for under £50 here in the UK and this is an absolute bargain if the meter is working. I suspect many people don't realise how good this meter is and assume it is just another old and obsolete meter with nothing to offer in 2021. This is still one of my most prized and respected pieces of test gear despite the low cost.

author: JohanH
author_rank: Frequent Contributor
timestamp: December 27, 2021, 07:12:06 pm
content: Quote from: theHWcave on November 16, 2021, 10:31:17 pmThe caps were fine (checked ESR after desoldering) and looks to be very high quality, but I anyway changed the electrolytics (they could very well work another 25 years, but why take the risk?). C1, C2 to Rubycon 105 ℃ long life and C31, C46 to Nichicon 105 ℃ long life polymer. As these are power supply caps, any good quality should do. I used wire end sleeves (cut to size) on the snap-in capacitor legs (C1, C2) to get a good solder connection.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: March 16, 2022, 01:28:21 pm
content: I too have just acquired an ex-MOD 9300 which was dead on all ranges 1V and above.It was a simple reed relay fix. For me it was RLA that was faulty - on testing the coil was energising, the reed was clicking but the contacts had no continuity.Having removed it, I used the usual fix of cycling the coil on 5VDC with 0.5A (limited) across the switch contacts. A couple of cycles and it cleared its contacts and once reinstalled the meter appears as spot on as one could want on all ranges.QUESTION: does the IEC inlet assembly include filter caps? (I haven't tried to pull the whole back off to extract the unit from the plastic boot). If so are they magic smoke ones or not?For completeness, here's the inside of the 9300 with RF screen removed.

author: JohanH
author_rank: Frequent Contributor
timestamp: March 16, 2022, 05:06:11 pm
content: Quote from: 6SN7WGTB on March 16, 2022, 01:28:21 pmYes, it has the typical X Y caps between phase, neutral and ground. Don't know how common it is that they fail, but I changed the inlet anyway. YMMV.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: April 08, 2022, 03:20:35 pm
content: Quote from: JohanH on September 19, 2021, 05:34:03 pmDid you do this?I've calibrated mine and as you say you've only got to sneeze in the next room and the setting changes.I've also noticed a little drift - I wonder is this too is really the pots given how sensitive they are?

author: JohanH
author_rank: Frequent Contributor
timestamp: April 08, 2022, 06:44:41 pm
content: Yes, I did it and now it is really easy to adjust.There is a small imperfection with some of the ranges that can't be adjusted. I think it was the 10 dB and 20 dB filters that aren't perfect. So it affects mainly 0.3 mV, 1 mV, 1 V and 3 V ranges that can't be adjusted with presets (except for the total gain that affects all). To adjust them, would require tuning the RC filters. As all of the ranges use the filters in various combinations, it affects many ranges, so it would be a lot of work to get everything better (could easily end up worse, too). It is relatively within spec, though, so I can't really complain. And it could be up to component tolerances, so another meter could be totally fine.I haven't measured drift, but for my usage the meter is perfectly fine now.