solartron 7075 (or: will I be able to reassemble it ?) - Page 1
Forum section: electronics / general

author: muvideo
author_rank: Frequent Contributor
timestamp: March 23, 2012, 08:55:55 pm
content: Hello, some time ago tekfan posted the photos of his solartron 7075.Recently I won this instrument on an ebay auction (edit: not tekfan's one, it's another one), sold as broken, tekfan was very kind to sendme a copy of the maintenance manual for the instrument, if somebody wants this manual it's nowhosted on ko4bb site:http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/Solartron/Solartron_7075_Service_Manual.zipI received the instrument in a fair condition, the only evident abuse was a bump on the rear.After powered on the first thing evident was that the display had problems, the positive thingwas that the instrument responded to test sequence, so the logic was at least partially ok.The display is built with 4 panaplex modules, 3 model sp-353 (3 digits) and 1 sp-354 (2 and half).One of these modules was evidently broken, fortunately the displays are mounted on sockets.Now I would be curious to know how would be possible to break that display when mountedwithout crushing the caseThese displays light up when excited with a high voltage, around 160V are needed.I searched in the junk box a transformer with a high voltage secondary and fitted arectifier and a 1MOhm series resistor, to obtain about 200V dc limited to 200uA.With this I tested all the segments and are all ok, only the broken display didn'tlight up at all. Lukily I had some old pcbs with the same displays fitted, so I replacedthe broken one with another old display.The -18V was also absent, the PSU is interesting, it's a switching psu with a workingfrequency of around 25kHz. The -18V are obtained from +5V with a voltage inverterand capacitor multiplier, but the capacitor wasn't there, the terminals seem to bebroken: this instrument must have been subject to some rough handlingI refitted the capacitor and rechecked the display, now all the digits light up butthere was a problem with the cathode drivers: in the first image it's clear.There are two cathode drivers that multipex trough the display, lighting two digitsat a time.Apart the missing block, each two digits are lighted by the two cathode drivers, one ofthem is faulty and can be seen by the fact that one the digits per couple is only partially lighted,fortunately the board I used to extract the spare displays also had some cathode drivers,and the luck is that these Beckman DD-700 are perfect replacement for the broken DS8880.I noticed a pair of strange things, I don't know if these are from factory or modificationsfrom someone:the first thing is that the rear "External Reference High" terminal is disconnected fromhis wire and connected to the analog pcb to "Ohm guard" pin.the second is that some of the single pin connectors are soldered, many others arenot soldered, it's a pain to disassemble the soldered pins without damaging the boardSo now the display is fixed and I decided to disassemble all the thing,after all this is the funI washed all the case components, but I didn't wash the boards, I have fearto have problems, maybe distilled water can be used?The question is:will I be able to put all together?More to come.Fabio.edit: trying to show the images inline...

author: tekfan
author_rank: Frequent Contributor
timestamp: March 23, 2012, 10:04:22 pm
content: If you wash the boards in water you won't have any problems if you let them dry completely. Put them in sunlight for a day and they should be fine. Blow some air under the ICs to check that it is completely dry. Or speed up the process with an air compressor.I would check all the relays for good contact. Some of the relays use a field coil mounted on the top side of the board and switch multiple reed switches on the bottom side.Make sure that the field coils are still OK and check the reed switches with an ohmmeter. Do the same for the two big relays on the main board. I did this and it saved me quite a few hours of troubleshooting. Don't forget the small relays in the metal cans. Also check for shorted flyback diodes across the relay coils.I've also replaced the four LEDs in the chopper assembly. The original ones were so dim that they couldn't turn on the LDRs so there was a large offset with the leads shorted.I also found that some of the SN74190 reversible counters were partially dead. I only found this out when I was calibrating the meter and the third least significant digit was always 1 too high. I also found out that when the meter was in ohms mode with the leads opened it autoranged to the highest ohm range but it didn't display a flashing 14000.000 like it should but some random number. I ended up replacing all of the counter ICs. They seem to be very sensitive to heat and fail if you're soldering the IC next to it.There are 7 counter ICs, one for every digit. The DISPLAY switches select how many of the counter ICs are used and thus display less digits.If you really wanted you could make this multimeter into a frequency counterYes, this meter isn't a true 7.5 digit (20000000 count) meter but actually a 14000000 count meter.The wire going to the external reference plug definitely shouldn't be there. In my meter there are also a few bodges like cut traces and two diodes on the underside of the main board. The diodes are in the schematic so it doesn't look like a user mod.You're lucky to have replacement Panaplex displays available. They seem to be more expensive than the meter itself!Also you can reverse the leads on the decimal point neon bulb so that you use the other electrode. It is much brighter since it was never used.

author: muvideo
author_rank: Frequent Contributor
timestamp: March 23, 2012, 10:08:54 pm
content: More internal details:Display assembly:Display Panaplex sperry or beckman, the glass is blackened:Logic boards, no cpu here (or anywhere in this instrument)Switching PSU:the big ferrite is the actual transformer:Analog assembly:Analog board with all the interesting stuff:Next post reference details and chopper assembly.Fabio.

author: muvideo
author_rank: Frequent Contributor
timestamp: March 23, 2012, 10:26:28 pm
content: Reed relays fields:Contacts:Chopper assembly opened:Chopper ldr :Heated reference:Here an interesting discussion of Mickle T., with a pic of this reference disassembled:http://www.radiokot.ru/forum/viewtopic.php?f=10&t=52829&start=60Fabio.

author: muvideo
author_rank: Frequent Contributor
timestamp: March 23, 2012, 10:56:33 pm
content: Quote from: tekfan on March 23, 2012, 10:04:22 pmI'll think about it, the dirt on the boards is not much, does the compressed air bring electrostatic problems?Quote from: tekfan on March 23, 2012, 10:04:22 pmThank you for all, I'll check all this.Did your had many of the wire connectors soldered, like mine?What does the meter do with the leads disconnected? Mine just lockswith busy flag on, but it performs the test correctly.Did you test electrolitics and resistors around?Most of the resistors are carbon composition, and the ones under the display are way off value,I'm just thinking about changing some of them.Quote from: tekfan on March 23, 2012, 10:04:22 pmDefinitely a big luck, here we say "Una botta di culo!", they were just there, in the "strange things box" waiting for the 7075Don't know where these come from, probably some old medical equipment a friend of mine had around, we dismantledall and kept all that seem unusual. Searching for these displays I was shocked by the price, later I found some asking 14usdfor these, I have also some incandescent seven segments similar to minitrons, now I learn that these had shocking prices

author: tekfan
author_rank: Frequent Contributor
timestamp: March 24, 2012, 12:10:51 am
content: QuoteOnce the MOS elements are connected to other components, the risk of static electricity building up is very low. Unless you rub your feet against a carpet and hear sparks when you touch things you'll be fine.QuoteYes. Many of the traces in my meter are cut, but checking the schematics it all matches up. Yours looks to be in much better condition than mine. I can't see any big cuts in the tracks from your pictures.Your meter seems to be from 1978 and mine is from 1976. Looks like they made some revision changes to the board to eliminate the cut tracks.The wire between the chopper assembly and the reference zener looks the same as mine. There is actually a track that runs between the components, but for some reason they decided to cut the track and put in the wire instead. Maybe because of leakage?Interestingly they mounted the reference zener horizontally.QuoteWith LOCAL, DC, 100mS and REP selected the meter powers up usually in the 10V, 1V, 100mV or 10mV range and then the reading drifts since the input is very high impedance on those ranges and the meter is picking up noise from nearby electrical fields.The V symbol is on. The BUSY light should not be on in the REP mode. If you use the SINGLE button the BUSY light will come on for each press of the button untill the reading is completed.QuoteI tested the electrolytics on the power supply board just to check if they are all OK. The power supply in my meter was working well within voltage and noise specs so there really isn't any need to change the caps.The other boards all use solid tantalum capacitors which are probably better than electrolytics that you can get today.I also checked the resistors and found one that was burned and open (low ohm resistor near TR204 and TR205). I still don't know what caused it to burn but I think it was a shorted flyback diode for one of the relay coils.Also be sure to check that none of the resistors and diodes are touching eachother since they are all mounted vertically on the main board.QuoteWhen measuring the resistors on the display board be sure to unplug the two plugs connecting it to the display driver board. It may give false readings. Those resistors are all 10% tolerance and are not very critical. The worst that can happen is that one of the digits will be slightly dimmer or brighter. The actual brightness is set by the resistors on the display driver board near the DI220 display driver ICs.I think it's nice that these rather obscure meters now have well documented teardowns. One year ago I couldn't even find the specifications!

author: SeanB
author_rank: Super Contributor
timestamp: March 24, 2012, 10:56:33 am
content: I have similar Panaflex displays from an old NCR cash register.

author: muvideo
author_rank: Frequent Contributor
timestamp: March 25, 2012, 06:04:18 pm
content: I washed all and started checking components, for now I found two reed relays faulty, oneopen (about 10-20kOhm) and another closed. Diodes and relays are ok.I don't know much about reed relays.For now I put two relay contacts scavenged from old PBX devices, the original part is a bit difficultto find, one is alma 18V 3012022-01, the other a bare reed contact.Will I have problems with these old reeds i put in, should I search for something better?Now the true moment will be powering up after assembledQuote from: tekfan on March 24, 2012, 12:10:51 amProbably later, many components have late 79 datecode.Quote from: tekfan on March 24, 2012, 12:10:51 amYes, that's definitely a goot thing.The 7075 is so different from the fluke 8520, they have about the same age, fluke seem very well built, has computingcapabilities, self test routines and documentation are a piece of art, but is full of selected or custom components,this solartron compared to fluke seem very simple, but it's specs are very high.The impression is that fluke used the "no compromise" approach, while solartron extracted the best from simpler design.Perhaps there were also patent problems, for example solartron used a Voltage to time adc converter while fluke a more obscureRecirculating Remainder converter.Talking about obscure meters, there is prema, seem that there is not much info around, but they sold somehigh end meters for some time (for example 6048 and 5017 resp. 8 1/2 and 7 1/2 digits).Fabio.

author: muvideo
author_rank: Frequent Contributor
timestamp: April 06, 2012, 01:59:45 pm
content: Here is a little update for the 7075, it was hard but I wonThe first problem after reassembling was a bad smoke out from the high frequency transformerAfter a little checking i fount a shorted coil in the transformer, so no choice, I had to open it up,I desoldered the wires, and heated the ferrite on a hot plate, hoping to not crack it.Fortunately the glue softened and it split open without damage.The coils are three:- primary- earthed secondary- floating secondary.each guarded with a copper foil.The earthed secondary was the culprit, opened it up and found that the wires were shorted onthe output, I must have caused the thin insulation fail moving the wires, so fortunately therepair was easy.I replaced some components (a pair of logic ic, one resistor, two reed contacts, and a pair of diodes).Finding the faulty logic ics was painful and time consuming.Some notes of interest:The converted value is represented by a pair of digital signals called GLUG+ and GLUG-.These are fixed fequency 1kHz, variable width pulses, the converted value is encodedas pulse width, GLUG+ is positive value, and GLUG- is negative.The logic sequence is very simple, roughly:- select an integration time- reset the counter and wait the first glug signal- start counting, GLUG+ makes the counter go up and GLUG- makes the counter go down- when the integration time expires display the result.The logic also checks GLUG pulse lenght and counter result to determine if the autorange system mustgo uprange or down range.Earthy logic is connected to floating logic by a simple interface, 5 optoisolated signals,tree going from Earty logic to Floating:- Clock 1.6MHz it's generated by a pll multiplier from mains frequency- serial Clock- serial Data this with it's clock transfer an 8-bit word that sets the working mode and rangetwo Floating to Earty logic, both 1kHz non overlapping PWM:- GLUG+- GLUG-The mode word is encoded as follow:bit 7bit 6bit 5bit 4bit 3bit 2bit 1bit 0(first)(last)M2M1Range4Range2Range1RatioSC2SC1M2 M11 1 Volt DC1 0 Volt AC0 1 Ohm0 0 Self CheckRange 4:1 (for dc volts, havent checked the ac and ohm)111 10mV110 100mV101 1V100 10V011 100V010 1000VRatio1 off0 onSC2 SC10 0 normal function or reference check0 1 Zero check1 0 10V AC check1 1 10kOhm checkIt wouldnt be too much difficult to interface a microcontroller to this, and extract data on a serial lineThis is the chopper amplifier glow, it's orange beacuse i had a some very brightorange leds and replaced the 30year old red ones.

author: muvideo
author_rank: Frequent Contributor
timestamp: April 06, 2012, 02:15:50 pm
content: Now I have a question regarding the reference:it's voltage is about right, but it seems strange, is this silicone? It seem opened and leacky.I measured the heater resistance and R380 that is in serieswith the heater, both are 100Ohm, tekfan have you checked theheater resistance value?P.S.My instrument PSU is diifferent from the schematic in the service manual(this one [url=http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/Solartron/Solartron_7075_Service_Manual.zip ),also R380, in series with the heater, isn't in the schematic.

author: T4P
author_rank: Super Contributor
timestamp: April 06, 2012, 04:50:35 pm
content: Quote from: muvideo on April 06, 2012, 02:15:50 pmThat's a polypropylene cap that audiophiles pay oh so much ( like 200$ for a cap , really ? ) .The normal "non-audiophile" ones doesn't cost even remotely like that .

author: muvideo
author_rank: Frequent Contributor
timestamp: April 06, 2012, 05:04:38 pm
content: Quote from: Dave.S on April 06, 2012, 04:50:35 pmIf you are referring to the last images,it's an ovenized zener, simply a zener with a resistance heater around,I found also a photo of someone that took it apart:From this discussion:http://www.radiokot.ru/forum/viewtopic.php?f=10&t=52829&start=60Fabio.

author: T4P
author_rank: Super Contributor
timestamp: April 06, 2012, 06:01:35 pm
content: ... Well i am very mistakenIt looks like that ... So sorry for the blunder , i didn't know they made such zeners ( that is oven stabilized )

author: tekfan
author_rank: Frequent Contributor
timestamp: April 06, 2012, 09:42:13 pm
content: Good to see the meter working once again.Looks like the white stuff that can be seen around the leads of the zener oven is thermal paste. If I remember correctly the heater resistance in my meter is around 60 ohms. I'll check it again once I have the time. It also doesn't have the resistor in series with the oven, so there are slight differences between old and new meters. And as you said the power supply is also different.I hope the power supply in my meter doesn't crap out any time soon. I quickly loose my patience when troubleshooting switchmode power supplies.As you said, it would be quite easy to program a micro to control the meter through the back multipin connector. I find the lack of manual ranging sometimes very annoying.I've seen a few meters on ebay which had a box with a GPIB connector that plugged into the big multipin connector. Sadly the schematic for that is probably lost forever.

author: muvideo
author_rank: Frequent Contributor
timestamp: April 07, 2012, 12:41:30 pm
content: I played a little with the oven.The series resistance permits to easily disconnect the heater circuit, so my curiosity couldbe satisfied.The heater is definitely a PTC resistance.The heater is in series with the 100 ohm resistor and both are connected to 0V and -21V rails.So I isolated the resistor from -21V rail and connected it to an power supply and checkedthe resistance in vivo, it started at 100ohm and stabilized to 420, i verified also the response tovariation of the supply voltage:I tried to estimate the working temperature, heating the case of the oven externally, andmeasuring the resistance. Heater resistance started climbing rapidly around 80°C andit reached 370ohm at around 87°C. Now I don't know real internal temperature, so maybethe internal ptc temperature was lower.Quote from: tekfan on April 06, 2012, 09:42:13 pmProbably you measured it when it was colder than mine, the value seems about right.QuoteThat transformer seem quite delicate, but the overall circuit is reasonably simple, it's a buck to 65V followed bya fixed frequency half bridge.When mine gone shorted, the buck went in protection and tha damage to components was very little, only somerectifier diodes easy to test.Anyway it's matter of personal preferences, I'd like to check 10 psu instead of 1 board full of logic icQuoteYou are right about the interface, but I had to disconnect one of the connector to the interface, connector "C" on top eartylogic board, when connected increases the noise of the readings.I was thinking a crazier idea:use a micro to measure and record the glug signals, with an good micro (i'm thinking of some stm32 boards I have around)one can use the 1.6MHz as pll timebase for the micro, multiply it x8 or x16, and use the internal timers to collect the glug data.One could collect statistical data on these signals to speed up the reading.

author: mianchen
author_rank: Regular Contributor
timestamp: August 20, 2012, 09:05:18 am
content: sorry to dig out this old thread. I've got myself a same unit off ebay for £29 shipped. it appears to be still alive, so I'll attempt to restore it to fully working when I have a bit more time. thanks for the tear down and link to the manual!!my question is where can I find a connector for the test probes?

author: muvideo
author_rank: Frequent Contributor
timestamp: August 20, 2012, 01:13:49 pm
content: Hello, the connector is a Fischer S 104 A053, it is shared by others solartronmeters, but usually the meters on ebay lack the leads.I built mine using old bnc connectors pins, the central pin mates well withthe receptacles, if I remember correctly they are about 1.3mm diameter.I think I've seen the auction but the delivery was high for me.Mine still has some problems but didnt had the time to find the cure.Fabio.

author: wiss
author_rank: Frequent Contributor
timestamp: July 27, 2014, 06:58:49 pm
content: (Should I start a new thread for this? If I reply here muvideo and tekfan will see my question)Hi!In the pdf-manual I have for the 7075, IC406, the chopper-integrator, is spec'd to be a AD506L (hard to read the text), I can not find a data-sheet for that one.On one of my 7075's it is an AD545J (Vos 25 uV/C and 3 uV LF-noise).This will affect the noise and stability of the input-amp, right?I can, in test-mode, see a AD-zero stability of +-1 count at 10 s integration, reference stability (in test-mode) seems to be +- a few counts.But the daily drift seems to be at the limit of the specification and the noise quite a bit more than the AD and reference-noise.How much noise to other see in their 7075's?Could an upgrade of IC406 to, say, OP07 help?

author: Vgkid
author_rank: Super Contributor
timestamp: July 27, 2014, 09:59:58 pm
content: http://www.datasheetarchive.com/dl/Scans-005/Scans-00101960.pdf1mv Vos, 5 pA ib max(2 typ). Jfet typ, 1Mhz, didn't see anything about it being a chopper amp.When looking for parts i would try looking for less bias current, low offset v will follow.Looked up wrong opamp, specs are similiar. EXCEPT AD545J has 2pA Ib max opposed to 5pA.http://pdf.datasheetarchive.com/datasheetsmain/Databooks-1/Book231-651.pdf

author: wiss
author_rank: Frequent Contributor
timestamp: July 27, 2014, 10:38:01 pm
content: Thanks!It is part of the chopper.Better than the 545 but op07 still seems like a better choice here, lower noise and drift

author: Vgkid
author_rank: Super Contributor
timestamp: July 28, 2014, 06:00:43 am
content: The op07 will have to much input bias current.pure jfet options:http://www.digikey.com/product-search/en?pv161=2&FV=fff40027%2Cfff800dd%2C1000001%2C1140050%2Cf380012%2Cf380016%2Cf38001e%2Cf380033%2Cf380036%2Cf380042%2Cf380045%2Cf380046%2Cf380047%2Cf38004a%2Cf38004f%2Cf38006f%2Cf380070%2Cf380073%2Cf38009d%2Cf38009e&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25everything elsehttp://www.digikey.com/product-search/en?pv161=1&FV=fff40027%2Cfff800dd%2C1000001%2C1140050%2Cf380012%2Cf380016%2Cf38001e%2Cf380033%2Cf380036%2Cf380042%2Cf380045%2Cf380046%2Cf380047%2Cf38004a%2Cf38004f%2Cf38006f%2Cf380070%2Cf380073%2Cf38009d%2Cf38009e&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25I excluded choppers(3 items), and instrumentation amps.

author: wiss
author_rank: Frequent Contributor
timestamp: July 30, 2014, 08:53:58 pm
content: IC405 is oscillating!I will start a new thread about my 7075's any day now

author: Vgkid
author_rank: Super Contributor
timestamp: July 30, 2014, 10:26:32 pm
content: I will look forward to it, the 7075 is one of my dream meters. I own it more modern little brother 7065, teardown soon.

author: tggzzz
author_rank: Super Contributor
timestamp: May 20, 2016, 07:09:52 pm
content: To save creating a new topic and referring back to this one, I'll try to resurrect this conversation.I'm pulling a 7075 apart, and I'd like to know how the OP dismantled part of it: the two central analogue boards. I can dislodge the top PCB, but I'd also like full access to the lower PCB, in case I need to replace electrolytic capacitors. The OP's picture "board_r.jpg" shows the two analogue boards still mated to each other, together with the wires to the back panel Lemo connector.How was the Lemo connector removed from the back panel while still leaving the connections intact?Are there any other similar cables, e.g. to the front panel Lemo connector?Are there any other useful tricks?Thanks.

author: wiss
author_rank: Frequent Contributor
timestamp: May 20, 2016, 09:10:25 pm
content: I never ran it disassembled, i measured the lower board from the bottom...

author: tggzzz
author_rank: Super Contributor
timestamp: May 20, 2016, 09:22:42 pm
content: I don't think it would be practicalor beneficial to run it while disassembled; my interest is in replacing capacitors, and that would be easier with the boards out of the frame.

author: wiss
author_rank: Frequent Contributor
timestamp: May 20, 2016, 09:29:48 pm
content: From memory: remove all screws, unplug all ribbon kables, pull out the chopper led contacts, unplug the 3 coaxes (if present), lift out upper board, done.

author: tggzzz
author_rank: Super Contributor
timestamp: May 20, 2016, 10:19:23 pm
content: Quote from: wiss on May 20, 2016, 09:29:48 pmI presume the coaxes are removed at the board end, and not by soldering (cf the delay line in Tek 465).I assumed the two PCBs should be removed as one unit, then separated, worked on, then reunited, and the unit reinstalled in the case. In my 7075 there are two sets of contacts, the chopper LEDs (B5-B8) plus four (B1-B4) near the short edge. The latter are completelybetweenthe two boards, which makes it bizarrely difficult to position them correctly when the bottom PCB is attached to the case and the top board is being lowered inside the case.I know software engineers can be, um, intellectually challenged, but I've usually found mechanical engineers to be a little more, um, realistic.

author: wiss
author_rank: Frequent Contributor
timestamp: May 20, 2016, 11:28:45 pm
content: Quote from: tggzzz on May 20, 2016, 10:19:23 pmOut of my 6 7075 5 have coax and there is a connector, on the 6th those signals are in the ribbons.QuoteI removed the boards separately.QuoteYou mount the boards first, then you insert those two interboard connectors, much easier.Quote

author: tggzzz
author_rank: Super Contributor
timestamp: May 21, 2016, 08:13:30 am
content: Quote from: wiss on May 20, 2016, 11:28:45 pmI don't see a ribbon for that, so I'll assume there is coax in the cable,QuoteOK, I'll file my fingers down to 3mm, so they will fit through gap between the upper board and case. Seeing the position of the B1-B4 is more tricky; I don't know of a camera that small, so I suppose I'll just have to use my x-ray vision or put my eyes on stalks.Whycouldn't they have provided the same inter-board connector as used for B5-B8 chopper? Sigh.More seriously, thanks for your time and advice!

author: wiss
author_rank: Frequent Contributor
timestamp: May 21, 2016, 11:56:52 am
content: Quote from: tggzzz on May 21, 2016, 08:13:30 amI think you're on to something nowQuoteAren't they very similar? It did take a few tries, make sure that they are perfectly straight first, and then it is possible to feel if they go in or not...More seriously, thanks for your time and advice!

author: tggzzz
author_rank: Super Contributor
timestamp: May 21, 2016, 09:21:46 pm
content: The important difference if that the b5-b8 connecting pins protrude above the upper PCB and can be removed when the two PCBs are still screwed together. The b1-b4 connecting pins are completely between the two PCBs and cannot even be seen until after the PCBs are separated, let alone removed.See the OPs picture of the boards.