149 - Very large and very broken: The Wayne Kerr B642

hi there I bought this giant monster here because of its pretty spectacular specs and it was not expensive because it's broken and was sold for parts it's a r car auto balancing Universal bridge b642 this thing is basically an LCR meter a bit like my Marone TF 2700 but quite a bit more advanced and possibly easier to use judging from the manual the right side is called C term and for capacitance and inductance while the left side is called G term which is the resistance and conductance part so t

his instrument can measure resistance and reactance at the same time which means it's really measuring impedance and the auto balance in the name means it can be used to look at changing values here are the specs that convinced me to get it even if it's just for looking and maybe harvesting Parts capacitance from one fto to 10 fars inductance from from 1 Nano Henry to 10 mega Henry resistance from 10 Micro ohm to 100 GM wow that is amazing I've never seen any claims like that by the way the rang

e with a cute upside down Omega is conductance G which is the reciprocal of resistance it was called Mo which is the reversal of ohm but we call it nowadays cement so 10 Pico cement is the same as 100 G ohm and 100 kilo cement is the same as 10 microohms okay the next part tones the specs down a bit if you want 0.1% accuracy the ranges are more like what you would expect one picofarad to 10 microfarad 1 M Henry to 10 kiloh Henry 10 ohms to 100 Meg ohm but hey 0.1% accuracy is actually quite a cl

aim by itself and not to be sniffed at then we have a table with the actual range settings this table presents what the unit will show you directly in the ranges from 4 to 10 with it G and C terms this is actually for complex impedances with capacitance and resistance Parts but of course we can use it to measure just capacitors or resistors the effect is that for a good capacitor the G term will be nearly zero meaning very high parallel resistance while for good resistors the C term will be near

zero the range for capacitance is from 10 microfarad down to 1 p farad and the first dep column shows the resolution so for example in the Range Five you could measure a capacitor between 100 nanofarad and 1 microfarad with 0.1 nanofarad resolution to 0.1% accuracy reciprocal values for G are 10 ohms to 100 Meg ohm and in the same Range Five the value could be between 100 ohms and 1 kiloohm the fact that it reads conductance and one has to take the reciprocal to get ohm HS is easy today but in

days before electronic calculators more of a headache I imagine ranges one to three are for direct readout of more low impedances but you can use it for measuring inductances from one microhenry to 1 mli Henry or 10 mohms to 10 ohms but all only with 0.3% accuracy unsurprisingly this range requires to use four wire connections because of the low ohms and even then the manual explains that the correction Factor has to be applied to account for the wiring of the bridge itself the last table is the

one that surely caused the most headache when this machine was used the reason is that to measure these resistan and inductances you need to do more maths resistance is straightforward namely taking the reciprocal of the displayed conductance for the inductance you have to use this formula to convert the C term Omega is of course 2 * pi * frequency cunningly the built-in generator uses 1 15915 Hertz that makes Omega 10,000 so to convert one has to take the measured C term multiply it with 10,00

0 squared or 100 million and then do the reciprocal of course if you use external frequencies then Omega needs to be calculated for each case the maintenance manual has a block schematic and a lot of the secret source is in these complex Transformers on a high level we have an oscillator on the left with a magic frequency of 1, 15915 Hertz to get a nice and easy Omega of 10,000 that sine wave is used for measuring voltage and current through the unknown impedance at the same time the sine wave i

s formed into two square waves with 90° face shift as references to the G and C detectors I have seen more modern LCR meters to do away with the Transformers but kind of us use the same principle and do the needed math for each read out in a processor so much for the theory of that thing let's see what I received and maybe it's even repairable and we are off to a very bad start the class of the C meter has been pushed in and squash the needle onto the scale that met with its minus1 to + 10 scale

is not something found easily the G meter at least is dirty but seems okay the needs a clean but it does not look too bad there are two outputs to connect plotters for C and G but can also be used to drive two dvms if these work this is excellent news because that means the broken C meter could be replaced by a DVM read out from this port apart from the DC output we have a screwdriver operated switch to use internal or external frequency and some input bncs on the right is the voltage selector

set to 200 to 250 volts which is fine for the UK a fuse and a very ancient three pin socket this is the same as the one I found in the last video on the mystery high voltage box finding a matching plug or power cord will be tricky the cover is held by these four spring-loaded latches that need just a half turn to disengage pulling the cover off reveals a solidly built inner chassis did I mention that this thing weighs a ton the rear is held by screws in the upper Corners that are not the origina

ls because they are the wrong size and very loose the screws someone used here are metric and this machine obviously uses Imperial standards the rear falls down and I'm strongly reminded of old style TV repairs by the dimensions and build I also don't like the look of a half empty PCB the next Revelation there's no power supply just a cut cable where the power supply used to be another look at the insides of the machine it is hard to convey how big this thing is and how much space it takes on th

e bench a bit of good news I checked the service manual and it appears that the empty spaces on the PCB are normal so at least on first clance nothing was removed the same cannot be set for the power supply which is AAL it should be on the rear panel right above the power Inlet but only the hex standoffs are left so much for me woring about a matching power cord it appears instead of a power supply the previous owners added some termal blocks probably to keep the now often power supply wiring ti

dy that loose cable is in fact connected to one of the terminal blocks so it's something that the previous owner added not something left over from the original the power Inlet connector still has its ground and life fires attached but neutral has been cut off two caps provide some filtering testing the the connectivity of the black round wire of the power Inlet connector reveals that it's connected to all the black wires on the two terminal blocks I summarize the testing of the conductivity of

the red life wire it is connected to the fuse and from the fuse to the voltage selector and it ends there it is not connected to anything in the termal blocks even though it uses exactly the same thin red wire that is used there also not shown the red life wire from the power Inlet goes first to the onoff switch on the front and then to the fuse a safer way would be doing it the other way around anyway as this is all that's left of the main wiring it means the terminal block wiring must be outpu

ts from the now missing power supply this is confirmed in that the red wires in one of the terminal blocks are connected to the plus 9vt input of the PCB and the purple wire in the same termal block is connected to Theus 6V in put on another PCB this leaves the question where the red wires of the second termal block go when I suspected the indicator lamps there are nine indicator lamps in this meter and the easiest accessible ones are the ones for the sensitivity switch in the Middle with one PP

E connected to the termal block I check conductivity and yes this is the plus 200v input for the Neons now that I understand what power goes where I could try powering this externally but before that the broken C meter has to be fixed or at least disconnected I struggled for some time because the area is so packed that it's hard to get a proper grip on the ring securing the meter at the end I did manage without breaking anything including my fingers off camera I got the glass out and bent the ne

edle back so it doesn't track over the scale anymore and amazingly the meter still works pretty well I tried to open it to glue the glass back on but so far I have not found a way to open it nondestructively I connected a 200v power supply temporarily to the second terminal block and turning just the 200 volts on nothing else the lights come on the upper ones indicate the sensitivity of the bridge maybe one can live without these working the lower ones are driven by the range switch and indicate

where the decimal point of the readout is these patterns look pretty complex without working lights one would need to always reference a lookup table so if I get this bridge working making the lights work too is probably worth doing I attached 9 Vols for my bench power supply which should be enough to get the sign generator working it's an oscillator known as a wian bridge there's an exhaustive article about it in Wikipedia and I refer you to it sadly I could not detect any type of sine wave si

gnal coming out of this Circuit by the way in case you wonder that thing that looks like a small tube or valve is in fact a special thermister similar to a light bulb to stabilize the oscillator output the circuit diagram of the oscillator of note is that this circuit works either as an oscillator when the in X switch on the rear is set to in or as a buffer amplifier for an external frequency input when set to X so a problem in that switch or wiring could be the reason for not seeing any oscilla

tions a closer inspection of the INX switch and Associated wiring and indeed one of the tabs had broken off at a switch I managed to soter the tab back on the failure mode of this tab is quite obvious originally the cable must have been sticking out slightly forcing the tab to bend every time the rear panel was closed or opened until the tab finally broke sadly that broken tab did not fix the oscillator but by chance I noticed that one of the transistors was wobbly all transistors are mounted on

some black holder so you can't see the legs but it feels as one of the legs of the VT 26 is broken there was no indication of anything wrong on the underside of the PCB and they folded the legs making it quite hard to desolder but one of the legs had definitely brok earlier before my desoldering the transistor itself a BC 109 tested okay I debated putting it back in without the black disc as the legs were too short but in the end I decided to replace it with a new BC 107 B for my parts spin but

still decided not to use that black holder disc and yay the first sign of life in this old machine the oscillator is working but not quite at the right frequency it should be be 1 15915 HZ but it's only 1,578 HZ I worry about that later to test more I need to be able to power this thing while the rear is folded up to have more space on the bench so I probably added what the previous owner did namely an extension to connect the + 9 Vols and - 6 Vols from the outside optimistically I connected a

test resistor via BNC to alligator clips to be measured but not much is happening at least I can see there's a signal on the resistor and it's changing with range so that's encouraging but nothing of the rest works I did not connect the 200 volts so that's why there are no lights driving this thing from two external power supplies is already complex enough so the oscillator signal is at the test object and then it gets lost apparently if you remember from before the signal from the probe has has

to go through a feedback amplifier before it reaches the face sensitive detector and instruments so I traced the signal and I did video that but I simp to have somehow lost those files sorry I could see the signal in the scope getting into vt8 but nothing came out on vt12 it was still at c17 measuring diode Junctions across transistors quickly narrowed the problem down to VT 29 another BC 109 I desoldered it and it was another Case Of Broken pin with a transistor actually fine I replaced it wit

h another BC 107b and now the signal got through here's an image of the board after I fixed it v29 is the new transistor without that silly black disc I wonder if these mounts are actually stressing the legs somehow anyway with that the GM meter showed some movement but not the the C meter the phase sensitive detector is the board left from the wian oscillator it drives the meters and also contains the square wave reference generators I can see a nice Square wave on the Square wave generator of

the G term but nothing on the one for the C term clearly there's something wrong on this board too the circuit diagram shows that the G and C parts are nearly identical which is very nice because it allows me to compare measurements between the G part that works at least to some extent and a c part which doesn't sadly I came to the conclusion that the amplifier chip for the C term was defect that chip an SL 702c is a strange Beast not your standard op amp and certainly very vintage luckily I was

able to find it on eBay as New Old Stock I had to buy two for a total of £9 but okay at least I got a spare okay compared to the G term chip my replacement install looks decidedly sloppy but I just wanted to see if it works and TIY later it's a strange way of installing having it floating on pins but at least I could do it all from the top without having to disconnect everything to remove the board and it works very well a beautiful Square wave now on the C term as well I was a bit worried beca

use with eBay chips especially with New Old Stock you never know but it looks similar in amplitude and shape as the one from the original chip for the G term and now at last I can actually follow the setup procedure in the manual that one has to do at the beginning of using the bridge I even rigged up a third power supply for the 200v light circuit for this occasion with a range set to four I turn the sensitivity to three which is the highest setting then use the trim knobs for C and G to zero b

oth meters first a c term and then the G term the two May slightly influence each other but it seems to work just fine and then I set the sensitivity to calibrate and adjust the C meter to full scale that is 10 using the FSD knob and adjust the G meter to zero using the zero knob again the two adjustments slightly influence each other and may have to repeat these steps there now the sensitivity switch to Auto and the instrument is ready connecting a 100 nanofarad capacitor that are previously me

asured to be just over 90 nanofarads and now I'm supposed to increase the range until either meter shows more than one the C meter wins showing nine the next step is to set each of the first knobs of each decade control to the corresponding value the meter shows for G that is zero because it's less than one and for C that is N9 now the meter shows the second digit and sensitivity has increased automatically to two if more accuracy is needed repeat the step set the next digit on the G term to zer

o as well and the next digit on the C term to one and the sensitivity is now automatically increased to three the Met show now the third digits and so on you get the idea one can stop at any time if the needed accuracy is reached we have a g term of 00 4.5 micro cement equivalent to 222 kiloohms and a c term of 91.6 nanofarad excellent this bridge is back in business in playing some more I discovered some more FS courtesy of the previous owner the decayed knobs are not identical and some in the

C term are obviously wrong like the one on the most left in this picture which is actually the second digit clearly there are no markings behind the minus one even though this rotary switch has more positions the most right knob controls actually a potentiometer not a rotary switch and that has only a 270° range so these two knobs have been accidentally swapped and I need to undo this by taking the cap off off losing the nuts until the knobs can be pulled off swapped and then tightening everythi

ng again without losing the red zero position which is essential for the working of the bridge so the second decade now has a digit read out for every switch position and the knob for the potentiometer now doesn't have a digit read out for position that can't be reached but then I noticed that the most left knob remember that's a second decade switch because there is one more to the the left should really have a 10 position at least the procedure in the operator manual assumes that it has so the

left and middle knob here have to be swapped too while the maintenance manual Wars that the knobs for the decades are not interchangeable they did not bother to print a little reference that tells you which knobs goes where I bet that accidentally swapping knobs producing wrong readouts was a common problem with these Bridges at least the G term seems to be okay and now the C term knobs match the order of the G term as I mentioned before this bridge has DC outputs for the meter readings these n

eed to be calibrated so that when the meter shows 10 you get one volt to do that I need to remove the protective caps of the trim pods and of course connected DVM that was no issue for the C term output but I was not able to pluck anything into the G term output on closer inspection the reason why nothing can be plugged in is a broken of piece of banana plug stuck in the socket this bridge really must have had a hard life considering all the problems I found so far that stuck plug could have bee

n an issue forcing me to replace the socket but it turned out because the broken plug was a holot type it was easy to grip and pull out we have come a long way the next thing is obviously to replace the missing power supply adjust the oscillator and maybe Place some more with the bridge but that will be in the next part if you like my videos don't forget to subscribe there are many more projects repairs and reviews coming up and it would be great if you decided becoming a patreon that would real

ly help this channel thanks for watching