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Eico 950A Resistance-Capacitance-Comparator Bridge

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Post by Guest July 28th 2019, 2:47 pm

Not being fully content with the circuit, results and performance of the late model EICO 950A cap checkers (high serial number), I've been thinking about a way to separate the power supplies for the 1629 eye tube and leakage tests. Stopping short of replacing the power transformer I came up with the idea of adding a small transformer just to power the 1629 tube triode plate and target circuits. The current demand is small-not likely to exceed 5 mA. The voltage can be anything from 100V to 250V DC. A small 125VAC 15mA power transformer could do the job. These are still around and not so expensive. Not having one on hand but wanting to go ahead with the change I used a 12.6V filament transformer wired up backwards so the usual 117V primary now becomes the new HV secondary winding and the transformer's 12.6V filament winding goes to the 12.6V filament connection of the EICO power transformer. I used a 1N4007 rectifier diode and 47uF 160V filter cap mounted on a terminal strip to complete the new 1629 tube power supply.

The HV power supply using the 6X5GT tube continues as before except that the cathode of the 6X5GT is grounded directly to the chassis. This puts the B+ at ground and the negative end of the HV supply goes to the output control pot as usual. In the EICO 950A a 1 meg Ohm carbon pot is used for P3 and since the B- going to it will be on the order of 720V I decided to replace the 30K Ohm series resistor with a 220K Ohm dropping resistor instead. This reduces the voltage the pot will see to about 550V DC. I also wired (2) 22uF 450V electrolytic caps in series for the HV filter cap. I used the same 220K 3W bleeder resistor in the HV supply as before. I plan to replace this resistor with a 5W 220K Ohm resistor soon as I can get one (5W film resistors are rated for 700V DC).

One note-I have noticed before that some makes of 6X5GT tubes are not up to handling the rather high voltage winding (565 VAC) of the EICO power transformer. In particular, National Union and RCA tubes with the overlapping plate construction are likely to arc over when the unit is powered up. I suggest these be avoided and prefer to use the newer GE style 6X5GT tubes. They can take it.

Well, the changes are working good and I like the results. Finally, one of my ideas pan out hi,hi.

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Post by FrankB April 3rd 2019, 11:22 pm

Thanks for the info on the 950B, esp . the removed resistor. that explains why one I have is missing it and does not match the schematic.
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Post by Guest March 31st 2019, 11:11 pm

Recently, I was successful in getting (2) good used RCA 1629 (VT-138) eye tubes at a reasonable price. I still needed one for the Eico 950A model (serial number 8086). The (3) used Soviet/Ukainian 6E5S tubes I had bought for it were a disappointment. (1) arrived with an intermittent open heater and the other (2) were too dim to use. At least they didn't cost much. By the way, weak eye tubes don't just get dim with age and use. They don't respond well to signal changes similar to that of old B&W and Color picture tubes. Anyone that has spent much time watching an old CRT TV with a worn out picture tube will know what I am talking about.

I did come up with a simple way to feed 6.3 volts to the 6E5S by running another heater/filament wire to the octal socket of the eye tube. Then I could choose between 12.6V or 6.3V to power whichever type eye tube happened to be available. A jumper from pin (4) to pin (6) on the socket took care of differences in the base connections for the target electrode of 1629 vs. 6E5S tubes.

I installed all new 3 Watt carbon film resistors in the HV power supply. I used 75K for R8 and 220K Ohm for R14. I had these resistors on hand from my Supreme 504A-B tube tester project. I installed a 4uF 350V cap for C9 and a 10uF 600V cap for C1. I choose the best and brightest 1629 tube for the Eico 950A. The results are very gratifying. To finish up, I installed some new stainless steel screws to hold the front panel to the case. They look very good and resemble nickel plated hardware.

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Post by Cliff Jones January 5th 2019, 9:08 am

I watched Mr. Carlson's Lab a couple of days back and He was demonstrating His new gizmo to replace old Cap checkers. He made a mention of being able to check leakage as low as 1 micro-amp.
So I was thinking why not just a flashlight battery and a Digital Meter in series? And of course a resistor to limit current. I do have several Meters, two of them are Fluke bench Meters Models 45 and a Model 8840A/AF and an older Fluke,8520A.

I will have to review my user manuals for each, but I think I could use a series resistor for voltage drop say 100Kohms to find the current or find a 10 microampmeter (Really hard to find or a galvanometer).

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Post by Guest January 5th 2019, 12:07 am

It is time to put out another (somewhat shorter) report on the Eico 950A capacitor checker project hi,hi. While neither one is completely finished, I do have two working Eico 950A cap checkers. The first one is the late model 950A I wrote about previously. It works quite well except that I'd like to replace the 1629 tube with a 6E5S. This  is a Russian version of the 6E5 with an octal base. The change will require a minor rewiring of the filament circuit as the 6E5S has a 6.3V filament as opposed to the 12.6V filament of the 1629. Also, the target electrode on the 6E5S is wired to pin 6 instead of pin 4 so a jumper wire will be needed.  

The second 950A is the early model that I mentioned previously. It is also working quite well, well mostly. The eye tube on this one is very bright and impressive. This 950A does a good job on the bridge/power factor functions and on the electrolytic leakage test. However, the leakage test for tubulars, micas, and ceramic caps leaves a lot to be desired. The indications seen on the eye tube as you advance the test voltage is at best confusing and inconclusive. The eye opens up a bit wider as you crank up the voltage and then returns to its normal wedge appearance if the leakage is insignificant. This slight change is supposed to indicate a good cap. If I had bought this instrument back in the early 1950s I would have sent it back to the factory or distributor ASAP for a refund. 

Well, what I saw in the early model 950A no doubt applies to the original Eico 950 model. Worse, the 950 uses the same circuit for checking both tubulars and electrolytics. Note that there is only one position on the function switch for leakage testing. Results must have been very poor.

73, Rod  WB6FBF

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Post by Guest October 24th 2018, 1:11 am

Thanks for the explanations on this power supply fellows.  I appreciate your time and help.

Garrett Fulton

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Post by Guest October 21st 2018, 8:45 pm

Garrett,
Yours is a timely question. I'll try to answer it to the best of my current understanding of the circuitry in the Eico cap checkers.  The circuit looks amazingly simple and yet ingenious especially in the then new redesign used in the later production 950A and the 950B model that followed. This circuit became the standard that all the other producers followed in their checkers yet it appears to go back to the earlier Jackson model 650 produced in the 1940s. If you can find the schematics for these sets take a look and follow along with me. 

BAMA has manuals on the 950 and 950B. Data on the 950A models is a bit hard to come by online at the present. I have seen a manual for the early 950A on photobucket but it doesn't seem to be available anymore-not in full anyway plus they have a lot of annoying ads on their site to deal with. You can find schematics for both the early and late 950A from doing some searches in the Test Equipment forum on the Antiqueradios website until you find both of them. By the way, don't be fooled by all the 950 models you may see on eBay-they are really 950B models. Why Eico labeled some of them 950 on the front panels I don't know. Incidentally, if  you read some of the posts on Antiqueradios about these checkers you'll see how a lot of operators have been having trouble with their units too. This is especially true with getting the right output voltage range for the leakage tests and also a nice, bright screen on the eye indicator tube.

I got my 950A back in August around the 22nd. I started working on it right away. I had been looking up as much info on it as I could before it arrived. I choose the 950A because of the neon bulb feature for electrolytic leakage tests. I liked how it had worked on demonstrations of the Heathkit model C-2 that I had seen. Getting back to your question, the power supply circuit in the early model 950A is fairly straight forward and easier to understand. A reading of the manual for the 950 and the early 950A shows that the indications seen on the eye tube when running a leakage test are quite different from that of the late 950A and the 950B which became the standard in all the other checkers. I just bought an early model 950A this morning, which after it comes, I'll be able to verify this and will make a report on it later. 

Back to the circuit: the 950 and early 950A have a conventional half wave HV power supply with the negative side grounded to the chassis. R8 is a simple (100K) dropping resistor that comes off the cathode (B+) side of the 6X5 rectifier/C1 filter cap connection. B+ also goes to R7 (30K 1W) and on to the high side of voltage control pot P3 (1 Meg Ohm). The output voltage of P3 is a positive voltage with respect to the chassis. So far so good. The leakage current, if any, is sampled by R3 (100K)  and is applied to the control grid of the 1629 tube thru the function switch. This is seen on the 1629 eye tube as it gives a reading by the opening and then partially closing of the eye during the testing of tubular, mica, and ceramic capacitors. Leakage looks like charging except that the eye tube does not recover. This is what I have mentioned as having been described as what to expect in the 950 and early 950A manuals. 

Now this all was reversed with the redesign and change in production after serial number 7500 on the 950A. The change looks simple but it is not. It was probably made to get a better, more easily understood closing and opening of the eye tube i.e. the information that the eye tube presented to the user during leakage tests for tubular, mica, and ceramic capacitors. Remember that electrolytic leakage tests were made by changing the function switch to the electrolytic position with the neon tube used as the leakage indicator in both versions of the 950A.

Now for the changes and the results: R8 was repositioned, changed in value to 47K, and is no longer a simple dropping resister to the 1629 tube B+. Instead, R8 goes to the chassis ground. The cathode of the 6X5 goes directly to the 1629 tube. A new electrolytic capacitor, C9 was added across R8 to the chassis ground. Looking at diagram, one can see that the 1629 tube is now in shunt with R8/C9 or parallel with them in other words. This arrangement is significant as it effects the B+ voltage that the 1629 tube sees. Increasing the value of R8 tends to increase the B+ (target and triode plate supply voltage) seen by the 1629 tube. Increases in 1629 B+ voltage reduces available B+ voltage for leakage tests. Another significant new component is R14 placed across the HV electrolytic cap C1. If the value of R14 greatly increases or opens up altogether the 1629 tube will go out i.e. there will be no green glow on the conical screen of the tube. In addition, I initially figured that R8 going to the chassis essentially grounds the B+ to the chassis far as the leakage testing goes. I still think this is the case. Remember that the current flowing in the leakage circuit is fairly small, in the microamp range in fact for tubulars and micas. So a value of 47K or 68K for R8 is not going to be problem. How R14 factors into the mix I'm not quite sure. But R14 is not simply a bleeder for C1 as in conventional power supplies. I mentioned before that changing the value of R8 will affect the B+ on the 1629 tube. This to be expected as they are in parallel. However, as is often the case, there is some interaction among the various components in the later 950A models. There is R8, then there is R14, also there is the current flowing in the target (anode) to cathode portion of the 1629 tube's CRT section. The triode plate current flow of the 1629 tube is quite small but keep in mind that the triode's plate is directly coupled to the grid of the CRT section of the tube. This is show on some circuit diagrams. I only mention this because it might be a factor in the overall function of the circuit. Now the target current can be 3-4 mA at 200 volts B+ according to the tube manuals. This amounts to a resistance in parallel with R8.  Different 1629 tubes appear to give different results as far as brightness is concerned. Otherwise, the opening and closing of the eye of the tube does not seem to be affected by the above factors.

When the change over was made to the 950B model, R8 was increased in value (to 68K) and R14 (220K) was eliminated. P3 was greatly reduced to 100K Ohms. R7 was also reduced to 3.3K Ohms. Since the total series combination of R7+P3 (103.3K Ohms) is essentially in parallel with C1, R14 was apparently no longer needed.

Today, I think it is fair to say that the new owners of these vintage cap checkers might want to try different values for R8, R14, and/or a selection of 1629 tubes to get good brightness and proper output voltage range if stock values do not do the job. Just beware of the possible interaction of these various components.

I think this is about as complete an explanation as I can give at this time. Hope this helps. More to follow.

73, Rod  N4QNX

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Post by Cliff Jones October 21st 2018, 8:14 pm

Garrett!
this is what I found close to a split ground explanation.
It includes several old brand Cap checkers.

Hope this Helps.
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A quick view of the power supply schematic, shared by the Knight, Lafayette, and most other cap testers, revealed why that would happen. The Eico uses a high voltage supply of about 700 volts with two electrolytics in series. That voltage is split into two parts with the chassis in between the two parts. One electrolytic has its negative terminal to chassis, the other has its positive connected to chassis. The load of the tuning eye tube and the 82K ohm resistor balance the voltages so that the eye tube (and the one cap) can function at a design B+ of about +200 volts relative to the chassis. The other cap sees B- at about -500 volts relative to chassis which is used for leakage tests and for reforming electrolytics. If the 82K resistor opens or goes high, the B+ on the tuning eye increases. If the cap seeing the most voltage shorts, as happened to my 950A, all that voltage is dumped on the remaining cap with the tuning eye seeing up to 500 volts or more which can destroy it.

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Post by Guest October 21st 2018, 3:33 pm

"I read somewhere that after serial [You must be registered and logged in to see this link.], Eico changed the design for the 950A power supply from a negative ground supply as used in the 950 and early 950A models to a split ground system where the negative end of the HV is floating or above ground. The chassis is positive as far as the leakage tests go."


Sir. could you try to explain this business of the split ground for someone who was trained in '68 in Navy avionics school.  I've been away so long that I can't understand it.  Thank you for your time and any help.


Garrett Fulton

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Post by Cliff Jones October 19th 2018, 9:14 am

I understand that the eye-tube will degrade just from ambient light. so if you have any bright ones be sure to keep them in the original box to shut out the light

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Post by Guest October 18th 2018, 11:28 pm

I have had the Eico 950A for a couple of months now and thought this would be a good time to submit a report on it. It arrived somewhat banged up-the inexperienced seller/packer having used no packing material at all. The 1629 Cathode Ray Tuning indicator tube was obviously damaged but not destroyed so I set it aside for the time being. I had a spare 1629 which I installed for tests. I found out later that the replacement 1629 tube had its own problems. It is common for these tubes to be rather dim after long hours
of operation like an old picture tube. By the way, the 1629 tube is an octal based version of the 6E5. 1629s were WWII surplus and low priced at the time.

The unit I received was mostly original except that someone had wired in a .5uF 600V bathtub cap in there for some reason. One 
end of it had been grounded to the chassis at some point and the other end I couldn't tell where it went. The cap had come loose and was sitting in the bottom of the cabinet after I got the panel unscrewed and pulled the chassis out. 

Looking at the power cord it, it was shot so I put a new brown cord on it. I've been getting these cords from Walmart. They are 8' long having been made for rewiring table lamps. 

Besides why the extra cap was in there I was trying to figure out the power supply hookup. There are two different versions of the 950A that have a major difference in the power supply that also affected the operation of the tuning eye tube. The circuit of the early 
950A version resembles that of the older 950 model. So far, I have not seen an example of the original Eico 950. Must be scarce.

I read somewhere that after serial [You must be registered and logged in to see this link.], Eico changed the design for the 950A power supply from a negative ground supply as used in the 950 and early 950A models to a split ground system where the negative end of the HV is floating or above ground. The chassis is positive as far as the leakage tests go. This made a difference for the better in how the 1629 tube indicated leakage during tubular, mica, and ceramic cap testing. If you read the instructions on how to interpret the 1629 tube indications for the 950 and early 950A leakage tests you'll understand what a difference it made when they came out with the later version 950A and 950B.

Well, as I was saying earlier I was a bit anxious about what version I had on hand. The serial number on mine is 8086-a fairly low number but well above the reported 7500 production change. The mystery bathtub cap didn't seem to fit in. Most of the circuit did look like the later 950A models except that C8 which shunts the dropping resistor R8, was missing. C8 was new in the later 950A models along with a change in value and hookup of R8. This was carried into the 950B models along with a change in the value of R8. A change in the value of R8 affects the plate voltage on the 1629 tube. The voltage drop across R8 also affects the available B+ for leakage tests. (In my case, after some testing, R8 went from 47K to about 1 Meg Ohm and the plate voltage on the 1629 rose to 300 Volts.)

Well, I finally had to conclude that someone had removed the original C8, listed as a 4 uF electrolytic cap in the schematic (rated 150VDC in the 950A and 250VDC in the 950B), and used the .5 uF bathtub cap as a replacement. We'll never know why, hi, hi. 

As a final comment, I found that the value of R8 does affect the brightness of the 1629 tube especially on weaker tubes. 

Well, that's all for now. I'll follow up in the next installment.

73, Rod N4QNX

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