SUPER HEATH C-3 REBUILD KIT TECHNICAL DETAILS This rebuild kit designed to work with the components supplied. No changes should be made to the circuit. The 4.7uF 250V capacitor for the +180V eye tube may be damaged if you do not replace the 47K resistor with the zeners. Please read this document several times to obtain a good understanding before starting the modifications. The changes are highlighted in YELLOW on the schematic printout. It is suggested you perform the power supply modifications before implementing the amplifier upgrade. Obtain a comfort level operating the instrument with several known test capacitors after the power supply modifications. This will allow the best evaluation of the later amplifier modification. More is not better. Increasing amplifier gain above three can render the instrument almost useless unless you have a specific application testing good capacitors. The capacitance of some high leakage capacitors can not be measured. A number of extra parts are included that can be used for other experimental modification or used in other projects. Use an ohm meter to verify the value before installation. Color codes are getting harder to read because of the variance of color and some resistors may be the four band 2% type. TERMINAL POSTS The CENTER connector is POSITIVE when a LEAKAGE TEST is performed. It is also the terminal closest to the case ground potential. This defies what you would normally think and it is not marked on the case. It is suggested that you replace the center connector with a red one to act as a reminder of the polarity if you have one handy. Some HEATHKITS have red and black while others are just black. A reverse voltage on an electrolytic capacitor will show excessive leakage and may damage the capacitor. DANGER!!!!!!!! In circuit leakage tests are dangerous. The far right terminal can be up to -450V DC when a leakage test is performed. That means the metal case of the capacitor can is at a high potential. That also can mean the chassis of the device with the capacitor under test can become live at that potential to ground. LEAKAGE TEST Leakage Test can be performed at 25V, 150V ,250V, 350V and 450V. The 25V range is actually about 45V without a load. This appears to be by design to provide enough voltage for the eye tube to sense and compensates for the large series resistance of the voltage divider when testing electrolytics with leakage. KNIGHT KIT has the identical circuit and parts, but they are more honest and label the test voltage as 50V. If you test a lot of low voltage capacitors you should consider modifying the last two resistors of the divider. Remove the 10K resistor and replace it with the 18V zener (band to chassis) and a 2K resistor in series. This 2K resistor raises the voltage slightly to about 25V and prevents a gigantic surge when the voltage is switched from 150V to 25V. This results from the discharge of the .25uF (.22uF) through the lower 18V zener. This can burn out the rotary switch contact over time. See FIG1 for this wiring detail, SC3.pdf and FIG2 for schematic. If you want a lower test voltage of 18V, move that 2K resistor in series with the voltage selection wiper wire either at the rotary switch or power factor switch. NOTE: Some low cost DVMs have a 1Meg input resistance. Measured test voltage will be less than actual test voltage because of the voltage drop of the 220K resistor going to the .25uF filter capacitor. A second 18V zener is added in series with the next 22K resistor for the 150V range. Again the band should be going in the direction of the chassis ground. This 18V zener increases the 150V range back to normal voltage and returns the total divider current back to normal. Normal in this case is about 4.5 mA and is at the extreme end of what the 22K 1/2W resistors can tolerate. Switches can be delicate. It is easier to just clip the resistor leads close to the switch. At that time, it was taught to wrap the leads tightly and then solder. You risk mechanical switch damage if an attempt is made to remove the old component wires completely. They also sometimes used a higher lead content solder back then that had a higher melting temperature. Adding extra new solder and then removing the excess will thin out this old solder and leave a better finish. Eye closes at about 9 micro amps (4.2 mA with Power Factor on for elctrolytic test), eye sensitivity can be reduced by switching to a lower test voltage. Be sure to allow enough time for the internal and external cap to charge through the decade resistors and 220K. Electrolytics should be tested with the Power Factor pot switched ON. Be sure you switch it back to the position labeled Paper & Mica for smaller caps). This switch changes the current sense resistor from 470K to 1K because electrolytics normally have higher leakage. It also shorts out the 220K resistor in series with the test voltage switch for faster charging. Consider changing the 1K resistor to 2K to increase sensitivity 2.1 mA (2.4K = 1.8 mA). The later design of the HEATH IT-28 uses 2 mA for electrolytic leakage tests and the EICO 950B uses a 2.2K resistor (2 mA). Leakage currents in excess of one half mA make the selected test voltage meaningless because the resistance of the divider results in a considerably lower voltage. This current sense resistor, switch to ground is shown in the upper left corner of FIG3. Voltage is only applied to the terminals when the spring loaded switch is turned to LEAKAGE. There may be lethal voltages present at these terminals and current is not limited to safe levels. DANGER The far right terminal will have the high voltage potential to ground and is NEGATIVE. There have been comments that when large value & capacity electrolytics are tested for leakage, the dial pot may be damaged when the capacitors charge is dumped when the Leakage control is released. The suggestion was made that the dial be turned fully counter clockwise before performing a leakage test. Mid position would be easier to remember. I haven't seen it, but the practice couldn't hurt. If you want to protect against the possibility of forgetting, a 120 ohm 2W resistor is included. Connect this between the ground lug and the wiper terminal of the 10K pot. This reduces the surge current to a safe level that the 10K wire wound pot can handle. There is normally no current in this path and the resistor will not alter the calibration of the dial. ACCEPTABLE CAPACITOR LEAKAGE CURRENTS Small paper and mica capacitors should have leakage currents under 2 uA and miniature electrolytics 15 uA. Large electrolytics can have several milliamps. A general rule is to calculate the current in milliamps by the formula: I = (K x C) + .3 C is in uF and K varies by rated voltage. K(100V) = .01, K(200V) = .02, K(300V) = .025, K(450V) = .04 For a 40 uF 450V, the calculation would be (40 X .04) + .3 = 1.9 mA. That leakage may be a little high for modern electrolytics. Consider a current sense resistor based on the capacitors you test most often or add a switch. The 2K resistor is a better choice since the voltage divider can't handle that current. The IT-28 uses 2 mA for the high current range. Those with higher leakage current can be tested at a lower test voltage. POWER FACTOR Electrolytic Power Factor pot dial reads from 0 to 50%. This pot, 800 ohms or 1K, is in series with the 2uf reference cap (.1 to 1000uF, two upper ranges only) to add in an equivalent series resistance to the reference cap in the bridge. When the capacitance has been found, rotate power factor for widest opening or cleanest image of the eye and read the Power Factor. If the eye doesn't open move the pot to about 1/4 position and try again. Power factor is in % at 60 cycles. The equivalent series resistance would be ESR = Decimal per cent divided by (2 Pie x Frequency x Capacitance x .000001) A 2 uF capacitor at 6o Hz with a Power Factor of 5% is ESR = .05 divided by ( 2 x 3.14 x 2 x 60 x .000001) = 66.3 ohms. Once upon a time, a power factor of 50 may have been valid. If you have the components, change the 800 ohm POWER FACTOR pot to a more useful value like 200 ohms and make it a 10% control. Do you really want a capacitor that has 200 ohms or more of series resistance? Remember that the smallest resistance you can likely get with an 800 ohm carbon pot at the zero setting is 30-40 ohms. If the resistance in the OFF position is more than 25 ohms, add a resistor in parallel. This resistor is shown as RP in the schematic SC3.pdf above the Power Factor pot. Start with a 750 ohm resistor. Some earlier C-3 units had wire wound pots for PF and they actually do go to near zero resistance. With the better filtering of the power supply, a dual switch on the pot may not needed. A pot with a single switch may be all that is needed and is a lot easier to find. Try substituting a 36K resistor in place of the 220K and ignore that switch connection. Using a single switch to operate a relay is another possibility. The added sensitivity of the amplifier modification may cause the eye to never fully open on the highest two capacitor ranges if the pot can not obtain a resistance lower than 40 ohms. In that case a 100 to 750 ohm resistor RP in parallel with the PF pot will make it operational for lower ESR values. RESISTORS Heath used resistors in the Leakage Voltage selector (4 X 22K) as a load for the -450V power supply section. The half watt resistors were operated at .45 watt, close to the power limit. If the unit had been left on for long periods in its past, these resistors could have changed values. Resistance measurement to ground should be about 98K on the -450V line. The 22K resistors, if defective, can be replaced with the 27K resistors supplied to reduce the dissipated power of each one to 0.36W. The 47K load resistor is replaced with TEN (10) 18V zeners in series to produce an eye tube supply of +180V. These TEN zeners are indicated as Z1 through Z10 on the schematic SC3.pdf, FIG5 shows an example of the zener string mounting above the chassis. RECTIFIER TUBE HEATH used many parts from the surplus market. These were cheap and were generally from military sources. Designs were often a little oddball in order to use these parts. The C-3 uses a 1626 triode as a half wave rectifier. The rectifying function of this tube will now be performed by three 1N4007 diodes in series to achieve a very conservative PIV. An additional 270 ohm wire wound resistor is in series is to simulate some voltage drop of the tube and limit inrush current. See lower section of FIG2. Solid state rectifiers have no warmup period and voltages on the power supply capacitors may rise above safe levels until a tube warms up. The zeners on the eye tube cap prevent this from happening and must be used.. The 4.7uF capacitor will over voltage for a short period of time if the 47K resistor is not replaced with the zeners. All the connections to the 1626 triode will be removed except for the filament pins 2 & 7. The 1626 then can be used as an amplifier tube to make the bridge more sensitive. Schematic of power supply is FIG4. MAGIC EYE TUBE The eye tube plate in the factory C-3 typically operates on approximately 160V DC. Going to a solid state rectifier and TEN 18V zeners in series increases the voltage to 180 which helps out many of the old eye tubes. An extra zener is included to increase the voltage to almost 200V for very weak eye tubes. The eye tube is rated at 250V and many technicians have increased the supply voltage to as high as 600V without problems on weak tubes to keep them useful. Don't go over 200V unless you are about to spend $20 for another tube. Increased voltage will wear out the tube faster. Exposing the tube phosphor to direct sunlight will also shorten it's life. Something to keep in mind if you have other magic eye equipment. A weak eye tube from one instrument I own with a 150V supply was put in another piece of equipment which had a 250V supply. This produced acceptable brightness of the tube. EYE TUBE SUBSTITUTION The 1629 magic eye tube can be replaced with a 6E5 with changes to the filament supply. These tubes are becoming scarce and there are several types that may be substituted. A 1629 is 12 volt 8 pin tube and the same as a VT-138. The C-3 can be modified to use the 6 pin 6 volt 6E5 by changing the socket and installing voltage drop resistors. Two 10 ohm 2 watt or four 4.7 ohm 1 watt resistors in series will drop the voltage in half to produce the 6.3 volts needed for the tube. It will take slightly longer to warm up as the filament resistance increases. 1629 TUBE 6E5 2 FILAMENT 1 7 FILAMENT 6 3 1 MEG 2 4 PLATE B+ 4 5 GRID 3 8 CATHODE 5 10 MEG POT ADJSTABLE GAIN The optionl 10 meg pot can be used in place of the 6.8 meg resistor at the grid of the amplifer tube. This option is only sugested if a gain of greater than three is used. The wiper of the pot is connected to the grid. The ground connection of the pot may be connected through a 670K ohm resistor so the signal gain never drops to zero. Excessive gain can prevent the user from seeing the eye open with a leaky or high internal resistance cap. An alternate method to reduce the tube sensitivity is mounting a normally closed switch in the back of the cabinet. This switch would be in series with the signal to the amplifier tube with an additional 6.8 meg resistor in parallel with the contacts. This would operate much like the beam finder push button on a scope by reducing the gain in half. POWER SUPPLY The 500V AC secondary of the transformer is floating above chassis ground. One end of the secondary winding is connected to the -420V power supply caps. The other winding is half wave rectified to produce the +180V DC for the eye tube. The eye tube needs about 1 mA of current to operate. The 300K resistors in this kit modification serve to drain off the capacitor charge when the unit is turned off and provide additional load. The 98K resistance total of the Leakage Voltage selection switch form the voltage divider with about 4.4mA going through it. FIG3 shows the component placement and FIG4 shows the schematic of the power supply. No changes are needed if the 22K resistors are replaced with 27K resistors. The old -450V supply capacitor is replaced with two 220 uF 250V capacitors in series. This creates an equivalent withstand voltage of 500V in the Leakage Test supply section. The two 300K resistors across each cap to balance the voltage evenly on each cap. An additional 300K from the -420V power to ground provides an additional load for more stable voltage. These 300K resistors look small, but they are rated at 0.9W. See FIG2 and SC3.pdf for schematic with more details. There is a terminal strip on the bottom of the chassis under the eye tube. This terminal strip serves about the same purpose as having to add eggs to an instant cake mix. This just gave the kit builder something to do and feel like they were really doing something. If you run direct wires to the eye tube, this terminal strip is not needed. Also remove the ground lug attached to this mounting screw so it can be used above the chassis to mount the zener string. The 220 uF 250V capacitors fit nicely in these terminals. Rewiring the line voltage terminal strip will also provide a convenient center terminal for one of the high voltage secondary transformer wires and the 270 ohm current limiting resistor. The See lower right of FIG5. The 270 ohm current limiting resistor can be placed in series with either lead of the high voltage winding. The three diodes that rectify the high voltage were placed in heat shrink tubing on the other high voltage lead. See FIG6. This eliminates the need for an extra terminal strip if you don't have one. Note, the schematic shows this resistor as 300 ohm. The capacitor for +180V eye tube supply is 4.7 uF 250V. A 300K resistor is placed in parallel with the cap to provide bleed off since the zeners won't conduct once the voltage drops a little. The clip for the 2uF reference cap holds the capacitor securely and the single terminal strip isn't needed. This terminal can be used for +180V supply to mount the 4.7uF and the 300K resistor. 180V ZENER REGULATION A string of 18V zeners in series is used to regulate the +180V eye tube supply instead of the 47K resistor in the eye tube supply. The recommended number is a string of 10 zeners in series (Z1 through Z10), the band end goes to the eye tube pin 4. This number can be increased to 11 zeners for weak eye tubes (200V). These can easily be soldered together in a string of less than 8 inches. This can be soldered to a lug above the chassis that mounts the transformer on the left side. Include the ground wire for the eye tube with the zener string. Then heat shrinking tubing on the zener string and wire will provide a lot of rigidity. See FIG5 for a wiring example. The outside covering from the new line power cord being installed can also serve this purpose. 0.25 uF CAPACITOR This capacitor performs an important function of filtering the selected leakage test voltage in combination with the 220K ohm resistor. If the old .25 uF capacitor is leaking, the test voltage will be lower than expected when a Leakage Test is performed. Be sure the case connected lead of the capacitor goes to the chassis. Without this capacitor, any power supply AC ripple is fed directly through the test capacitor to the eye sense resistor. This AC ripple current would make a good capacitor appear to be leaky. The improved power supply will have less ripple and will allow increasing the 1K current sense resistor to 2K for newer capacitors with less leakage. 2 mA leakage sensitivity is more appropriate and allow you to spot capacitors that will have future problems. Even when the Power Factor switch is closed there is always at least 22K resistance between the negative power supply and the .25 uF capacitor to provide some filtering. A .22uF may also be supplied as a replacement and will work just as well. 2.0 uF REFERENCE CAPACITOR This capacitor may be supplied in a variety of forms from a single capacitor to two or three in parallel. If the capacitor is in a metal case, it must be insulated with plastic tape. While the capacitor is insulated, the metal case will have a some stray capacitance that can affect readings on the low pF range. Insulating the metal capacitor with tape will greatly reduce this effect. Check with an ohm meter to verify that the capacitor case is still isolated after installation. It is important the this capacitor have a very low ESR value. All capacitors supplied have been individually tested to verify capacitance and ESR. All capacitors are not alike. The 2uF supplied is significantly better than the new capacitor that Heath supplied with the kit. 0.02 uF REFERENCE CAPACITOR In some cases this capacitor will not have to be replaced unless it is a very old wax paper type or known to be defective. Make sure you use the selected capacitor with the closest value to .02uF if more than one .02uF capacitor is supplied. This capacitor may be made up from two .01, a single .02 or a selected .022 uF. Manufacturing of capacitors lately has become a precise endeavor. When you buy a +-10% capacitor today, it is generally always on the low side of the specification. A little foil saved on each capacitor makes extra capacitors they can sell. LINE VOLTAGE MODIFICATION The .05uF cap from the line hot to ground should be removed for safety reasons and to prevent possible tripping of GFI breakers. Capacitors to the chassis of the value Heath used do not conform to new UL leakage standards. Remove the existing capacitor to ground. An alternate wiring with a 0.1uf type X2 UL line rated capacitor is made across the primary of the transformer, Line to Neutral. The added fuse is rated at one half amp and should be placed before the switch on the hot side of the line. You can mount this just under the eye tube using the screw that mounts a terminal strip. See FIG5 for mounting. The BLACK wire (BROWN if IEC code) is hot and should go directly to the fuse holder. Strip the cord long enough so you don't have to use the center terminal that needs to be used for the high voltage supply. Making a cut around the cord at about 8 inches will provide a plastic sleeve for the zener string. Before you wire the power cord in, make sure that the plug end will go through the hole of the cabinet. About half the new power cords have a plug that won't fit through the case. Use a ty-wrap to secure the cord from pulling out the grommet. Grounding the case with the 3 wire power cord will make the readings less temperamental by reducing noise pickup. The HEATH manual had you reverse the non polarized power plug for the lowest noise. RUBBER FEET The old rubber feet should easily break off. Pull the new feet through the mounting holes while moving the tang from side to side to seat in the groove. I've never had any trouble, but WINDEX is a good lubricant for installing rubber parts. The extended tang may then be cut off to make installing the chassis easier. If your unit doesn't have holes, the Heath mounting hole measured 0.238 inch. That would be a 15/65 inch drill or a 7/32 with a little slop. These feet also seem to fit the EICO products that I have worked on. If you paint the case, be sure to tape around the front edge of the cabinet. This will insure a good electrical ground after painting. CLEANING POTS Some of the older pots have a back metal cover with a metal tab half way between the outside edge and the center. Prying that tab up will allow the cover to easily come off. Spraying in WD-40 works as good as anything and is helpful with those stuck pots. Just give it time to soak in. Many EICO pots have a back cover that just pries off. Older pots with front tabs can be bent back without the pot flying into pieces. It is like they expected someone would be taking them apart to clean them. SUPER C-3 AMPLIFICATION MODIFICATION The 1626 triode is used to provide a gain of about three to the eye tube. This gives a much tighter notch for finding a value and observing Power Factor. Three doesn't sound like much, but the results are amazing. Any higher gain and it would be difficult to find the notch. The eye would also never open with a capacitor that has a high series or parallel resistance. Remove the end of the original .01 capacitor that goes to the terminal of the Leakage switch. Input from center front panel terminal is fed through a 0.01 uF to the grid pin 5. This pin also has a grid resistor to chassis of 6.8 Meg. The cathode pin 8 is connected to chassis ground through a 36K resistor. In parallel with the resistor is a 100uF 10V electrolytic, positive to pin 8. In operation, the voltage from pin 8 to ground is between 4 and 5 volts. See FIG7 or SC3.pd for schematic. If supplied, .022uF capacitors may be substituted for the .01uF capacitors. The power for the tube is provided by the +180V filtered through a 36K resistor to a 4.7uF 250V connected to the chassis. The nominal voltage is about +150V on this cap. This voltage connects to the plate of the 1626 tube pin 3 through a 300K resistor. Nominal plate voltage for pin 3 is about 30-60 volts and can change with signal level and tube condition. Another 300K resistor feeds the amplified signal from the plate through a .01uF ceramic disk to the pin of the NORMAL/LEAKAGE switch. This is the same pin that connects to pin 5 of the eye tube. The existing .01uf capacitor from the original design was disconnected from this pin. This capacitor may still be used as the input cap if it is not an old wax type. Connect a wire from the removed end of the capacitor to the grid pin 5 of the 1626. FIG6 shows the layout and FIG7 shows the amplifier schematic. The 1626 circuit is operated to produce maximum gain for this tube. An exceptionally hot tube may require the 100uF cathode capacitor to be disconnected. The 100K pot may be connected in series with the 100uF 10V capacitor to adjust the gain slightly lower. This signal out of the tube may be fed to a RCA or BNC connector on the back of the chassis and viewed on a scope or AC meter. Use a separate .01uF disk from the plate to the connector if this option is desired. This would help with problem capacitors that overload the eye tube. Just adjust the dials for minimum reading. Be aware that the design of the power transformer is winding over winding. With the bridge winding floating, this allows any noise on the power line to be capacitively fed to the amplifier with no common mode rejection. Devices such as light dimmers will show up as spikes. Optional filtering of the signal can be performed with an additional 300K resistor and a .001 or .0047uF to the chassis. See the schematic in figure FIG7. The .001uF cap seems to work best. WHAT HAPPENS IF THE 1626 TUBE IS DEFECTIVE A 1626, not listed on many tube charts, can be tested on tube testers as a 12J5. It tests the same on my Jackson tube tester even though the gain should be lower than a 12J5. The 1626/VT-137 is still available from sources. One source is www.vacuumtubesinc.com which is more reasonable than most. It started out as a transmitter tube and recently audio enthusiasts have been building small audio power amplifiers with them. It is a very poor small signal amplifier with a gain of about 5, that makes it ideal for this application! A 12J5 (one half of a 12SN7) is the exact same pin out as the 1626, but will require the component changes of the 12SN7. The common 12SN7 will also work as a replacement if pin changes are made. Jumper a wire from pin 2 to pin 8, this provides filament voltage (pins 7&8). The 15K cathode resistor that goes to pin 8 should be changed to 27K and moved to pin 6. There is no need also to move the capacitor to that pin and should be removed. The 12SN7 has higher gain than the 1626 and removing the capacitor will lower the gain of the tube by providing negative feedback. If adjustable gain is desired, a pot can be connected between the 220uF capacitor and pin 6. A 50K or 100K pot would be suitable. Approximate gains are 0 ohms = gain of 9, 5.1K ohms = gain of 6, 100K ohms = gain of 3. This can vary with the condition of the tube. The input/grid connections are moved from pin 5 to pin 4. The plate connections are moved from pin 3 to pin 5. Parallel another 300K resistor with the resistor that goes from the plate, pin 5, to the 4.7uF power source. This will increase the tube current and produce more negative feedback in the cathode resistor. If the amplified output is fed to a scope or the gain is higher than three, it may be worthwhile to balance the filament winding with a 300 ohm resistor from each heater pin to the chassis to reduce AC pickup. FACTORY MANUAL & SCHEMATIC The factory schematic and manual is available at the web site www.bama.sbc.edu Many manuals for other vintage test equipment can be found there. A copy of the manual can also be downloaded FREE. There are compatibility problems at times with this site. Don't be discouraged, the schematic may take several attempts and work better on other computers. Some have reported that FIREFOX2 works well as a browser. Read the FAQ section thouroughly and try using the mirror site EDEBRIS. Many documents are in Deja Vue format and this reader can also be downloaded. Be sure to copy this document and the pdf to permanant media. Many emails are automatically deleated after 30 days.