Eico 666/667 Meter Power Cicuit Description Mike at MDBVentures.com 14 July 2017 (Updated 31 Mar 2019 - Revised 31 Dec 2023) http://www.MDBVentures.com - Great prices on great tubes! The important part of the 666/667 tube testers is the meter power circuit and how that power is distributed to the meter through the various test configurations. This article descibes how the meter power circuit works and the various problems that can occur if there is a problem. A common blame for meter calibration failure is to blame it on the C1 capacitor. While it can have an impact, it is actually a low probability. The capacitor used in the tester design is rather robust and it has to be seriously bad before it causes trouble. Another source of blame is the CR1 selenium rectifier. While it is possible for the rectifier to go bad, it is actually rare. Some people just don't like the selenium rectifier and will replace it with a modern silicon diode. While that does no harm, there is really no need to replace the selenium rectifier if it is still working. The C1 capacitor is directly tied to the output of the CR1 rectifier. Any leakage current in the capacitor that would be enough to prevent the circuit from working would damage the CR1 selenium rectifier. An open capacitor could potentially cause problems because the output of the rectifier would no longer be filtered, which would result in a lower average output voltage from the leak cal circuit (which is a part of the line cal circuit) and thus the calibration would fail. The line cal circuit is extremely simple. It consists of the leakage cal circuit which consists of R7, R8, C1 and CR1, and the Line adjust resistors (R18 and R19). The line adjust just sets up a current flow through the meter of 100uA which is the center position of the meter. C1 and CR1 form the DC supply voltage for the meter circuit. CR1 is directly tied to the 50V tap on the power transformer and C1 is directly tied to the output of CR1. That provides a 70VDC power source to the leakage measurement circuit and the line adjust. R8 (270K) and R7 (100K) form a dropping resistor chain that act in series with the tube element under test and the meter to measure leakage in the tube. R18 and R19 form the remainder of the line adjust and add another 270K and 100K resistor chain in series with the leakage adjust resistors to obtain the 100uA line adjust current for the meter. Normally as a capacitor ages it will reduce the leakage currents as the electrolytic chemicals dry out. Corrosion or dirt on the terminals can cause increases in leakage when installed in the circuit, but again since the C1 capacitor is directly connected at the output of CR1, even moderate leakage will not affect the operation. The most important aspect of C1 in the circuit is the capacitance, and having more (up to twice as much) is not a problem since it is being used as a peak filter. An excessive amount (more than twice as much capacitance) is only a problem in that it may damage CR1. Note: If you decide to replace C1 (10uF 150V), use the same size capacitor. In this case, a larger capacitor (such as 50uF) is a bad idea as it can damage the rectifier. A smaller one (such as 2uF) is bad in that it reduces the available capacitance margin, which may result in weak readings. Like the little bear's bed, 10uF is just right. An open capacitor circuit (or significant loss in capacitance caused by electrolyte loss) can cause the average DC voltage in the circuit to drop. When it is being filtered properly by C1 the voltage will be around 70VDC across C1. The adjustment range provided by R7 and R18 is about +-12volts (50000 * 0.000236). The meter is 1000 ohms (which means it has little contribution to the voltage drop) and has a full scale deflection at 200uA (100uA at half deflection = line adjust point). Note: For testers with a selenium rectifier you will see around -66VDC +-4Volts when the Line Adjust is properly set. For testers with a silicon diode you will see around -70VDC +- 4Volts. The AC supply should be 50VAC +-3Volts. The lower voltage for the selenium rectifier is because they have more voltage drop than silicon rectifiers. These voltages are perfectly fine as long as the supply voltage is between -62VDC and -74VDC (when the Line Adjust is properly set) the calibration and leak test will work correctly. Without C1, the average voltage presented to the line adjust circuit will be 50V (The peak voltage is no longer being held up with C1, so the meter only sees the average voltage). That is 20Volts less than the proper voltage and below the available adjustment range. This can be one of the causes of not being able to get the line adjust to work properly. However since it is unusual for the capacitance to fall to the point of not being usable (it would have to fall below 2uF), C1 failing sufficiently to prevent the meter calibration from working is actually rather rare. The more likely cause of a problem with C1 is a bad solder joint. If the CR1 rectifier has gone bad and shorted out, it can destroy the C1 capacitor. Also a fumble fingered technican can accidently short out the connections to the CR1 rectifier resulting in C1 being destroyed as well. If C1 has been destroyed, then it is probably a good idea to replace the CR1 rectifier as well just to be safe. The positive end of the C1 capacitor attaches to an ear on the bracket that CR1 is attached to and the negative end of C1 attaches to the anode of CR1 (where the 270K resistor attaches). The cathode end of CR1 attaches to the 50 Volt tap on the power transformer which is obtained via a wire connected to the filament switch 50V position. If you are installing a silicon diode (such as a 1N4004), then you should consider installing a terminal strip to connect the CR1 rectifier and C1 capacitor along with the R8 270K resistor and the wire from the filament switch (these are normally connected to the ears on the CR1 rectifier). If you use a terminal strip, you can use the mount hole for the CR1 recifier to install it and connect the positive end of C1 to the terminal ear that attaches to the CR1 mount hole which is a lot easier to solder to since the tab that it normally solders to on the mount bracket is a massive heat sink and a common cause of a bad (cold) solder joint. Although it is preferable to run a wire from the terminal strip ground to the mount bracket tab as well so that if the terminal strip ground comes loose or gets corroded the ground will still be intact through the wire. Another potential problem is that the 270K resistors can increase in resistance with age. These are carbon composite resistors which have the problem of increasing resistance with age if they are not properly constructed. This is especially true if the tester has been exposed to large temperature swings as would happen if it was stored in a shed or the attic. If the resistor has gone bad, you can replace it with another carbon composition resistor or a newer metal film resistor. If the resistor still has the correct resistance there is no need to replace it. The single most common cause of meter calibration failure is the R7 and/or R18 calibration pots getting leaky over time. This is especially true if the tester has been sitting in a shed in the back yard, the attic, or an unfinished basement for a few years, or operated by someone who smokes. Although even in the best of conditions, the pots can get leaky. The problem with the leaky pots (R7 and R18) is that gunk can build up on the internal parts and short the resistor track to the chasis which the case of the pot is connected. That diverts current away from the meter. Since we are only dealing with 200uA full scale on the meter (100uA half scale), it doesn't take much contamination to degrade the measurement circuit. As little as 10Meg ohm of leakage in the pots can prevent the circuit from being calibrated. That is why it is so important for them to be free of contamination. Note: It is critical to adjust the leakage calibration first because it provides the required defined resistance/current source for the line cal adjustment. If you don't get the leakge cal adjusted properly, it throws off the line adjust cal as well as the leakage cal. The meter power circuit is only used during the Line calibration test (when you push the "Line" button) and for the leakage test. For the main test (when the Merit lever is pulled), the tube under test itself becomes the rectifier for the meter circuit (CR1 is no longer in the circuit). The leakage calibration must be done first, because it sets up the 200uA reference for the Line calibration. The Line calibration then adds additional resistance to set the 100uA current reference point for the Line calibration at the meter center scale. Once calibrated, the Line Adjust control on the frontpanel will accurately reference the line voltage applied to the tester. (ie If you push the Line button and adjust the Line Adj control to get the meter needle to center on the Line Adj mark, the Line Adj knob will be pointing to the value of the AC power voltage. So you can actually use it to find out what the AC power voltage is.) The purpose of the control is to insure that the tester circuits receive the correct AC voltage supply so that the test results will be consistant regardless of the power voltage the tester is attached to. It does this by a boost/buck configuration which controls the field strength of the magnetic field presented at the transformer's primary so that the voltage at the transformer's secondardies will always be the same when the Line Adj control is correctly set when the Line button is pressed. The Line Adj boost/buck configuration is a little inefficent, but extremly simple and reliable. Because a tube that draws a lot of current is reflected to the current draw from the primary of the power transformer, and thus the voltage presented at the primary, the voltages presented to the tube can sag if the tube draws a large amount of current. That is why it is important to readjust the Line Adj control after a tube has been installed in the tester and warmed up. A tube will draw more filament current when it is cold because the filament increases in resistance as it warms up, thereby reducing the current drain. How much the change in current draw by the filament is dependant on the tube. Most small tubes have little or no noticable change. Most high power tubes will have an impact on the voltages. Even with the adjustment of the Lin Adj control, when the merit lever is pulled to test the tube, the current through the tube (Plate current), will also affect the voltages applied as well. Again, for small tubes this is usully unnnoticable, but for power tubes, it can impact the voltages. There is little that can be done about that unless more circuits were to be installed to provide a constant independant power voltage measurement. Some testers like the Hickok 539 provide an independant power meter, but that adds expense to the tester. So instead, what most testers do is simply ignore the issue and the test values read reflect the test being done at the lower power voltage caused by the tube's added current drain. So for testers that don't allow continuous monitoring of the power voltage, there is no need to adjust for the change in the power voltage since it is compensated for in the specified test results. For information on how to calibrate the EICO 666/667 tube testers download and read the restr666.txt file. For information on how to troubleshoot and repair the EICO 666/667 tube testers download and read the eico666-667-repair.txt and eico666tester-meter-check.txt files. Also see the companion files: http://www.fourwater.com/files/restr666.txt http://www.fourwater.com/files/666-667-mod.png http://www.fourwater.com/files/eicotesting.txt http://www.fourwater.com/files/eico666-667-repair.txt http://www.fourwater.com/files/mutualconductance.txt http://www.fourwater.com/files/eico666meter-power-notes.txt http://www.fourwater.com/files/eico666tester-meter-check.txt http://www.fourwater.com/files/how-eico666-works.txt http://www.fourwater.com/files/eico666parasitics.txt http://www.fourwater.com/files/testertypes.txt http://www.fourwater.com/tubeinfo.htm Visit us at http://www.MDBVentures.com - Great prices on great tubes!