AVO_CT160_rht维修电路原理图.pdf

This document is a collaboration between Martin Forsberg, Sweden, and Euan MacKenzie, Australia. Link to Mr. Yutaka Matsuzakas website used with permission. Copyright Martin Forsberg each part discusses a circuit, with a group of components that relate to each other, not only function wise but more so electrically in the VCM. The first chapters describe the Mk III, Mk IV, CT160 and CT160A and the last chapters describe the VCM163. Schematic drawings have been included at the end of this document for the AVO Mk III, Mk IV, CT160, CT160A #1, CT160A #2 and VCM163, as well as for the AVO TT537 transistor tester. Corrections have been incorporated in the schematic drawings where differences have been found in real testers compared to AVOs original schematic drawings. Some of these corrections have been discussed in earlier articles written by me and my friend Euan MacKenzie. Parts of the schematics have been cut out and are shown in the corresponding chapter together with the discussed components and equations. I have also included simplified schematic drawings for the AVO CT160 which makes it easier to understand how each test circuit works and therefore how each test is performed. I have cleaned the schematics from the hand drawn diagrams I was given as scanned photo copies, and also modified them and in some cases added some details to make them clearer and easier to understand. The parts being explained in detail are listed below; each chapter is complete with mathematical equations, diagrams, photos and tables. AVO Mk III, Mk IV, CT160 so that the calculations will be correct. There are two more interesting AVO patents that you can read. Patent GB510098 which describe how the valve panel and roller selector at the top works for the AVO Two panel valve tester and patent GB735865 which describes the wiring beneath the valve panel, the wire loops, which removes oscillations of valves under test. If you are unfamiliar with the conversion from sinusoidal AC RMS voltage to its equivalent Mean DC voltage after rectification, then I suggest that you read “Appendix 1: RMS and Mean” first. The first step in getting correct readings from any AVO VCM is, of course, to make sure that it is in perfect working order. You will have to be sure that every component is within tolerance, if just one is out of tolerance, you will get at least one measurement that is wrong, but probably quite a few. So before you try to repair any section of an AVO VCM I suggest that you check all components. You should start by checking the meter, and only after that start to check the components and each circuit, this as you cant get correct readings without a correct meter (within certain limits). This checking might mean that youll have to de-solder some components or wires to be able to measure each component by itself, but it is well worth it, because only then will you know that the tester is in top condition. RadioFans.CN 收音机爱 好者资料库 This document is a collaboration between Martin Forsberg, Sweden, and Euan MacKenzie, Australia. Link to Mr. Yutaka Matsuzakas website used with permission. Copyright Martin Forsberg or any voltage that it is possible to select with the range of the SET rotary switch. This means that readings that were taken at a certain time can only be reproduced at another time if the primary voltage is the same, or if it can be adjusted with the SET rotary switch so that the needle lines up with the calibration mark once again. If you cant set the needle at the same point as was used during the calibration, then your measurements will be a bit out compared to the calibrated values. Figure 1: The nine steps for the SET switch can be seen in the picture above Seven of the nine steps correspond to either an increase, or a decrease, of 2.5V from the centre voltage of the primary voltage, which is selected via the fuse inside the VCM. The two outermost positions on each end are connected together so there will be no change in settings between these two positions at each end, this can be seen in Figure 2 below. RadioFans.CN 收音机爱 好者资料库 This document is a collaboration between Martin Forsberg, Sweden, and Euan MacKenzie, Australia. Link to Mr. Yutaka Matsuzakas website used with permission. Copyright Martin Forsberg this is to protect the meter during overload conditions. You cant replace these silicon diodes with a pair of germanium diodes as those will have a voltage drop that is to close to the required 97.5mV and they will then introduce a non-linear behaviour, the high reverse current through a germanium diode will also result in the wrong reading as it shunts some current from the meter. Early models of the Mk IV only had one silicon diode in the direction of MR7 and no capacitor. The AVO Mk III, CT160 and CT160A do not have these diodes or the capacitor. In the AVO CT160 and CT160A the meter does not have any of the diodes nor the capacitor, but AVO incorporated another simple protection. This protection consists of a resistor, 22K, and a switch which shorts this resistor when the mA/V dial is turned to the SET ZERO region, and onwards. When the mA/V dial is at its resting position this resistor is inserted into the meter circuit and limits the current through the meter circuit, albeit to a maximum current of approximately 400A which is 10 times the FSD current for the shunted meter, but it is much better than the 4mA which could otherwise flow through the meter circuit. This setup also gives you the possibility of fine tuning the Ia reading as you can fine tune the mA dial for the 0-10 range when the mA/V wheel has been rotated to the SET ZERO region. You will have to be careful though as you have just started to add a delta voltage to the grid, which can affect the reading if it is not calibrated correctly (it should be close to zero). RadioFans.CN 收音机爱 好者资料库 This document is a collaboration between Martin Forsberg, Sweden, and Euan MacKenzie, Australia. Link to Mr. Yutaka Matsuzakas website used with permission. Copyright Martin Forsberg this is not recommended, as there is insufficient polarising voltage, so they rapidly become leaky. Note: there is nothing magic about the value of 8F, just remember it was a standard value in the industry at the time. The time constant is 3,250 /10k x 8F = 20ms; so using a modern 10F will not make any discernible difference. The diodes can be the same Silicon diodes as recommended elsewhere in this document, such as the BYW96E, as these will work just as well here”. Other manufacturers like Hewlett Packard used the same setup with a capacitor and diodes across the meter connections and they used values up to 100F, and some manufacturers even beyond that, to dampen the movement. If you have a meter that seems to have a fast movement which is poorly dampened, you should check C3 and replace it if necessary; this is not always an error in the movement itself but can be due to a faulty C3. RadioFans.CN 收音机爱 好者资料库 This document is a collaboration between Martin Forsberg, Sweden, and Euan MacKenzie, Australia. Link to Mr. Yutaka Matsuzakas website used with permission. Copyright Martin Forsberg the red calibration line is painted by hand at the correct place on the scale on each meter. This place might differ somewhat from the centre position of the black area for the SET region, like in Figure 5 below. This current of 35.775A is exactly the current that needs to flow through the calibration resistor for the meter to reach the red line at the SET region. The calibration resistor is made up from two matched resistors as AVO calls them in their component lists, with a total resistance of 2.96M in the AVO Mk III and Mk IV and 1.32M in the AVO CT160 and 1.22M in the AVO CT160A. The voltage drop across the combination of the shunted meter and the 2.96M resistance in an AVO Mk III or Mk IV will then become 2.96M x 35.775A + 87.75mV = 105.98V. This Mean DC voltage must be present at the junction of calibration resistor and the SET Vg potentiometer, in order to reach the red line at the SET region on the meter! For the AVO CT160 the same calculation results in a voltage drop of 47.3V, 1.32M x 35.775A + 87.75mV = 47.31V. This equation is not the same for the CT160A as the circuit has been split and a separate voltage/current ratio is used, however the same current is flowing in the circuit as the same calibration spot is drawn on the meter dial. This circuit results in the following calculation 1.22M x 35.775A + 87.75mV = 43.73V. Figure 5: Meter from an AVO Mk IV, note that the red calibration line is not exactly at the centre of the black calibration area How do we get these Mean DC voltages at this junction across the grid Volts circuit then? For the AVO Mk III, Mk IV and CT160 you will first have to open the link Vg Link, and set the voltage for the Grid Volts control to the required calibration voltage of 52.5V with the SET Vg potentiometer, and then close the link again, and now adjust the SET potentiometer to set the voltage to the desired level to get the meter needle to indicate at the red line at the SET region. The calibration procedure is described in detail in the AVO Service Manual. RadioFans.CN 收音机爱 好者资料库 This document is a collaboration between Martin Forsberg, Sweden, and Euan MacKenzie, Australia. Link to Mr. Yutaka Matsuzakas website used with permission. Copyright Martin Forsberg this has been done since AVO would otherwise have had to make two scales for the insulation and leakage scales, as those rely on the voltage in the grid volts circuit. In the CT160A you can double the grid voltage with the X1 / X2 switch and this means that you would have to have one scale calibrated for the 40V grid voltage and another scale for the 80V grid voltage. This could have been avoided by adding a voltage divider that supplied the calibration resistors with the same voltage no matter what maximum grid voltage had been selected, but my guess is that this would have been more costly and more complicated than simply splitting the circuits. The fact that AVO split the Grid Volts circuit and the calibration circuit in the CT160A shows that these two circuits are not connected in any way and work perfectly well without being connected together like they are in the Mk III, Mk IV and CT160. The CT160A still reports the same measurements for a valve as the CT160 does, with these circuits split. Figure 6: Calibration circuit in the AVO CT160A with RV6, D2, R41 and R3 RadioFans.CN 收音机爱 好者资料库 This document is a collaboration between Martin Forsberg, Sweden, and Euan MacKenzie, Australia. Link to Mr. Yutaka Matsuzakas website used with permission. Copyright Martin Forsberg as those two controls are connected in parallel. The total DC Mean voltage you set for the SET position is not used for valve measurements in the TEST position, only that part of the half wave rectified voltage that is in phase with the Anode because then the VCM would be out of calibration, and therefore no longer making correct measurements. What then would happen if the resistance in either the Grid Volts control or the mA/V control was not correct, when you do the calibration of this voltage across these two controls? You would end up with both controls being in error when they were used, as the current flowing through each circuit would not be correct, and it would give rise to the wrong voltage, at either the Grid Volts control, or at the mA/V control, or at both controls. Therefore it is very important that both of these controls have the correct resistance, so that the combined resistance is the exact resistance for the circuit to work properly. However, since one resistor out of tolerance can cancel the effect of another resistor out of tolerance, it is important that every resistor has the correct value and not just the total resistance. In the Mk IV there are a few extra resistors in the circuit, which are not present in the CT160 or in the Mk III, i.e. resistors R41, R42 for the Grid Volts control and R20 and R43, plus R46 for the mA/V control. In the newer CT160A, some of these functions have been incorporated in the resistors R39, R4 and potentiometer RV5 for the extra 80V range of the Grid Volts control. Resistors R41 the total resistance changes from 25,855 to 25,898 when the range is switched from the 1 - 10 mA/V range to the 8 - 60 mA/V range. There is a small difference in the total resistance when these values are calculated but that change can be minimized by choosing resistances for R20, R41, R42 and R43 that match the other resistors in the circuit. This then shows the importance of the correct resistances in these two circuits. The simplified circuit for the CT160 can be found in Figure 7, this circuit is the same in all test positions after the calibration has been performed and the Vg Link is closed. RadioFans.CN 收音机爱 好者资料库 This document is a collaboration between Martin Forsberg, Sweden, and Euan MacKenzie, Australia. Link to Mr. Yutaka Matsuzakas website used with permission. Copyright Martin Forsberg apart from checking that each resistor is within tolerance. By first looking at the schematic drawing for the AVO Mk III and then the AVO Mk IV it will be easier to see how the Grid volts control is connected. Figure 10: The Grid Volts control in the AVO Mk III, in the position shown the base voltage is +40V In Figure 10 above the base voltage added to the Grid Volts dial is +40V. The current flowing in the Grid Volts circuit in this position passes resistors R7, R8, RV5, R15, R13 and R14. The resistors R7 R14 are used to set the added base voltage. Figure 11: The Grid Volts control in the AVO Mk IV, in the position shown the base voltage is +80V and the range used is 0-21V In Figure 11 above the base voltage added to the Grid Volts dial is +80V and the range used is 0 21V. The current flowing in the Grid Volts circuit in this position passes resistors RV5, R15, R11, R12, R13 and R14. The resistors R7 R14 are used to set the added base voltage in the same manner as in the AVO Mk III. RadioFans.CN 收音机爱 好者资料库 This document is a collaboration between Martin Forsberg, Sweden, and Euan MacKenzie, Australia. Link to Mr. Yutaka Matsuzakas website used with permission. Copyright Martin Forsberg the reason for this is that the extra resistance above 2,500 Ohm is used as a small overlap for each range the control is marked 5.2V and 21V, not 5V and 20V and this difference of 0.2V and 1V makes up for the extra resistance. The potentiometer can be turned past both the zero position and the 5.2V / 21V position as it has a range with zero resistance at the start (and end) and only part of the travel is used for the 0-21V or 0-5.2V range. Another equally important thing to note is that you will have to use the DC mean voltages printed on the scale of the Grid Volts potentiometer when performing any calculations for the Grid Volts and mA/V controls an