AVO_CT160A维修电路原理图.pdf

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1、RadioFans.CN 收音机爱 好者资料库 Page 1 of 17 This document is a collaboration between Martin forsberg, Sweden, and Euan MacKenzie, Australia. Copyright Martin Forsberg and also explain why there is a slight modification to the circuit as a consequence. It will also explain how you can improve the calibratio

2、n, by making a further slight modification to the calibration circuit. It will also explain how you can place a Silicon diode in the Anode voltage circuit and a small modification that this entails. All of these modifications have actually been tried out on an AVO CT160 (Serial No. 4087 YF) and the

3、modification do not change the results in any way; you get exactly the same results when comparisons to a standardised valve (a CV455) are made, both before and after the modifications were performed. There is also a short description of the function of R14 and SW3, which are used in the SET ZERO po

4、sition on the mA/V dial. The protection of the meter is also discussed and a comparison with AVOs other Valve Characteristic Meters is made. This text is a collaboration between me, Martin Forsberg, and my friend Euan MacKenzie, where I have written most of the text and Euan have made most of the me

5、asurements and all of the modifications in his AVO CT160. Euan has helped me with proof reading of the text and also supplied indepth information about components and mathematics. It is quite long, but I really hope that you find it worthwhile to read through to the end! There is more to come in the

6、 future with a look at the Dutch Military special edition of the AVO CT160 named AVO CT160A. The AVO CT160A might have been used by others too but it has been referenced to as the Dutch Military versions in a few places and I have found no other mentions of it. Part 1: Anode Current Controls; and re

7、placing the CV140 valves with Silicon diodes:- In the AVO CT160 the anode current is measured by means of a Potentiometer, which is a laboratory instrument for the precision measurement of an unknown voltage. If you did Physics in the sixth form at school, you would have encountered the Potentiomete

8、r; in its simplest form it is comprised of a 1m length of Nichrome wire, alongside a 1m wooden ruler; driven by a 2V lead acid cell and calibrated using a galvanometer in series with Weston Standard cell, to detect the null. It is a common misconception that the CT160 is a Bridge; it is not, because

9、 a Bridge has four arms, whereas the Potentiometer only has three. In the CT160, the anode current flows through a 200 sensing resistor, the voltage drop produced across this resistor is then compared to a known voltage; which is developed in a separate circuit, comprised of a constant current flowi

10、ng through nine switched resistors, R15 to 23, and one variable resistor, RV4; which are labelled the ANODE CURRENT controls. The anode current controls perform the same action as the backing-off controls do in the AVO Mk III and Mk IV, they effectively balance out the large standing anode current,

11、which is flowing through the 200 resistor in the anode circuit. This makes it possible for the meter to show the small current change produced by the mA/V control, when the mA/V measurement is being performed. In the CT160, the anode current controls are designed in such a way that you have to read

12、the anode current, which is flowing through the valve, on the scales provided on the controls, not on the meter. Whereas in the AVO Mk III and Mk IV, the backing off controls are not provided with any scales, and the anode current is indicated on the meter. The anode current controls (or backing-off

13、 circuit) act as a power supply, which is designed to produce exactly one volt for each step from 0mA (0V) to 90mA (9V) on the rotary switch, or coarse anode current control; plus the variable 0mA (0V) - 10mA (1V) from the fine anode current control; (in fact, the maximum on the fine anode current c

14、ontrol is actually 11.25mA, or 1.125V). Thus makes it possible to balance out a maximum of 10.125V; which corresponds to an anode current of 101.25mA. The small extra voltage on the fine anode current control is intended to produce a small overlap on each range; since it is possible to RadioFans.CN

15、收音机爱 好者资料库 Page 8 of 17 This document is a collaboration between Martin forsberg, Sweden, and Euan MacKenzie, Australia. Copyright Martin Forsberg because the valve works as a half wave rectifier for the applied AC voltage to it. Now 50mA flowing through 200 produces a voltage drop of 10V across the

16、 anode current sense resistor; which is then compared, using the Potentiometer, to the voltage produced by the anode current control circuit. In the coarse anode current control, the voltage drop in each step of the rotary switch must equal exactly 1V; since each step has a resistance of 80 , then t

17、here must be a constant current of 12.5mA flowing through the circuit. Now this value exceeds the manufacturers maximum value of 9mA for the CV140/EB91/6AL5 valves, which AVO used for rectification in the CT160, and is likely to lead to a shortened service life. These particular valves were actually

18、 designed as FM detector diodes, not as current rectifiers! As you will have no doubt noticed, the maximum available screen voltage in the CT160 is 300V; this is due to the valve manufacturers PIV limit of 330V, (now referred to as Repetitive Reverse Maximum, RRM). Fitting suitable Silicon diodes wi

19、ll remove this limitation; so if you do wish to utilise a higher screen voltage, you could disconnect the 125V wire from the screen voltage selector switch; as this is the odd one out, in the sense that 125V is not available on the anode voltage selector switch. Then move the higher voltages back on

20、e tag on the screen switch wafer, and then connect the last tag to the 400V tap on the HT transformer. AVO marked the taps on their HT transformer with the mean voltage, so it is labelled as H400 (instead of using the customary rms value, ie 440V). The screen selector options will then be exactly th

21、e same as on the anode voltage selector. As for suitable Silicon diodes; dont fit any old diodes that just happen to be lying in your scrap box; it is well worth fitting soft recovery diodes, which are designed to minimise circuit switching oscillations; for example Philips BYW96E, which is rated at

22、 3A and 1kV Vrrm. Admittedly, we dont require a forward current of 3A, however the lower current soft recovery diodes dont seem to have a sufficiently high enough Vrrm; 800V would be sufficient. If you are going to replace the valves in the CT160 with Silicon diodes, you will have to alter the serie

23、s resistance R6 (730 according to the legend, but in reality 750 was used) which is used to obtain the correct current flowing through the anode current controls, so that the voltage across each 80 resistor will still be exactly 1V. If you place a standard Silicon diode in place of each valve diode

24、(remember the CV140 is a double diode, so one Silicon diode will be needed for each diode) then you will have to increase the series resistor, R6, to approximately 935 , in the anode control circuit. The calculation for this is as follows:- the anode current control circuit is supplied from a transf

25、ormer winding of 50V rms; which is equivalent to a mean voltage of 45.02V; (the ratio between rms and mean voltages being 1.1107 for a sinusoidal waveform; which gives us 50V RMS 1.1107 = 45.02V Mean). Now the approximate forward voltage drop for a Silicon diode is 0.7V; however for a CV140 it is 2.

26、2V (as measured at the manufacturers maximum current of 9mA); so the value of R6 must be increased. The anode current control circuit requires an exact current of 12.5mA (derived from the 1V volt drop across each 80 resistor, 1V 80 = 0.0125A or 12.5mA). After rectification by the Silicon diode, you

27、then have a voltage of 45.02V 2 - 0.7V = 21.81V DC mean. (The 45.02V has to be divided by 2, because it is a half wave rectifier; the mean voltage following half-wave rectification is obviously halved, and the figure of 0.7 Volt comes from the forward voltage drop of the Silicon diode). The total re

28、sistance in the circuit will then have to be 21.81V 12.5mA = 1744.8; from this we must subtract the existing resistors in the anode control circuit = 9 x 80 + 90 = 810; thus the new value of R6 = 1744.8 - 810 = 934.8. You could, if you prefer, use a small variable resistor in series with a larger re

29、sistor, so that you can adjust the mean current to precisely 12.5mA. Alternatively, you might choose to RadioFans.CN 收音机爱 好者资料库 Page 9 of 17 This document is a collaboration between Martin forsberg, Sweden, and Euan MacKenzie, Australia. Copyright Martin Forsberg or in series, eg 910 + 24, to adjust

30、 the total current. If you wanted to measure the current, you could insert a current meter in series with the circuit, but you would then have to adjust the circuit for the additional resistance introduced by your current meter, as it will also produce a voltage drop. It is therefore easier to measu

31、re the voltage drop across each resistor with a digital multimeter, as that will have a high input resistance, typically 10M, which will not affect the circuit as much as the current measurement would do quite apart from that, you do not need break the circuit for voltage measurements! This also mea

32、ns that if you need to replace the 90 anode current potentiometer, with another value, say 100 (since 90 will be hard to find nowadays) you can do that, but then you will need to reduce the series resistor, R6, by the additional resistance in the new potentiometer, in order to keep the current at 12

33、.5mA through the backing-off circuit; in this example, by 10. However you will need to make a new scale for the anode current potentiometer, and you will get a larger overlap on each range, but it will still give the correct measurements for anode current. The practical minimum value will be close t

34、o 90, as it was originally. If for any reason, you need to go lower than 80, then you will have to lower each 80 resistor, and increase the current, to maintain exactly 1V. A typical reason for replacing it would be that the old one is open circuit, or perhaps has become non-linear, due to wear, or

35、is otherwise damaged. Making a new scale is quite easy if you use a 360 protractor, together with a multimeter to measure out each step of either 0.1V, or 8, and mark them on the protractor, then transferring them to a paper scale. Or, alternatively, you can drill a hole through the centre of a fair

36、ly large protractor, or a piece of PCB, then fasten the potentiometer in the hole and using a large knob on the potentiometer, and as you turn it, mark each point on the protractor/pcb, which you can then transfer to a paper scale. NB:- There is one case that has not been checked thoroughly so far,

37、and that is whether the gm measurements will be affected, if the potentiometer is changed to any value other than 90! There is only a slight risk of that, since each of the three 240 resistors, R24 to R26, are used to compensate for the anode current control resistances - this should be investigated

38、 further, before I can recommend changing the potentiometer for another value; but my guess is that the change doesnt matter, as it is the voltage delivered between the two points that form the backing-off circuit, is what the measurement is compared with, and since that is unchanged, so then is the

39、 current and resistance in that circuit path. However, there also the possibility to put a potentiometer with a higher value in, and then shorting out the last part of the track above 90, and then make a new scale to fit the new potentiometer; then this will work just as well as the old one, except

40、that the new scale will be more cramped! For the anode current control measurements to be accurate, you must ensure that the remainder of the components in the AVO CT160 are within tolerance, and also that the tester is calibrated; however for the anode current controls in themselves to be accurate,

41、 you must ensure that a voltage of 1V Mean DC is developed across each of the 80 resistors. The other diodes in the CV140 valves, V1 this then means that the diode in the CV140 RadioFans.CN 收音机爱 好者资料库 Page 10 of 17 This document is a collaboration between Martin forsberg, Sweden, and Euan MacKenzie,

42、 Australia. Copyright Martin Forsberg which in itself, is a necessary prerequisite for the gm measurements to be performed. When the large standing current has been balanced out by the Anode Current controls, (or backed-off) the gm measurement can commence. When the mA/V dial is in its resting posit

43、ion, ie turned fully anti- clockwise by the spring mechanism, the switch SW3 is closed and R14 is shorted. When the mA/V dial is moved to the SET ZERO region, the switch SW3 is opened, and the resistor R14, 22 k, is inserted in the Anode Current circuit; this results in a very small voltage drop fro

44、m the current then flowing through the circuit, usually below 1A. For example, a current of 1A will result in a maximum voltage drop of 22mV, which corresponds to a standing Anode current of 0.22mA. If the balancing out of the Anode Current controls has been exact, then no current will flow. However

45、, if this is not the case, any residual current flowing in the circuit can now be balanced out by fine adjustment of the Anode Current control, thus enabling the balancing out procedure to be more precise. This will ensure that when the gm measurement is performed, when the mA/V dial is turned into

46、the gm measurement zone, the measurement will be more exact. You will also be able to get a more precise measurement from the Anode Current controls, after this fine adjustment have been performed. This fine adjustment procedure in no way effects the large standing current flowing through the valve

47、during the testing, as the large Anode current is still flowing, as long as the tester is in the TEST position; as only in this position the Anode voltage is applied to the valve under test. So releasing the dial and letting it go back to the resting position still leaves the large standing Anode cu

48、rrent flowing through the valve; the only difference in the position of the mA/V dial, is how much the grid voltage is changed, from zero to 0.260mV - this is the extra voltage necessary for the gm measurement to be performed. Part 3: Calibration resistors: To make sure that it is possible to calibr

49、ate the CT160 (and this also applies to the AVO Mk III and Mk IV) you will need to ensure that all the components are within the tolerance levels that AVO specified; not only the calibration resistors, but it is obviously worthless to have the correct value for the calibration resistors, if the rest of the tester is not within tolerance (especially the meter, anode current controls and 200 resistor in the anode circuit). The calibration resistors in all of the AVO CT160, Mk III and Mk IV family are equally important as they provide the same function in