AVO 160A Operating Instructions And Maintenance Manual Download | Manualshive

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Summary of Contents for 160A

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Page 2: ...d R37B as per component list on page 18 of the Service Manual consisting of one 13 Ω selected resistor each SH6 Ground connection for tags 2 5 was missing from original CT160A schematic but can be found in later CT160 schematics and also in the original CT160A CT160 testers WIRES COMPONENTS Moved for clarity The third schematic has been modified in the following areas All of the corrections above ...

Page 3: ...ore detail in this article The AVO CT160A has been referenced to as the Dutch Military versions in a few places and I have found no other mentions of it Please also understand that I have not had access to an AVO CT160A myself for this article only to second hand pictures and information plus photocopies of AVO manuals However the modifications are not complicated once you understand how an AVO CT...

Page 4: ...n all other aspects the modification status of the AVO CT160A corresponds to the last modification status of the AVO CT160 up to resistor R36 for the Grid to Cathode connection Modification A C Replacing the valves with Silicon diodes meant that the extra heater winding is not necessary any more and it has been removed from the schematics but since no actual AVO CT160A have been available for chec...

Page 5: ...as more expensive to have it there and have it changed in step with the grid volts switch or maybe AVO found that it was not necessary to have that voltage there Modification G Here AVO have changed the calibration resistor circuit introducing a potentiometer RV6 for calibration purposes and also changed resistor R3 to 1 22 MΩ AVO also changed the circuit by introducing a resistor R4 wrongly named...

Page 6: ...2 Modifications done by AVO to the over current relay protection Here is some more new information as I have studied the soon finished AVO CT160A manual in some more depth By finished I mean that I have soon finished cleaning up all the pages While AVO was changing the current range for Diodes and Rectifiers by adding the 180mA range they also changed which voltages that are used for testing Diode...

Page 7: ...result the factor between the numbers is exactly three and one third 3 333 That was what I meant with the fantastically well correspondance between the AVO Mk IV and the AVO CT160A here in this circuit Is this just a coincidence or something that AVO planned My guess is that it is planned but the resistance for the resistors and RL3 will of course vary between each tester but in the AVO CT160A ser...

Page 8: ... replacing 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 Potentiometer in its simplest form it is comprised of a 1m length of Nichrome wire alongside a 1m wooden ruler dri...

Page 9: ...on diodes don t 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 3A and 1kV Vrrm Admittedly we don t require a forward current of 3A however the lower current soft recovery diodes don t seem to have a sufficiently high enough Vrrm...

Page 10: ...ter 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 and that is whether the gm measurements will be affected if the potentiometer is changed to any value other than 90Ω There is only a slight risk of that since each of the three 240Ω resistors R24 to R26 are used to compensate for...

Page 11: ...ent of 0 22mA If the balancing out of the Anode Current controls has been exact then no current will flow However 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 turn...

Page 12: ...V the Anode voltage is already rectified by a silicon diode The voltage from the 66V rms winding is only used to keep the grid negative during the half cycle where the anode of the valve is driven negative by the AC Anode voltage and is not used for any measurement purposes in the TEST and GAS positions According to the AVO Patent No 606707 this is to ensure that no current is drawn through the di...

Page 13: ...nts though firstly it is difficult to access the anode voltage selector switch without having to remove a lot of the mechanical structure so the easiest place to fit the diode is at one of the tags for the anode current relay coil RL3 It is the middle row of the six tags and the anode coil has a much lower resistance than the two other coils between 5 and 6Ω Secondly as AVO discovered it is necess...

Page 14: ...act FSD value of 30µA is done via the magnetic shunt which is located under the RHS needle stop It is a hinged plated steel arm which is mounted on a neoprene bush it shorts out some of the magnetic flux across the pole gap It is or it should be cemented to the existing ferromagnet you will need to break the cement in order to adjust the shunt and then re cement it when the adjustment is correct I...

Page 15: ... the AVO CT160 has a total internal resistance of approximately 3250Ω So to protect the meter from overload by a too high current you will need to have a total resistance in the measuring circuit that will only allow a maximum current close to 30µA to be drawn With a standard 1 2V NiMH that means a total resistance of 40kΩ when the meter resistance is subtracted that leaves 36 750Ω With the follow...

Page 16: ...lypropylene type or failing that a non polarised electrolytic just be sure to use a good long life model with very low leakage AVO originally used a small 8µF 12V electrolytic this is not recommended as there is insufficient polarising voltage so they rapidly become leaky Note there is nothing magic about the value of 8µF just remember it was a standard value in the industry at the time The time c...

Page 17: ...ng adjustment of the magnetic shunt arm inside the meter or it could have been stored in a place where the meter has been exposed to a powerful magnetic field which have affected the magnet although this is not such a probable cause In this case it is also better to send the meter to Herts Meter Co Ltd than to fiddle around with it yourself as they are so easily damaged The magnetic shunt arm is s...

Page 18: ...node circuit measuring resistor at 200Ω as that affects all of the resistors on all ranges as that resistor is the resistor used for measurement purposes Appendix The calculation of the total resistance of the Grid voltage and mA V control circuit The meter has a total internal resistance of 3 250Ω which is in parallel with the shunt resistor R9 which has a resistance of 10kΩ results in a total re...

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