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Tek A6909 - repair and modification

Repair and modify Tek A6909 insulation amplifier for a standard probes.

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A6909 come with a non standard probe connector design for the obsolete P850 Tektronix high voltage probes. The original probes are 10 Mohm that create a 1:31 divider with the internal 333 kOm resistor. A 666 kOhm resistor is added to achieve the 10 MOhm input impedance, and a 5-30 pF trimmer capacitor to compensate the additional resistor.
The output BNC connectors are the original once, just moved to the front panel, and the input connectors are new and they should be insulated.

Development and repair of switching mode power supplies , battery charger, motor drives and many others required use of oscilloscope with isolated channels. Unfortunately those  oscilloscopes are relatedly expensive even if you buy an used one like TPS2xxx series. The TPS2012 can be found on ebay for about $1500 and having on mind that this is only 2 channel 100 Mhz bandwidth scope the purchase make no sense except for people that need scope with isolated channels on daily base but can't afford a new one.  

For people like me that do not use it on daily basis, and already have other scopes, buying such a scope is not the best option, and here the Tek A6909 come in to the game. This is relatively old Sony-Tektronix isolation amplifier produces in two configurations, A6909 - 2 channels and A6907 - 4 channels.

The amplifier is well designed and fully repairable, no custom or potted parts with available service and repair manual /can be found on the link/ 

Those amplifier has two issues, first is the non standard probe connection, the same used in the old portable Tek 2xx scopes. Find such a probe is a challenge, and the easiest solution is to reworked it for a standard 10x probe.  The second problem is the leaking electrolytes. I both 2 A6909 amplifiers, and all 4 channels had this issue, so most probably you will need to replace them even the unit seems working. 

The probe connection rework is relatively simple task. On the 2 channel model both input and output BNC connectors can be installed on the front panel, on the 4 channel model the output connectors should be kept on the back of the unit.  

When replacing the connectors keep on mind that this is isolation amplifier, so you should provide a reliable isolation from the enclosure, and between the channels themselves.  Fortunately the front panel is design around isolation, and the rework is very simple. There is small half flange around the connector that need to be removed to allow proper installation of a standard BNC connector. 

I used a Molex 0731315013 insulated BNC connector soldered to a small prototype board. The purpose of the board is not so much to take the resistor and the trimer but to provide a reliable attachment for the coaxial cable. Keep on mind that the original cable is with very thin central wire that can be broken very easy. 

Since the amplifier impedance is 333.3 kOhm the original probes are with 9.66 Mohm internal resistance and for a proper operation a 666 kOhm resistor need to be add in series to the probe. Hawing on mind that a full calibration will be required anyway the value of the resistor is not so important until is in available for the adjustment range. In my first version i used 1 Mohm resistor, and planing to change it with a selected 665kOhm 1%. The 1% tolerance giving you +/- 6.6 Kohm and there is a good chance to find easy one that is close to the 666.66 kOhm required. A parallel capacitor is required to compensate properly the added resistor. With 1 MOhm resistor a 11-12 pF capacitor is OK, but the best option is to add a 5-30 pF trimmer capacitor. 

Here are the resistor Vishay  RN55D6653FRE6 and the trimmer capacitor Knowles Voltronics JR300 i did order. 

To fix the leaking capacitors you will need the following values :

82 uF /35 V Nichicon UPJ1V820MED1TD, 3 pcs per channel on A10 board 

22 uF / 35 V Nichicon UPJ1V220MDD1TD, 2 pcs per channel on A10 board

22 uF / 25 V Nichicon UPW1E220MDD, 1 pcs per channel on A10 board

10 uF / 25 V Nichicon UEP1E100MDD, Non Polar 1 pcs per channel on A10 board

10 uF / 16V Nichicon UPS1C100MDD1TD, 4 pcs for A6909 and 8 pcs for A6907 on A30 board

220 uF / 25 V Nichicon UPW1E221MPD1TD , 6 pcs for A6909 and 12 pcs for A6907 on A30 board

47 uF / 16 V SMD tantalum  Vishay TMCHC1C476MTRF , 2 pcs for channel on A20 board

220 uF / 35V SMD  Nichicon UWT1V220MCL1GB, 2 pcs for channel on A20 board, plus 2pcs on A50 board....

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A10.pdf

The A10 board schematic. I didn't draw the power supply part of the board, but it's very simple few 78xx, 79xx regulators. The 220 n decoupling capacitors are not measured, I change some of them and the measured value was around 200 n so i set the same value for all of them.

Adobe Portable Document Format - 575.03 kB - 08/22/2023 at 18:53

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  • 1
    Amplifier calibration

    After few months I notice a strange behavior in one of the units, it become that one of the ceramic capacitors was damaged by the electrolyte leak, either I didn't see it, either didn't clean the board very carefully after I replaced all the electrolytes. never the less this was the reason to start digging in to the schematic. Beside the issue the service manual calibration process looks strange and didn't give a good results, so I wanted to find what exactly those trimmers are doing. The 251 and 210 was obvious set the gain and the offset, but 430 is not well described. So after spend two days reversing the schematic, testing and calibrating i get to the following calibration process that is the only one really working. 

    1. You need to open the amplifier, you will work only with A10 board. You need to have an access to the TP300, everything else is a compromise. 

    2. Let the board works for at least 20 min, longer is better, I think the optimum is 30 -60 min. It's important, calibration cold unit wild not work.

    3. You should select 1V , DC range and have 0 V input signal

    4. Adjust R210 until you get few mV on TP300. The perfect will be to have +/- 2 mV but you should keep on mind the next step.

    5. Switch to 100 mV, DC range and check the offset, if the offset larger than +/- 40 mV you should try to adjust it in that range. 

    6. You should do few iterations between steps 5/6 until you get a  reasonable offset on bot ranges. The worst board i have is -30mv at 1 V and + 40mv at 100 mV range.  Those values are still in the range the autocalibration can solve with a good margin left. 

    7. Connect your probes to +5 V. Please not that if you do not use the original probes but 1:10 instead, you should add 655k resistor in series, and 5-40 pf capacitor to compensate the input. 

    8. Using R251 you should set the TP300 to be 500 mV. 

    9. Set the input to 0 v and double check the offset,.

    10. Assemble the amplifier, and let it works for another 20-40 min. 

    11. Set the range to 1V DC, set the input to 0 V and the output, keep on mind that the output should be a 50 Ohm terminated. 

    12. With R430 adjust any offset you have. 

    13. Switch to 100 mV DC and check the offset, if is more than +/- 40 mV try to adjust it using R430 and distribute it equally between the 1V DC and 100 mV DC range. 

    14. Set to 1V DC range and connect the input to a 5 V

    15. Using R251 adjust the output reading to 0.5 V plus the offset you have. For example if your offset is 10 mV you reading should be 510 mV, if your offset is -10 mV your reading should be 490 mV. 

    16. At 1V DC range connect the input to a 1 Khz 10 Vpp source, and the channel output to the scope.

    17. Compensate the probe, at this point you may not be able to have it fully compensated.

    18. Compensate the amplifier input with C100, if you with standard 1:10 probe this will not be enough for proper compensation. 

    19. Adjust the trimer capacitor in your extra compensation network /655 k + 5-40 pF/

    20. You may need to do some iterations between 17-19 until you get the max signal flatness. 

    21. Press Ch1 down + CAL to initialize the memory

    22. Press CAL to start the calibration. If all step correct the calibration should pass successfully. 

    Step 22 calibrate only the offset and the gain for 1V and 100 mV ranges. Further you may perform a full calibration CH2 DWN + CAL.

    R430 set the LED current, since the amplifier use amplitude modulation, the LED current on A10 appear as an offset on the A20 output. You want to have the A20 output with small DC offset when the A10 is properly balanced. There is no way to do this without opening it. 

    Also on two of my amplifiers i notice an issue with the trimmers, mainly R210  / 430. They are very sensitive to a touch, it's very hard to calibrate those amplifiers, especially one of them is impossible, the moment you lift the screwdrivers it changes with 20-30 mV, some times more, so they need to be replaced. 

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