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Taylor 130C Insulation tester question

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Marconi_MPT4
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I have a question about adjust zero setting.

Zeroing on the 100k ohm range the control has adequate adjustment and correct setting tends to be just over the pot half way position. Switching to 1000M ohm range the pot has to go fully clockwise and will only just get to the zero mark. This has not changed since acquiring over seven years ago. Is this difference normal for the 130C? I am thinking adjustment is a bit marginal but does anyone have a diagram or service info handy?

Very little data appears to exist for this insulation tester. Chris(OTT), would you like a scan of operating instructions for the technical library?

Cheers
Rich

 
Posted : 21/06/2013 5:48 pm
Marconi_MPT4
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Curiosity finally got the better in the absence of anything else.
The four case screws had never been removed and still factory sealed. Attached to the side was an original Taylor Electrical Instruments Ltd inspection label with serial number.

Inside on one side of PCB are two parallel connected SenTerCel K8/30 selenium rectifiers together with a Welwyn C25 10M ohm 2W 1% 1500V resistor for the 1000M ohm range. Rectifier spec is 2000V PIV with forward current of 5mA.

Viewing the other side we see..... a Hunts 0.25uF 750V capacitor. Did not expect to find one here! Below is a TG 258 120k ohm 30W and a smaller 400 ohm Welwyn C22 1% resistor divider for the 100k ohm range. The 270k is connected via a push button to rapidly charge capacitor connected to the test terminals.

Resistor values measured using Keithley 2000 DMM as: 120.6k (OK), 399.3 ohms (OK), 325.4k (out of spec), 10.34M ohms (3.4% out). The 10M and 270K resistors will need to be replaced.

Voltage measurements show rectifier AC input as 489V with the DC output across 0.25uF Hunts as 449V giving a low test voltage, either leaky capacitor or the rectifiers have a higher forward resistance. A bit more digging is needed.

Cheers
Rich

 
Posted : 25/06/2013 12:39 am
Marconi_MPT4
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Discovered the Hunts capacitor was leaky at about just under 1Meg ohm with 500V test, no surprises there but it was not the cause of low HT. Substituting D1 and D2 with a temporary test diode to hand (BYM26) HT increased to 530V. With test probes shorted the Adjust Zero pot pointer (VR1) is now just left of center. Test current measures just over 52uA.

As the original diodes had significant forward resistance which affects calibration, getting a modern replacement with appropriate PIV rating would need a series resistor to bring HT down to just over 500V with 4mA flowing through 120K(R1)+400R(R2) by select on test.

Attached reverse engineered circuit showing a simple arrangement but took longer to draw out on CAD !

In summary both rectifiers, Hunts capacitor and 10M resistor are out of spec and will be replaced.

Cheers
Rich

 
Posted : 30/06/2013 9:36 pm
Refugee
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From the circuit it looks like the test voltage for the 100K range is much lower that it is for the 1000M range.
Is this correct?

 
Posted : 30/06/2013 9:46 pm
Marconi_MPT4
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From the circuit it looks like the test voltage for the 100K range is much lower that it is for the 1000M range.
Is this correct?

Yes according to instruction sheets, the test voltage is approximately 1.5V purely to measure resistance. No leakage test available on this range. Out of curiosity just measured the open circuit terminal voltage at 1.66V

Hmmm, pressing Push TO Charge button reverse polarities meter as would be expected with 270k wired from 500V HT to output terminal. An undocumented feature! Some 30 odd years back I recall seeing this happen on the workshop tester.

Rich

 
Posted : 30/06/2013 10:11 pm
Refugee
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It should drop to 1.5 volts with 100K connected to the probes. The DMM will be 10 meg.

 
Posted : 01/07/2013 1:17 am
Marconi_MPT4
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Hi Ref,
Drops to just over 1.5V and then continues to fall slightly. Directly measuring resistors one or two are starting to fail in the 100K circuit! These are Welwyn C22 which according to data sheet are high stability panclimatic protected (to DEF-5112) rated at 0.5W 350V.

The DMM used for measurement has an input resistance of >10G ohm on 10 Volt range, so providing minimal circuit load.

Rich

 
Posted : 01/07/2013 8:51 pm
Refugee
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What is showing incorrect on the resistors. Last time I had trouble like that it was a faulty probe on the meter I was measuring it with.

 
Posted : 02/07/2013 12:59 am
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