UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Comark Early 1905 Insulation Meter Checks

WME_bill · 21st Jan 2019, 11:04 pm

Comark 1905
The voltages across the test terminals should be close to that specified for the range with an infinite load. On the 1000v range, the impedance is 100Mohm fed through 2.2M, so a 10M voltmeter across the 100M will read 10/12.2 of the 1000v. The differences will be worse on the lower voltage ranges.
But your very low voltages on all the ranges suggest that the transformer has failed.
I assume as you talk of a RMS AC voltmeter that you are measuring the DC voltage.
If you are actually finding AC, then you have more troubles with the high voltage rectifiers and the EHT smoothing capacitors as well.
Have you checked it over with the circuit diagram.
I will get mine out of the attic and see what the voltages are, though that won't give you vital information. The results you have show clearly enough that it need some work done on it.
wme_bill

21st Jan 2019, 11:04 pm	#6
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Comark Early 1905 Insulation Meter Checks Comark 1905 The voltages across the test terminals should be close to that specified for the range with an infinite load. On the 1000v range, the impedance is 100Mohm fed through 2.2M, so a 10M voltmeter across the 100M will read 10/12.2 of the 1000v. The differences will be worse on the lower voltage ranges. But your very low voltages on all the ranges suggest that the transformer has failed. I assume as you talk of a RMS AC voltmeter that you are measuring the DC voltage. If you are actually finding AC, then you have more troubles with the high voltage rectifiers and the EHT smoothing capacitors as well. Have you checked it over with the circuit diagram. I will get mine out of the attic and see what the voltages are, though that won't give you vital information. The results you have show clearly enough that it need some work done on it. wme_bill Last edited by WME_bill; 21st Jan 2019 at 11:14 pm.