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04-13-2022, 10:30 PM
Hi Rick
As a reference, touch the leads of the ohmmeter together to see what your meter says is "zero", then subtract this from low-R readings you make. Obviously, if the reading is in tens of Ohms or more, then the probe resistance is insignificant.
The winding DCR on large PTs can be pretty low. According to the single data sheet I could find this PT is about 262VA total. The plate winding is 190VA and with solid-state rectification could support a 70-80W amp.
What is the recent history of the application? Is this in a restored PSU using oil-filled or paper caps? The latter are complete garbage with limited life and should be replaced with plastic if one wants to avoid electrolytics.
Of course, you should use a Power Limiting Safety Socket to test this unit. The PLSS uses lamps to limit power,; a variac provides no protection at all. Test without tubes initially, and a low-wattage lamp. Pre-connect the meter to high-voltages prior to powering up. A PT with an open primary can often pass the unloaded test and provide output voltage, but then even a tiny load will show up its flaw. You are measuring low resistances, so your concern would be more of winding-to-winding shorts. This might happen with a paper-bobbin PT that has been in a moist environment.
The PT is not necessarily the problem here, so just be methodical and control the power input while doing baby-step-tests.
Have fun
As a reference, touch the leads of the ohmmeter together to see what your meter says is "zero", then subtract this from low-R readings you make. Obviously, if the reading is in tens of Ohms or more, then the probe resistance is insignificant.
The winding DCR on large PTs can be pretty low. According to the single data sheet I could find this PT is about 262VA total. The plate winding is 190VA and with solid-state rectification could support a 70-80W amp.
What is the recent history of the application? Is this in a restored PSU using oil-filled or paper caps? The latter are complete garbage with limited life and should be replaced with plastic if one wants to avoid electrolytics.
Of course, you should use a Power Limiting Safety Socket to test this unit. The PLSS uses lamps to limit power,; a variac provides no protection at all. Test without tubes initially, and a low-wattage lamp. Pre-connect the meter to high-voltages prior to powering up. A PT with an open primary can often pass the unloaded test and provide output voltage, but then even a tiny load will show up its flaw. You are measuring low resistances, so your concern would be more of winding-to-winding shorts. This might happen with a paper-bobbin PT that has been in a moist environment.
The PT is not necessarily the problem here, so just be methodical and control the power input while doing baby-step-tests.
Have fun
