fini Posted January 26, 2005 Share Posted January 26, 2005 In a stand-alone power transformer (where I know what the voltages should be), how can I check to be reasonably sure it is in good working order, using a multimeter? I would assume you're never 100% sure until it's in a circuit, that's why the "reasonably" part. Quote Link to comment Share on other sites More sharing options...
Marvel Posted January 26, 2005 Share Posted January 26, 2005 fini, Transformers work in ratios. If a power tranny has a 117v pri. and a 234v sec., this is pretty obvious, a 1 to 2 ratio. If your wall/line voltage is higher, you will get a higher voltage out of the sec. That is the ideal. If there is no load on the transformer, this should work or be very close. A load will start to give you slight variations on this. This doesn't give you any idea what the current (ampere) rating is, but you can check the voltages pretty accurately. It is the same with output transformers too. A 10 - 1 output tranny connected to an 8 ohm load would mate well with 800 ohms on the prim. side. This is where the OPTs are made for specific tubes and their plate resistance for a given speaker load. Marvel Quote Link to comment Share on other sites More sharing options...
fini Posted January 26, 2005 Author Share Posted January 26, 2005 Dumb question: So, I must apply line voltage to check it? Quote Link to comment Share on other sites More sharing options...
3dzapper Posted January 26, 2005 Share Posted January 26, 2005 If the primary measures say 100 ohms and the output voltage should be 500V with 100V in the secondary should read 500 ohms. In other words the ratios still hold. This method of measuring is more applicable to see if a known good trannie will work for a given useage. This is inexact and does not test for dynamic shorts but will tell you if it is open or GROSSLY shorted. Your "bad" MarkIV trannie would probably measure "good" by this method. Rick Quote Link to comment Share on other sites More sharing options...
JJkizak Posted January 26, 2005 Share Posted January 26, 2005 Remember that with the circuit energized you will get the normal voltage readings even if the transformer is open. So disconnect the wires and use an ohmeter. Also check for smoke and a nice burned smell. If its the output transformer you might have to sell your grandmother to get another one as they were almost designed for the particular amp in power handling and frequency response capability. JJK Quote Link to comment Share on other sites More sharing options...
Marvel Posted January 26, 2005 Share Posted January 26, 2005 JJK, What do you mean by "tansformer open"? The power tranny will not give you normal voltage readings with an open winding. fini, Ohms if you do it without power and AC Volts with power. Marvel Quote Link to comment Share on other sites More sharing options...
jtkinney Posted January 26, 2005 Share Posted January 26, 2005 The resistance of the primary and secondary will not follow the turns/voltage ratio. The resistance is dependent on the wire size and length, and there will not be the same size or length of wire in both primary and secondary. Fini, this is where you need that variac. It is a lot easier and safer to apply a low voltage to one side (primary or secondary) and measure the voltage on the other. This is especially true for the plate power supply, which may run hundreds to thousands of volts. I have a couple of Ham Radio amplifiers that run 3500 to 4000 volts at 1-2 amps. I don't have anything that will measure that high a voltage, and I'm too chicken to try anyway. Quote Link to comment Share on other sites More sharing options...
JJkizak Posted January 26, 2005 Share Posted January 26, 2005 Marvel: Yes it will. Many a tech has been caught on this one. Voltages check ok but the dingus no workie. JJK Quote Link to comment Share on other sites More sharing options...
Daddy Dee Posted January 26, 2005 Share Posted January 26, 2005 How does a novice check a transformer? tehehe... When I saw the thread title, I was expecting a punchline.... especially when fini is the poster. Quote Link to comment Share on other sites More sharing options...
D-MAN Posted January 26, 2005 Share Posted January 26, 2005 ---------------- On 1/26/2005 2:12:35 PM Daddy Dee wrote: How does a novice check a transformer? tehehe... When I saw the thread title, I was expecting a punchline.... especially when fini is the poster. ---------------- My first thought was the infamous "SMOKE TEST". Remember SMOKE = BROKE. Or "if it smells bad, it IS bad"... DM Quote Link to comment Share on other sites More sharing options...
BUDA Posted January 26, 2005 Share Posted January 26, 2005 Very Carefully!!!! looking very intensely at the windings and with a ohm meter check to see if it changes if no change is detected,it has not transform.In this case follow instructions given by Marvel. Quote Link to comment Share on other sites More sharing options...
BUDA Posted January 26, 2005 Share Posted January 26, 2005 Very Carefully!!!! looking very intensely at the windings and with a ohm meter check to see if it changes if no change is detected,it has not transform.In this case follow instructions given by Marvel. Quote Link to comment Share on other sites More sharing options...
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