Kriesler 11-81 transformer temperature
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Location: Melbourne, VIC
Member since 19 May 2014
Member #: 1577
Postcount: 101
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Hi,
I got a Kriesler 11-81 and have replaced all the paper and elect capacitors.
However, I noticed the temperature of the transformer goes up from about 25C to 50C in an hour.
Is this normal for this radio?
Any easy ways I can test the main transformer for shorts?
Thanks.
Kevin
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Administrator
Location: Naremburn, NSW
Member since 15 November 2005
Member #: 1
Postcount: 7290
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50° does seem a bit warm, though some transformers will get warm depending on the load applied to them.
First thing to do is power off and unplug until the transformer is stone cold. Remove the valves and use a megger (insulation tester) to test for shorts between windings and the core and also shorts between windings. The result should be a few MΩ.
Then do a resistance test on each winding. Use Ohms Law to calculate what the correct resistances should be (results can be slightly different for each model of radio). Compare the actual results to the calculated ones.
If all is good, unplug all the valves (and remove diodes if any exist for rectification of HT) and then power up the radio (do this in a child-free area of course and making sure the radio's chassis cannot fall into your lap). Leave for half an hour. If the transformer still gets hot then it's stuffed and will need to be rewound. If it remains cold then something in the radio's circuit may be consuming too much power. Either that or this model has a transformer that normally gets warm under load.
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A valve a day keeps the transistor away...
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Location: Melbourne, VIC
Member since 19 May 2014
Member #: 1577
Postcount: 101
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Thanks Brad,
I don't have a megger so I did a quick checking of the resistance.
The primary is 61.2Ω, one half of the secondary is 319.6Ω, the other half is 336.8Ω.
The secondary has a centre tap.
Is the measurement make sense?
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Administrator
Location: Naremburn, NSW
Member since 15 November 2005
Member #: 1
Postcount: 7290
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There should be two secondaries, one for the valve filaments & dial lamps and the other for the HT, the HT one will be the one that contains the centre tap. The difference between the two secondary readings you have doesn't seem significant enough to warrant further testing.
61Ω is probably a little low. I don't have the benefit of a circuit so I cannot confirm whether this is correct for this model or not.
I'd follow up now with the no-load test on the transformer.
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A valve a day keeps the transistor away...
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Location: Wangaratta, VIC
Member since 21 February 2009
Member #: 438
Postcount: 5239
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A lot of circuits do not give a primary resistance: We are dealing with inductance there one Philips here is 27 ohm & I think I have seen transformers in the 40 Ohm area. Might have to measure a few.
Marc
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Location: Melbourne, VIC
Member since 19 May 2014
Member #: 1577
Postcount: 101
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Administrator
Location: Naremburn, NSW
Member since 15 November 2005
Member #: 1
Postcount: 7290
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Personally I can't see how that can test for shorts in one of the windings. It might be able to test for an open circuit but I wouldn't be prepared to endorse it. Neons can work under the weirdest conditions and give false positives and negatives because the current flow to make them glow is very small.
One other thing - if you put a 1.5 volt cell across the dial lamp winding it'll momentarily pump around 160 volts out the HT winding and you won't want your fingers anywhere near the tails of that winding. Each time you connect and disconnect the battery the HT winding would produce a flow of current. If you leave the battery connected it'll also put a dead short across it and ruin the battery or even cause it to overheat, as transformers are designed only to work on AC where the voltage constantly changes with the waveform.
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A valve a day keeps the transistor away...
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Location: Melbourne, VIC
Member since 19 May 2014
Member #: 1577
Postcount: 101
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Thanks Brad, I will remove all tubes and do the testing tonight.
By the way, do I need to remove the two dial lamps too?
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Administrator
Location: Naremburn, NSW
Member since 15 November 2005
Member #: 1
Postcount: 7290
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Yes, good idea to remove all dial lamps at the same time.
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A valve a day keeps the transistor away...
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Location: Wangaratta, VIC
Member since 21 February 2009
Member #: 438
Postcount: 5239
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Albeit a 115V transformer I note a US one at 10.5 Ohms (listed on schematic).
If there is a short the transformer will often growl.
You cannot test the resistance of the secondary with good globes in circuit.
50 degrees is not over hot. Run the transformer for a while unloaded (smoke = fail)
In order to insulation test for inter winding leakage both CT & LV earth has to be lifted, do earth leakage at this time as well. If it is bad on the primary an RCD will trip if you earth the chassis.
Marc
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Location: Melbourne, VIC
Member since 19 May 2014
Member #: 1577
Postcount: 101
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Hi Brad,
Did some test with all five valves and two dial lamps off.
22C at start.
27C 30 minutes
32C at one hour.
33C at one hour and half
33C at two hours
it seems get to some kind of themal balance after an hour.
what do you think of the test reault?
Hi Marcc,
do I need a megger to do the insulation test or there are other ways to do it?
Thanks all for your help.
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Administrator
Location: Naremburn, NSW
Member since 15 November 2005
Member #: 1
Postcount: 7290
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A megger is the only way. You need the high voltage they produce to produce an accurate result.
It's odd that the transformer heats up with no load. If you know someone you can loan a megger off then I'd be grabbing the chance. It doesn't matter whether it is a crank job or a modern solid state one, just as long as it can do the tests.
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A valve a day keeps the transistor away...
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Location: Melbourne, VIC
Member since 19 May 2014
Member #: 1577
Postcount: 101
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Hi Brad,
At three hour it is 34C.
If I try to get a merger, what voltage should it support, 1000V ones are ok?
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Administrator
Location: Naremburn, NSW
Member since 15 November 2005
Member #: 1
Postcount: 7290
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500v should be fine, most meggers output that though the more expensive ones go to 1000v. Either way, just remember that they bite if the probes are handled wrongly whilst operating. As Marc said before, the best way to test the transformer is to disconnect all the tails - make sure you label them before disconnecting - so no false results come from the tests.
The rule of thumb is low resistance for continuity tests and high resistance for insulation tests.
If these tests pass, my next move would be to reinstall the transformer and do voltage readings on the two secondaries with the multimeter - make sure the multimeter can handle what will likely come from the HT winding, usually 350-0-350v or thereabouts. If your meter only handles up to 600VAC then avoid testing across the whole HT winding, go between one end and the centre tap and then test from the other end to centre tap. The other winding should put out about 6.3v. If voltages on both secondary windings are down then it is likely there is a short somewhere on the primary winding and perhaps heat being generated by minute arcing when the transformer is under load.
You will need a circuit diagram (hopefully with voltages quoted) to determine what the exact secondary voltages should be.
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A valve a day keeps the transistor away...
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Location: Blue Mountains, NSW
Member since 10 March 2013
Member #: 1312
Postcount: 401
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It's a long shot but it might have moisture built up in the windings. Leaving it with nothing connected but power on for a few hours might dry it out. This once worked for me. Don't leave it unattended though!
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