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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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8th Jan 2022, 9:59 am | #1 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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Transformer - very odd readings
Just wondering if I could pick your brains
I am currently building an amplifier and have a transformer rated 5v, 6.3v and 250-0-250v which came with the chassis. It's perfect for my needs. It has 3 voltage settings 200/215 220/230 240/250 The voltage is selected using a selector wander plug type dual connector. I don't know if this has a proper technical name. You can see the transformer in the picture. I took some readings to check the voltage output with a 5v4gt rectifier in situ, but no other valves as the majority of the wiring isn't done yet. 240/250v transformer setting: Rectifier filament 5.8v AC under load (2A) - should be 5v Instead of 500v AC between the 250-0-250 I'm getting 620v, but this is with no load present. DC resistance of primary 22 ohms. Readings confirmed with 2 different multimeters Mains AC 242v 200/215v transformer setting Rectifier filament spot on 5v AC under 2a load 500v AC spot on between 250-0-250 transformer output (no load) DC resistance of primary 28 ohms 220/230v setting Rectifier filament 5.4v AC under 2a load (ie in between the above two) Based upon these readings, am I correct in my assumption that the transformer voltage labels are in reverse? i.e the 200/215 setting is actually the 240/250 and vice versa. I've never come across this before. Thanks Gabriel |
8th Jan 2022, 10:09 am | #2 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,173
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Re: Transformer - very odd readings
Hi Gabriel, it does sound a bit like it. To confirm, with 240v (measure) applied to the 240/250 setting, measure the mains outputs on the other taps and see if they are greater or less than the inputs.
To avoid any saturation effects it might be better to do this test at 200v if you have a variac; radios between the tapping will not alter. As a double check try using an analogue meter (Avo etc) as some DVM's can misread, especially on resistance ranges and distorted AC waveforms on inductive devices Ed |
8th Jan 2022, 10:38 am | #3 |
Banned
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
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Re: Transformer - very odd readings
As Ed has already stated.
BUT with only a single loading of the rectifier valve your readings " are very " close to what I expect for a transformer of that type. Build your amplfier NEATLY and then measure your voltages. I will bet they come within a few percent of requirements. REMEMBER::: Voltages within +/- 10 % are perfect for valves. read ANY valave manual!! ALL of them state quote: Voltages within plus or minus 10% is perfectly normal Unquote. Joe |
8th Jan 2022, 10:49 am | #4 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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Re: Transformer - very odd readings
Hi ed, thanks for the prompt reply.
I've tried both the digital and analogue multimeters as I don't trust the digital one especially at low AC readings. The readings are consistent So, with the connector between common and 240/250v, the voltage readings are as follows 278v on the 220/230 setting 300v on the 200/215 setting This confirms that the voltage labels are reversed right? The transformer was humming a bit more than I expected which prompted me to check - running it on the other voltage settings cures this. Gabriel |
8th Jan 2022, 11:09 am | #5 | |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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Re: Transformer - very odd readings
Quote:
Last edited by Gabe001; 8th Jan 2022 at 11:23 am. |
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8th Jan 2022, 1:45 pm | #6 | |
Rest in Peace
Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
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Re: Transformer - very odd readings
Quote:
I had a similar anomaly where a transformer fitted to a motor controller was wired with the two different secondaries interchanged, so one rectified voltage was too low and the other was too high. No idea how long it had been like that, I only opened it because oil had leaked into it and discovered the electrolytics on the rail that was too high looking very distressed. |
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8th Jan 2022, 1:54 pm | #7 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Transformer - very odd readings
I would have thought that a simple resistance reading across the mains primary would confirm if the voltage taps have been wired incorrectly. J.
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8th Jan 2022, 3:54 pm | #8 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,173
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Re: Transformer - very odd readings
Hi, to absolutely confirm reversal of voltage tappings, and in the absence of a variac, check resistances with an analogue meter. the should progressively increase as you go up wards.
OR With a lamp limiter connected measure the voltages on all the voltage taps, with mains applied between 0 and 200V. (voltages will be LOW, but this protects the windings). If the connections are reversed the voltages will now gow down as you go up the tappings Ed |
8th Jan 2022, 3:55 pm | #9 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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Re: Transformer - very odd readings
Lucien, I suspect it was like this from new. It doesn't appear like it's been meddled with
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12th Jan 2022, 4:01 pm | #10 |
Dekatron
Join Date: May 2010
Location: Greater Manchester, UK.
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Re: Transformer - very odd readings
Could you please post the resistance measured across the pins of the mains plug corresponding to the three positions of the mains selector?
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12th Jan 2022, 6:05 pm | #11 |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
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Re: Transformer - very odd readings
While +-10% is perfectly normal, Philips tube manuals usually state that for an optimal life, the valve heater voltages (currents in case of series heater strings) should be kept within 5% of their nominal value.
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12th Jan 2022, 8:29 pm | #12 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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Re: Transformer - very odd readings
Graham, I did post resistances in my first post.
On the highest voltage setting, 240/250, resistance 22 ohms On the lowest voltage setting, 200/215 resistance 28 ohms The middle voltage reading gives a resistance someone in between the 2, but I can't remember what it was. The labels are definitely wrong. The voltage readings also confirm this. I wonder if anyone has come across it before?. Easy to miss if you just measure HT, especially if the rectifier is past it's best. |
13th Jan 2022, 1:06 pm | #13 |
Dekatron
Join Date: May 2010
Location: Greater Manchester, UK.
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Re: Transformer - very odd readings
Sorry for not reading your initial post properly.
I can imagine a scenario where the paxolin was put into whatever held it for screen printing the wrong way round, and this went unnoticed because of the symmetry. I wonder how many others were made?
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-- Graham. G3ZVT |
15th Jan 2022, 12:13 pm | #14 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,013
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Re: Transformer - very odd readings
Wouldn't it more likely be an error by the wireman?
I would think it should be possible to lift the end cheek and swap over the two wires. The middle one is already in the right place! |