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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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9th Sep 2021, 5:29 pm | #1 |
Rest in Peace
Join Date: Jun 2012
Location: Milton Keynes, Buckinghamshire, UK.
Posts: 109
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HMV 656 transformer
The original mains transformer is overheating.
I have another transformer of similar specs and good windings Having removed the laminations to check the wiring insulation that emerges next to them The laminations which are U and E shaped have a white-ish insulating coating one side and flat grey on the other, there is also some rust present. Some of the coating has gone in places How should I proceed with re-furbishing the laminations? Your comments appreciated Thanks Jim |
9th Sep 2021, 8:42 pm | #2 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,171
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Re: HMV 656 transformer
Hi Jim, I suspect that the oxide coating on the laminations will have little effect on the heating.
If the lams were not butted together closely mag current will increase and this can cause overheating If uninsulated clamping bolts for the core are used this can have the same effect. It could also be a short or low resistance on the sec of the transformer. If the 2 transformers are indeed the same, try some "compare and contrast" measurements Ed |
10th Sep 2021, 12:10 pm | #3 |
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Join Date: Jun 2012
Location: Milton Keynes, Buckinghamshire, UK.
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Re: HMV 656 transformer
Thanks Ed I will re-assemble and check it as suggested
Jim |
10th Sep 2021, 12:43 pm | #4 |
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Location: Seaford, East Sussex, UK.
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Re: HMV 656 transformer
Did you check it out if circuit? Do you have an insulation tester?
Transformers will get warm to touch even when not loaded. Ed can explain better, but to minimise the number of turns required, the primary is wound with a compromise inductance resulting in some primary current that dissipates as heat due to the winding resistance. |
12th Sep 2021, 1:30 pm | #5 |
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Join Date: Jun 2012
Location: Milton Keynes, Buckinghamshire, UK.
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Re: HMV 656 transformer
Thanks PJL and Ed
Back in the shed soon to complete the work, will report back |
16th Sep 2021, 8:38 pm | #6 |
Rest in Peace
Join Date: Jun 2012
Location: Milton Keynes, Buckinghamshire, UK.
Posts: 109
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Re: HMV 656 transformer
Well I am sure the problem with the transformer is now found
I have an old megger with a winding handle that produces 600volts, a real beast. All of the windings had infinite resistance to earth but unfortunately the secondary centre tapped high voltage winding was leaking to one of the 6 volt secondary windings. The fluke multimeter read it at 500 ohms. The current drawn from the mains, when no loads applied was 115 mA that was with open circuit secondaries and the fault had no effect. Jim |
17th Sep 2021, 8:19 pm | #7 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
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Re: HMV 656 transformer
Hi Jim, quite often the HT sec CT is connected to earth / common. One side of the 6.3V heaters are often also so connected. There may not be a problem, but cross check with the schematic.
500R is quite a bit high for a deliberate connection 115mA is also quite high for the mag current bit not impossible for a large transformer. This can also happen if the transformer is badly constructed with the lams not closed up with minimum gaps. Ed |