UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 9th Sep 2021, 5:29 pm   #1
jmedley
Rest in Peace
 
jmedley's Avatar
 
Join Date: Jun 2012
Location: Milton Keynes, Buckinghamshire, UK.
Posts: 109
Default 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
jmedley is offline  
Old 9th Sep 2021, 8:42 pm   #2
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,171
Default 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
Ed_Dinning is offline  
Old 10th Sep 2021, 12:10 pm   #3
jmedley
Rest in Peace
 
jmedley's Avatar
 
Join Date: Jun 2012
Location: Milton Keynes, Buckinghamshire, UK.
Posts: 109
Default Re: HMV 656 transformer

Thanks Ed I will re-assemble and check it as suggested

Jim
jmedley is offline  
Old 10th Sep 2021, 12:43 pm   #4
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default 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.
PJL is offline  
Old 12th Sep 2021, 1:30 pm   #5
jmedley
Rest in Peace
 
jmedley's Avatar
 
Join Date: Jun 2012
Location: Milton Keynes, Buckinghamshire, UK.
Posts: 109
Default Re: HMV 656 transformer

Thanks PJL and Ed
Back in the shed soon to complete the work, will report back
jmedley is offline  
Old 16th Sep 2021, 8:38 pm   #6
jmedley
Rest in Peace
 
jmedley's Avatar
 
Join Date: Jun 2012
Location: Milton Keynes, Buckinghamshire, UK.
Posts: 109
Default 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
jmedley is offline  
Old 17th Sep 2021, 8:19 pm   #7
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,171
Default 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
Ed_Dinning is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 5:35 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.