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Heatercathodeshort · 15th Jun 2021, 6:33 pm

I forgot to mention that I added a 60m/a 1 1/4" fuse in a plastic holder from the hot end of the EHT winding to the HVR2 anode. Don't use a 20mm one. The gap when blown is small enough to create an arc over. The mains transformer is the complete transformer supplying EHT, HT and valve heaters. It is very robust and must have been a very expensive component to manufacture. No doubt war time production experience helped to a manufacture a reliable transformer at reasonable cost. Most mains EHT transformers of the period broke down in service.

Recently a few have broken down due to the peak voltage across the rectifier, all of 12kv punching through the heater winding insulation destroying the transformer. Mike Barker can engineer a very good rewind that no doubt will out live it's owner. [Other guys rewinding transformers are available]

The 60 m/a fuse will protect the transformer should a fault develop in the EHT smoothing capacitor or if the EHT rectifier decides to commit suicide.

I'm always a bit nervous when I post a receiver that has mains derived EHT. You really have to great care when working on chassis of this type. Make sure the chassis is rigid on the bench backed up with wood blocks. make up six or so, you will find them very handy when working on record autochangers etc. A scrap length of 2X4 cut into 6" lengths or so will make service work easier and at the very least may prevent damage to the equipment being serviced and in the case of mains derived EHT systems, your life!
You must prevent the chassis from falling on yourself trapping your hands or arms. Keep all pets, children well away from your work area. Take your time checking everything and if your not sure what you are doing, switch off at the very least and get some advice from this Forum.

Sorry if this sounds a bit Nanny but knowledge is better than ignorance. Take great care. Regards, John

PS Thanks for the info Eddie. I have not seen the EF50 story. I write everything from memory! J.

15th Jun 2021, 6:33 pm	#13
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: PYE D18T 9" table model 1946. I forgot to mention that I added a 60m/a 1 1/4" fuse in a plastic holder from the hot end of the EHT winding to the HVR2 anode. Don't use a 20mm one. The gap when blown is small enough to create an arc over. The mains transformer is the complete transformer supplying EHT, HT and valve heaters. It is very robust and must have been a very expensive component to manufacture. No doubt war time production experience helped to a manufacture a reliable transformer at reasonable cost. Most mains EHT transformers of the period broke down in service. Recently a few have broken down due to the peak voltage across the rectifier, all of 12kv punching through the heater winding insulation destroying the transformer. Mike Barker can engineer a very good rewind that no doubt will out live it's owner. [Other guys rewinding transformers are available] The 60 m/a fuse will protect the transformer should a fault develop in the EHT smoothing capacitor or if the EHT rectifier decides to commit suicide. I'm always a bit nervous when I post a receiver that has mains derived EHT. You really have to great care when working on chassis of this type. Make sure the chassis is rigid on the bench backed up with wood blocks. make up six or so, you will find them very handy when working on record autochangers etc. A scrap length of 2X4 cut into 6" lengths or so will make service work easier and at the very least may prevent damage to the equipment being serviced and in the case of mains derived EHT systems, your life! You must prevent the chassis from falling on yourself trapping your hands or arms. Keep all pets, children well away from your work area. Take your time checking everything and if your not sure what you are doing, switch off at the very least and get some advice from this Forum. Sorry if this sounds a bit Nanny but knowledge is better than ignorance. Take great care. Regards, John PS Thanks for the info Eddie. I have not seen the EF50 story. I write everything from memory! J.