Post by briano on Jul 7, 2022 17:19:39 GMT 12
I have written further below about the preparation and switch-on of my Grandmother's circa 1939 radio. Some unfortunate news. I conclude that I have a faulty mains transformer which overheats and starts crackling. I have these six questions and would really appreciate your continued help:
QUESTIONS
In regard to my findings further below:
1. Do you agree with my conclusion that the transformer is faulty?
2. Are there any other tests you can recommend?
3. Where should I start looking for a replacement mains power transformer?
4. Is a brand-new transformer ex AliExpress an option, such as the following:
267W, secondary: 320V-0-320V 0.27A; 0-6.3V 4A; 0-6.3V 4A; 0-6.3V 2A; 0-5V 1A
www.aliexpress.com/item/4001118737572.html?spm=a2g0o.seodetail.topbuy.1.6b296bb545NEbq
200W (LAIDYS), secondary: 380V-330V-0-330V-380V 0.20A, 5V/3A, 0V-2.5V-6.3V/3A
www.aliexpress.com/item/4001162964097.html?spm=a2g0o.seodetail.topbuy.1.50c92833SsS0yi
(For the above transformers, filament current rating would be okay. For the HV secondary current, my calculations, at the very bottom, suggest a total valve draw of 0.1A. Although need margin for resistor voltage dividers. And surge current when reservoir capacitors charge at switch-on).
5. Should the 6U5 magic eye have behaved properly on those voltage readings for it in my first diagram?
6. If the 6U5 magic eye is faulty, where would I source a replacement 6U5/6G5/VT-98 magic eye?
COMPONENT CHANGES
Okay, this is what I had done to prepare the radio for its switch-on:
POWERING UP THE RADIO
Light Bulb Test
Transformer Insulation
Radio Performance
SECONDARY TERMINATIONS REMOVED
Voltages
DC Resistances
RESEARCHING POSSIBLE TRANSFORMER REPLACEMENT
My Calculations - New Transformer Requirements
Total Draw 5v Filament = 2.0A
(80 valve = 2.0A)
Total Draw 6.3v Filament = 2.45A
6SK7 = 0.3A
6SA7 = 0.3A
6K7 = 0.3A
6Q7 = 0.3A
6F6 = 0.7A
6U5 = 0.3A
Dial lamp = 0.25A
Secondary Winding
Up to 385V
Valve Current = about 0.1A
6SK7 = 13mA + 4mA = 17mA
6SA7 = 14mA
6K7 = 9.5 + 2.7mA = 12.2mA
6Q7 = 1mA
6F6 = 38mA
6U5 = 4.5mA + 0.2mA = 4.7mA
QUESTIONS
In regard to my findings further below:
1. Do you agree with my conclusion that the transformer is faulty?
2. Are there any other tests you can recommend?
3. Where should I start looking for a replacement mains power transformer?
4. Is a brand-new transformer ex AliExpress an option, such as the following:
267W, secondary: 320V-0-320V 0.27A; 0-6.3V 4A; 0-6.3V 4A; 0-6.3V 2A; 0-5V 1A
www.aliexpress.com/item/4001118737572.html?spm=a2g0o.seodetail.topbuy.1.6b296bb545NEbq
200W (LAIDYS), secondary: 380V-330V-0-330V-380V 0.20A, 5V/3A, 0V-2.5V-6.3V/3A
www.aliexpress.com/item/4001162964097.html?spm=a2g0o.seodetail.topbuy.1.50c92833SsS0yi
(For the above transformers, filament current rating would be okay. For the HV secondary current, my calculations, at the very bottom, suggest a total valve draw of 0.1A. Although need margin for resistor voltage dividers. And surge current when reservoir capacitors charge at switch-on).
5. Should the 6U5 magic eye have behaved properly on those voltage readings for it in my first diagram?
6. If the 6U5 magic eye is faulty, where would I source a replacement 6U5/6G5/VT-98 magic eye?
COMPONENT CHANGES
Okay, this is what I had done to prepare the radio for its switch-on:
- The two power supply electrolytics were replaced with 10µF 600v capacitors
- All wax capacitors were replaced with metallised polyester film 600v capacitors
- A 1-Amp fuse was wired in series with the phase line terminating on the transformer primary
- The tone circuit was rewired, and the 250k-ohm first audio plate load resistor replaced, as per advice from Peter Walsham (270k resistor substituted)
POWERING UP THE RADIO
Light Bulb Test
- A 70w incandescent mains light bulb in series with the mains supply runs brightly
- When the secondary windings of the transformer are disconnected, the 70w incandescent light bulb in series with the mains supply runs at about half brightness
Transformer Insulation
- When the radio is run for a minute or two, the insulation between the windings and the chassis remains good
- When turned off, the 10µF electrolytics hold their charge
- A DC resistance test showed no break-down to chassis. (Unfortunately I do not possess a megger to test when cold).
- The "80" rectifier plates did not run red indicating not a severe current draw on the HT line
- If the radio was left turned on a while, the transformer heated up (over 95°C using infrared surface thermometer), started crackling, both primary and secondary winding broke down to chassis.
- When turned off after transformer overheated, the 10µF electrolytics immediately lost their charge
- A DC resistance test showed 1M-ohm to chassis, which slowly rose to infinity when cooled.
- The 1A fuse in the phase line remained intact and not stressed throughout all tests
Radio Performance
- Once the valves warmed up, the radio was lively and tuned okay on both MW and SW. Sounded great!
- The 6U5 magic eye was extremely dim. Had to 'cup' the valve with the hands, and even them could only just discern a faint green glow.
- The 6U5 heater voltage was only 5v, and presumable other valves were only running with 5v and not 6.3v, lower than expected.
- With the radio running, before the transformer overheated, the HT measured 200v, lower than say, up to 240v DC I would have expected.
- Please see first diagram for other voltages
SECONDARY TERMINATIONS REMOVED
Voltages
- On the secondary, there were uneven AC voltages between CT and each 385 terminal (230v AC / 340v AC)
- Please see second diagram, voltages in blue
- The AC voltage from each side of the secondary to chassis were not the same
DC Resistances
- With no mains applied, uneven resistances between CT and each 385 terminal
- Please see second diagram, resistance measurements in red
- The secondary winding appears to have a short on one half
- Primary winding resistances measured and probably normal
RESEARCHING POSSIBLE TRANSFORMER REPLACEMENT
My Calculations - New Transformer Requirements
Total Draw 5v Filament = 2.0A
(80 valve = 2.0A)
Total Draw 6.3v Filament = 2.45A
6SK7 = 0.3A
6SA7 = 0.3A
6K7 = 0.3A
6Q7 = 0.3A
6F6 = 0.7A
6U5 = 0.3A
Dial lamp = 0.25A
Secondary Winding
Up to 385V
Valve Current = about 0.1A
6SK7 = 13mA + 4mA = 17mA
6SA7 = 14mA
6K7 = 9.5 + 2.7mA = 12.2mA
6Q7 = 1mA
6F6 = 38mA
6U5 = 4.5mA + 0.2mA = 4.7mA