27-07-2020, 01:45 PM
(This post was last modified: 27-07-2020, 01:55 PM by Mike Watterson.)
The HT caps in the PSU (and transformer) are perfect. The 3000uF measures only 168uF. I find generally it's the low voltage electrolytics die more often than HT. The HT often reform. Even at the start the HTT leakage at 30V was less than 0.5mA and rapidly dropped to under 150uA for both in parallel.
The LT caps ominously had almost no leakage and the 3000uF felt very light!
The Westinghouse Selenium rectifier. has the LT regulator, pair of LT rectifiers and HT rectifier all one stick. I presume Selenium. It's very high resistance. I've seen few Selenium rectifiers that have been OK, or if OK that didn't fail in a few months of use. I'm not fussed on originality under the chassis on 1950 and later gear unless it's something very rare/special. I'd rather leave a pre-1929 set unmolested and display only. 1929 and early 1930s I might leave parts in place and snip one end.
Now to find a mains cable and test on dummy loads and no loads. I've made some changes:
Small 1000V 2A bridge on HT instead of halfwave.
Change centre tap of LT to be 0V instead of +LT and fit a TO200 cased dual Schottky rectifier. I've maybe a 100+ 35V PIV parts and a sack of 10V PIV parts (maybe for a 3.3V switcher); big box of parts given to me 20+ years ago. I've fitted a 35V part. The winding might be a 6.3V centre tapped or a 12V centre tapped, I'm not sure. I'll measure. Increased the 21 Ohm to 33 Ohm (30 on schematic) and the 1000uF -> 2200uF and 3000uF to 4700uF. Selenium shunt regulator is replaced by 2 x 1N5408 which can dissipate more than the 1N4007. Lower voltage rectifiers tend to give a lower forward drop when used as a regulator. The voltage rises as they heat up, so the 1N5408 don't rise as much from self heating having the big fat legs.
I'm tempted to replace the pea sized pilot lamp across the full AC of the LT winding with a tricolour LED. Red for too high, blue for too low and "white" for OK. Colour vision impairment friendly. Except that would need an LM393 and some diodes.
Dummy loads:
12 Ohms for LT
10K for HT, or maybe 8.2K Ohms.
Also always test with no load. There is no HT bleed resistor to limit HT when radio is off. However, if it's less than 150V it's probably fine. About 80 to 85V is fine for HT and 1.3V to 1.37V for LT on a PSU on load. No more than 1.5V LT with no load (consider if the O/P valve fails, falls out or is removed?).
The LT caps ominously had almost no leakage and the 3000uF felt very light!
The Westinghouse Selenium rectifier. has the LT regulator, pair of LT rectifiers and HT rectifier all one stick. I presume Selenium. It's very high resistance. I've seen few Selenium rectifiers that have been OK, or if OK that didn't fail in a few months of use. I'm not fussed on originality under the chassis on 1950 and later gear unless it's something very rare/special. I'd rather leave a pre-1929 set unmolested and display only. 1929 and early 1930s I might leave parts in place and snip one end.
Now to find a mains cable and test on dummy loads and no loads. I've made some changes:
Small 1000V 2A bridge on HT instead of halfwave.
Change centre tap of LT to be 0V instead of +LT and fit a TO200 cased dual Schottky rectifier. I've maybe a 100+ 35V PIV parts and a sack of 10V PIV parts (maybe for a 3.3V switcher); big box of parts given to me 20+ years ago. I've fitted a 35V part. The winding might be a 6.3V centre tapped or a 12V centre tapped, I'm not sure. I'll measure. Increased the 21 Ohm to 33 Ohm (30 on schematic) and the 1000uF -> 2200uF and 3000uF to 4700uF. Selenium shunt regulator is replaced by 2 x 1N5408 which can dissipate more than the 1N4007. Lower voltage rectifiers tend to give a lower forward drop when used as a regulator. The voltage rises as they heat up, so the 1N5408 don't rise as much from self heating having the big fat legs.
I'm tempted to replace the pea sized pilot lamp across the full AC of the LT winding with a tricolour LED. Red for too high, blue for too low and "white" for OK. Colour vision impairment friendly. Except that would need an LM393 and some diodes.
Dummy loads:
12 Ohms for LT
10K for HT, or maybe 8.2K Ohms.
Also always test with no load. There is no HT bleed resistor to limit HT when radio is off. However, if it's less than 150V it's probably fine. About 80 to 85V is fine for HT and 1.3V to 1.37V for LT on a PSU on load. No more than 1.5V LT with no load (consider if the O/P valve fails, falls out or is removed?).
