PSU III Pre and Phono

[bikerhifinut]

220uf suggests they have been "upgraded" from the original 100uF.
When you say the voltage rating isnt right Bob, am I thinking these are possibly only 350V rated? Which I would agree is way too low as until the preamps start conducting HT, the voltages across the HT capacitors could well exceed their rating.
Personally I'd go no lower than 450V, plenty of choice at that or 500V.
I recapped my PSU3 with the standard value 100uF High quality F&T450V wkg, and also added the extra capacitors allowed for on the board with 100uF F&T.
This gives a far greater smoothing effect than a couple of 220uF due to the extra RC filter, explained very well in Morgan Jones' well known tome.
Given a choice I'd always try and obtain the F&T electrolytics, proven and very high quality at a reasonable price.
You may prefer other flavours and that's fine by me Bob, you'd never fit a dodgy part I am certain.
My feeling is that the choke has been damaged due to abuse from a previous owner/bodger and as you imply, perhaps the HT caps have come out in sympathy.

I've just done a quick back of an envelope sum and made a couple of assumptions, namely the primary of the mains transformer is wound for 230V being the nominal UK mains Voltage, and that the actual Voltage is up at the maximum legally allowed of 253V which we get close to here. That would give an off load HT DC of around 390V give or take. So for me even a 400V rated HT capacitor is too close to feel 100% secure.
Andy.

[bob orbell]

I agree Andy, Matthew will be sending me a new board and choke, the latter is a very good price, four cheap 100uF 450v capacitors will be more expensive. On stripping every thing out, I found that the HT diodes are 1N4007's may be Peter C ran out of the faster recovery diodes', who knows, I shall replace with HER108, as I have some, but the heater diodes are Schottky, strange goings on. Bob

[bob orbell]

The new choke and PCB plus four diodes arrived this morning from Matthew, all fitted and the voltage is back up to normal, well a good 12% above normal, so it was the choke that was at fault, the resistance measured on the meter is the same as the old one but the voltage is so much higher, also a very annoying hum has now disappeared completely. Thank you to all who commented and a big thanks to Matthew. Bob

[Richard]

Originally Posted by bob orbell

The new choke and PCB plus four diodes arrived this morning from Matthew, all fitted and the voltage is back up to normal, well a good 12% above normal, so it was the choke that was at fault, the resistance measured on the meter is the same as the old one but the voltage is so much higher, also a very annoying hum has now disappeared completely. Thank you to all who commented and a big thanks to Matthew. Bob

Excellent result Bob (and Matthew!) glad it's sorted. Just a thought Bob, did you change the diodes at the same time as the choke, could it have been a faulty diode causing hum and low voltage? Have you tested the diodes you took out?

[bob orbell]

Hi Richard. What I did was to assemble the new board with all new components, test and measure, all very good but a little too high, then wire in the old choke and measure, the voltage collapsed, wired in the new choke and, bingo back to high voltage, clearly the choke was the problem, and no hum. Bob

[Richard]

Very good Bob, yes strange things these chokes! As I also found certainly we can't take their simplicity as meaning they will be reliable.