PSU III Pre and Phono

[bob orbell]

Hi all, I have been correcting a Phono III for a chum after they drilled the board holes out to fit bigger components , New PCB put this right, he then got back to me about a lot of hum when using Phono and CD, so I have all three units here on the bench and for two days been scratching my head, with nothing connected to PSU the voltage took around 9 seconds to reach 370 volts, under load Pre was 209 volts and Phono was 211 volts, I chatted to Matthew questioning the resistor values as I don't have a PSU III circuit diagram, any chance of supplying me with one Greg, thanks, any how I think I have found the problem, the choke, I removed the choke and fitted a 510 ohm resistor across the pins on the board, voltage now rises almost instantly with too much at Pre and Phono. Has any one had a choke break down like this ? it measures just over 500 ohms. I there any better test to confirm good or bad. Thanks'. Bob

[bikerhifinut]

That's a weird one and no mistake Bob.
The choke DCR we can measure against a good un, that should give you an Idea.
500R seems a bit high to me if thats what your choke measures. If you have the means check the choke fora nominal 15H inductance.
Anyway if the PSU has been abused, and I'm certain you've checked the rectifiers< Heater Reg and Mains Transformer for issues, I think I'd swap the HT reservoir and smoothing caps in case theres been some issue, or at least measure capacitance, leakage and ESR. Again I'd expect you to have done that.
As a get you going fix for your pal, There is no electronic reason why you can't use a C R C filter rather than a C L C, and you can have another R C filter downstream by using the redundant capacitor spaces on the board which adds another stage of filtering.
The choke has a current rating of I think 30mA, looking at the size of it I'd say thats an absolute maximumas it's very small for a 15Hchoke, and it may be a previous owner has allowed this to be exceeded.
So Bob, try the simple CRCRC filtering, probably cheaper than a new choke and if it's satisfactory off you go and see how your pal likes it as it's his kit after all.
After that and here's where yours and my experience fits in, you can either get a choke as a spare from Matthew.................... Or get a Broskie regulated board at whatever price they are now. I hamfistedly blew the pass device on mine due to a loose wire inside a 4 pin connector, I forgot the name for now so went back to the dodgy PSU 3board and solved all my previous problems with a bunch of new high quality F&T 100uF electrolytics and added the extra RC filter while I had the board on the bench. I'm actually well satisfied with it now so havent bothered repairing the regulated PSU board as I find it very fiddly to remove and replace those devices without wrecking the pcb traks.
That said we both know the regulated board works really well. I found by setting the output volts to the highest of the two required and then by a calculation/measurement of the dropper resistor inside whichever amplifier needs the lower HT it stayed steady as a rock as you'd expect and never felt the need for extra supplies. The transformer is well over specified for the HT and copes well with the Heater demand of 5 x ECC8* types. Runs a bit warm mainly the regulator heatsinks so I never worried.
Andy

[bikerhifinut]

Daft question Bob, when you measured the Voltage with a Resistor to replace the choke and it was too high, was that off load or under load?
As you know, if no current is being drawn then there'll be no voltage drop across any resistors in the HT line.
Mainly as a pointer to the less experienced constructor here.

I know many of you would recoil in horror at the idea of a simple RC filter but it can be very effective and especially if you're dropping Volts anyway the CRCRC style filter can really attenuate ripple hum very effectively indeed.

Andy

[bob orbell]

Thanks' for the reply Andy, the voltage with resistor instead of choke was, no load 380, with load just down a tad on that, as the Phono and Pre started conducting, it was very high at about 360, didn't' leave it switched on too long, so transformer and diodes all good, I will change all the large capacitors and am waiting for a price from Matthew for the choke, I would like to leave this as is, by the way, I now have two Broski regulated PSUs' to power Pre and Phono and all is good. If you don't' want your Reg. board, as I find working on them is no problem, please give me first refusal. Bob

[bikerhifinut]

Sadly Bob the reg board is on its way to a friend of mine locally.
A very highly qualified (Postgraduate degree) and experienced ex BBC and telecomms engineer, in the absolute true sense of the word. One of the Local radio Amateurs round here, we've got a really good local club, no rules no membership fees and all very welcoming and no looking down on a recently licenced novice like me, I'm working up to full licence exam from intermediate at the moment. It's a good associated hobby for me and I do like the real world science involved. At HF things like skin effect and velocity in coaxial cables and even just a length of wire have measurable and real effect.
He's interested in it and will no doubt deconstruct it, iron out any remaining bugs and design and create his own professional PCB if I know Ron.
He's getting into thermionic technology again as a contrast to his usual thing of very complicated Silicon circuitry scattered with logic and computer technology.
On the other hand, I may get some benefit of Ron playing with this PSU if he feels he can improve on it.

Anyway its good to see you're keeping others on the right path with the practical help Bob, I know from my own experience you have done a lot of really good stuff for me.

Andy

[Richard]

Originally Posted by bob orbell

Hi all, I have been correcting a Phono III for a chum after they drilled the board holes out to fit bigger components , New PCB put this right, he then got back to me about a lot of hum when using Phono and CD, so I have all three units here on the bench and for two days been scratching my head, with nothing connected to PSU the voltage took around 9 seconds to reach 370 volts, under load Pre was 209 volts and Phono was 211 volts, I chatted to Matthew questioning the resistor values as I don't have a PSU III circuit diagram, any chance of supplying me with one Greg, thanks, any how I think I have found the problem, the choke, I removed the choke and fitted a 510 ohm resistor across the pins on the board, voltage now rises almost instantly with too much at Pre and Phono. Has any one had a choke break down like this ? it measures just over 500 ohms. I there any better test to confirm good or bad. Thanks'. Bob

Hi Bob,

The following may not be your fault but even a simple choke can give some strange problems. I had a Glasshouse 300BSE which was quiet from switch-on for about a minute then started a gentle hum. Repeatable every time for weeks.

It took a while to track it to the choke which was going short some way into the turns. As it warmed up it lost its choke properties and effectively became just a wire link.

I found it by checking the choke resistance cold (correct value) and warm after the amp had been humming (much less resistance, near short). A new choke fixed it instantly. Try a new choke if you can, 500R sounds about right but who knows what it might be doing.

The last version of PSU3 I have (maybe changed again I don't know) uses the LM1084 Adjustable version with 2 resistors to set voltage. The pcb has WDPSU 3 v3.1 printed on it. I can email you the circuit for that version if it is the one you have. Please send me your email address as I've lost most of my old contacts inc yours.

[bob orbell]

This is all interesting, one thing I did not mention was, at switch on, with the choke fitted and with Pre and Phono as the load, the voltage took some time to get to about 370 then fell much lower than it should, about 190, then after about 30 seconds it started to rise then fell back even lower, then removing the two loads the voltage very slowly came back up, all voltage readings were taken from the choke wire as it renters the PCB. Thanks' for the offer of the CD, yes, that is the same, I will PM my email although I think you will find it in the members list. Bob

[bob orbell]

Just opened my emails and Greg has sent the circuit diagram. Many thanks Greg. Bob

[Richard]

Originally Posted by bob orbell

This is all interesting, one thing I did not mention was, at switch on, with the choke fitted and with Pre and Phono as the load, the voltage took some time to get to about 370 then fell much lower than it should, about 190, then after about 30 seconds it started to rise then fell back even lower, then removing the two loads the voltage very slowly came back up, all voltage readings were taken from the choke wire as it renters the PCB. Thanks' for the offer of the CD, yes, that is the same, I will PM my email although I think you will find it in the members list. Bob

As said the choke may be changing properties with temp but it also sounds like it may be a capacitor fault or a poor connection/umbilical, something not grounded etc.

[bob orbell]

Still waiting for Matthew to get back, I will replace the board and fit new capacitors as the ones in at the moment are 220uF 400volt, I am not happy with the voltage rating. Bob