Phono3S unstable........again.

[bikerhifinut]

I have an issue with the Phono3S again in that its showing an audible oscillation which sounds like a faint, fastish "motorboating" effect that "Pulses" at around 1 to 2 seconds intervals.
It's suggesting to me that its capacitor(s) related as if its charging up and then discharging with a time constant of around 1 second or so, combined with another faster oscillation.
I have eliminated any PSU issues coming in, the regulated PSU is rock steady and I swapped out the bypass/decoupling capacitor across the HT after the resistor from the power socket.
Valves were changed out and I can say that the CF output stage appears stable, and changing V1 and V2 had no effect.
The noise occurs on both channels and that's baffling as if an HT decoupling capacitor had failed or gone fauty you'd expect any problems to follow the dodgy capacitor on the relevant channel, unless the noise can get past the filters made by the opposite channels decoupling caps and the main HT decoupler.
I reflowed the solder on the valve bases as on inspection they looked a bit sparse on the solder, just in case there was dry joint somewhere.
So I'm left with the choices of replacing all 4 10uF HT decouplers in case its one of them, I'll have to buy a batch in as I only have 4.7uF High voltage caps in the toybox, they'd probably do at a pinch but I figured 10uF was specified for a reason for the filters.
It seems to be scattered with what look like fairly ordinary Carbon film resistors throughout but whether one of those has broken down I wouldnt be able to tell as yet, and if i have to start replacing them I might as well just order a new PCB of Matthew and rebuild the whole thing.
For the record, I didnt assemble this kit, I bought it used from a dealer and he'd been told it had been "professionally" built for the original customer. Those of you who can remember that far back will know I had no end of issues with the kit which were partially rectified once we'd realised the grounding arrangements had been totally screwed up by the original builder. Things like the lift resistor not being in circuit and the mains earth being run to signal earth which I think was done with some sort of misplaced belief that the unit would be somehow unsafe if the signal earths weren't electrically connected to the PSU chassis which of course is safety bonded to earth.
It's never really been "right" and this again makes me wonder if thats why I dont have the same regard for the Phono3 as most of the guys on here, it's quite a noisy beast in my view, the background noise is much more than a faint hiss and theres always been odd rumbles and rustles and Valve noise, no matter what valves were in it.
Or to put it another way, I prefer my Homebrew silicon based MM Phono stage in just about every regard.
I don't think would be being fair to the Phono3 to write it off completely though, as said before it's never really worked properly, and the only way I cured the initial very slow sub 1Hz instability was to replace the PSU3 innards for a regulated HT PSU, which really suggests to me I treated the symptoms rather than the illness. Having said that with the original PSU3 I suffered quite bad LF instability with any preamplifier i used so maybe there was an indiced fault in the original build there. I've got a spare unused PSU3 PCB in my posession and I just might build it up in another PSU and see what I get.
Any ideas guys? Do I cut my losses and get a new PCB and rebuild with nice new quality parts, most of which I have in stock anyway, or do I try and trace the issue down?
Andy.

[bob orbell]

Andy, do you still have the 100uF capacitor and the 1K resistor fitted in Phono ? if you have, remove them, I had a very similar experience with mine after building the second regulated supply. Regulator at 250v and power direct to Phono PCB via cable and plug socket. Bob

[bikerhifinut]

I'll give it a go Bob, but why that would make any difference makes no sense electronically as the purpose of the 100uF is to decouple the HT rail coming in off the 1k resistor which acts as an RC filter, the idea is to prevent hum and instability being coupled in via the PSU. Which is also what the 10uF high voltage capacitors on the board are doing.
It needs a lot done to it anyway. And i have a feeling that its very sensitive to valve noise, I have unused Valves here that are very noisy when used in the Phono3 at the mV level of sensitivity, the best set seems to be a little used pair of EH 12AX7EH from Russia.
Even the original JJ Valves are bad, these go berserk in the Phono3 and hoot like eckmondwike.

I will be on the dog and bone to Matthew with a view to pricing up a new board and some other bits I'd like to do a bit of work on the Transformer output Pre4 clone.

I just have a feeling that this Phono wasn't right to begin with and to save myself a lot of wasted effort tracing enigmatic faults I figured a rebuild of the board would be the least painful way to sort it out.

I intend to use high quality but standard passives like 1% Metal film resistors and decent polyprop caps where appropriate, given that like you Bob I think the original fitment "Orange drop" Polyesters on the CF output are about as good as it gets.

Andy

[bob orbell]

Andy, my Phono now has Wima MKP10 capacitors the .1 and 2.2uF, they are quite pricy but they are very neutral with no harshness to the sound, also I now have Charcroft Z-Foil resistors in the first stage, bloody pricy but worth every penny, it is very quiet. Bob

[bikerhifinut]

Well there's a turn up for the books.
Would you believe it was the bloody Squeezebox touch causing the problem.
I think its either starting to develop a failing component thats causing bursts of RF or it could be the WiFi, I eliminated the SMPS it uses so there it is, cured by moving it a couple of feet horizontally. But weird as its never been a problem beforeand doesnt interfere with the Silicon based RIAA stage. Mind you the homebrew Silicon stage using an old Hitachi chip that was deliberately designed for use in Feedback based EQ systems is housed in an all enveloping hammond extruded case. I had thought the WD metal case would shield from that kind of thing but apparently not. I think its getting in directly via the valves themselves rather than down the wires but who knows.
I've done a quick valve roll too, the Amperex Dutch made ECC83 have gone noisy and I eventually found the EH and a couple of JJ were the quietest as were also my collection of US made 5751. It's running a pair of GE grey plate 5751 at the front with a Black plate 5751of unknownmanufacture except its known to be a US made job so it'll likely be Sylvania or some such like.
I quite like the 5751 there its a nice lively sound playing Groundhogs "Blues Requiem" remaster which shows up any flaws in the higher frequencies.
I think I might leave them in for now.
All this after replacing the 4 10uF HT decoupling capacitors which was a pain and I had to do a wire bodge on the PCB track on one as the through plating had disappeared so I put a wire link across to the other end of the 1k resistor off the HT.
I may yet do a full rebuild as theValve sockets are frankly not very good and one definitely makes a racket if you gently waggle the valve, its either a lousy contact or a dodgy solder joint, again through plated , but they arent as good as the Gold plated ones that Matthew and others supply.
All grist to the mill, and if I do rebuild it I will make it a fairly basic version, I never use the LF filters anyway and I am with Bob on the debate over the 7.5uS correction.
Panic over for now guys.
Andy

[bikerhifinut]

Originally Posted by bob orbell

Andy, my Phono now has Wima MKP10 capacitors the .1 and 2.2uF, they are quite pricy but they are very neutral with no harshness to the sound, also I now have Charcroft Z-Foil resistors in the first stage, bloody pricy but worth every penny, it is very quiet. Bob

Your money, your choice Bob. If it floats your boat and you can hear the difference that's all that matters, my experience and my opinions are only relevant to myself.

I wouldnt have thought WIMA MKP poplyprop jobs were that expensive, you live and learn, I use a lot of the smaller WIMA polyprops in stuff like EQ circuits etc and never thought them expensive in the same way that a specialist polyprop can be. Anyone for a pair of huge WIMA 0.1 rated at 1250V?

Personally I doubt my cloth ears can tell the difference between a cooking Metal film and one of those fancy ones and I'm perfectly happy with the supplied capacitors, Ive had my time spending money on boutique parts and know where I personally stand on it now.

Most of the noise on the Phono3 comes from the Valves themselves and thats a can of worms, suffice to say you can keep your overpriced "NOS" Mullards etc as most of the ones I have bagged in recent years have turned out to be less than optimal. I'll stick with new production and say it softly but I have a pair of no name Chinese 12AU7 in the Broskie CF stage that I got from RS and I reckon theres nothing to choose between those and my stash of ex BBC Brimar 12AU7.

Cheers.

Andy.