WD phono/PSU III clone

Graeme · Post by **Graeme** » Mon Dec 15, 2008 10:12 am

The 8.2 c14/15 are 1%, c16/17 are 2% as i already had them.

Ill get some metal films, and maybe some new caps for c16/17 (they were taken out of an old radiogram

) and see how things are then.

I may order the PCB, i may not for now. It will need rebuilding as its getting a bit messy so ill rebuild with new resitors either PCB or hardwired.

Ill order up tonight and hopefully they will arrive soon. Will give me something to do over christmas!

Mike H · Post by **Mike H** » Mon Dec 15, 2008 12:51 pm

Graeme wrote:Ill have a closer look, see if i can trace it or detect any voltage changes at V1 and report back .

It will be so small at that point I doubt whether you'll be able to see it with a multimeter. Usually needs a 'scope which then needs to be wound right up into the tens of millivolts ranges... don't forget that V2 is amplifying it by maybe X50 - 70 times

Sould there be any voltage at all at the phono outs with no input connected?

Should be zero DC.

Post by **Nick** » Mon Dec 15, 2008 1:01 pm

Also don't trust DVM's without a bit of caution. I have seen DVM's inject their own sampling noise into the circuit under test.

Mike H · Post by **Mike H** » Mon Dec 15, 2008 1:06 pm

Graeme wrote: The arm and wires arent earthed or screened, hard to do that with an air arm. Doesnt seem to matter though. As for oscillation, i know nothing about that but yes, there are 1k grid stoppers.

Cartridge and arm are best earthed through their respective phono leads and the pre-amp sockets, imho. Nowhere else

There is also quite significant roar which remains even without the cart connected to the phono. Both noises die slowly at switch off.

If you sit and listen to this roar, can you detect any speech or music in it? Might be AM radio pick-up, not difficult since local AM stations can be quite powerful!

I assume i can just short out 2 old phono plugs and put them in the inputs?

That'll do nicely

I have no phono chassis to earth, at the moment its basically still breadboarded.

Mmm, in my experience very sensitive pre-amps like this do rather prefer a metal box around them to keep all the crud out (including radio ~ have you had the taxi parked outside your house using his transmitter burst-through effect yet?

)

Ill try and trace the voltage changes some more and try shorting the inputs and see what happens. When we had a scope on it my mains was rock solid.

Aren't you lucky! Or was it just a 'good day'?

Mike H · Post by **Mike H** » Mon Dec 15, 2008 1:22 pm

Graeme wrote:I keep thinking if i'd just be better off buying the PCB. Would this be worthwhile do you think?

Personally don't think would make much if any difference, unless it's simply wired up wrong. ...?

PCB's arent great for component swapping.

No they're not!

Are the kiwame worth the money?

I think so, and do use 'em

You are having problems aren't you? Actually quite bizarre, not seen the like before to be honest

Here's an experiment to try ~ disconnect V1 output cap tp V2 grid, i.e. break the signal path at that point, by isolating V1 from V2 completely, and see what that sounds like. Does the roaring noise and fluctuations disappear when you do that?

Mike H · Post by **Mike H** » Mon Dec 15, 2008 1:56 pm

Now wait a minute you said that the HT on the input side of the choke is rock steady, but on the B+ (o/p) side it fluctuates? Which is what led you to suspect the choke in the first place

This must be low frequency oscillation then, because then the V2 ECC83's when they 'turn on' they increase the supply current hence drop the B+, then when they 'turn off' the current falls so the B+ rises. The circuit's still LF oscillating then. The low freq. is determined by the large values of the supply caps

Did you insert that resistor in the supply between V2 anode resistor and V1 anode resistor? And with the 10uF remaining on the V1 anode resistor side?

I also notice V2 anode resistor is '330k', seems a bit large to me, you don't get much more gain from a triode by simply upping the anode resistor value. Also it goes from there straight into a second EQ network, while it's o/p impedance is very high; I'm wondering whether it should actually be 33k

Post by **Nick** » Mon Dec 15, 2008 3:38 pm

No, its not 33k, the valves are running with bugger all current.

Mike H · Post by **Mike H** » Mon Dec 15, 2008 5:30 pm

Yes by my calculation 330k on V2 gets a gain of about 80.5, and if B+ = 250V and anode = 100V it's 450µA, and makes the cathode @ 1 Volt

V1 with 100k makes X55.5, and if anode is at half HT say then should make 1 - 1.5V on the cathode (if around 1mA)

Post by **Nick** » Mon Dec 15, 2008 5:58 pm

Yep, that all sounds about right from when I built one.

Graeme · Post by **Graeme** » Mon Dec 15, 2008 6:21 pm

You are having problems aren't you? Actually quite bizarre, not seen the like before to be honest

Here's an experiment to try ~ disconnect V1 output cap tp V2 grid, i.e. break the signal path at that point, by isolating V1 from V2 completely, and see what that sounds like. Does the roaring noise and fluctuations disappear when you do that?

Ill give this a go and see what happens.

Graeme · Post by **Graeme** » Mon Dec 15, 2008 6:26 pm

Mike H wrote:Now wait a minute you said that the HT on the input side of the choke is rock steady, but on the B+ (o/p) side it fluctuates? Which is what led you to suspect the choke in the first place

This must be low frequency oscillation then, because then the V2 ECC83's when they 'turn on' they increase the supply current hence drop the B+, then when they 'turn off' the current falls so the B+ rises. The circuit's still LF oscillating then. The low freq. is determined by the large values of the supply caps

Did you insert that resistor in the supply between V2 anode resistor and V1 anode resistor? And with the 10uF remaining on the V1 anode resistor side?

I also notice V2 anode resistor is '330k', seems a bit large to me, you don't get much more gain from a triode by simply upping the anode resistor value. Also it goes from there straight into a second EQ network, while it's o/p impedance is very high; I'm wondering whether it should actually be 33k

Yes, HT before choke rock steady, fluctuating after choke.
The PSU did this even with the phono stage disconnected.

I have replaced the choke but dont know if the voltage still fluctuates as i dont have a 'scope.

Replacing the choke vastly reduced the volume swinging when the phono is run into 5687 but were ignoring 5687 for now.

Replacing the choke made no difference to the random swinging voltage at the phono's outputs.

I fitted the resistor between r29/30 and c12/13, made no difference (that i can tell)

Graeme · Post by **Graeme** » Mon Dec 15, 2008 6:30 pm

No speach in the roar, we are about 100m from a quiet road.

The cart/arm earths. 4 leads from cart to phono's at base of arm, interconnect to phono stage. Thats all, arm isnt earthed, signal wires are not shielded. Its not possible.

It will be getting a metal box soon

Graeme · Post by **Graeme** » Mon Dec 15, 2008 6:35 pm

Oh, ive said it before (i think) but ill say it again. Thanks for all the help on this guys, really appreciated

Mike H · Post by **Mike H** » Mon Dec 15, 2008 7:42 pm

Graeme wrote: Yes, HT before choke rock steady, fluctuating after choke.
The PSU did this even with the phono stage disconnected.

even with it disconnected ...

What the blinking flip is going on there then

...?

Dodgy capacitor?

I fitted the resistor between r29/30 and c12/13, made no difference (that i can tell)

Okay but leave it in anyway

Graeme · Post by **Graeme** » Mon Dec 15, 2008 8:04 pm

I dunno, could be.

They were given to me but i assume they were working fine.

Any way to check?

I could replace them but its a £15 'stab in the dark'

audio-talk

WD phono/PSU III clone

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