WD Phono 3 - DC on Output?

[moley]

Thanks that's fantastic.

Strange thing is, when I looked in my parts bin to see if I had a 450v 100uf caps as per the Phono 3S schematic I found two and suddenly remembered these were left over after I had built the kit originally (Phono 3 XL).

I have ordered some 3 watt 1k resistors and will fit these with the caps as per the schematic. Would metal oxide be preferred here?

I just did a quick test with a 5.6K ohm dummy load and the power spike only registers at 2 volts DC when the PSU3 is switched on. Moving the scale down to mV range I am now measuring at -5 to 7 mV DC. The corner frequency calculates as 12.92Hz with this load and 2.2UF caps currently in place.

Will carry out more tests when the resistors arrive and will also try valve swapping. I did find some cheap 0.22UF and 0.1UF poly caps in my spares bin so can try these in place of the 2.2UF clarity caps also.

[bob orbell]

Hi Moley, question, why do you have two capacitors left over ?, and what power supply are you using ?, if the power supply is the WD PSU III are you using a Pre III also, if the answer is no,then the resistor you mention should be 10000 ohms, amusing it is a WD PSU III, have you built it correctly, the Phono 3, or 3S runs at 250 volts DC, have you checked the voltage at the Phono ?. BOB

[moley]

Hi Bob,

I built the kit (Phono 3 & PSU 3) about two years ago and it passed all of the tests for both units as detailed in the instruction manual. I have not had any issues with it in use over the last two years and this has only come to light as I have tested the outputs for DC prior to potentially purchasing an amplifier which is DC coupled at its inputs. I don’t run a Pre 3. (well not yet anyway!)

I recall checking the PSU 3 at the time when I suddenly found I had two 100UF 450V caps left over. I am going to check this when I get it all on the bench but I recall coming to the conclusion that I had been given two pairs of Caps for C1 in the PSU III. I will check again the voltage coming into the Phono 3 also.

I have attached two images of the top right corner of the Phono 3 schematic, the version that I had when I built my kit and the updated version with the additional resistor (R35) and Cap (C17).
Can I just check, are you saying that R35 should be a 10K 3W rather than the 1K shown?

Thanks for your help.

[moley]

Sorry scrap my last message!

I'm reading back through the instructions and clearly it talks about R35 and the options (1K and 10K) I remember carrying out this step, I also remember installing the capacitor C17 100uF also. Again this is detailed in the build instructions that I worked from. I am going to check all of this but I am quite confident that those parts are already in place.

What had thrown me is when comparing the schematic I received with my Phono 3 kit to the Phono 3S, the only differences I can see the addition of R35 (1K / 10K 3W) and C17 (100uF 450v) so I assumed that I had not installed these but they are detailed in the instructions.

Having gone through both schematics, it looks like the only difference between my build and the 3s is the filter options and the fact that C13, C14, C15 and C16 are now 450v rated vs 400v rated in my build.

Is there something else that I am missing? Should I replace these caps with higher rated caps?

[bob orbell]

Hi Moley, there is no difference now, between the 3 and the 3S, they are the same board, it is just the first stage coupling capacitors, Phono 3 uses .1uF 250 volt in C1 and 2 position, 3S uses .033uF, 250 volt in C1 to 6. C13 to 16 are 10uF 450 volt, your 400 volt would still be well inside the working tolerance, and yes R35 for you should be 10000 ohms. If you have been running it with 1000k, or nothing, you may have too much voltage all round, please check. BOB

[moley]

Thanks Bob for the advice, unfortunately I have had to delay getting some time with it on the bench but hopefully I will be able to do this at the weekend coming.

[moley]

Hello folks,

I finally got chance to get my PSUIII and PhonoIII on the test bench.

I swapped all the ECC83 (TJ Full Music) valves around and tested the output of the Phono 3 measuring voltage with a 50K dummy resistor load connected across the output.

The large spike at switch on still occurs and shortly after switch on I still have
an ever changing reading dancing from -40mV to +90mV.

I do have the 100UF cap and correct resistor installed in the Phono III and I do have close to 270V (267v) coming in to the Phono III.

I then ran through all of the checks detailed in the manual. It passed everything apart from the following two tests :


- Check the heater voltage at the heater pin points on the top of
the PCB (HTR+). It should read close to 45 volts DC.
= I get 56 volts

-Remove the negative lead of the multimeter from the earth
terminal and check the heater voltage across the positive and
negative pin points on the PCB (HTR+ and HTR-). It should be
between 5.9 volts and 6.7 volts DC.
= I get 6 volts here so does that mean the 56 volts measured above is inconsequential since I have the correct HTR- & HTR+?

Check the Cathode voltage on pin 8 and pin 3 of the output valve
(junction with R23/R24). It should read about 110v DC.
= I get 118 volts and 124 volts here! Is this potentially linked to my issue of DC offset at the outputs?

I haven't yet ventured to changing the 2.2UF output caps and trying a smaller value to increase the corner frequency of the RC filter.


As always, any help or suggestions gratefully received.

[moley]

I have just tested some of the resistors around the output valve V3a/V3b to check and cross-reference their values against the schematic.

All are OK apart from R27/R28 (1Mohm ) one resistor reads 1Mohm as expected and the other reads 55Kohm!? I can see from the colour coding it is correct and is the same as its partner on the other channel which is reading correctly at 1Mohm. I have checked and re-checked and it still reads 55K. I can't understand why its resistance is so low.

Obviously I need to get in and change it, what effect would this resistor (R27) failing / losing resistance, have on the Phono III?

[bikerhifinut]

Originally Posted by moley

Hello folks,

I finally got chance to get my PSUIII and PhonoIII on the test bench.

I swapped all the ECC83 (TJ Full Music) valves around and tested the output of the Phono 3 measuring voltage with a 50K dummy resistor load connected across the output.

The large spike at switch on still occurs and shortly after switch on I still have
an ever changing reading dancing from -40mV to +90mV.

I do have the 100UF cap and correct resistor installed in the Phono III and I do have close to 270V (267v) coming in to the Phono III.

I then ran through all of the checks detailed in the manual. It passed everything apart from the following two tests :


- Check the heater voltage at the heater pin points on the top of
the PCB (HTR+). It should read close to 45 volts DC.
= I get 56 volts

-Remove the negative lead of the multimeter from the earth
terminal and check the heater voltage across the positive and
negative pin points on the PCB (HTR+ and HTR-). It should be
between 5.9 volts and 6.7 volts DC.
= I get 6 volts here so does that mean the 56 volts measured above is inconsequential since I have the correct HTR- & HTR+?

Check the Cathode voltage on pin 8 and pin 3 of the output valve
(junction with R23/R24). It should read about 110v DC.
= I get 118 volts and 124 volts here! Is this potentially linked to my issue of DC offset at the outputs?

I haven't yet ventured to changing the 2.2UF output caps and trying a smaller value to increase the corner frequency of the RC filter.


As always, any help or suggestions gratefully received.

I wouldn't worry about any of those measurements. All in the "ball park" i reckon. There are factors that will cause a small variation in those values ranging from the actual mains voltage you experience at home, this can vary by around 40 volts depending on where you live, to variations in valve parameters. The Phono3 circuit and PSU3 circuit are relatively simple and thus variations in mains voltage will affect the HT voltage, and component tolerance variations and valve parameters will take care of the other deviations from the nominal design parameters.

My feeling is that you are getting a small amount of very low frequency instability which explains the varying DC figures you get (maybe) and I'd also check for leakage from the DC blocking capacitor (coupling cap) on the output. You should have a resistor from the output end of the coupling capacitor to earth which will bleed any stored charge away. I would be concerned if I measured any DC voltage at all across the output of a capacitor coupled amplifier. You could try sticking a cap in series with the output of your phono3 and the input of your pre or integrated amp. That might at least tell you if the output cap is leaky. I'm afraid that i am a bit suspicious of certain "boutique" capacitors and certainly on initial build of gear i use standard but good quality plastic film capacitors and then i might experiment with other coupling capacitors. Theres some snake oil out there to beware of. Any plastic film caps supplied by matthew/WD will certainly be fit for purpose.
I have to say I use cheap and cheerful "Vishay" 630V DC polyester film capacitors when doing first builds and they havent let me down yet. I do experiment with other brands but always start with a known quantity.

Another thing that springs to mind is that some modern test equipment is just a bit too sensitive, its quite possible to see a small varying voltage in the tens of mV from sources like your own skin creating a voltaic reaction with meter probes (Unlikely here as you ain't daft enough to touch any exposed metal on live gear) down to thermocouple or other galvanic effects from dissimilar metals in contact with each other.
Back in the day when the industry standard tool was an AVO 8 moving coil meter with a 20k/V sensitivity, a lot of the stuff we can measure now just wouldnt have cropped up. Even now I use an old fashioned analogue 20k/V meter when measuring voltages in older Valve kit as thats what the original specs were measured with.

Andy.

[bikerhifinut]

Originally Posted by moley

I have just tested some of the resistors around the output valve V3a/V3b to check and cross-reference their values against the schematic.

All are OK apart from R27/R28 (1Mohm ) one resistor reads 1Mohm as expected and the other reads 55Kohm!? I can see from the colour coding it is correct and is the same as its partner on the other channel which is reading correctly at 1Mohm. I have checked and re-checked and it still reads 55K. I can't understand why its resistance is so low.

Obviously I need to get in and change it, what effect would this resistor (R27) failing / losing resistance, have on the Phono III?

Bugger all i'd say as its the bleed resistor across the output cap. make sure you check it out of circuit as, for instance, a leaky output coupling/dc blocking cap will have the effect of a resistor in parallel. Theres a small amount of attenuation as it forms the bottom leg of a potential divider with the cathode stopper resistor on the output valve. It would lose you roughly 2% of the available signal into the input of the next stage, doubt you'd hear that.
It is possible if unusual for modern resistors to change value from the extremes of short circuit to open circuit and if it does measure low when tested OUT OF CIRCUIT then its prudent to change it anyway as its likely on the way out.

Just realised, even if the 2.2 uF was a dead short the effect of having 1.1meg in parallel with 150k would only get you down to about 130k on the cathode which wouldnt upset the DC conditions fatally.

Daft idea: check the output socket wiring for any solder debris or other contamination that might offer a lower resistance path from the output to earth? And another daft question, did you check the resistance with the output disconnected from the preamp/input of your main amp? because you'll get the effect of its input resistance paralled with the output resistance of your phono3.

andy