Help - Phono 3 Heater voltage Measurements

[moley]

Hello folks, newbie here seeking some help / direction.

I have just completed a build of the Phono 3 and PSU. It *seems* to be working ok in that I have had it in my system and listened for a whole night. It sounded great and improved as the night wore on.

However there was one measurement on the Phono 3 as per the build manual that didn't tally up and I still can't understand why or where I could have gone wrong especially given that the unit is functioning.

When I test voltage from chassis ground to HTR+ I get 3.4volts and on HTR+ I get -3.1volts. I should be getting around 45 volts at HTR+ as per the manual and the PSU tests fine as does the PSU cable and the pins coming into the Phono 3. I have double checked the socket and can't understand how I am measuring 53 volts coming into the bottom left pin (as seen from the solder lugs) into the grey wire and 60 volts on the bottom right pin (as seen from the solder lugs) going into the brown wire yet when these reach the HTR+ and HTR- PCB points I am only measuring 3.4volts and -3.1volts which is of course no where near the reading of 45 volts as per the manual.

How is this working given the difference in the readings, have i missed something obvious and am testing incorrectly. Measuring between the pins of course gives the correct reading but not from the HTR+ or HTR- to ground.

All other readings appear to be fine in line with the build manuals.

Sorry for the ramble! Many thanks in advance for any help / advice anyone can offer.

[pre65]

From what you have said it sounds like everything is OK.

A more knowledgeable expert might come along soon and confirm that.

[John Caswell]

Hi
What you are describing does not make sense.
If the readings on the socket on the back of the Phono III are correct i.e. 53V and 60V DC then as you only have two wires grey and brown going from the socket to the PCB heater +/- and you say that the heater volts are correct on the heater pins then I suggest that you are measuring incorrectly.
I assume that you are measuring this with the DMM set to 200V DC and not AC (which will give all sorts of strange readings.)
I also assume that you have definitely got the earth pins on the PCB returning to the earth pin on the main smoothing capacitor in the phono III.
Initially I suggest the you switch off, remove the umbilical and all phono III valves and measure continuity across the heater +/- pins on the PCB. It should read open circuit, plug one valve in and the reading should drop to a low figure indicating that the heater wires are continuous (which they must be as you say the Phono III is working OK.
Have a look/check and report back.

John

[bob orbell]

Hi Moley and all I would check your PSU again, is C5 the correct way round, is R2 150k, is R3 22K and is R5 100 ohms. Your voltages are a tad high too. It would be advisable to double check everything. What is the HT value in the Phono at the juction of R29/R31. Do also make sure you have a good connection to the chassis. BOB

[moley]

Hello folks,

Thank you very much for your replies and advice on what to investigate.

-As I have put heat shrink on the back of the binder power socket I can't take a voltage reading from the back of the socket but I have checked the continuity of the pins on the other side to the correct places and there are no shorts between the pins.

-I have low resistance between HTR+ and HTR- until the valves are removed then it becomes open circuit.

-I checked again the voltages coming from the PSU cable by measuring at the pins in the cable and these are correct with 405 volts top left, zero top right, 53 volts bottom left and 60 volts bottom right.

-Both earth connections of green wire show continuity back to the smoothing cap.

- If I measure the junction between r29 AND r31 at the HT+ point I get 403 volts withouth the valves in but with the valves in this becomes 270 volts in line with the manual.

- still measuring the HTR+ to PSU chassis ground (with DC mode on the multimeter) I get
3.49 volts and -3.13 volts on HTR- to PSU chassis ground. This is without the valves in. Reading across HTR+ and HTR- I get 6.62 volts.These readings are repeated across the other two sets of HTR+ and HTR-.


- I checked that there are no shorts between the pins of the PSU cable coming from the PSU and oddly there seems to be a tiny amount of continuity between the bottom two pins which connect to the brown and grey wires in the phono 3. The multimeter gives a tiny very short beep but doesn't give a reading like there is a small connection that comes and then goes. If I leave the test probes on the continuity is gone but if I lift them and then place them back I get a very short quiet beep again.

I removed the PSU cable from the PSU and tested this and the shorts were gone so I got inside the PSU and tested the grey and brown heater PCB points leading to the two PSU sockects and again I get the tiny amounts of continuity that appear, disapper and only reappear if I lift the test probes and place them down again. Is this at all normal? Is this due to continuity that exists within the PSU circuit but my multimeter DC signal is then blocked by some of the components in there?


- With regards to other checks inside the PSU. C5 is the correct way around with the stripe against the negative connection. My voltages look ok looking at the schematic which shows 53 and 60 volts output respectively. R5 is 100 ohms. R2 and R3 look correct per the coded bandings, although putting a multimeter on them to check, the reading keeps slowly increasing. In the case of R2 it climbs from 30k slowly up past 150K to around 169K where is slows right down and stops. Similarly R3 behaves this way too going from around 5k to 20.6K where it stops. Other resistors test as normal with a correct reading straight away.

Many thanks again in advance for any advice on this!

[bikerhifinut]

Originally Posted by moley

Hello folks,



- still measuring the HTR+ to PSU chassis ground (with DC mode on the multimeter) I get
3.49 volts and -3.13 volts on HTR- to PSU chassis ground. This is without the valves in. Reading across HTR+ and HTR- I get 6.62 volts.These readings are repeated across the other two sets of HTR+ and HTR-.


- I checked that there are no shorts between the pins of the PSU cable coming from the PSU and oddly there seems to be a tiny amount of continuity between the bottom two pins which connect to the brown and grey wires in the phono 3. The multimeter gives a tiny very short beep but doesn't give a reading like there is a small connection that comes and then goes. If I leave the test probes on the continuity is gone but if I lift them and then place them back I get a very short quiet beep again.

I removed the PSU cable from the PSU and tested this and the shorts were gone so I got inside the PSU and tested the grey and brown heater PCB points leading to the two PSU sockects and again I get the tiny amounts of continuity that appear, disapper and only reappear if I lift the test probes and place them down again. Is this at all normal? Is this due to continuity that exists within the PSU circuit but my multimeter DC signal is then blocked by some of the components in there?


- With regards to other checks inside the PSU. C5 is the correct way around with the stripe against the negative connection. My voltages look ok looking at the schematic which shows 53 and 60 volts output respectively. R5 is 100 ohms. R2 and R3 look correct per the coded bandings, although putting a multimeter on them to check, the reading keeps slowly increasing. In the case of R2 it climbs from 30k slowly up past 150K to around 169K where is slows right down and stops. Similarly R3 behaves this way too going from around 5k to 20.6K where it stops. Other resistors test as normal with a correct reading straight away.

Many thanks again in advance for any advice on this!

I am a bit puzzled by this measurement to earth from the heater, it seems to indicate to me that the heaters are referenced to ground via a centre tap, and not lifted via the potential divider R2 and R3.
Because WD use DC heaters there is no need for a centre tap style reference, it is sufficient to lift and referemce from the -ve rail of the heater supply. what you should get is a steady 6.3Volts, give or take depending on the tolerance of R9 and R8 I wouldnt worry if the voltages are between 6V to 6.6V as these are within the tolerances of the ECC83 heaters. This is measured across the heater supply which should be kept ISOLATED from the chassis/earth on the preamplifiers and only connected at the PSU via the voltage reference network.
Furthermore I would double check that the chassis (case) is "lifted" by the 100ohm resistor R5. ON NO ACCOUNT should any wires be connected from the earth (negative HT rail) direct to chassis. This is quite easy to do by mistake. There should only be ONE earth wire in the PSU case going from the pin at the BACK from the 100 ohm resistor R5 to the chassis connector. I had an issue with excessive hum and noise on my Phono3S which was due to the previous owner/first constructor running the earth wire from the front of the PCB straight to the chassis and thus creating an earth loop.
To recap, measure across the +ve and -ve heater outputs and that should be around 6.3V plus or minus 5%. Then measure from the negative side of the heater supply to the negative side of the HT rail (Signal ground) and that should be around 40V ish, dont fret if its a few volts either side its not that critical.

Did you use a DVM or a moving coil meter when measuring resistances in circuit? DVM's in particular can give erroneous readings on the resistance ranges due to the resistors being shunted by the electrolytic capacitors in the PSU. Plus measuring resistors in circuit is only practical if they arent shunted by other resistors etc, and in the case of the potential divider R2 R3 if youtrace the circuit back you'll see its shunted by the bridge rectifier and the transformer secondary and has the choke in series along with R1.

[bikerhifinut]

Can you take a photograph inside the PSU and PHONO3 showing the wiring and PCB, that could be a big help in diagnosis.

Andy.

[moley]

Hello thanks for the reply.

Yes it is a DVM, thanks for clearing that up for me.

I also had a reply from Matthew at World Audio Designs today advising me that I am perhaps losing some of the heater supply to ground.

I shall double later check to see if I have any continuity between HTR+ / HTR- and chassis ground. I guess having a voltage on chassis earth and it not being at ground potential would be why I am getting the strange readings between HTR+ / HTR- and chassis earth.

I have checked continuity from inside the PSU at the PCB right through to inside the Phono at HTR+ / HTR- on the PCB and have continuity and low resistance to the correct places. I don't have any shorts on the PSU cable either so I guess that means that if there is an issue with heater supply going to ground it must lie within the Phono, possible a short on the PCB.

I shall investigate and if still stuck get some images.

Thanks again.

[moley]

Sorted!

The LED negative leg was soldered to LEDG rather than LED2 and this was the cause of the heater supply anomaly.

Thank you very much for all of the replies!

[John Caswell]

Hi Moley,
Well I am glad it appears to be fixed, I am still puzzled though!

John