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Old 16th November 2014, 11:43 PM
bikerhifinut bikerhifinut is offline
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Join Date: Jan 2006
Location: Penrith, Cumbria
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Default Re: Help - Phono 3 Heater voltage Measurements

Originally Posted by moley View Post
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.

Last edited by bikerhifinut; 16th November 2014 at 11:58 PM. Reason: clarification. R5
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