PSU heater voltage at 1.3V?

[MrKettle]

I had last night, but today there is a different problem. The AC voltage is ok at the input to the heater circuit, but the voltage at the led take off is 1.8v so it is not working again.

[bob orbell]

Hi mr K, reading your posts you have a very early PSU 3 and if you still have the two diodes, the regulator should be a 78S05, this fitted to a heat sink top side of the PCB, look for 2 4mm holes, you can use LM1084, but you will need to mount on the chassis and must be inslated from the chassis, a kit is available , the diodes are now not used and need to be changed with two resistors, 560R & 130R the position of the resistors is also different to the diodes. You must look at the circuit diagram. BOB PS the two regulators have different connections, do not fit the LM1084 in the PCB, it will not work.

[Richard]

There have been 3 arrangements for the PSU3 regulator.

1) Originally as developed from PSU2 it came with 5V 78S05 reg and 2 diodes to lift ground to give 6.3V. The reg was moved from the pcb and bolted to the alu case under the pcb. PCB was "WAD PSUII Issue 2"

Then a problem was sometimes found when powering both Pre3 and Phono3 if the mains voltage was low. The regulator could start to drop out (not enough over-voltage for it to work within) which showed as hum so,

2) A low drop-out reg 5V LM1084 was used instead still with the 2 diodes to lift ground. This was a direct swap for the 78S05 with just the pin-out wires needing swapping over. This fixed the problem and the first 4 pictures in the first post of this thread show how this version should look when assembled,

http://www.world-designs.co.uk/forum...ead.php?t=4131

3) Peter then re-designed the PCB around the LM1084 regulator but used instead the adjustable version using 2 resistors to set voltage instead of the diodes to lift ground. PCB was "World Designs WDPSU3 v3.1"

So,

i) check for sure if yours has 2 diodes or 2 resistors to set voltage

ii) check which version of LM1084 you have. They both say LM1084 on them but one has 5.0 and the other ADJ. 5.0 goes with the diodes and ADJ with the resistors

iii) re-check pin-out wiring and that the reg is firmly fixed to the case but electrically isolated from it

[Richard]

Link in that thread now fixed to the shottky bridge made for the development "version 2" of PSU3, http://www.world-designs.co.uk/forum...ead.php?t=4158

Note that the latest version of PSU3 now includes a new PCB with LM1084 ADJ regulator, resistors to set the heater voltage, and schottky diodes so all this is sorted

(Note also that version 2 with LM1084 and schottky bridge works perfectly well btw. I have a new PCB Version 3.1 but have never felt the need to change it.)

[bob orbell]

Thank's for the information Richard, I did not know that there was 3 versions, maybe Mr K is using the ADJ chip!. BOB

[Richard]

Hi Bob, Mr K,

Yes possibly ADJ instead of 5V, possibly pin out, possibly case shorting, etc.

I don't know if any kits were supplied as arrangement 2 but Mr K said he has diode ground lift and then mentioned ordering LT1084 so we needed to clarify which version he has as certainly some earlier ones would have been updated or built as per that thread.

The PCB my PSU3 was supplied with was "WAD PSUII Issue 2" and the later one Peter sent after re-designing them has on it "World Designs WDPSU3 v3.1"

[MrKettle]

It is one of the earlier ones with the diode lift and the original bridge rectifier. I haven't (yet) changed the rectifier to Schottkys.

The regulator I was trying was the one from Farnell reference 9559787 and when I checked there wasn't any connection from the tab of the regulator to the earth post.

[Richard]

Hi Mr K,

The schottky bridge would give another 1V headroom, and I would do it later, but that 5V LT1084 should be fine so long as it's wired pin to pin as in the picture below. You'll see the wires/pin positions cross over from the original 78S05 the PCB was designed for,

[bob orbell]

Hi all, that Farnell no is correct for the 5v, are you still having trouble or is it now OK. BOB

[bob orbell]

Richard, your reg. seems to be mounted on an isolator, could this be Mr K's problem?. Forget this post.