PSU heater voltage at 1.3V?

[MrKettle]

This evening, I turned the system on to be greeted by no LEDs and no relays. A prod around in the power supply gave a reading with nothing connected of 1.3v at the heater output pins. All other voltage readings, including the AC voltage supplying the heater supply were fine. Any ideas?

[bikerhifinut]

Originally Posted by MrKettle

This evening, I turned the system on to be greeted by no LEDs and no relays. A prod around in the power supply gave a reading with nothing connected of 1.3v at the heater output pins. All other voltage readings, including the AC voltage supplying the heater supply were fine. Any ideas?

the obvious thing is to trace the voltages stage by stage. You say the AC output from the mains tx is fine? Then check that it is ok at the input to the bridge rectifier (module or discrete diodes as i dont know what amp or rect you are using). if ok there the wirings ok that far. Then check the output of the bridge. I suspect your rectifier may have failed but there could be other reasons.
Do this with the valves out of their sockets first, and check the wiring to each valve.
Trace the circuit from start to finish until you find the fault and don't assume that the rectifier failed on its own if it has blown. you might have a short or low resistance path inside a valve or a loose wire etc etc.
I realise i may be a bit wooly here but if you think it through it should make sense.

[MrKettle]

Just to be clear, that reading was at the pins on the circuit board of the PSU3 itself.

[Richard]

Hi Mr Kettle,
To be even clearer are you getting that reading across the pins of the heater-out wiring on the pcb? If so you'd expect 6.3V as you know.
Is your regulator lifted from ground by 2 diodes or is it the later one with 2 resistors to set voltage?
If it's lifted by 2 diodes I get a feeling (the voltage may just be coincidence) the regulator may have failed and your reading is for the 1.3V difference caused by the diodes.
Put probes on the input to the regulator (top of C6) and ground looking for 9V going in then move the one on the reg input to the reg output to double check it is just 1.3V and if that is the case it sounds like the reg.

[MrKettle]

The measurements were on the heater output pins on the PCB at the back of the unit, and it is one of the ones with two diodes, so a new LT1084 regulator (and a spare as part of the minimum order) are on their way from Farnell.

Just thinking about it, the relays on the Pre have had a habit of chattering on first start up for a little while which would presumably be consistent with a regulator on the way out.

[Richard]

Mine have always chattered and I put it down to pulsing when the caps are filling at switch on.

I suggested the check for 9V in the input side to the reg to be sure the TX winding and diodes were ok.

[bob orbell]

You may find that the relays will still chatter when sorted, mine did but a change of capacitor C6 from 4700uF to 10000uF stopped the chatter. BOB

[MrKettle]

Update from having replaced the regulator with an LT1084 and cursed whoever put the PCB pins for the regulator so close together, all details unloaded: Voltage at the input to the regulator is 12V as expected, but the output from the heater circuit starts off at 1.2V and rises very slowly over a minute or so to 1.8V.

Is it a good job I got two regulators or is something silly going on?

[MrKettle]

Such as, of course, the LT1084 regulator having a different pin out arrangement. Normal service has, I think, been resumed...

[John Caswell]

Hi,
Have you fixed the problem, not quite sure from your comments

John