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Edwar65 8th November 2012 11:05 AM

lm 317t voltage reg
Hi , can anyone advise procedure for testing an in-circuit lm317t voltage regulator please? I have an arcam cd73 cd player and am in process of checking the output voltages of all vr's as I have getting no disc error on display, laser is not emitting any light,replaced laser but issue still exists?
The circuit diagram for the laser supply shows this vr to be the regulator,however when testing via a multimeter, I am getting readings of approx 1.2v between pins 1-2 and 2-3, the diagram shows that these should be 3.6v and 5v respectively as this supplys the laser circuit and pll. Am I doing somthing wrong?
any help much appreciated!!!

Mooly 8th November 2012 11:21 AM

I remember the other thread :)

I haven't got the circuit on this PC but it sounds like you need to be looking further back (before the regs). Those voltages are to low. If the regs are hot then it could be a short somewhere after these but I suspect not.
Did you do the checks here,

PlasticIsGood 8th November 2012 08:10 PM

1.25V is the over-current shut down voltage of an LM317t.

Considering this regulator is by design operating perilously close to drop-out I assume there is another voltage regulator supplying the 5V, and this may also be a LM317. One scenario to consider is the upstream regulator has shut down due to over-current.

The 317 is pretty bomb-proof. I would check the voltage on the pins of the regulator supplying what should be 5V: if you have plenty volts on its input, and only 1.25 on its output, then there is probably a near short somewhere downstream. If it were a total short there would be no voltage at all.

Check the upstream regulator's regulation pin isn't shorted to ground.

May be something else altogether. Just ideas.

Erratum: the instant I posted this I realised that you were measuring pin to pin rather than to ground. The regulation pin is not grounded, so your voltages may add up nearly right. Since I'm not staring at a LM317t datasheet I don't know which pin is which. So...measure voltage of each pin to ground, to avoid more confusion, please.

dacen 8th November 2012 08:29 PM

Here is the service manual for Arcam CD73. Just download. Whith this one you are in a much better position to do the faultfinding. Good luck.

Categorized Schematics and Service Manuals for free download A-C

Edwar65 9th November 2012 12:00 PM

hi,many thanks for the reply, voltages are from pins 1 to ground and 3 to ground and voltages are around 1.2 - 1.3v, took the ground reading from the body of reg,same as pin2 looking at schematic. Pin volts to ground as per schematic should be 3.6 and 5v?

Edwar65 9th November 2012 12:06 PM

hi,sorry last post was meant to read pins 2 ground and 3 to ground, looking at schematic again pin1 is ground,same as body of regulator,
sorry for confusion,

Xoc1 9th November 2012 12:34 PM

The Lm317 is not grounded as it is an adjustable regulator. The voltage reference is 1.25Volts between pin 1 (adjust) and pin 2 V out, so this is correct.
You are not measuring the voltages with reference to 0V. You will need to look at the schematic and find a 0V Point to measure the regulator input voltage on pin 3 and output on pin 2.

sofaspud 9th November 2012 12:40 PM


pin1 is ground,same as body of regulator,
This is incorrect. Pin 1 is Adjust, the tab is pin 2 Output.

Mooly 9th November 2012 01:00 PM

3 Attachment(s)
Tom... did you check the fuses first.

Look at the PSU on page 8 of the manual.
Connect the negative lead of your meter to any ground point such as the audio output ground on theaudio output phono socket. Yes :)

Now measure the plus 11 volt supply which you can find on one end of D521. The other end of the diode should have 8 volts.

Thats your first step. Are those two voltages correct.

If they are, then you need to see if the 11 volts is reaching U502. Again we can easily measure on the diode D516. There should be 11 volts on one end and 5 volts on the other.

Again if that is OK then we now move to U507 and again by measuring on diode D525 we should see 5 volts on one end and 3.6 on the other.

Report back with your readings :) (and don't forget the fuses)

AndrewT 9th November 2012 01:57 PM

Clip your black probe to audio ground, either inside the player or at an RCA.
Leave that there.

Using one hand with the red probe, go around carefully taking voltages with reference to that ground.

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