Allen & heath idr 8 d-out Expander psu

[Antwan]

Hi all new to the page so forgive me if I miss anything out on my post I'll make sure to add it as requested.

So.... Iv picked up an Expander for the allen & heath idr 8 (the out module) the psu is dead as can be no rails at all not even the main 48v rail that feeds the other 4. Iv had a little look and I can chase power to the switching transformer at around 150v bit otbing the other side Im thinking maybe a bad transformer but non of the windings show anything which is unusual.

Could it be a bad switching driver not putting out the right frequency? I can't find a schematic anywhere.

I do have a working supply from another unit. A scope and other tools so I can check your suggestions

Thanks in advance

[Jon Snell]

Replace the electrolytics and check the secondary diodes for short circuit; (a pulsing or chirping sound).

[Antwan]

Thanks for the reply iv replaced the caps on the primary side as there known for failing. Diodes all appear OK in circuit. It's not a complex psu to be honest, but as an amature I'm sure I'm missing the obvious.

As I say iv got a second psu that works I cam compare it against. I wish I could find the schematic.

[Antwan]

Sorry also there's no chirping so it's not cycling I'm praying it's not the the transformer

[Enzo]

SO the unit has an SMPS? The actual transformers in those rarely fail. SMPS usually directly rectify the mains. On the primary side, the control circuitry is not referenced to earth, but instead to the negative DC from that mains rectification. DO NOT ground your scope to it. However, to see if the things is oscillating, you can hold a scope probe right next to the parts and the strong field will couple.

A bad switching transistor won't likely alter the frequency. The control circuits handle that. Bad switchers usually short and take out fuses, or they open leaving it dead.

DID you ask A&H if they would provide drawings?

[Antwan]

Hi Enzo yes I'm getting the rectified mains for sure iv not yet scoped it as I'm wary of doing damage but I'll try your suggestion of holding it near by and listening for a switching signal I'll update you tomorrow thanks for the advice

[Enzo]

In the biz, we use an isolation transformer to power up an SMPS for service.

[Antwan]

In the biz, we use an isolation transformer to power up an SMPS for service.

well its on my list to be honest so i may have to invest as im repairing more and more of these type of faults as of late

[Jon Snell]

Thanks for the reply iv replaced the caps on the primary side as there known for failing. Diodes all appear OK in circuit. It's not a complex psu to be honest, but as an amature I'm sure I'm missing the obvious.

As I say iv got a second psu that works I cam compare it against. I wish I could find the schematic.

A&H do not supply schematics for power supplies that are low value and may cause a safety issue if incorrectly repaired.
The transformer will be fine. It is the output electrolytics that go low value or high ESR and cause it to fail. You must not attempt a repair on a safety item like this without an isolating transformer!!!
By chirping, it will be above human ear range, if you scope the output, you may see the supplies starting and then failing, if that is the case, check the shottky diodes on the output side.
If the input side has failed, it is likely to be catastrophic and there would be skid marks from escaping pixie dust.

[Jazz P Bass]

Can you identify the controller ic for the high side supply?
Also, which mosfets are being used?

[Antwan]

Great suggestions I'll check them all when I get in from work. Thank you

[Jon Snell]

The power supply in these consists of a full wave bridge rectifier, four 10u 400v tank or smoothing capacitors that feed 380volts DC to the transformer primary winding. The power flows through the winding to the Drain of the FET. A high value resistor from the 380v line trickles voltage to pin7 of the UC3842 driver chip. When the voltage reaches a few volts the chip will oscillate producing a current output on pin 6 through the 100R resistor and onto the Gate, turning it on momentarily. This triggers the FET causing high current to flow through the primary. The current stops after a few microseconds and as the current stops the magnetic field collapses causing induction to the secondary windings.
If all is OK, no shorts on D1, D7, D6, D8 or D9 it will try again and continue until D3 builds up a steady DC voltage on pin 7.
During this process pin 3 of U9 monitors the voltage across the resistor on the Source of the FET for over current, if the current across R24 & R29 exceed the rated amount, pin 3 switches the 3842 off. This causes the system to try and start again.
Under normal conditions over current does not occur and the power supply FET will work hard until the voltage on C35 feeds the opto isolator pin 1 and subsequently U4.
C36 supplies the voltage to U4 and when acheives breakdown voltage switches U5 on, causing the comparrator of U9 to reduce its drive and therefore regulate the output from the closely coupled secondary windings, which means the produced DC voltages required to run the expander circuits, namely; +5Vd, +5Va, +15v, -15v and +48v supplies.
If any of the shottky diodes mentioned go short circuit OR there is a short circuit on a supply line within the expander, you will get what apears to be a dead power supply.
Proceed at your own risk and be extremely careful of either hurting yourself or killing the sensitive chips that run the power supply.

As I mentioned before, these are not safe to work on without the correct test ans safety equipment. Rectified AC mains at 380volts will cause you to die!
Don't touch it!!!

For your information the document is the DR88 Expander PSU. Drawing C4780.

Edit; https://www.ti.com/product/UC3842

[Antwan]

thank you all for your help i used the idea of the scope to pick up the switching signal to find it wasn't in fact switching, So i followed the advice to check caps although id done the main larger caps id missed 1 small one 47u 64v and low and behold that was the issue.

Iv also ordered my sefl a isolation transformer based on your advice so thank you all once again

[g1]

How big of an isolation transformer did you order? They are fairly expensive for anything that has a substantial wattage or VA rating.

[Antwan]

1000VA "apparently" You know how ebay can be but to be honest I usual just purchase audio gear that need faders buttons etc i know some electronics but I want to learn as i go especially if i can repair more stuff as I go I wont be plugging anything in to hefty like amplifiers etc