Chipset for Astec AA20040 power supply - Page 1
Forum section: electronics / repair

author: ingalopez
author_rank: Contributor
timestamp: November 08, 2022, 08:18:27 pm
content: Greetings!. Ive got an Astec AA20040 power supply to repair. Blown fuse, bad mosfet. Yet, after replacing and looking for further damage, nothing happens. No Stby supply!. Unfortunately the chips on this unit have been sanded (!?¡). The stby chip is a DIL16 type. Any one knows?? Thanks

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 08, 2022, 09:50:32 pm
content: Hello!I'm sure you'll get more help if you add a couple of photos (top/bottom), the SMPS gurus will chip-in with some layout based suggestions.

author: mariush
author_rank: Super Contributor
timestamp: November 08, 2022, 10:20:28 pm
content: Post some pictures?Maybe you're confusing the 16 pin chip with the main pwm controller ... a common chip in those days was SG3525 or variations of it ... it's available in dip16 or soic 16

author: amyk
author_rank: Super Contributor
timestamp: November 09, 2022, 05:25:00 am
content: It could also be a TL494. Agree with the above, post pictures (both sides) for more info.

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 09, 2022, 11:52:53 am
content: The 3rd commonly seen PWM IC in computer PSU's is the KA7500.

author: ingalopez
author_rank: Contributor
timestamp: November 09, 2022, 10:14:44 pm
content: Thank you there!.But no, this chip is a self contained PWM pulser. It drives the tranformer (the small, stby one) directly. It even receive the input DC (+300V), very few other external components. A couple of pins have been removed (not drilled on the board). It appears to be in high impedance, on the high voltage sector, but some 400 ohm on the electrolitic capacitor , where the Vcc is expected to appear, but not showing.Attached are some pics of the board.The PWM main controller, is mounted on the vertical pcb, near the power elements. The offending chip, is the one on the main pcb.Both have been erased. What a nasty practice!There is another vertically mounted pcb, (green) on the edge of the main board, where the voltage supervision is, I guess.Hope the potato camera is enough!

author: fzabkar
author_rank: Super Contributor
timestamp: November 09, 2022, 10:22:01 pm
content: Could there be a high value resistor between the rectified 300VDC supply and the Vcc pin of the IC? These resistors often go open circuit.You may need to trace the circuit around the IC if someone is to identify it for you. There may also be markings on the underside of the IC.

author: ingalopez
author_rank: Contributor
timestamp: November 09, 2022, 10:59:47 pm
content: Nope.Nothing on the underside. And no resistor either!. I believe this chip uses internal hivoltage fet to generate its VCC, without the proverbial high value resistor. Like some TOPs and other families do.

author: Chris56000
author_rank: Frequent Contributor
timestamp: November 09, 2022, 11:14:15 pm
content: Hi!There is one PWM that operates off the HT+ line direct in a 16 pin DIL package – it is the Microchip/Supertex HV9120PG, but unfortunately it's e.o.l. and supplies are getting very scarce, and its very likely Astec used that one!Unfortunately it's internal startup feed from the HT+ input at pin 1 does have a habit of going open–circuit internally, resulting in the chip sitting there with 300 V on pin 1 and dutifully ignoring it !I came across these in some large French made battery charger units that were made for railway rolling stock, and it puzzled me for days that there was no obvious means to start these units up internally until I twigged the startup was inside the device itself!You might be able to find it in 16 pin SOIC and use an SOIC–DIL adapter tho!The AA20040 is a standard format 145W ATX PC type power supply unit and by the time you've found a '9120 it'll have cost more than TWO of those complete units new!!Chris Williams

author: ingalopez
author_rank: Contributor
timestamp: November 09, 2022, 11:22:46 pm
content: Thank you Chris!That seems promising, since the hiV is actually applied on pin 1, as in your case, and seems to be internally disconnected, as well!Little hope of finding such rare chip down here, but I´ll check the datasheet to see if it fits.Alejandro

author: Chris56000
author_rank: Frequent Contributor
timestamp: November 09, 2022, 11:30:37 pm
content: PS!https://www.ebay.co.uk/itm/402028588296?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=M_wP2V8nR8K&sssrc=2349624&ssuid=qNpFa7hUTgy&var=&widget_ver=artemis&media=COPYPostage might be expensive tho!Chris Williams

author: coromonadalix
author_rank: Super Contributor
timestamp: November 10, 2022, 03:04:42 am
content: i think this is the same ic datasheethttps://www.ibselectronics.com/ibsstore/datasheet/Vishay/si9120.pdfattached too

author: fzabkar
author_rank: Super Contributor
timestamp: November 10, 2022, 07:58:15 pm
content: I'm wondering whether the failures in this IC could be a consequence of a failure in the bootstrap VCC supply, eg a dried out electrolytic.Quote

author: Chris56000
author_rank: Frequent Contributor
timestamp: November 11, 2022, 07:36:33 pm
content: Hi!Yes, very possibIy!The Vishay Si9120 is electronically similar to the HV9120 so if you can locate one it should get it going!Worth saving a search on eBay in case another the same comes up cheap as a "parts mule" (add filter "for parts or not working" so you don't have to plough through thousands of listings of new units) !Might be worth de–soldering the vertical sub–panel from the main PCB so you can check the components on it more easily!It's also trying a search for "AA20040 UL Report" – a "UL Report" which stands for the Underwriter's Laboratories", have to receive a report on each new model from the OEM detailing how the proposed design comies with UL safety requirements, so the UL can then test and certify these supply units before they go into quantity production – these reports sometimes have a circuit diagram of the unit in an appendix at the back!Chris Williams

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 12, 2022, 01:18:43 am
content: Depending on whether the transformer has an auxiliary winding to supply the PWM:Go for the cheap and nasty 12V zener between Vcc pin7 and GND, then appropriate high value resistor to rectified mains or rectified mains/2 depending on capacitor configuration. Or capacitive divider from AC. That should run in all cases.If there is an aux. winding and you want to be less energy wasteful add in an extra diode and capacitor from the aux. supply and use that to switch a high voltage depletion MOS on the above setup closely emulating the original internal function.

author: ingalopez
author_rank: Contributor
timestamp: November 13, 2022, 11:54:51 pm
content: I´ve looking at the 9120 specs, but it doesn´t include the power mosfet to drive the Stby Xfmr, as the offending chip in my board.As you said, this power supply is kind of a standard ATX power supply. Perhaps is just a bin case, after all.

author: ingalopez
author_rank: Contributor
timestamp: November 14, 2022, 12:22:46 am
content: I may try that, out of desperation.But I don´t think the rest of the chip is active... measure a few hundred ohms on the VCC line, it may be deceased altogether.

author: ingalopez
author_rank: Contributor
timestamp: November 16, 2022, 07:33:07 pm
content: Find another such power supply and the chips aren´t blanked!The chip for the stby power supply is the MC33363P, a Motorola design. It includes the switching device (700V 1 A) and the start up mosfet.The primary side PWM controller is an Astec AS2214.Now I will check if those are available .Than you all!