Onkyo TX-DS797 Standby Transformer substitution help

[kumasan]

I have an Onkyo TX-DS797 av amp that I am trying to repair. We had a 600v power surge thanks to local power company that took out the units standby power supply. Unit was not on so I think the main transformer and guts is ok but the standby power supply is always on to feed the microprocessor and work the relay to enable the main power supply.

Upon debugging I found the primary coil of the standby power supply blown.
Im having a challenge to find a suitable replacement transformer as the original model seems to be an unlabeled proprietary unit made by tamura:

Original Transformer:
onkyo npt-1111d (2300670A)
From looking at tamura website I was able to find a standby transformer design factor with same dimensions that shows it is ~2.5VA model

Since it is blown I cannot measure the output voltage. From the service manual it seems the output of the power supply is labeled as 13v. The schematic for the standby supply shows it is a simple full wave power supply.
Basically the transformer feeds diode bridge 4xRL1N4003 and a 220uf cap and 4.7k resistor in parallel with the output.

If I do a basic calculation for the required transformer VAC:

Vtran = (13 + 0.7*2)/1.414 = 10.1838

Where I am a little challenged is in understanding how to account for the regulation factor of the transformer.

If I purchase a 10vac transformer with a 25% regulation factor as I find on digikey:

http://www.digikey.com/scripts/DkSea...16101542782866

Im guessing that the minimally loaded standby mode may be as high as 12.5v instead of the 10v resulting in a supply voltage of 16.6v instead of the target 14v which is substantially higher.

From google it seems that a more realistic minimally loaded transformer will have 15% higher voltage and the 25% is too pessimistic I might consider using a 9v transformer. This would give a 13.23v result, but would potentially drop lower if the standby power uses closer to the 2.5watts.

Dont have another similar amp that I can plug in to see how much power it draws at standby. I would appreciate any tips/pointers in helping decide on a suitable replacement.

[Khron]

Re: Onkyo TX-DS797 Standby Transformer substitution help

Got a picture of the stand-by power supply? And/or the part of the schematic where the wires from the stand-by supply go to?

I'm willing to bet a large wad of cash that, wherever those wires go to, they go straight into something like a 7805 or a 5v or 3.3v regulator of some sort You can bet your boots the micro does NOT work on 13v

Most likely the 13v is so it's enough to (also) drive the relay that couples the AC input to the main transformer, when you turn the whole thing on, and usually, relay coil ratings are a bit on the loose side (+/-10% if not more).

[kumasan]

Re: Onkyo TX-DS797 Standby Transformer substitution help

Thanks for the reply, much appreciated.

The schematic for the supply card is very simple, just the full wave rectifier and relay circuit. I tried to trace the supply output through the schematics but when it goes into the first block they abstracted the signal into a bus and it is not clear where the signal disappears.

Based on your reply and some more thought im thinking that the supply is not regulated so it must have some downstream rectification and tolerance for higher voltage.

After tracing through schematics for similar models transformers I was able to find another onkyo transformer for 14v supply on ebay. It is similar (same width/length but slightly thicker core stack). It was a little more expensive than a digikey generic 10v unit but Im thinking there is less guesswork involved in sizing the voltage.

If it is indeed same voltage with slightly higher amperage then Im guessing it should have similar (perhaps slightly higher) voltage output since it wont be driven as hard. Perhaps it will burn a little more power in standby mode, but Im hoping this over-voltage is not enough to cause problems.

We had a electric company transformer down the street short between the input/output and I think it was like a 600v surge into the unit. Im hoping that the surge took out the transformer primary before it had a chance to blow out the regulators or processor downstream, but I dont know how much these components can take before frying.

Keeping my fingers crossed and will find out when the transformer arrives.

[Khron]

Re: Onkyo TX-DS797 Standby Transformer substitution help

Well, especially in small transformers, the primary wire *IS* indeed pretty thin (since you need a lot of turns, AND they don't need to pass much current), so... if you're lucky, the stuff on the secondary just might've survived.

In the service manual i've found, there seem to be a couple of the schematic pieces missing, but on the board views, i saw that +13v going to a smaller board, and what looks like some voltage regulator.

[kumasan]

Re: Onkyo TX-DS797 Standby Transformer substitution help

Replaced the transformer and now I can turn on the unit and work the controls. Looks like the input side is working as I can select tuner or external optical in and see the digital modes change when I apply/remove a valid signal.

Problem is Im not getting any sound at all not even a click from the speakers.

Checked voltage from standby supply and it seems ok, think problem is probably not due to transformer issue.

When I power on the amp I get the first standby relay click and a few seconds later another click (which I think is supposed to be amp power) but no sound whatsoever.

Im kind of stumped how to proceed tracking things down from here. Im wondering if it could be preamp. .... Will keep on trying to debug...

[stj]

Re: Onkyo TX-DS797 Standby Transformer substitution help

any hum?

[Agent24]

Re: Onkyo TX-DS797 Standby Transformer substitution help

Are all the main rails coming up?

[kumasan]

Re: Onkyo TX-DS797 Standby Transformer substitution help

Checked the mains voltages and they all looked ok.

Im not exactly sure what was happening but It now seems to be working.

While checking the mains I disconnected the power connector from the power regulator board to the mainboard that the back cards plug into while in standby mode. I made the mistake of plugging it back in while in standby mode and the main power supply relay and display were flashing/sparking away. After it got plugged in I rechecked the voltages and everything looked ok.

I went back and connected up some speakers and the unit seems to work ok now. It had been a while since I last used the unit and it seems like you cant hear anything on the low settings of the volume and then suddenly it kicks in what seems like 50/100. It is possible when I was testing it after replacing the transformer I did not turn up the volume enough so it could be heard.

Thanks again to all the help and suggestions, another device comes back to life with help from badcaps forum.

[Agent24]

Re: Onkyo TX-DS797 Standby Transformer substitution help

If you don't get any sound until halfway up the volume control, it sounds like bad connection somewhere. Check for cracked solder joints on the boards.

Would help to post some photos.

[lphelps]

Re: Onkyo TX-DS797 Standby Transformer substitution help

I have an Onkyo TX-DS797 av amp that I am trying to repair. We had a 600v power surge thanks to local power company that took out the units standby power supply. Unit was not on so I think the main transformer and guts is ok but the standby power supply is always on to feed the microprocessor and work the relay to enable the main power supply.

Upon debugging I found the primary coil of the standby power supply blown.
Im having a challenge to find a suitable replacement transformer as the original model seems to be an unlabeled proprietary unit made by tamura:

Original Transformer:
onkyo npt-1111d (2300670A)
From looking at tamura website I was able to find a standby transformer design factor with same dimensions that shows it is ~2.5VA model

Since it is blown I cannot measure the output voltage. From the service manual it seems the output of the power supply is labeled as 13v. The schematic for the standby supply shows it is a simple full wave power supply.
Basically the transformer feeds diode bridge 4xRL1N4003 and a 220uf cap and 4.7k resistor in parallel with the output.

If I do a basic calculation for the required transformer VAC:

Vtran = (13 + 0.7*2)/1.414 = 10.1838

Where I am a little challenged is in understanding how to account for the regulation factor of the transformer.

If I purchase a 10vac transformer with a 25% regulation factor as I find on digikey:

http://www.digikey.com/scripts/DkSea...16101542782866

Im guessing that the minimally loaded standby mode may be as high as 12.5v instead of the 10v resulting in a supply voltage of 16.6v instead of the target 14v which is substantially higher.

From google it seems that a more realistic minimally loaded transformer will have 15% higher voltage and the 25% is too pessimistic I might consider using a 9v transformer. This would give a 13.23v result, but would potentially drop lower if the standby power uses closer to the 2.5watts.

Dont have another similar amp that I can plug in to see how much power it draws at standby. I would appreciate any tips/pointers in helping decide on a suitable replacement.

What was the transformer that you used to replace the bad NPT-1111D?? The digikey link you posted didn't reveal the actual transformer you used..

Thanks!!

[Andrew F. Ali]

Re: Onkyo TX-DS797 Standby Transformer substitution help

I think any small 12V transformer will work. 0.5-1.0 Amp transformer.

[lphelps]

Re: Onkyo TX-DS797 Standby Transformer substitution help

I think any small 12V transformer will work. 0.5-1.0 Amp transformer.

I kinda need the right one that will fit on the board.. I believe the primary side has 3 pins and the secondary has 4 pins.

I can't believe I can't find any cross reference guide to match a NPT-1111D to an identical replacement..

[budm]

Re: Onkyo TX-DS797 Standby Transformer substitution help

The board is made to accept 5 pins on the primary and 4 pins on the secondary.