Hitachi 57F710E Power Issue

[navagator]

I have a Hitachi 57F710E Rear Projection TV that was working great until about a month ago. I came in and tried to turn it on and it was just dead. So, I unplugged it and plugged it back in. Nothing, not even the slight click that it makes when you plug it in. So, I removed the power supply board (It's a vertically mounted board, I believe model HA01312) and inspected it visually for bulged caps or burnt or overheated components, anything that didn't look right and couldn't find anything.

I noticed on the bottom of the board there was a row of solder joints with voltages printed next to them (10.5v, Gnd, Gnd, 5.6v, 5.6v, Gnd, 16.5v (not used), Audio Gnd, Audio), which are coming from the large transformer on top. I checked all of these voltages and they're all reading 0 volts DC.

There are 3 glass fuses that I checked and they're all fine, and 2 pico fuses (E940, E941) on the secondary side of the board that I checked for continuity while in circuit and there was continuity, there may be more fuses but I don't see any. While checking for DC voltage off of the small bridge rectifier (D901) on the primary side, I short circuited it and had to replace it. But there is approximately 108 to 115 volts DC coming off of the new rectifier, it's fluctuating real bad. If I remove the glass fuse inline right after the DC positive leg of the rectifier, I get a reading of 113.3 to 113.4 volts DC with very little fluctuation. There is approximately 120 volts AC going into the rectifier. So, I assume there is something wrong after the rectifier. There is a larger capacitor after the fuse and I believe some kind of switching chip or IC offline switch flyback after that (I901) chip # A6159. And a few other caps, resistors, and a small transformer. Perhaps the problems lies within one of these components.

I would really appreciate any help anyone could give me in fixing this issue. It's an old TV but it still has a great picture and I would prefer to fix it if I can.

Thanks

 

[kellys_eye]

There is approximately 120 volts AC going into the rectifier. So, I assume there is something wrong after the rectifier.

You should get approximately 170V DC after the rectifier. Potentially missing one leg of rectification with such a low reading. Change it.

I901 (chopper controller) would be the next suspect - can't see a device number on it though. Got a schematic?

 

[navagator]

Thank you for your reply.

I checked the rectifier in circuit and got these results: while in diode mode I touched the + lead of my meter to the DC negative leg of the rectifier and the - lead of my meter to the other 3 legs of the rectifier one at a time and got readings of approx .520 at each leg. Then I reversed that process and got 3 readings of .OL. Shouldn't the reading between the DC + and - legs of the rectifier be doubled? I only got a reading of .520. Could this be an incorrect reading because I tested the rectifier in circuit or is it bad?

I don't have a schematic but if I'm reading it correctly the full # for I901 is:. A6159, SK582, 990C.

 

[kellys_eye]

A6159 is a switching regulator device and the datasheet shows a 'typical' connection arrangement like this:


Based on this you can trace the circuitry and operation as required.

With 120V on the AC legs of the bridge you should see ~170VDC across the smoothing capacitor. If it isn't 170V~ and is 'pulsing' as you said in your original post it could be an indication of the controller going into protection mode due to a fault on the secondary side (over current). Check C940 and the device beside it (regulator?)

 

[navagator]

I'm getting the same fluctuating reading across the filter cap as the DC side of the rectifier and 0 volts DC at both C940 and the input leg of the voltage regulator.

 

[kellys_eye]

it could be an indication of the controller going into protection mode due to a fault on the secondary side (over current).

..or a defective controller IC.

Break the DC path on the secondary side to remove the load and see if the SMPS starts up. You may need to add a small load (resistor) across the secondary side to allow regulation to start properly. If it doesn't start then the fault is on the primary side (probably the regulator IC itself).

 

[navagator]

I would love to do that but unfortunately I don't know how. LOL.

What would I need to do to break the DC path on the secondary side?

 

[danadak]

Lift one leg of the diode connected to secondary....


Regards, Dana.

 

[navagator]

Thank you for your reply. I disconnected D949, it's connected to pin 6 of the small transformer on the secondary side, I hope this was the correct diode. But nothing has changed on the primary side.

 

[navagator]

I think I'll go ahead and change out the regular IC and see what happens, I might go ahead and order a few new caps as well to change out.

Thanks

 

[navagator]

My order arrived from Digikey today. The first thing I did was install the regulator IC. Unfortunately that didn't fix the problem. So the next thing I did was replace the filter capacitor (C908), after that I noticed that the DC voltage coming out of the bridge rectifier was now reading a stable 167 volts. But nothing else changed, still no DC voltage on the secondary side. I then changed out C912, C911, C909, C905, and I904. There was still no change. R908, R909, R910, R906, D106, and D907 test ok.

What could be left, perhaps the small transformer? There is a stable 167 volts DC going to pins 3 and 5 of the transformer.

Thanks in advance.

 

[kellys_eye]

The first thing I did was install the regulator IC.

Bad move. If there was a fault prior to the IC then that fault remains and could have destroyed the IC again. Given that the DC voltage is (was) incorrect coming from the BR you should have sorted that issue first.

So the next thing I did was replace the filter capacitor (C908), after that I noticed that the DC voltage coming out of the bridge rectifier was now reading a stable 167 volts.

Proving my point.....

There is a stable 167 volts DC going to pins 3 and 5 of the transformer.

If the switching IC was working you'd measure ~170V DC on the 'top' of the transformer and a LOWER voltage on the 'bottom' (where it is switched by the 'D' pins of the controller IC). A test meter might not be the best device for measuring accurately though (too high a frequency) but since it's showing THE SAME voltage top and bottom then the IC is dud - again.

 

[navagator]

Thank you for your reply.

If I understand correctly, the filter capacitor was bad/weak which caused the low fluctuating voltage. This in turn caused the switching IC to fail. So, I should have installed the filter cap first or at least installed all the components I decided to replace at once before applying power to the board.

I ordered two switching IC's, so I have an extra one to change out. When I remove the switching IC I just installed, is there a way to test it with a multimeter?

 

[navagator]

I installed the new switching IC and applied power to the board and checked for standby voltage but still nothing, everything appears to be the same as before. I'm about to the point of trying to find a used Hitachi TV with the same power supply board to swap out.

Unless there is something else you can think of?

 

[navagator]

I did make some progress today. First I removed the voltage regulator just off of the switching transformer on the secondary side and tested it, there was a a steady 5 volts DC coming off of the output leg, so I reinstalled it. Then I removed capacitor (C940) and replaced it with another capacitor I had of the same capacitance but a slightly higher voltage spec. and applied power to the board, I then had a voltage reading of approx. 7.75 DC coming off of C940 and a steady 5 volts DC coming off of the voltage regulator, however I didn't hear any of the relays click in.

I reinstalled the board in the TV in hopes the issue was resolved but unfortunately it wasn't. Still nothing.