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Need help with Sansui TV

I would appriciate any advice I could get on my next step with troubleshooting a Sansui TVM2500C "Version A" 25-inch television set. I should declare right now that I'm a two-way radio guy, not a TV repair man. I have good general troubleshooting skills but I don't have the equipment or knowledge to work on circuits specific to the picture tube, flyback transformer, etc. This repair is being done as a favor to a friend.

The unit was brought to me in the following condition, so I have no information on its behaviour prior to this point. When first powered up, the screen lights up with normal empty-channel snow even though a station should be present on that channel, there is no volume, the channel up/down buttons work properly (the numbers change on the on-screen display, anyway) but the volume up/down buttons do not - one volume button causes the channel to change and the other doesn't do anything.

after the TV has been running for a while, a popping and hissing starts to come from the flyback transformer (at least it sounds like it's coming from there) and a TV station starts to appear erratically on the screen for whatever channel It's tuned to. This behaviour goes on for a while, a pop or click whenever the TV station appears or disappears, and when the TV reception is flickering rapidly the transformer is hissing. Eventually the signal settles down, the noise stops and the station is displayed but there is no color. The volume buttons continue to operate improperly as described above.

I have checked most of the electrolytics and they're fine. Although I do not know the exact voltages that should be present during operation, the levels I see seem reasonable for the rating of each of the capacitors I've checked. Oddly enough, I do not detect any voltage fluctuations on the circuit board during the unstable period of operation. Whether there's video on the screen or not, everything I measure seems to be the stable. I'm only checking the low and medium voltage points of the board at the base of each capacitor so I don't accidentally encounter any high voltage related to the picture-tube, and I'm also checking certain points in the power-supply section that I feel are safe for me to meter.

I've only been able to find data on two of the chips on the circuit board, so checking ICs for voltage and logic levels according to the published data isn't going too well.

Due to the noise coming from the flyback transformer during the erratic period of operation, I suspect this component. There are two low-voltage points silk-screened on the bottom of the transformer, a +25 volt and a lower one I can't recall right now. At no time do I see any voltage whatsoever at either of these points, whether the channel is being displayed on the screen or not.

Basically, I want to know if there's anything else I should be looking at or if I should be telling the owner that the flyback transformer is shot and it's out of my hands. It just seems odd to me that the flyback transformer should be affecting the user controls and that I can't find any unstable voltages. I keep thinking there's something I've missed.

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          From the keyboard of Alan - VE3NNM (Kenora, ON, CANADA)
                    E-mail: ve3nnm AT lycos DOT com
               http://mywebpage.netscape.com/kenoraradio
Reply to
Alan Stevens
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At this point I wouldn't worry too much about the noise from the flyback. I would check for bad connections to the micro or the crystal. Also, given that symptoms are worst when cold, and seem to stablise, I would replace the secondary electrolytics in the power supply to be sure.

electrolytics), your description of the problems with reception sound like this area may be of importance here.

In one case, I was working on a Thomson chassis and there were similar symptoms to yours (poor tuning, jumpy picture, loss of reception, snow etc,) coupled with intermittent user controls. Inspection showed that data and clock lines went round the tuner on the pcb, these traces had been cracked due to someone forcing a plug into the antenna jack (integrated in the tuner.) Due to the tuner being soldered directly onto the mainboard, there was not enough support for this heavy component and the board got cracked, lifting the tuner earth points and the data and clock lines. A few moments with a sodering iron effected a repair. good luck, Ben

Reply to
b

On Mon, 01 Aug 2005 06:59:12 GMT, Alan Stevens put finger to keyboard and composed:

If the volume control works correctly via the remote, then I would suspect a problem with the uP's key matrix. I have seen this kind of behaviour (in other TVs) when water has got into the set. IME it's the glass IN4148-type signal diodes that are usually affected.

- Franc Zabkar

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Reply to
Franc Zabkar

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