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Great Mystery Magnavox TV 27MS3404/17 Problem

Hi,

I have a Magnavox TV model 27MS3404/17, chassis L01.1UAC-7638 (mfg. 06/05) in my service shop that comes on, picture shows for a few seconds, then the TV goes "off". Actually the horizontal sweep shuts down. The SMPS is still "running" at "full power" (Reg B+ is 130V). Note: The HV was around 27KV with picture showing.

If I use a pattern generator (color bars, cross hatch, etc.), the TV stays on!!!!! However, with a normal TV signal, it goes off in a few seconds after the picture shows. With no signal (snow), the TV stays on much longer but it still goes off. It only stays on with the pattern generator!!!!

Note: I had tried different "Screen" (on FBT) settings.

Any idea what is the cause of this weird problem?

Thanks in advance, Brad

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Reply to
Brad
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Is the beam current less with the pattern generator signal? Might be a clue there.

Reply to
Charles Schuler

Hi Charles,

I thought the beam current might be causing a shut down. I have Sams Photofact 4968 for this model TV, but it leaves out a lot of information. No pin out info on the jungle IC 7200, TDA9588H/N1/1133, and I couldn't find it on the web. I assumed pin 54 may control shut down by excessive beam current so I opened that circuit and used a 100K, then 200K to ground resistor which set the voltage close to what was on the schematic, but the TV still shuts down. Another pin 55 on that IC relates to "BCI", but I didn't experiment with that circuit. I have used a lot of time trying to fix this mystery problem.

Brad

Reply to
Brad

Hi Charles,

Correction: IC7200 is actually a TDA9577H (Sams listed it as "TDA9588H")

I thought the beam current might be causing a shut down. I have Sams Photofact 4968 for this model TV, but it leaves out a lot of information. No pin out info on the jungle IC 7200, TDA9588H/N1/1133, and I couldn't find it on the web. I assumed pin 54 may control shut down by excessive beam current so I opened that circuit and used a 100K, then 200K to ground resistor which set the voltage close to what was on the schematic, but the TV still shuts down. Another pin 55 on that IC relates to "BCI", but I didn't experiment with that circuit. I have used a lot of time trying to fix this mystery problem.

Brad

Reply to
Brad

Sorry Brad ... I tried a Google for the part no. but could not find a data sheet out there. If you run the brightness all the way down, does it still shut down?

BCI is intriguing ... beam current ... ?

Good luck!

Reply to
Charles Schuler

Just did one of these... it's a little 25v 47ufd cap a couple of inches away from your HOT. I don't remember the position, but it's the only little cap right off the HOT (a couple of inches.) Change it out and you're probably good. Good luck.

Kevin S.

Reply to
Kevin S.

Hi Kevin,

I had already replaced that cap, C2455, which has a different symptom associated with it, I have seen in the past. The symptom is, you can't turn on the TV or it is hard to turn on the TV (several attempts). This cap has nothing to do with the problem I have now.

Brad

Reply to
Brad

Hi Charles,

Lowering the brightness (very low) allows the TV to stay on. I suspect the problem relates to beam current sensing circuit or TDA9577H (no info). I notice that on pin 55 of TDA9577H, the voltage changed apx. 2 tenths of a volt between pattern generator and TV signal (before lowering brightness).

Note: No beam current (if you remove CRT pcb) will also cause a shut down. Brad

Reply to
Brad

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