Sanyo CPT3240 no image troubleshooting

[ostlar]

I just got this Sanyo 14", model CTP3240, and it has a major issue.




Turned it on and it lit up to a proper snow picture, everything seemed fine, however after barely touching the potentiometer for the horizontal size (?) marked [<-->] seen on the second picture, the flyback lost power and image turned off. Adjusting it seemed to cause the flyback to turn on and off like it had a bad connection. After turning the pots for a while, it didn't want to kick on the flyback at all. Only some screaming noise from the speaker as if it was an LW radio out of tune.

I opened it up, dusted it out and started looking for cracked solder joints. Noticed that the front potentiometers were not soldered onto the board, but in a plastic cage with wires running from the pots to the board. The connections seemed fine, so no cracked solder joints to blame here.

Cleaned out the pots with contact cleaner, lots of turning, and dried them out with compressed air. Did this twice. Now it stopped making any noise, just a faint 50hz hum.



The big resistor on the image above get insanely hot, so I haven't dared to try the TV on for more than a couple of seconds while testing. I can smell it, and even after being unplugged for a couple of minutes, the resistor is too hot to touch.

Could an IC or a transistor suddenly die due to the crusty horizontal size potentiometer? Where would I start to diagnose this without the schematic?

My first thought is to disassemble the potentiometers and measure them, but I'm not sure if I know what resistance they should be.

I would really like to resurrect this set.

[vol.2]

It's probably in HV shutdown because there's some kind of a short on the board. I think that the H-Size pot was just a coincidence, and the reason it stopped working has more to do with something giving up the ghost. You might have a short that has blown up a resistor or a fuse or a thermistor or something like that.

I would A) completely disconnect the pots you think were a problem and just test them with a multimeter. It's very easy to do, you just measure across the pot and turn it from side to side. It should ramp up and down fairly smoothly without any obvious gaps or weird values.
B) Once you've verified the pots are okay, (with the set turned off) start testing continuity between the power rails and ground. You are looking for anything that is at or very close to zero ohms.
C) You can test all the electrolytics. I would be extremely surprised if there were no bad or high ESR lytics in the set. That could be the cause alone, although they may have taken out another component or more when you tried to power it up.
D)You will likely need the schematic, so I hope you have it. With the set hooked up to an isolation transformer, all of the power rails need to be verified.

If any of this is outside of your comfort zone and/or ability to handle lethal voltages safely, go learn how to be safe with it before proceeding.

[ostlar]

Originally Posted by vol.2

It's probably in HV shutdown because there's some kind of a short on the board. I think that the H-Size pot was just a coincidence, and the reason it stopped working has more to do with something giving up the ghost.

I'm not so sure it was a coincidence... The image stayed on for long on first power on, touching the h-size pot made it die, then turn on if I turned it a bit more. It stayed on until I touched it again, went dark, and stayed off until I touched it again. Each time i touched the pot it flickered and the sound of the flyback did too.

Thanks for the tips! I will try these next time I am able to. I also will try to do a thermal image of the main board to see if it's anything else than the big resistor that gets hot.

[vol.2]

Originally Posted by ostlar

I'm not so sure it was a coincidence... The image stayed on for long on first power on, touching the h-size pot made it die, then turn on if I turned it a bit more. It stayed on until I touched it again, went dark, and stayed off until I touched it again. Each time i touched the pot it flickered and the sound of the flyback did too.

Something could have been right on the edge of blowing up and messing with the h-size pushed it over. Changing the h-size effects a lot of the deflection circuitry, so if something was wrong, then twiddling it could have a big effect on the current and/or voltage at different nodes in the circuit.

I think it's likely that it would have happened anyways if you just left it alone for long enough, possibly even worse if it was allowed to stay on with a short, heating things up.

But that's just my instinct about it. Either way, if the H-pot checks out, you know it wasnt to blame for anything. It just being dirty wouldn't blow up your set.

[Username1]

.

That big resistor is part of the power supply, And strangely under it should be a 8pin
Single Inline Chip that regulates the voltage. For that set it may be like 120V to
150V Don't remember... That transistor that is in the picture is probably the
regulator, emitter output. As for the Horiz Width you turned - It may be dirty
And shut down the H Osc. because the wiper had no connection to the
resistor. So first clean those pots, then try it again. And yes that big
resistor running hot is normal. Those were good tv's back in the day....

.

[ostlar]

Thanks for the tips guys, I will try to troubleshoot a bit more this weekend. I'm going to start with the potentiometers and work my way back, check the main voltages and fuses. I haven't found a service manual for it yet, I'm hoping I can find a chassis number and see if there's any info on it.

I will update this thread with details, might be useful for others having the same or similar issue.

[vol.2]

Originally Posted by ostlar

Thanks for the tips guys, I will try to troubleshoot a bit more this weekend. I'm going to start with the potentiometers and work my way back, check the main voltages and fuses. I haven't found a service manual for it yet, I'm hoping I can find a chassis number and see if there's any info on it.

I will update this thread with details, might be useful for others having the same or similar issue.

It's a cute little set and worth fixing.

Probably is basically just working and needs to have solder joints reflowed extensively, pots/switches cleaned, and some bad electrolytics. The lytics can be tricky in sets from that era. Sometimes you will find one value or brand in a set that every one has gone bad, and other times it will be all of them. Usually, if it's just specific location caps, it's stuff that's close to heat sources. Anything near a big heat sink or power resistor or power transistor could be dried out and useless just from sitting there next to the heat

[ostlar]

Okay so I've done a bit more troubleshooting.

Started with measuring the potentiometers:



This is viewed from the front of the TV. The resistance shown is with the potentiometer turned that way, meaning left is counter-clockwise, right is clockwise and middle is... middle-ish, the highest value i measured on those two. Which was kind of strange.

All potentiometers seemed smooth, no sudden jumps or anything.

I've measured all the large, marked resistors, and they all have the correct values.

I've also measured the voltage out from Q901, main power regulator it seems. Voltage was +225V DC, so it seemed to be okay.

While measuring voltages, the large white 20W resistor (R322) got so hot it started smoking, so there is definitely something shorted or gone open circuit, meaning this resistor dissipates too much power. It's almost like a convection heater.

No fuses are broken either.

I'm suspecting that there is an issue in the main power area, seen here:


Or might there be something on the main board shorting the power supply?
It's such a pain trying to diagnose this without the schematic, and I'm quickly losing interest each time I try.

The model CTP3240 yields no results, and 79P-B8BF00 is the chassis number. I get some schematics from searching just 79P chassis, but it seems too different to be of use.

I've checked it thoroughly for cracked solder joints four times now, I see nothing.

Here's a high resolution image of the bottom of the board, if it's of any use: https://i.imgur.com/XVnwNwG.jpeg

And one from the top: https://i.imgur.com/0hIUEX3.jpeg

Next thing would probably be to check all the caps. There are some electrolytic caps marked "NP" which I'm guessing is non-polar?

[vol.2]

It could be a component in the power supply or something downstream loading it down. Either is possible.

Originally Posted by ostlar

I've checked it thoroughly for cracked solder joints four times now, I see nothing.

That means nothing. Many, possibly most, cracked solder joints can only be seen with a microscope. What happens is that they heat up and expand as and then start to develop a layer of corrosion at the interface

You could have cracked solder joints all over the place and end up chasing your tail around looking for a ghost

There are some electrolytic caps marked "NP" which I'm guessing is non-polar?

Yes. NP or BP both mean non-polar or bipolar. Same thing.

I think you need to hunt for the schematic. You need to see what's connected to that resistor that's smoking and test it all

I tried to find a manual and struck out as well. They might know something over on the antique radio forum.

Another thought is that this chassis is probably used by a different US retailer like Sears or some department store, and you would have infinitely more luck finding that service manual, if you could figure out what the alternative model/chassis number is. I took a quick look on Sam's for something that might match and couldn't find it, but maybe someone here or elsewhere might recognize it

without the schematic, I wouldn't be too much use for you doing remote troubleshooting. Username1 and Zeno are the heavy hitters here, and they might have some things to try

Best I can suggest at this point is to look what the resistor that heats up is connected to by traces and start testing everything you find. There's definitely a bad part in there if it's smoking

[ostlar]

Originally Posted by vol.2

That means nothing. Many, possibly most, cracked solder joints can only be seen with a microscope. What happens is that they heat up and expand as and then start to develop a layer of corrosion at the interface

You could have cracked solder joints all over the place and end up chasing your tail around looking for a ghost

Fair point, I could try to reflow all of the solder joints. It would be a tedious task, but an important one when diagnosing. I might need to print out an image of the board to mark the solder joints I've reflowed, so I don't miss any.
I've tried giving it the percussive maintenance by lightly tapping the board with the back of my screwdriver while powered on to see if there is any cracked joints. Probably made more cracked joints by doing that.

I want to get this thing to work again, it's just a matter of not giving up. For most people it's not worth the time and effort, but for me it's a learning experience and a way to save a CRT which I think is important. I've fixed several other CRTs, but they have had clear symptoms and usually have had a simple fix like a visible cracked solder joint or a leaky capacitor.

Thanks for the moral support, it really helps!

Next thing would be to focus more on getting the schematics. Someone somewhere probably have them in paper format, I might have to ask around.

EDIT: Oh and I forgot to mention that I could hear the degaussing coil on a cold startup, so that works at least.

[ostlar]

Doing some more digging, I was able to find a model number on the board that says "MCL-427F". Googling that results in a cap replacement kit for arcade monitors. That's interesting, but not much else to find, and the board in question doesn't look similar to mine.

However, I found an eBay listing of a CTP4100 service manual which also has a P79 chassis, and the power board looks quite similar to mine. Luckily the seller has taken an image of the schematics, so I might be able to use this as a reference. I will do some cross-checking tomorrow.

[vol.2]

Originally Posted by ostlar

I might need to print out an image of the board to mark the solder joints I've reflowed, so I don't miss any.

Typically, the best place to start is the heavy components like transformers, power transistors, power resistors, anything with a heatsink, and with anything that takes physical stress of any kind like a connector, switch or board-to-board wiring.

If you reflow all of that, you have probably covered your bases. Sometimes you'll find the odd loose part that you don't expect, but it might not be the best way to spend you time doing the entire board.

[ostlar]

Did some thermal imaging of the TV today, I can see that R326 and R322 get absurdely hot after about 10 seconds of power on. These are in the power supply part of the TV.



R326 is the one at 127°C on the left, and R322 is the large one on the right.

I could also see that R493 got somewhat hot on the other side of the board, it looks to be near the vertical deflection circuit, so there's power there:



It went up to around 80°C until I turned it off, as I didn't want to keep it powered on with this fault for long.

Nothing else gets noticeably hotter than ambient temperature after 10-20 seconds of power on.

I also went and reflowed a lot of solder joints, especially in the power supply circuit, but the issue is still the same.

I am working with a guy in the UK to source and scan the P79 chassis schematics, that will make this so much easier.
In the mean time, I have this screenshot of the power supply schematics, the hot resistors are circled.
I've just tested that the power regulator transistor Q901 works with a multimeter.

[Username1]

.

Good Job getting help with a schematic... WOW I don't think I worked on one with a 225V
Power supply, All the ones I remember were around 120V, 150V. Anyway... With
no pic, and that resistor smoking I think maybe you got a shorted Horiz. Output
transistor, or possibly Vert. Output Shorted. So get checking on those parts.

OOpsie - I see Your in Norway.... Is your wall outlet voltage 240V? I see the
schematic also shows 240V on the plug...... Interesting.... Funny how
that regulator IC is floated above ground... I think they used that
same basic circuit in 240V area and in US 120V.

Good Luck!

.

[vol.2]

Originally Posted by ostlar

I am working with a guy in the UK to source and scan the P79 chassis schematics, that will make this so much easier.
In the mean time, I have this screenshot of the power supply schematics, the hot resistors are circled.
I've just tested that the power regulator transistor Q901 works with a multimeter.

Are you getting 220V B+ at Test Point B1 (TP B1) on the schematic?