Need help with repair of an early 2000s TV. zombified after LOPT replacement

[special_K]

Hello all. After two years of searching I've finally got hold of a large screen colour CRT TV*, a 28" from Schnieder, Model STV-2802T, Chassis "TV9". It came in with the LOPT arcing to the nearest heatsink, causing immediate shutdown.

The first thing I noticed is that it'd had a replacement LOPT in the past already, but it was fitted with the incorrect one, HR8031 vs the correct HR8320. The only difference being that the 8031 has lower voltages on some of the secondaries.

I've fitted a new HR8320 and now the TV powers on to a gray raster with retrace lines. There is no OSD, neither the front panel nor remote produce any kind of response. Many TVs of this era have automatic switching to AV input when connected to something via SCART, this also doesn't respond.

I've never seen a TV in this state before and it has me stumped. My first thought it that the microcontroller isn't running, but I don't want to make any more assumptions than that.

 I have a copy of the service manual for the chassis here. It's crusty and split across multiple A4 pages so I'm going to spend the next couple of days going through it.

In the meantime I'd appreciate any thoughts from any old TV repairmen out there with a better understanding than me.

*They are extremely rare above 14" in the UK, and usually need a 12 hour round trip to collect. I don't have any better option than to repair this.

[special_K]

Not much progress made today, just getting the information that I do have access to into order.

Checking the numbers on the PCB I found I'm dealing with the version 2 design, so I pulled the pages for the schematics from the PDF and stitched them together.

Video Jungle IC is a VDP3108B. Microcontroller is a ST92R195, got the datasheets for both.

As I'm sorting through them, service manual, schematics, chip datasheets being added to https://winged-horse.fyi/broken_tv/

[special_K]

I'm trying to suss out the cause of the white screen with retrace, but so much is left unsaid by the VDP datasheet that I can't draw any conclusions.

What I know is it's responsible for controlling the horizontal and vertical deflection, and the TV has horizontal and vertical deflection. What I don't know is how an undamaged VDP is supposed to behave in a default state, if it has no valid video input and the microcontroller is not communicating with it.

we've got this grayish white screen, right? I've never seen this before, but it's neither bright white nor black, so it's in between. I would assume the RGB outputs are left floating, and that's just what that looks like?

We've got retrace lines even when the flyback SCREEN voltage is set very low, which means vertical blanking isn't being done. That COULD be the jungle being broken, but it could also just be something left floating without a video signal?

So I seem at a crossroads: Either the VDP is fine, and this is expected behavior without a microcontroller setting it up with a signal source... or the VDP is damaged.

If any ex-TV people have any thoughts?

[Phaseseeker]

This might help you to check if the various analog outputs of the VDP are in the right range: https://ksp-electronics.com/media/62675/vdp31xx_2.pdf

[special_K]

Thank you, that will be one of the things I test, when I'm ready to start testing things.

I still need clarification, though, on what the default state should be. The VDP has four CVBS inputs, Vin1 through 4. You'd think Vin1 would be the default, but it actually goes to the SCART socket, and it's not accepting input from there.

There's two lots of RGB in pins, each with their own Fast Blank pin so they can hijack the VDP's RGB outs. But I don't see an equivalent set of pins to rotate through Vin1, Vin2 etc. It makes me feel like source selection is mostly through I2C?

It makes me feel better in a way, i'm now leaning more towards microcontroller being the problem, rather than both microcontroller and jungle chip. But at the same time it makes it much less likely I can get the jungle to operate without also having a working micro.

[Phaseseeker]

According to the datasheet, default is input VIN3, PAL mode, no scaling. But the RGB cutoff and drive values will also be the preset values, not what it's actually stored in the EEPROM.
Edit: Yeah, input selection is done through the I2C registers. You could possibly try sniffing the bus to see what the MCU is configuring the chip for... Looks like the VDP even has a colorbar test signal generator mode, but again it's only accessible through the I2C commands apparently

[special_K]

I've not the equipment to read the bus, (nor the budget to buy any) so that's a non starter. The most specialized thing I've got here is a 10Mhz single channel scope. I don't think the microcontroller is even running (no response to any controls), though it does have power.

Before bed I buzzed the VDP RGB outs, they're at 5V (supply voltage).

Will keep poking around.

[Phaseseeker]

You don't really need specialty equipment to interface to the I2C bus, pretty much any cheap microcontroller board (Arduino ecc) should be enough. Even a PC with a parallel port can do it in a pinch.
BTW, did you verify if the screen voltage does indeed change when adjusting the potentiometer?

[special_K]

I can't afford an arduino board or anything like that, and I just sold my last parallel port PC to cover half the cost of the new LOPT.

The voltage definitely does change when twiddling the SCREEN pot, this is a brand new part which came with it turned all the way down, which produces no raster. I have it set as low as will give one, turning it up makes the display brighter.

[Phaseseeker]

Your oscilloscope should have more than enough bandwidth to at least confirm there's some kind of activity on the I2C bus. This can help you verify whether the microcontroller is running. Also, looking at the schematics, the whole address+data bus of the main CPU are exposed, so you can check those for activity too. Also, how are you turning the set on? The power supply looks like it's controlled by the microcontroller...
If, on the other hand, the VDP is actually never configured, it looks to me as though the default configuration would be input VIN3, which is connected to the tuner's video output and to the PIP module. You could potentially try to disconnect those two circuits and inject a video signal through a 150R resistor (look at the way the SCART inputs are connected)

[special_K]

I assumed that the power supply was only controlled by the TDA4605, though now I you point it out I see a line marked "Power off" going to the base of Q101, which then triggers a relay in the power switch. Doesn't it require "Power off" to be high in order to cut power? If the micro isn't alive it couldnt send such a signal.

Turning the TV on goes as normal for anything it's age. I hit the latching power switch, I get a second or so of standby LED before it wakes and gives a picture. It's the non-response to the controls (remote or front panel buttons) that makes me think the micro is dead.

You have a good point about using the scope to at least see bus activity. It's going to be tricky to clip the probe anywhere so I'll probably solder a small wire to a pin and attach off of that.

I'd also thought about feeding VIN3 (one of the earliest things I ever did to a TV was adding a composite input, years ago...), I just haven't done it yet as it's one of those days where the cold's gotten into my bones and I don't want to do anything much at all. such is november.

I'll let you know what I find.

[raftronik]

Hello,
My English isn't very good, so I won't say much,
 but the screen with white return lines is caused by incorrect voltage on the second grid of the picture tube. You can temporarily adjust it using the G2 (SCREEN) knob on the high-voltage transformer, setting the G2 voltage to the point where the return lines just disappear and screen goes black.

The service manual describes the procedure for adjusting the G2 voltage.

Here's a link to a better-quality service manual https://archive.org/details/manual_STV2802T_SM_SCHNEIDER

[special_K]

No it is not. As we have already discussed in this thread, G2 voltage is OK. White screen with retrace lines here is caused by RGB OUT on VDP being stuck 5V with no vertical blanking.

Thanks for the better service manual.

[special_K]

Turns out the microcontroller IS alive, it was just fooling me because the remote needs very hard button presses. Can go into and wake from standby.

With an RF modulator on UHF21 I can get audio, quietly, but it's there. Retrace lines become much more numerous and flickering.

Mute button works. Switching between AV and RF input does not work. The user manual suggests pressing button 0 but while that does wake from standby it's not switching from RF input audio to scart audio.

Probing the i2c is very difficult as I don't have anything to hold it in place. But I can at least see there is at least something there on the data pin. I haven't probed the clock.

[Phaseseeker]

According to the service manual there should be some kind of service/test header where the bus is exposed and easier to probe; anyway, with the MCU alive, I2C bus activity (though the amplitude looks wrong - are you using a 10X probe?) and most of the CRT drive signals being generated by the VDP, I'm inclined to think it's the one at fault... The only other thing I see that has an influence on the RGB signals is the PIP module, so you could try disconnecting it and see if there's any difference

[special_K]

Good news: this TV has no PIP module... It's an optional feature.

I also agree with you now we're looking at a bad VDP. Luckily, they're available and don't cost too much money. I'll budget it in for next month, alongside a resupply of flux paste and a better desoldering sucker. My existing one lost it's suck.

Before replacing it though, I must scope VIN3 and see if what's going in is a correctly formed CVBS signal, just to rule that out. I have a little 5" TV with composite input and I'm tempted to feed it the output of the Schnieder's IF.

I have to say, even though I'm not finished (or really even started) fixing it, having you replying to this thread and giving me something to bounce thoughts off of has been a massive help though the process of understanding what's going on. Without having someone to discuss it with I would probably just be stuck in place.

[Phaseseeker]

Happy to see my help's been useful.

[special_K]

Replacement VDP-3108B arrived, fitted, problem NOT solved.

RGB output pins now read 4.7V (previously were 5V), screen is now dark grey instead of light. No other change to the TV.

Back to the drawing board.

[Phaseseeker]

Well, that sucks. Is the TV still refusing to change inputs? Otherwise you could inject some test pattern into the VDP inputs and then use the scope to see what it's putting out on the RGB drive outputs... Unfortunately it appears most of the CRT signals are generated by the VDP based on what the micro tells it via I2C, so unless you can get to the service mode you can't really control any parameters externally.
Also, if we assume the VDP is now working and it's being configured with reasonable settings, the only other thing I can think of is that it might be getting wrong values into one of its feedback inputs, but I don't know how you would even check that...

[special_K]

Yeah it still refuses to change inputs between RF and AV. Channel changing on RF still works, though.

I've been successfully feeding it colour bars and audio via RF and seeing the correct waveform on VIN3. I ran a wire from that pin to the composite input of a small battery powered TV and it displayed the image there. (Audio comes out of the Schnieder's speakers).

the VDP itself does react to the presence of a video signal, the retrace lines appear moving, as though it's swapping between odd/even fields. There is no waveform when I scope the RGB output pins.

I'm at a point where between the LOPT and the new VDP I've spent as much money on this unsuccessful repair as it would cost to replace the entire TV chassis with a generic China TV Chassis. I'm inclined to fit one and forget the whole mess.

[Phaseseeker]

Yeah, I agree it's frustrating, however I doubt a cheap Chinese replacement board would've worked well here, looking through the datasheet it looks like the original board supports quite a few advanced features that a replacement wouldn't implement (scan speed modulation, scaling, various geometry adjustments, 4 RGB inputs ecc).
Regarding the VDP: looks like you can bypass quite a bit of the digital processing by injecting your own analog video into the analog RGB inputs and activating the fast blanking inputs; the VDP still has some control on the outputs that drive the CRT, but it might give some useful info...
IMHO part of the problem in troubleshooting this is that the VDP chip is pretty much a black box and we have no idea about what the thing is doing...

[special_K]

I wouldn't discount the Chinese TV boards. They have less features, yes, but I don't want any features. I just want to feed RGB (or component) into it and have it be displayed. I believe it does have service menu geometry adjustments but I'm equally at home doing them by replacing passives.

In comparison this "fancy" German TV is full of black boxes, digital data busses controlling what in older TVs was simply a pin held high/low, cluttered with optional support for useless gimmicks like PIP, through-hole chips surrounded by densely packed SMT... no wonder it is difficult to diagnose.

It's getting late, I'll try setting off the fast blanking tomorrow.