Continuing my series on component-modding CRT TVs, this time I'm revisiting a 21" TCL TV manufactured around 2012.
This TCL has a board layout
similar to the China TV I've posted about previously, but uses a different jungle chip, a Trident TDA11135PS/N3/3. The modding procedure is very similar, but this time I'll introduce input protection diodes. Below is an overview of the virgin chassis vs TV with DVD input after modding.
This mod process is shown in two stages - Stage One covers what I did in January 2022, before frying the jungle. Stage Two covers what I did recently, replacing the jungle IC and installing input protection diodes.
INPUTSInput pins for Pb/Y/Pr are 49, 50, 51. There are empty spots on the PCB where extra parts can go to upgrade, but no advice provided on values. I found pinouts for this jungle and a schematic for an another TV that uses it (attached), but no actual datasheet. The schematic showed each input with 75R termination, then 100R and 100nF in series. This lined up with the unpopulated spots on my TV, and how I've done other mods, so I initially just went with that.
Below are before and after pics of the main areas to work on. Jungle IC on side. The four capacitors circled in yellow are all 100nF ceramic - yes they look somewhat like resistors but are not. Three of those caps, marked with blue, are for our video signal inputs. They will be fine for now, though I'll end up replacing them eventually.
Six resistors are also needed (3x100R; 3x75R). The spots for five of them are circled in turquoise. One resistor (100R) is already present, on the Y input. I'll add another two 100R, either side of the Y, for Pr and Pb; and then two 75R terminations, each side, for the same signal inputs. The Pb signal input can also "land" directly here, as there is nowhere better.
The remaining inputs (Y, Pr, ground) can come in using unpopulated spots for s-video inputs. They are even conveniently marked, "Y" for luma and "C" for chroma, or in our case Pr/Cr. For our final resistor, there is already a 100R termination resistor present for the Y signal, I replaced that a 75R (small and with blue body) for consistency.
JUNGLE CURRYThe TCL PCB also had unpopulated spots for zener diodes, used in some TVs for input protection. You can see the spots right there in the pic below. I had no idea on values, and the schematic (for same jungle, different TV) didn't include any zeners, so I decided to ignore them. I came to regret that.
Why? Just as I was finishing the mod,
I managed to fry the jungle's Pr/Cr channel. How? Checking carefully, I found a random tiny blob of solder on a spot connecting Pr/Cr input directly to ground. Causing or allowing a voltage/current spike to hit the IC. My own fault, blob probably fell from my iron while I "tinned" it. That blob had barely stuck on, and came off easily with a fingernail, but the damage was done.
Still had picture, just very little red (if it was no red at all I'd suspect the red drive or CRT gun). Connections all good and removing/reseating Pr cable had no effect.
Confirmed the jungle was fried by checking the jungle pins for Y/Pr/Pb with a multimeter on ohms, measuring resistance from to ground. The resistance was many megaohms (~10-20M) for Y and Pb, normal, but only 330R for Pr. So yeah, Fried channel.
So, I ordered a replacement jungle IC. Knowing that would take weeks or months to come in, I promptly moved onto other projects and forgot about it. That was in January 2022, three and a half years ago now. I've only come back to it now because I've been trying to sort through my CRT collection, and realised that I had the replacement jungle (TDA11135) now.
Finally got around to it, and replaced the jungle IC the other day. Removing the old IC was easier than I feared, despite the 64 pins. I have a desoldering station and have gotten quite proficient with the hand-held solderpult/solder sucker. just took a little patience and 5-10 minutes work. New jungle IC on left, old jungle on right.
Curiously, with the new jungle installed and no video inputs it displays the ROWA brand!
INPUT PROTECTION DIODES (ZENER)Obviously, I decided to put in some zeners but remain a bit lost as to what values. Scouring schematics for other TVs show that makers sometimes use 12v or 11v zeners, but sometimes lower values as well. Zeners are a bit tricky to use because while they are great for regulating over-voltage in reverse-biased application, they can also be quite "leaky" to low current, below the actual rated avalanche breakdown voltage, due to quantum tunnelling effects. This must be taken into account, thus why some manufacturers choose to use 12v instead of 2v rated zeners for protecting a 1v max signal.
What all that TL;DR gobbeldegook means is: whenever I saw a lower rated zener used, like 2.4v, or even 5.6v, they would be paired with another diode, perhaps a 1N4148 or anther zener, to limit current leakage at lower voltages (<0.7v). The lowest rated zener I saw used without a paired diode was a 6.8v zener, so I chose to use that as I only had space for a single protection diode on each input. For now that seems to work fine.
Grateful for any input on this choice (Scott??). I may change my mind in the future, maybe just try 12v to see if that makes any difference.
So, below is a pic after I've replaced the jungle and added the 6.8v zeners (1N4736).
I also replaced that bank of weird-looking 4x 100nF (104) ceramic caps with some old-skool ceramic disc-style 104s, because some looked damaged and I just happen to have a box full of the retro 104 caps.
Pins marked in blue are the Pb, Y, Pr input pins (49,50,51)
Parts list:
3 x 100nF ceramic capacitor (104)
3 x 75R resistors
3 x 100R resistors
3 x 6.8v to 12v zener diodesCOSMETICSOpening up this TV again was a real pleasure, in a way, because I couldn't spot my original modding work until I had the back off! Being an older mod of mine from 2022, I hadn't installed a mini-PCB for the new inputs, and ran wires directly from the PCB to a 3x RGB gang connector. This connector screws directly onto the TV back cover at an unused spot for component inputs, so it looks "factory" from the outside. Connecting wires are neat and just long enough, yet not too long. For a moment I was impressed with my younger self, yet my feelings of swelling pride are short-lived. Here are some closeups.
COLOURS AND GEOMETRYLooking at those closeups, I just now realise that my younger self got the red & green inputs flipped around, compared to the external case markings. This is the kind of mistake one would only notice on a reviewing a closeup photo! They are right up the back, "where the sun never shines", and the markings are black on black. Well, I may flip them around later, it is just a cosmetic detail. While it is tempting to just turn the whole 3x gang upside-down, then the screw holes won't line up <sigh>.
Anyway, all is fine and below are some pics showing the TV back in action.
All the colors look good and image is crisp, though I feel it could be better.
There are some minor geometry issues (see the slight wobble to vertical lines on the right side), but I can probably live with that for now.
The picture is too large vertically, a bit too wide, and needs to be centred.
For example, on the geometry test picture below, the circles in the corners should be all or mostly onscreen. However, we can barely see one quarter of each on the left, only half of each circle on the right.
All of these things can likely be improved by tweaking the service menu - if only I could access it.
REMOTE CONTROL AND SERVICE MENUMuch of this can probably be adjusted out with the service menu, which brings us to the final issue: accessing the service menu and the remote control. I need the remote control working properly to access the service menu, and the remote seems to be on its last legs. TV is very slow to respond to the remote, even with new batteries installed.
Simply turning the TV on/off with the remote, changing AV mode, is possible with some patience. I must hold down a button for several seconds before the TV responds. Entering the complicated codes needed to access the service menu seems impossible.
I checked the remote's infrared output using my smartphone camera. Human eyes are incapable of seeing infrared, so a digital camera/older camera viewfinder/smart phone camera is needed to see it. I found that the LED was very slow to light up, like several seconds, and then slow to turn off again.
I pulled the remote apart, there's really nothing to it, just two caps and an IC. Replaced the capacitors, but nothing improved. The IC obviously "works", so why the delays? Then, on a closer look, I noticed corrosion on the PCB - PCB rot had set in, and some traces were disappearing!
PCB rot is some kind of corrosion that eats away at the copper in the PCB. I'm not sure what causes it, but we are in a hot, humid climate. I've lost a TV chassis to it previously. This TCL TV has been otherwise unaffected AFAIK, so it is surprising to see it in the remote control, which I have never opened previously. I suspect it may be from contaminants like salt or sugar, possibly from people with contaminated fingers, that catalyzes the oxidation. How that gets into a remote that has never been opened, I don't know, unless it came from the factory workers themselves? Looking at photos, maybe some liquid might have come in via the LED hole at front?
Reviewing my photos, it is easy to see some of the areas where corrosion seems to have settled in (circled in yellow). Some of the stuff is just gunk, but some definitely seems to be corrosion, perhaps associated with the gunk. Cleaning it off with isopropyl alcohol did not seem to improve things or remove the corrosion.
Unfortunately I can't just bridge these corroded areas with wires, as the button "mat" must lie flat, directly on top and against the PCB. There is no access from the other side of the board.
So, I will have to wait a few days for some copper tape to come in, then I can maybe attempt to replace the corroded traces. I'm not sure it'll work and I've never attempted this kind of repair before, so wish me luck! More on this soon.
I will also probably get a universal remote, hopefully it is sufficient to access service mode.
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