Matsushita TM-202g color issue/overheating resistor

[nd3738]

Hi all, first time poster here. Also first arcade machine I've ever worked on, so still learning a lot.

TLDR: TM-202g monitor has green tint, transistor swap didn't fix, power resistors on neckboard are stupid hot.

I'm in the middle of repairing/modifying a Millipede upright cab, and was hoping for some electrical help regarding the monitor. It's a Matsuhita 19" TM-202G, which seems to be original for this cab. I've completely recapped it, but when powering on the monitor has a strong green tint to everything (see attached pic). I tried adjusting the R and B Drive pots (no green one present), along with the brightness and RGB Low Light pots, but can't seem to get the colors quite right no matter how much I tweaked it. Clarity and geometry seem plenty fine, just colors are off. I didn't get to mess with it for very long, as you'll see shortly. Also, FYI I did set the B+ voltage to spec (123V) according to the service manual, if that helps at all.

So based on some googling, I tried replacing the three RGB drive transistors on the neck board, as I read that they can cause color issues if they go bad. No change at all, so I swapped them back to the old ones. Used NTE157 transistors as replacements. My multimeter doesn't have a transistor tester built in, and I was struggling to test them with voltages/resistance, but since there wasn't any change I don't think they are the issue (could be wrong though).

Then I noticed some smoke rising from the neckboard, and that the R351-R353 Power Resistors were burning hot to the touch. So hot that they were melting the solder their legs were mounted with. I assume that isn't normal at all. The resistors still measure 4.7k ohms as rated, so I think they're okay. So my current assumption is that some other component that feeds them is bad? I didn't notice it before I tried replacing the resistors, but it's very possible that it was doing the same beforehand (and they still smoke with the old transistors put back in).

I ain't an electrical engineer and still learning how to read circuit diagrams, but it seems that these power resistors maybe get 180V from the main board? Then the signal meets with the transistors, goes through R354-R536 and into the tube? So would I check the voltage from the main board next, and I can do that with a basic digital multimeter? Or does anyone have any experience with similar issues, and know where I should try troubleshooting next?

Service manual here for reference (I think the schematic for the neckboard is page 25): https://www.arcade-museum.com/manuals-monitors/Atari Monitor TM-202 1st Printing Matsushita 19in.pdf

Appreciate any help or advice y'all can provide!

 

[Plymelk]

Cool cab. In the classic era community (i.e., KLOV) the Matsushita that came in Millipede and other Atari titles of similar vintage is looked down upon as failure prone. I haven't had the opportunity to work on one of these myself, but I have seen them looking very nice when operational.

I would put the original transistors back in and remove the NTE157s, as it seems likely the problem was at least accelerated by this change. Reflow all solder joints on the neckboard (and perhaps the power section on the mainboard too). Id next measure for 180v at pin 10 of the neckboard (per the manual you linked). Usually, the transistors are amplifying the RGB voltage at the neckboard and the 180v is the rail they seem to be relying on to get there. Verify your work and double check component orientation against pictures and the schematic.

Also, most transistors can be checked using the diode function of a digitial multimeter. You are looking for a "voltage drop" really and for them not to be shorted or open. PNP and NPN transistors are measured differently. The values for all 3 neckboard transistors should roughly all be the same.

As to the colors, Ive seen some older monitors only use a Blue and Red scale for drive/cutoff like you have. I would reflow these pots and verify the values against the manual. A shotgun approach would be to replace all color pots on the chassis. If that fails id start looking to that amplification daughter board and repeat. Best of luck!

 

[nd3738]

Thanks for the advice here, appreciate it!

I put the original transistors back in, reflowed the neckboard and main board, and checked the voltage at pin 10 (it was 172 volts, so not quite the stated 180). Still no difference; after a minute of being powered on, R351 and R352 get hot and melt the solder, causing some smoke.

So I'm guessing that its some component on the neckboard that has gone bad, if the input voltage is roughly correct. Guess I'll start testing resistors and such.

Kinda tempted to see if I can find a different 19" monitor as a replacement, since the Matsushitas get some flak online. We'll see.

 

[Plymelk]

Measure your horizontal windings (red and blue wires off of yoke?) and vertical windings (other two wires) and report the results. I think this is a high impedance chassis (similar to G07 or K4900) but that is speculation. Inductance is the best measurement but resistance will likely give us a general idea. If you wanted to scrap the 202, i have a hunch the g07 or k4900 would be a suitable replacement without a yoke swap but that is TBD based on the readings.

 

[nd3738]

94.2 ohms between the yellow and green wires on the yoke, and 1.9 ohms between blue and red wires on yoke. Used each probe of the multimeter for each color pair on the yoke end, assume this is the right method. See attached pic for where I probed.

 

[Plymelk]

Yea thats a high resistance yoke but its even higher than expected. G07 or K4900 would be around 50 ohm on the vertical. This is basically double that. Looks like you measured correctly.