Author Topic: PCE LT No Display  (Read 177 times)

coda

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PCE LT No Display
« on: February 14, 2024, 10:53:59 AM »
Hello all, have an LT here that is giving me fits and I'm posting in the hopes someone has some hidden knowledge on the display circuit to help me out.  Simply put this LT arrived to me and was fully functional, but in need of a recap.  After performing the recap I suddenly lost the display, the backlight comes on but there is just a black screen and the brightness does not respond to the brightness wheel.  Everything else is functioning (audio and controls still work fine, game loads just fine etc).  Thankfully I do have another, and fully working, LT that I pulled the mainboard from and verified the display was still good to rule it out.  Probing around the board (using some help from Herr-g's  thread with similar issues),

i noticed that my 30v and -20v rails for the display are missing but I don't know why.  Probed all the diodes/transistors in diode mode and the readings felt normal and matched between the good and bad boards.  The only other things that stand out are I am seeing about 1/2 the expected voltage on the output side of T500 (around 3v), same with a small mostfet directly on the other side, and I am also seeing voltages on parts that don't have a voltage on my good board.  The other big thing i noticed is that on the good board underneath that same mosfet I have a 75ohm resistance to ground, but on bad board its either OL or in the megaohms.  I did pull the cap and 75ohm resistor from beneath the mosfet with no change.

https://imgur.com/a/s0SdU6Z

I drew this up from memory as I am away from the bench (also please ignore the corrosion, this pic is actually from the good board right before i cleaned all that crap up).

Yellow boxes are from the big Vreg on the top side of the board, all voltages agree here.  I also get the 5v readings on top of the 8.2k resistors between both boards.  First difference is the 4v reading on the south side of the resistor on the bad board, on the good board this is 0v for me.  That line leads to a small diode/transistor (not sure what it is tbh) which is out of frame which again on bad board has 4v on the connected leg, but 0v on the same leg with the good board.  To the east on the bottom we have the mosfet i mentioned earlier.  I expect to see around 6v on the top part, but I am only seeing half that on the bad board.  Directly north of that is the transistor which should be giving me a 30v reading on the top leg, but i get virtually nothing on the bad board.

https://imgur.com/HKJon6P

On the top side of the board we have T500 which is also giving the same bad readings on the output side.  The cap the west of it on the negative side SHOULD be giving me my negative bias rail for the display, but I get mV instead.  Once again where I am expecting to see 6v I am getting ~3v.

Sorry for the long thread, trying to provide as much detail as I can.  Will be happy to provide any more readings when I can if folks know how to narrow this down.  The board does get warm (im guessing due to a higher resistance dragging rail down) in the power area, but nothing you can't hold your fingers against without pain, but warm enough to be noticeable and picked up on thermals (good board shows no heat).  However everything that does show its heating up (the two 470 ohm resistors west of the mosfet and the two passives below said mosfet) meters out fine. Thanks for any help y'all can offer, its definitely been driving me up a wall, figured this would just be a recap and clean up!
« Last Edit: February 14, 2024, 11:49:55 AM by coda »

MasterofBiscuits

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Re: PCE LT No Display
« Reply #1 on: February 16, 2024, 06:21:26 AM »
I haven't worked on one of these personally but I know that area of the board is particularly vulnerable to leaking electrolyte. I would focus on very very thorough cleaning, testing all the vias in the area, and resoldering all the components. If it was working before the recap it makes me think there is some electrolyte still on the board somewhere.

Keith Courage

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Re: PCE LT No Display
« Reply #2 on: February 18, 2024, 03:19:49 AM »
I've worked on a few of these that had left over leakage from bad caps under components that had to be removed, cleaned, and then re-installed.

Also look on the bottom side of the board in that same area you have pictured for components possibly shorting from capacitor leakage getting there as well.

coda

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Re: PCE LT No Display
« Reply #3 on: February 19, 2024, 02:17:35 PM »
Thank you both for your replies.  Issue ended up being a faulty T500 transformer which was generating massive internal resistance and dragging the rails down around it.  Thankfully i had a dead PCE GT board (which i was planning on repairing) with a working T500 that I was able to transfer over and bring the display back to life.  Of course it has to be the one literal unobtanium part that dies on me, just my luck.  Over on the retrosix discord we think we identified a possible susbstitute, but its not been verified yet.  If i ever get around to working on the GT board again I am planning on installing one to see if it actually works or not.

For any other tech that's curious out there, the display circuit is similar in many ways to the GT.  T500 is hooked into a single BJT NPN transistor on the opposite side with two passives connecting to ground via T500 (should read the 75 ohm resistance to ground on the passives), this is the same part for Q900/901 which is used to drive the display circuit on the GT.  So if anyone else out there is witnessing similar symptoms, check out T500 and make sure its well. I'm just glad to be able to button this one up and move on, I hate working on these damn boards due to their rarity lol

For anyone that may be curious or want to try out the T500 sub themselves: https://www.digikey.com/en/products/detail/pulse-electronics/PA1133NL/5174310

Soldering instructions here (though the pin orientation is meant for GT not LT, so make usre you get pin 1 right): https://imgur.com/a/zKv0imP



MasterofBiscuits

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Re: PCE LT No Display
« Reply #4 on: February 22, 2024, 04:49:34 PM »
Glad you got her fixed! :thumbsup: