Another NeoTec NT500DX (pincushion issue) FIXED

[lilshawn]

pincushion issue on NT500DX. i've blasted basically everything in the pin circuit

IC601

C745
C774
C773

Q717
Q714
Q715
Q709
Q708
Q707
Q706

D707
D708

checked resistors

nothing. screen looks fine except for the damn pincushion.

EDIT:

i'm probably going to go ahead and re-replace all the transistors and fets. (probably next week when I have time) I pulled these from a non-working nt500 chassis that had the burn hole of death. I figured they are probably fine, but you never know. trannys are a dime a dozen these days anyways.

Q706/Q708 - 2SC945P
Q714/Q715 - 2SA733G
Q707/Q709 - IRF640B
Q717 - TIP122

[lilshawn]

had a look at the datasheet for the TDA9115 regarding the E-W output.

I noticed that for the pincushion control IC outputs a specific waveform for each scanline...



then this is used to drive a few transistors and with power from the 12v supply... biases the Horizontal deflection timings to even out the scanline times. more voltage, the pic gets narrower...when zero, it's wide since the sides are caved in, this means either the wave coming out of the IC is flat or the 12v voltage is dead...

seems the 12 volt works AND a wave is being output. something is stealing our biasing voltage.

notice a cap, C727 is tied to ground after our biasing voltage has been produced and goes off to the horizontal deflection circuit...



I'll bet this is toast and shunting our bias voltage to ground. sure enough... no good. replaced it and it's good to go again.