Hantarex MTC-900 Issues

[SirPeale]

I have to empty my storage facility so I brought this home to poke around on again.

So I'm metering resistors in the brightness/contrast circuit before I re-read this thread to acquaint myself with the issues again.  Forgot about the missing 12V until I reread.

Anyway...I got to R203 and R204.  Resistors that should have read 22K were reading 8K.  I was ecstatic!  I thought this was the problem.  Didn't have any on hand, so I managed to get myself to Radio Shack and purchase them.  I even bumped them up a bit from 1/4W to 1/2W.

I removed the first one and metered it...and of course it's reading 22K!  Same with the other.

What causes a resistor to read wrong in circuit like that?

[SirPeale]

I was metering the voltage between SP5 and SP8 and still getting nothing.  Then I got a bright idea: what if I metered between SP5 and the heatsink that IC4 is on.  Presto!  I have 12V.  So somewhere the ground is being dropped.  Now to figure out why and where.  I was thinking about taking an alligator clip and attaching one end to the heatsink and the other to SP8, any reason I should not?

There was also some funky wiring on the underside of the board.  I'm not sure if it's factory or not, but I untacked the ends and put some electrical tape on them.  I know where they go so if push comes to shove I can put them back, but those wires are not on either of the 900E chassis I have.

[grantspain]

maybe you have hairline crack on the track somewhere

[SirPeale]

I'm thinking something similar, but I'm not sure where.  I've traced the ground connection back quite a ways but I've gotten lost.

[SirPeale]

Jumping it didn't help.  Didn't hurt either, luckily.  Instead of the voltage being 12V it was like 10.18.  Something's pulling it down.

[SirPeale]

This is driving me crazy; most of it from my own inexperience with this level of troubleshooting.

I've been using the schematic extensively.  Wherever there's a ground point on the schematic I /SHOULD/ be able to test for continuity all over the board, right?   Say from SP16 I should be able to connect to the frame and have continuity, or to the neckboard, or wherever.

[SirPeale]

Using my 900E as a reference (since they're darn close) I determined that there's some kind of grounding problem going on.  On the good chassis you can hit any ground point and get continuity with any other ground point.  Not so with the bad one.

So I got the bright idea to alligator clip pin 12 from the flyback to the frame.  Horizontal Output Transistor blew.  Now I've got one on order.  Luckily they're not much, and I've got an order to place.  Anyone else need a cap kit?  K4600?  K7000?  K4900?  Sanyo EZ?

[Ken Layton]

Sounds like this chassis is going to be better if you convert this to a Wei-ya chassis.

[SirPeale]

I have a chassis that works already, I'm doing this for the challenge.

[SirPeale]

Now I'm not sure if the HOT is blown.  I removed one from a known good working unit, and it tests the same as the one I removed from my borked chassis.

[grantspain]

can you remove the B+ from the flyback and do the lightbulb trick then meter the voltages

[SirPeale]

Bumping this because I'm going to take yet another stab at this in the very near future.

[grantspain]

ok are will still with no picture but bright screen?
if so i had the same fault on a polo chassis this week,the 200v to the drive transistor was missing,traced it back and found a bad cap and open circuit resistor.
using the 900 schematic the same circuit will be c51,r145,r131 and d25,check for min 192 volts at r145

[SirPeale]

Last I can remember it was not doing much of anything.  Have not yet gone back to it.

[shagster]

Hi,i found this thread after a google search.Looks like you have the exact same problem i had with my mtc900 chassis,a red screen with retrace lines.Anyway i was lucky cause i have two mtc900s,one wont power on.Either would this one till i noticed the lopt had about four cold solder joints,once i reflowed them i had power
Then i just compared transistor values on each board with my dmm and found TR33 on the neckboard didn't read the same between collector and emiter as on the other neckboard.So i replaced it,powered it up and got a black screen,no more red retrace line screen
Put a game into it and,it didn't work
After a lil head scratching i noticed the game had no sound,tested the voltage on the harness and i was missing the 5 volts output,the 5 volts has a dodgy wire,gave it a wiggle and on comes the game.I got very lucky with this,but theres no better feeling when ya can get these things running again
Thats my third chassis repair
Anyway i just thought i'd share this with you as its the same problem.Hope you get yours sorted

[shagster]

Update,turned it on the next day,yellow tinted screen,no retrace lines.
Thought i'd just try to reflow the blue transistors,didn't work,plugged it in and now i got a blue screen with retrace lines!
Looked at the schematic and found the three transistors relating to blue on the neckboard,removed and tested the transistors and found TR26 was shorted,replaced that and now its working fine again,for now

Actually,i think this is a mtc 900/E chassis
Wonder what the hell the E stands for!