Dead AP-0428 chassis?

[ardarvin]

Hey,

First off, if anyone has any info on this chassis (picture below...ignore the green circles), an AP-0428 by A-One (apparently this is Orion?) I would be forever indebted to you.

So here's my problem:

- When I first tried the monitor, I got a working image for about 10 seconds, then the image died. I tried toggling power, got the same results, and then the monitor hasn't worked since.  No pops, no visible damage on the board, the chassis fuses are fine, and the flyback isn't cracked.

- I know the RGB-Sync input signals are correct from the game board, and the game works fine in "ghost mode"

- The chassis is getting the correct input voltage - 115V from the output of the iso transformer.

- The tube doesn't glow. If I increase the brightness (one setting on board, another on the flyback) still nothing. No raster lines, no white image, nothing.


I have no idea what a HOT is, or where to find it on the board.

Do I need a cap kit? If so, does anyone know where to find one for this chassis. Bob Roberts doesn't list this one.

Schematics or flowcharts for this chassis would be brilliant.

Thanks a lot!

[grantspain]

have checked for dry solder joints/blown fuse yet

[ardarvin]

haven't gotten to checking all the solder joints (will do tonight), but the fuses are all fine.

btw, any info on what the HOT is, and where it is on my board?

[grantspain]

its a horizontal output transistor
its bolted or clipped to the heat shield that surrounds the flyback transformer
i think its a little way back in your first photo as i can't see it
its a extremely common fault on all older monitors as well as bad caps
start with the solder joints though

[Ken Layton]

You have an early Kortek monitor.

Check for bad solder joints.

Install fresh new electrolytic capacitors.

Check for blown fuses.

Clean off all the dust/dirt on the flyback and examine it closely for cracks/burn marks. If any found, replace the flyback (should be common Kortek flyback).

Check horizontal output transistor for shorts.

[ardarvin]

Thanks Ken, once again your knowledge amazes.

How do I check the HOT for shorts?

[Ken Layton]

Unsolder it and measure the resistance between all three legs. There should never be any resistance less than 10 ohms between any legs.

[ardarvin]

I found a blown resistor connected near the AC input of the chassis. It's a 5w 15 ohm.

I'm wondering if I somehow blew this up. Correct me if any of this wiring is wrong:

- I have both my PSU and my chassis running off of the AC output from the ISO transformer.

- I have the PSU ground and the iso transformer output ground connected together at a common point on the metal bracket holding the CRT in place.

[Ken Layton]

I'm wondering if I somehow blew this up. Correct me if any of this wiring is wrong:

- I have both my PSU and my chassis running off of the AC output from the ISO transformer.

Never never never never never connect anything else to the output of the isolation transformer. Only the monitor should be connected to the output of the isolation transformer. See Bob Roberts' AC wiring diagram at his site for correct wiring.

You may have damaged the monitor main board.

[ardarvin]

Eeek. ok, so by saying only the monitor should be plugged into the output of the ISO trans, I'm assuming you mean that only the AC input of the chassis board is attached to the output of the ISO trans, correct?
(as I believe that is the only way to power the monitor).

[Ken Layton]

Yes, that is correct. The power input to the monitor comes from the output of the isolation transformer.

The way you had it hooked up no isolation occured.

[ardarvin]

OK, I rewired the whole thing...properly (thanks bob roberts diagram), replaced the blown power resistor, and got the same results...about 4 seconds of image, and then nothing.

I'm going to start switching out cap's...Question: Is it OK to just take the caps off, test their values (I have a cap tester) and if OK, put them back on? Are caps simply a pass/fail or do they slowly degrade?

Also...there is an unlabeled two terminal thing hooked to 2 of the terminals of the HOT (I looks kinda like a massive power resistor), any idea what this is?

[grantspain]

is it a mame jobby?

[ardarvin]

nope. it's a Neo Geo MVS. Original boards. I know the input RGB signal to the monitor is fine.

[ardarvin]

OK here's what I've done:

- installed fresh caps
- checked the solder joints
- checked the flyback for cracks
- checked the fuses
- checked the HOT for shorts (the resistance between terminals was somewhere around 1Mohm if I remember correctly).
- have a proper isolated 115V coming into the chassis board

Now when I turn it on, I get a dot on the CRT in the upper right corner...which fades to nothing in about 5 seconds. The tube never glows, nor does it buzz (I had a girl with good hearing listen) and I've got the brightness cranked on the flyback.

The tube does seem to be charging up (it gives a good pop every time I discharge it), just no image 

Any other suggestions (other than throw this thing in the garbage)?

[grantspain]

so you're saying you have ht(static on the tube) but no picture?
if so check for a bent pin on the neck card/check the g2 input on the neck card(i think there is a cap and safety resistor on the g2 line)

[Ken Layton]

Could be that the monitor is going into shutdown mode from the B+ voltage being too high. That would be due to a bad voltage regulator transistor or two in the power supply section.

Get out your high voltage probe and measure the high voltage underneath the rubber suction cup. The voltage should be steady and be between 19,000 to 24,000 volts. Anything less than 19,000 volts and the flyback is probably bad. If the voltage climbs to 30,000 volts then drops to zero then the power supply section of the monitor has problems.

[ardarvin]

heh, this is starting to get expensive... $141 for a high voltage probe. Well the good news is by the time I'm done, I'll probably be able to go into business.

So I measure the voltage while the suction cup is attached to the tube?

It might be cheaper for me to just replace the regulators...there seems to be big one which heat sinks itself against the metal shield around the flyback. Is this a common failure?