Neotec NT-500DX Repair

[Rocketeer2001]

There wasn't too many threads on repairing on of these monitors, so I thought I'd start one. Maybe that means these are so reliable that they don't break often?

Anyhoo, I just picked up a cabinet that has the 27" Neotec NT-500DX Trisync monitor in it. I need it to put into my Big Buck Hunter 2 with a failed WG D9200 EGA monitor in it.

I tried powering on the Neotec and it makes that recognizable 'toomph' sound and one single click, but then not much of anything else. No OSD or anything; just black deadness. There is a super high pitched whine, but I think that's just your normal power supply kind of sound.

After reading over some old threads and watching this guys videos, I've compiled a list of possible things to check:

http://forum.arcadecontrols.com/index.php?topic=99464.0
https://forums.arcade-museum.com/threads/neotech-nt-550dx-manual-and-schematic-needed.515427/
https://forums.arcade-museum.com/threads/neotec-nt-500-woes.408148/



Here's my list:

1. Check/replace the SMPS (power supply) driver chip IC501:  KA3842A
2. Check/replace the 5 caps in the power section: C525 (up its rated voltage to 25v or 35v),  C527, C519, C501, C502.
    The caps blow and the SMPS starts loading down till the FET blows and the SMPS control chip goes with it. Suggests “upcapping” the voltage rating on these caps in the power section.
3. Check/replace the Vertical IC 301 goes bad and takes out R623A & R623 (5.6 ohm, 2W, 5%). Replacing all ensures reliability.
4. The transistor that controls the power to the electronics Q501 (HSB772P) tends to heat up till it burns a hole in its place.
    Replacing it with an NTE185 and attaching a small aluminum heat sink to it to help dissipate heat really helps.
    This is a transistor that simply switches the monitor 15 and 12 volt supplies off/on using the monitors CPU.
5. Q504 (mosfet 2SK956) might be a blown transistor. Check if legs are shorted to ground.
6. When the SMPS is ticking, check R520 a 68 ohm resistor, and R526 a 0.15 ohm 2 watt 5% resistor.
7. Check R539

I've already written down all the caps and their values and will be replacing all of them. I've heard some success stories after simply replacing all the caps, so I'll start there.

Things I did find / test:
I found one lifted leg on C2A8 on the neck board. I reattached that bought I doubt that was the cause of it not working. Aside from that I've cleaned a bunch of gross black dust off everything.
I tested Q504, which I believe is the HOT, and it seems fine. (see pic)
Tested the big filter cap C504 and it's still very healthy. Tested C521 and it's still good too.
Tested R R623A & R623 and they're both good.


A couple helpful tips for the next NT-500DX lost soul:

1. You need to remove the metal cage around the neck board in order to get to the back of the board to desolder caps. You just need to desolder the 4 corner tabs and one big on in the middle and then the board will wiggle out.

2. I wanted to remove the high voltage wire from the neck socket so that I didn't have that dangling about when I'm flipping the chassis board around replacing caps. I couldn't figure out how to get the wire out of the socket until after I really cleaned it and desoldered it from the neck board to inspect closely.
There's a little tab you can pry away from the wire ever so slightly, and while holding that you push up on a little nubbin. It slides up about 1/4" and then you can pull the wire out. (see pic)

[lilshawn]

looks good visually. nothing stands out off the hop.

don't forget that some of those larger transistors and FETS used to switch the horizontal yoke or flyback will sometimes test good even though they are actually bad. It's due to the low voltages that testers use. the small 2 or 3 volts used to test won't be enough to flashover the gate junction... but when you apply full circuit voltage to it, it does.

so, I would say if it tests bad, its obviously bad... but if it tests okay, it could still be a 50:50 chance... only real way to tell for sure is to swap with a known good one, or devise a testing circuit that tests it at the circuit voltage (140v or whatever)

sounds like the monitor is kind of on, so a good place to start is by checking all the power rails and making sure they are all there. basically ground your negative lead and probe a device that is on the "out" side of the diode coming out of the SMPS transformer. there is a lot of rails in this chassis, but they all do something. hit up the "power" sheet and test them all. you may find one sagged down.

something else is do is use a different highlighter for each rail voltage... and go through the schematics and highlight any of the volt markers on other pages with the same color. makes isolating issues a little easier down the way. like, if the 8v rail is down to 6 you can easily find all the stuff running on 8v by finding all the pink highlighted volt markers.... and that way you don't have to read every single one, every time you look at the schematic.

[Rocketeer2001]

That's a good point about the low voltages of the testers. I could test the voltages coming off those transistors in-situ when the board is powered up though. I should be able to read if the correct voltages are coming out each leg then.

I'm going to do the voltage tests tomorrow, and I took your advice and highlighted all the voltage locations on the diagrams. That was extremely helpful! Had to go over the sheets a few times as I kept missing some.

[lilshawn]

I took your advice and highlighted all the voltage locations on the diagrams. That was extremely helpful! Had to go over the sheets a few times as I kept missing some.

nothing more frustrating that trying to find an issue and having to search all over hells half acre for a tiny circles only to find out the issue was on one you missed along the way.

i've started doing this to all my schematics that are broken upp like this. makes troubleshooting so much easier.

[Rocketeer2001]

Been some slow progress the last few weeks, but I'm back at it!

One improvement I made was at the connection at P903, which is for the degause coil wire that comes from around the yoke. The connector was just shoved onto some pins on the board. I think it should have had a JST socket for it to snap into. Maybe mine just fell off somewhere along the way. I salvaged one off a random board and added that in.

I tested the HOT with my multi-meter instead of my component tester and it might be half working? I put my meter in diode mode, put the negative on the center pin, the put the positive lead on either outer leg. To my knowledge, I should get around 0.4-0.5v on either of them, but I only got around 0.4v on one leg and nothing on the other. I tried juggling the leads around but got the same result.

Decided I should test this again when it's in the board and powered on, so I soldered it back in and reinstalled everything back into the cabinet. When I turned it on I was getting some strange results. I had the neg. probe of my meter grounded to the chassis and I was probing everything else with the pos. lead to check the voltages.
For the Q504 HOT I was getting:
Right leg (leads to L512) =  71VAC or -78VDC
Left Leg (leads to L513) = 71VAC or -74VDC
Center Leg (leads to L511) = inconclusive. Meter was going nuts beeping at me and auto changing rapidly between AC and DC.

Per the recommendation, I checked all the other voltage rails. I tried metering right at the pins coming off the main power transformer T501, but I was either getting a scrambled readings or none at all. If I probed farther down the circuit it was better.
On the AC side at pins 1,2,3,7 I get 71VAC. Pin 5 freaks out.

On the DC side I should have
this voltage @ this location = but I get:
51v @ R544 = 60v
15v @ C501 = 14.7v
12v @ C505 = 11.9v
7v @ C527  = 3.25v
5v @ C507 = 3.7v

I also measured some voltages at the neck board and I got:
7v = 0v
12v = 11.95v
140v (105v) = 146.6v

To me, it looks like I have an issue with my 7v and 5v supply. Also not sure if my AC volts should be 71v or if they should be higher, but it does appear to match the reading at the HOT.
Thoughts? Next things to check?

[princess prin prin]

I tested the HOT with my multi-meter instead of my component tester and it might be half working? I put my meter in diode mode, put the negative on the center pin, the put the positive lead on either outer leg. To my knowledge, I should get around 0.4-0.5v on either of them, but I only got around 0.4v on one leg and nothing on the other. I tried juggling the leads around but got the same result.

That's not the correct way to test an NPN transistor (the HOT here is a 2SC5411 so it falls into this category). NPN means it has two PN junctions (which behave like diodes): one is BC, the other is BE. So you put the red lead on B and the black on C and you get a reading (0.5V or close) and that's what you did. Then you keep the red lead on B and you move the black lead on C and you get a reading a little higher than the other. And that's it. Of course no readings if you reverse the leads and no reading on CE since it is not a junction.

[Rocketeer2001]

Ah, ok good to know. Went out and tried that just now and I got 0.5v and 0.68v. This is with the HOT still in the board, but not turned on.

Given that result, and the voltage readings I got when it was running, does that mean it's ok?

[Rocketeer2001]

Did some more testing tonight.

Everywhere on the board where there should be 7v, there is 0v, and everywhere it should have 5v, it has 3.6v.

I went back to pin 15 of the main transformer which is supposed to be outputting 7.2v and working my way along that circuit. IC 503 (AMC-76285) might be the culprit, but I have no idea what that is...maybe a voltage regulator? Maybe Q506? I tested it while on the board and I do get results instead of a short or zero's, so I'm inclined to think it's ok.

More than likely it could just be bad caps. There's only 3 and when I test them on the board with my ESR meter I get:
C525 - 16v, 1000uf = ESR 9.5 ohm (should be around 0.09 ohm) 
C527 - 16v, 100uf = ESR 2.5 ohm (should be around 0.7 ohm)
C507 - 16v, 100uf = ESR 0.63 ohm (should be around 0.7 ohm)

Guess I should just order some caps and recap everything and see what happens, unless anyone has other suggestions of what I should look at next.

[princess prin prin]

IC503 is a voltage regulator for sure albeit an obscure one (compatible with 7805??). It gets its input voltage from the cathode of D502 and outputs regulated 5V. The input must be at least a couple of volts higher than 5V and lower than the 16V rating of the smoothing cap C526. If you don't have a good voltage at D502, you could desolder the regulator, run the monitor without it and check if the input voltage is now OK. The regulator could be tested with an external voltage source. You could do the same with Q505. 7V is used as the heater voltage so no picture until you have that back.

[Rocketeer2001]

I'm thinking the IC503 voltage regulator is probably working correctly. I metered the voltage at the input side and it was 3.9V, and on the output side it was 3.6v. It is stepping down the voltage, I just don't have enough of it to begin with.

The voltage after D502 is also 3.9v, but the voltage before is unknown. My meter just goes nuts when I try to probe right at the transformer pins. Perhaps too much noise and it confuses the meter?

Kinda thinking that L509 or D502 might be my issue. Or perhaps even the whole transformer. Is there anything on the AC side of the transformer that would be missing up just the 7VDC side?

[lilshawn]

linear regulators need at least 2 volts higher (some 12+ volt regulators need 2.5 or even 3 volts) than their output voltage in order to work properly...

yes that part is a regular bog standard LM7805. it's output voltage is 5 volts, so you need at least 7 volts to get 5 out. (6 would be pushing it and it would technically work, but would be very prone to dropping out) since this is on a 7 volt rail and it's looking like it
s low... you might want to get the 7 volt rail a good looking at. something is dragging it down.

you cannot probe the output of the transformer directly, as it's output a very high frequency pulsed output that is smoothed out by the inductors (L5xx parts) and the diodes immediately after. the average layman should probe the cathode side of the diode for proper readings.

toasted caps can certainly cause rails to be low, and as these (and every other CRT monitor in existence), are all over 20 years old and can purchase their own alcohol now, should be replaced.

[Rocketeer2001]

Thanks for the suggestions and info, guys! Looks like I have some caps to order and then we'll see what happens. I'll probably get a spare voltage regulator and HOT while I'm at it, just in case. Stay tuned for an update in a week or two after I get the parts!

[Rocketeer2001]

I've come across a new hurdle; obsolete parts!

I found a LM7805 easy enough to replace AMC-76285 (IC503), but I can't find a replacement for the HOT (Q504) 2SK2648 (or 2SK956). I know I can use alltransistors.com to find something compatible, but I don't know which specs I can vary by whatever amount to choose something else.

Also can't find a 250v, 1uf NP cap. It's an oddball size and being bi-polar doesn't help. I could find a 1.8uf on aliexpress, but I don't want to use a cap from aliexpress, haha. I should just go test the ESR of those caps (there's 3 of them) and see if they're still ok. If they are, Ill just reuse them.

I was going to pickup some other spare parts too (for just in case), but similar issue with these:
IRF654B
HSB772P (or B772-Y). Shawn, I can't find a NTE185 that you recommended in an old Neotec post either.
KA7812
IRFU214A
KA3842A
TDA8172
C023M-15 (or CD023M or ES023M)
2SC5411 (or J6815 409)

[Rocketeer2001]

Decided to check all the major components coming off that pin 15 of the transformer. I removed them and put them in the component tester. These are good:
Q505 (mistakenly mentioned this previously as Q506)
D502
C552

I think IC503 is good, but it looks like a little PNP transistor rather than a big blocky mosfet like a LM7805. I don't think a LM7805 will fit in this space (see pics).

These caps are definitely bad:
C525 - ESR= 7.1 ohm, 574uf [should be 1000uf and ESR 0.09 ohm]
C507 - ESR= 4.2 ohm, 81.2uf [should be 100uf and ESR 0.7 ohm]
C527 - ESR 27pf [should be 100uf] (this one is so busted it doesn't even tell me the ohms or vloss %. THIS IS A REALLY BAD ONE)

I may have these caps kicking around in a parts bin somewhere. I could try swapping out just those caps and see if it fires up.

[lilshawn]

IC503 won't test with one of those testers since it's a power regulator. you'd have to feed power in excess of it's rated output (in this case 7v as we have discussed previously) and load the output to see if it's working properly.

i'd throw a new cap on anything 100uf and up. (they are the most susceptible to degradation that the smaller value caps.) and anything high voltage excluding the main DC B+ cap.