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Neotec NT-500DX Repair

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princess prin prin:

--- Quote from: Rocketeer2001 on September 01, 2024, 11:50:44 pm ---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.

--- End quote ---

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)  :o
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?

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