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Neotec NT-500DX Repair
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.
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