PROBLEM.....Or Not?

ARFA:

You are truly THE MAN. I wish I had half the knowledge and experience you have! Honestly, you have been a huge help to me with this issue. Thank you!

I am not a professional technician (did you guess that?), just a Nak enthusiast who doesn't have access to competent, reasonably-priced service in my area. So when I have a problem, I turn to the internet. It's great that there are folks like you who are willing and able to help out. It's a lot more than what I have where I am!

NOW TO MY LATEST QUESTION. Last night, the OTHER fusible resistor in this circuit blew. I have now replaced it and the display is OK once again. BUT....if the fusible resistors continue to blow, should I take a look at Q413's voltages (which is what I'm going to do anyway)? By the way, I have checked the two ceramic disc 0.1uF capacitors tied to the fusible resistors. They are both OK.

ANOTHER (RELATED) QUESTION: As I said before, this deck got hit with a power surge, causing damage to several components in the power supply (now fixed and stable). In order to explain why BOTH of the fusible resistors in this circuit have managed to blow...is it possible that both resistors were weakened by the power surge, and now that the deck is being used regularly once again, they just couldn't keep up the pace? Or...and I shudder to ask this....are they a symptom of a deeper problem, possibly with the transformer itself?

OK, something is definitely whacko, because I am only getting -4.97v DC at the emitter of Q413.

Looks like I have some more detective work to do.

Considering one blew, the other was probably weakened. I expect this is the last problem you'll have with the display.

Won't affect it.

Graham

Spoke too soon !

Graham

Sure looks to me like ZD406, R422 and Q413 regulate the -14.5V rail down to -5V which is applied to the center tap of the 7V winding.

-- Adam

Yep, you're quite right Adam. I must admit I didn't look at the value of the zener. I saw the label that said -14.5v and didn't notice the speck of flyshit that was the arrow pointing at the emitter of Q412 ! So -5v it is, and the

-4.97v that the OP is reading is correct.

As far as the resistors failing goes, I would suspect that they might have suffered some strain as a result of whatever nastiness was done to to the thing in the first place, or even just that the originals had aged and gone up a bit in value, which is not at all uncommon with these fusibles. If the value even doubles to 2 ohms, this can result in a significant increase in the dissipation in them. I would feel inclined to just go ahead and replace the one that's failed now, and then feel them both to see if you can detect any heat. I would normally expect them to run stone cold. You could also try measuring the AC voltage drop across each one, then work out the current and the power dissipation in each one. With a 1 ohm resistor, the voltage reading will give you the current direct (I=V/R) so if you read 0.4v you would have 0.4 amps flowing. The power in the resistor will be the current squared (P=IsqR) for a 1 ohm so would be 0.16 watts. This should give you a good idea if anything else is actually wrong. It's hard to see what could be, but an intermittent fault around Q412 is maybe a possibility. If your meter has a peak hold facility, might be worth leaving it hanging on the centre tap just to see if anything unexpected happens to the voltage from Q412.

Arfa

It's center-tapped. You're fine.

Peace, Paul