Realistic SA-100B Pre-Amp (maybe) Problems

[Blast]

Did you disconnect the transformer from the diodes to do that check?

[scootsity5]

nope. actually, that's a good idea, I'm away from the amp right now, ill try that in a few

[scootsity5]

I was thinking about it. You made an excellent point. That said, I don't want to remove the transformer from the board, it's really far back in there, and the solder globs are massive. Instead, I'm just going to go the other route, and replace the diodes. I've never seen a diode like that before, it's disc shaped. What diode should I replace them with? I'm also thinking that if I test it with the power switch off, such that the circuit is open and just check voltages between the switch V- and the V+, that should avoid the diodes and tell me if it's the transfo or the diodes. I'm thinking it's got to be the diodes, because what can possibly go wrong on a transformer, other than getting wet?

[Blast]

I'm confused. Isn't it the transformer connection to the circuit board the two blue wires seen in the upper left of this photo?



In my amp said blue wires are attached to the same pads that connect to the cathodes of the diodes. So if your board is similar won't you have to desolder these pads to remove the diodes anyway? Do as you wish but, can you not simply heat the solder up and lift the wire off the pad? No "desoldering" would be necessary.

I don't understand what you are saying about the test with the power switch off, V- and V+, etc. Sorry....

If you don't want to disconnect anything use the diode test on your DMM and test each diode. For instance, on my Fluke 12B set to diode test, on my amp I read .544 one way and, reversing the leads, open ("OL") the other way. Which tells me the characteristics of the diodes are similar to each other. One of your diodes could be leaky and this test would tell you so.

As to the transformer it could very well have a "semi-shorted" winding in the secondary. While it's possible it's not probable, in my book anyway.

If you still want to "shoot from the hip" go ahead with the diode replacement. As to what type of diodes I would think that any 1N400x would work.

[scootsity5]

In thinking about it, everything you say makes sense. Well, my wires are red, but whatever. I think my diodes both read 5V in one direction and open in the other, I'll check again in a few minutes. I don't have anything that fancy (in the way of multimeters), but it's should certainly do the trick. It's an extech EX330

I don't know why I was thinking the transformer was mounted differently, my mistake.

I'm trying to finish my MS thesis tonight, so no promises that I will get a chance to test everything tonight, but I'll try (sorry, you've been so helpful, I don't want to drag this out...but my thesis has to come first).

In my defense, for anything silly I say, I've been running on about an hour of sleep per night (average) for roughly a month now... but I'm in the final stretch!


EDIT: Diodes tested: Both reading 0.51V, well, one is 0.511, the other 0.509, that isn't enough of a difference to matter, right?

[Blast]

Originally Posted by scootsity5

EDIT: Diodes tested: Both reading 0.51V, well, one is 0.511, the other 0.509, that isn't enough of a difference to matter, right?

No. 0.511 and 0.509 are within reason. However, please confirm that you get get an "open" or OL in the other direction.

EDIT: I'm sorry, you did say "open" in your first sentence.

And if you do, then I'd call the diodes good. The next test is to check AC volts of each side of the transformer relative to the center tap. We need to find the voltage imbalance that you told me existed in post #19, above.

I'm glad you're on this because I'm on the edge of my seat here!

[scootsity5]

The reverse is open.

I'd hate to make you hold out any longer, but I can't pull out the transformer leads just yet. I need to get a few pages written. I'll try later on tonight, no promises.
Sorry, this was probably a really bad time to start this project!

[Blast]

OK. And, no problem. I'm just giving you a hard time!

I appreciate you inspiring me to open mine up and put the binding posts in it!

[scootsity5]

I found an STA-11 on craigslist today, I've been staring at these minimus 7 speakers so long, I just couldn't stand it. So, I got the STA-11, it's all right, lacks the character of the SA-100, and it's not quite as clear as my Yamaha or Sansui, but it will do until we get this sorted.

Sure beats the hell out of the creative computer speakers I've been using for years. It's funny, I have a great set up in my living room, but these days I spend all day at the computer, and my nearfield setup was crap. This at least brings me a step closer. I do have a phenomenal headphone set up here, though.

Anyway, updates when I can. I'm really, really hoping to finish my thesis up tonight, but well, I said that a few days ago, too.

The SA-100 is really neat, because I noticed while I was working in it, that everything in there either still is or was previously stocked at radio shack. When it was released, I bet you could build a point-to-point version of the SA-100 out of any radio shack store.

[Blast]

I hope the SA-100B doesn't know you're cheating on it.

[scootsity5]

I won't tell if you won't.
Sorry, busy with the holiday. I'll get back to it next week.

[scootsity5]

Sorry to drag this out further. Hoping to finish my thesis, defend, and finish school over the next week or two, sorry, this sucker is going to have to wait a little longer.

[scootsity5]

Blast, I hope I haven't lost you forever! I defend my thesis tomorrow, then a few more papers, some other school work and this puppy will have my undivided (or less divided) attention. I'd say a week or so yet! It's sitting on my shelf, staring at me, it will get worked on!

[Blast]

No problem here.

[Blast]

While I was waiting I recapped mine: