API 512b Schematic

[mitsos]

There is no trim pin on your board, that is your output. That resistor is some kind of load isolation and goes to your output trafo primary. (can you see the value of the cap next to it btw?)

I smoked one of those once on a clone (maybe clown is better?) but it had to do with a bad diode on the power rails or one I had put in backwards or something, not likely here since this was working before, but may want to check them anyway. I think diodes go open when they kick the bucket? Could be wrong.  Anyway, the fact that the 47R saw enough current to fry it, could mean something shorted to ground. Check the board and if there is a short in the opamp you may be able to check for it on the pins. Sam Groner is the opamp expert, maybe he could lend a hand here.


Greg, I think it's more likely rectification for the meter since it's a dual opamp. Don't wanna ask to see the bottom of the board. Or did I just do that?

 

[clintrubber]

API got back to me - and just as I suspected:

"I am sorry but I can't e-mail you schematic for 512b because it is very similar as current pre-amps (factory policy)."

Mmmm, so now instead of someone & a few of his friends having the schematic, now the full pics are on the Interweb as a result...
what would 'protect' them the most ?  :

But as was said, somebody can simply buy one anyway, reverse & clone.

As it looks like the signal-path is pretty much the same as 312, not ? Little ingredients to be different here, at least as far as topology is concerned.

 

[Harpo]

Sorry it took so long, here you go:

I darkened the area where the resistor burnt up. I thought it was connected to the trim pin because of this image I have

It looks like the value is printed on the circuit board, 47R.

That would be a 47 ohm 1% metal film resistor. I'd put a 1/2 watt-er in there to be safe.

As Mitsos already pointed out, could be an output load isolator in this spot. Any traces of burnt wire wound around this part ? Have a look at the Jensen JT-OLI-3 or the Hardy 990 paper (pg.6) to get the idea behind.

 

[mitsos]

I was just looking at the Eisen Audio schemo, and they have a 51 Ohm there they say is for short circuit protection.  So, one or the other..

What could cause so much current to run through this part?

 

[OneRoomStudio]

Well I don't know if it was a result of this thread or what, but I just got another email from API:

"Hello Peter

Yes this is output resistor and value is 47 Ohm, you must change op-amp in this unit because op-amp is reason why resistor is burned.

Best Regards"


I guess I'll give that a try. I really hope I don't end up with two blown opamps...

 

[mitsos]

well, good they at least helped out in the end.  Get an opamp kit from one of the guys in the white market. jsteiger has the 9 transistor version and mikefatom has the 10 transistor. 

 

[OneRoomStudio]

Yeah, I guess they just needed some prodding.

I already have a pair of Scott Leibers SL2520's and a pair of 990's. I'll probably stick one of the SL2520's in there and see what happens.


If all goes well, maybe this would be a good time to do a controlled DOA comparison...

 

[mitsos]

post back with results.

 

[Whoops]

any news?

 

[groselicain]

I know I'm resurrecting a very old, very dead topic, but I was hoping someone might be able to help. I've got a 512c here that won't pass phantom to the input jacks. 48v is getting into the board, but I'm very confused as to where it's all going wrong. I can find 48v headed into R1 (475ohm) but dropping to 42v as it exits that resistor. Replacing that had no effect.

Time for your close-ups. 

Relay, R8, input transformer stuff.

So this relay, how does it affect phantom power?

Thanks in advance, and apologies if this was a bad idea.

 

[PRR]

> ...48v headed into R1 (475ohm) but dropping to 42v ...

Picture the whole circuit. Do math.

48V-42V is 6V.

6V across 475r is 12.6mA.

The remaining 42V at 12.6mA figures as 3,325 Ohms.

Look again at the circuit. There is a 3,400 (or two 6,800) resistor to the jack. Allowing for some slop, this is *exactly* your error.

You have a short to ground at the jack.

 

[groselicain]

Thanks so much PRR. Novice would be giving myself too much credit when it comes to these kinds of things. I really appreciate your help.