Re: newbie solder question


"Dave D" <dave_d@xxxxxxxxxx> wrote in message
news:D8OdnbQDFvuHRFLeRVnygQ@xxxxxxxxxxxx
>
>
> Do you still have a short? If so that makes things very easy- a short is
> probably the easiest fault there is to diagnose. Simply trace the two PCB
> tracks which are shorted and find all other components across these
tracks.
> One of them must be shorted out. Is the capacitor in question on the
supply
> rail? Check that the new IC you fitted doesn't have any solder bridges,
and
> if it has a mica heatsink insulator, make sure it's not damaged. I doubt
> it's a shorted cap, but it is possible nonetheless.
>
I checked for solder bridges on the new IC VERY carefully when I put it in,
as I did also inspect the mica insulator.

> It's possible the 470uF capacitor is part of the decoupled supply to the
> preamp section, if so it may have a zener diode in parallel to give a
> regulated supply. Therefore, check for shorted zeners.
>
The balance/tone/volume IC (which is basically my entiere 'pre-amp section')
has a zener internally, BUT I see that it also uses an external decoupling
cap on the internally regulated voltage supply... must check which cap
performs this function.

> The output ICs no doubt are coupled to the speakers via large electrolytic
> capacitors, what is the voltage level on the IC side of this cap with
> respect to ground? It should be close to half supply potential. Compare it
> with the good side, but take care not to short anything out.
>
No, no output caps at all. Output pin of amp IC goes directly to RCA plug
via large trace.

> Are both output ICs getting a supply? Are they both getting a ground? Have
> you measured voltage levels on each pin and compared them with the good
> side? Have you downloaded a data*** for the ICs, found the input pin and
> injected a signal there to listen for output? Even touching a finger to
the
> input pin may give a buzz from the speaker, or a screwdriver may give a
> click. There's ways and means, even without test equipment!
>
> Dave
>
>


.

Quantcast