Amiga 3000 acting up resistor

[spudje]

I recently acquired a nice Amiga 3000 Desktop. I first had it checked and recapped. There it was noticed that R403 or R404 was burnt. So the resistor was replaced. However, after turning on the Amiga again the resistor burnt again straight away. The guy who did the fixing is very knowledgable on Amigas but couldn't find the cause.

I only quickly checked sound with the boing ball demo. It works, it might be a bit less clear than I'm used to and there seems to be some noisy echo. But that can all be due to bad cabling outside of the Amiga, or due to interference with other equipment since the box was open and other computers were running.

Maybe someone here knows what could be the cause of this? Thanks in advance!

[Blizz1220]

Well , try to check PSU for voltage values first.

Other case could be that something has broken and is
now draining too much power for resistor to survive.
Like some component getting reallly hot before resistor
gets burnt.

BTW is your Amiga fully functional this way (if you can
test it to boot I mean? ).If it is maybe better not repeat
the process too many times.Sound issues are almost always
connected to -12 V so peeking inside your PSU may not be
such a bad idea (or was it recapped too ? ).

That resistor is connected to cap that gives power to some-
thing and may be there to lower it or may be there as a simple
protection so the shock doesn't go any further).

I never really fixed any A3000s so this is all I know

[mechy]

I recently acquired a nice Amiga 3000 Desktop. I first had it checked and recapped. There it was noticed that R403 or R404 was burnt. So the resistor was replaced. However, after turning on the Amiga again the resistor burnt again straight away. The guy who did the fixing is very knowledgable on Amigas but couldn't find the cause.

I only quickly checked sound with the boing ball demo. It works, it might be a bit less clear than I'm used to and there seems to be some noisy echo. But that can all be due to bad cabling outside of the Amiga, or due to interference with other equipment since the box was open and other computers were running.

Maybe someone here knows what could be the cause of this? Thanks in advance!

I cant quite tell from the pic, but is the cap c491? installed with its polarity wrong?
The reason i ask is the trace leads to the burnt resistor.
barring that follow it on the schematic and board and see where it leads.. I assume the resistors were the proper values and ratings?
Without more info, i can only guess something is shorted or as blizz said.
is there any chance the plastic is missing under the motherboard between it and the case and the lead is shorting to the case?

[spaceman88]

I'd check the board for solder bridges, both on the capacitors that were replaced and also on the board in general (sometimes a drop of solder can fall when soldering or de-soldering and land just about anywhere causing all kinds of problems). Don't ask me how I know.

[spudje]

Thanks a lot guys! I have some pointers which I'll take with me when asking an "Amiga Guru" to look at it.

[Castellen]

That 1 Ohm resistor supplies DC to the audio stage.  You probably have a faulty op-amp (U402) which is drawing excess current, hence the resistor gets hot and essentially fuses.  Could also be a short in either C405, C406, C407, C408.

All of these parts are readily available.  You should be able to quickly verify the issue with an Ohm meter, then remove the parts I mentioned starting with U402 until you see something greater than about 1k Ohm from either of the op-amp supply pins (4 and 11) to ground.  You'll also need to replace that resistor again.


Edit: I should have read the entire post - yeah, if it's been worked on then there's a good chance there's a solder short to ground on either C407 or C408, check that first.

[rkidd7952]

One other thing to mention is to check the value of the resistor used to replace the burnt one.  R403 and R404 should be 10 ohms based on the Commodore schematics (A3000_19a.png in the A3000 service manual from http://www.devili.iki.fi/mirrors/4x4.hopto.org/).  The vector schematics from amigawiki (http://www.amigawiki.org/dnl/schematics/A3000.pdf) list those resistors as 1 ohm.

Robert

[spudje]

So what should it be then? 1 or 10 Ohm?

[rkidd7952]

I would go with the 10 ohm value from the Commodore schematic.  Both R403 and R404 have the same value.

From the seven resistors in a row, where #3 is the burnt one, it looks like #4 is a 10 ohm in series with a 2.7k (#6), which match the values of R403 and R407, respectively.  That makes the burnt one R404, which should be 10 ohms to match R403.

Robert

[Castellen]

So what should it be then? 1 or 10 Ohm?

It won't make any difference to the operation of the circuit which value you use, but to keep it consistent then I'd also agree to keep it the same as R403.

[Blizz1220]

@spudje

I would check if that cap isn't backward soldered first as
was suggested.That could be the reason for burning and
even if you put stronger resistor while the cap is backward
it will just swallow up.

On the side note , soldering job seems to be done well from
what I can see (without seeing back side of the board) , I
tend to make it messier :lol:

I had no idea A3000s were such a pain to fix

[spudje]

While it seems now audio is fine and it was just the boing demo sounding a bit distorted I might just keep it as it is. If I do decide to have it fixed you guys have provide a lot of valuable input.

I very much doubt the capacitors are causing this, as they have been replaced by someone who very much knows what he's doing, hence the decent soldering work.

He thought another possibility may be some (voltage) faults inside any of the custom chips.

I guess I'll also check with Haynie at Amiga30 this Saturday