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testing a crossover

hi guys and girls i have a
problem with my 3 way  wilmslow-audio mirrage speakers , I am hoping you can help me with the problem,.
Lately my power amp went pop and i sent it away for repair,when i
received it back {repaired} it went pop again,on sending it back to
the repair technician ,he has come back to me ,and asked me to take
some ohms readings on the speakers, on testing the speakers , i run
these speakers using 2 power amps{Arcam Alpha 10s,using one for low
pass{bass} and the other for MID/HIGH,this is the amp which keeps
blowing,on the high /mid,on testing the terminal the bass{low pass}
read {left speaker]7.9 ohms ,right speaker bass 6.8ohms, But on the
left and right speakers hi/mid ,both read 0.0 ohms,{which we all know
is wrong}
BUT the strange thing is,when the blown amp was away,i only used one
power amp to run both speakers in bi wire config,and they worked
ok,all being a bit lack of bass and tightness,but still sound great.
So i stripped both speakers and checked the drivers,all 6 drivers
where good no shorts reading approx correct ohms for age,{Speakers
disconnected from x over}.can any one help with this,i would be so much in debt.
kind regards Mark
likklegerry
pragmasi
212 posts
 
Are you measuring the resistance across IP- and IP+ on the PCB? It looks to me like IP+ routes to two resistors (2.2Ω and 3.3Ω), there's lots of clearance between the tracks and no obvious short. So even if there were a problem further down the line you'd expect to see at least 1.3Ω across the inputs. It's rare for resistors to fail closed circuit so I wouldn't go looking there just yet.
The 2.2Ω is in series with the 12µ cap which will block DC, I can't work out what the 3.3Ω connects to under that big cap, but I still can't see how there'd be 0Ω across IP- & IP+. I'm sure I'm missing something.
I can see fibres on the PCB conductors but assume they're hot glue or stuffing material. Even so I'd start by cleaning up the copper connected to IP- & IP+ and measuring the resistance across those if you haven't done so already. I'd also check the multimeter by measuring a known value e.g. a resistor that's out of circuit. You wouldn't believe the number hours I've wasted going down a rabbit hole only to find I've measured something wrong or missed something obvious.

I'm sure you'll get there in the end with the help of some others here... good luck.
fiesta75
1,378 posts
 
No NOT ac voltage, DC voltage. This is the offset voltage of the amp and I wanted to make sure they were low as THAT could be a cause of capacitor failure.
fiesta75
1,378 posts
 
Well if you have lifted one lead of each cap and they test good, I would say it's a short on the PCB. Check all the resistors just to be safe, most resistors open or go high when they fail. In one picture I saw a couple capacitor leads that looked very close together. On another note, you said the amp worked for about 3 hours when you first got it back. So I'm assuming they sounded good when bi-amped before it went pop? I'm thinking there could be physical contact between a couple components lead wires. Reason, it work for a while. Maybe vibration caused the components to get physically close enough to short. What is the value of the cap connected to the midranges in the bi-amp configuration?
fiesta75
1,378 posts
 
For some reason I can't access your crossover photos I saw yesterday. Anyone else have this problem?
likklegerry OP
30 posts
 
thankyou all again for this help,i uploaded a schematic of the crossover wilmslow have now sent me,i will test and check all readings and offset dc today,then i will get back to you all.
so many thanks
regards
mark
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