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Old 12th May 2009, 12:00 PM   #1
xn7 is offline xn7  United States
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Default newbie question - replacing caps

have a old aiwa solid state.. getting a pretty loud speaker/headphone hum.. level of hum is nondependent of volume control. not ground looping as far as i can tell. no interference from external sources.

so i think its the caps. from what i can see 2 of the larger caps are slightly bulged. 8200 µF 56v is what it says on them. turns out i can't find the exact cap.. all i've found are:

8200 µF 50v
8200 µF 63v

are either of these appropriate to use? im new at this.. lol. thank you
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Old 12th May 2009, 01:36 PM   #2
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I would go with the higher voltage rating. The lower voltage part may not have enough headroom above the power supply voltage to be safe. I would also check the rectifiers for leakage. At this point you don't know why the caps failed (or are in the process of failing). It could be just an old cap or it could be from AC leakage from a diode that is going bad.
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Old 12th May 2009, 02:36 PM   #3
flacer is offline flacer  Malaysia
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The 8200 µF 63v is appropriate to use. ( always can be replace by higher voltage rating )
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Old 12th May 2009, 02:42 PM   #4
AndrewT is online now AndrewT  Scotland
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Can you measure the voltage across the smoothing caps?
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Old 12th May 2009, 07:48 PM   #5
xn7 is offline xn7  United States
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thanks for the responses and clearing up the question regarding appropriate voltage of replacement caps. i did find the exact specification nichicon caps i need, but it is difficult locate a supplier that will sell in less than bulk quantities.

i looked around under the cover a bit more today, and saw a "toasty" area of the board underlying 2 resistors.. could this be the problem? i've also included pictures of the "bulging caps".

i'll see what i can do about testing voltage across the caps and troubleshooting the rectifier. regards

Click the image to open in full size.
Click the image to open in full size.
slight bulging of the larger caps to left of the picture, 2nd from left is shows most bulging.. am i seeing bulge where there is none
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Old 12th May 2009, 11:57 PM   #6
xn7 is offline xn7  United States
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did a little testing on the resistors, they're both green-blue-red-gold (5.6K ohm, 5%).

rear resistor measured 5.46K ohm, within spec
forward resistor was 6.44K ohm, a bit high(105% of 5.6k is 5.88k)

is this likely the culprit?

i haven't gotten around to the rest of the testing as this is all new to me and finding step by step instructions has been difficult. cheers
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Old 13th May 2009, 02:07 AM   #7
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Hi

Measure the voltage in terminals output (without load).
Must be less than <100mv
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Old 13th May 2009, 06:59 AM   #8
jaycee is offline jaycee  United Kingdom
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The capacitors dont need to be Nichicon. Any good brand will do. Elna, Panasonic, Nippon Chemicon etc. Use the 63v type. If dimensions permit you could even fit a larger capacitance, you might find a modern 10,000uF 63V will fit in the allowed space.

The bridge rectifier that powers the power amps will be the one with the heatsinking - this looks like an old M4C52 type. You can replace this with a modern GBPCW type. It may have 4 small ceramic caps across its terminals on the PCB - check or replace those.

The smaller BA40 bridge is a 4A bridge so that will probably be for auxilliary supplies and not the power amps.

The toasty bit of the board looks more like the small transistor is to blame than the resistors.
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Old 13th May 2009, 08:14 AM   #9
xn7 is offline xn7  United States
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tested both resistors again, both reading 5.4 or so now.

Quote:
Hi Measure the voltage in terminals output (without load). Must be less than <100mv
how do i best do this?
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Old 13th May 2009, 08:43 AM   #10
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Quote:
Originally posted by xn7

how do i best do this?
You need to identify your problem first....
its "Hum" can be DC offset in output, to test:Remove the connection of speaker and measure the voltage at the terminals, scale of the multimeter must be in DC
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