Sony ta-f570es eq board cooked resistors.

anderslober

anderslober

Yikes...not again!
Hi forum. This series of Sony amps is really nice. Excellent sound. I am doing a service/recap on the eq board. There is a series of resistors just before the regulators. They must run HOT! See picture.

The values on the board do not correspond to values indicated on the scat’. The two burned/cracked resistors r425/r426 are 2.4k ohm. On the scat’ they are 1k. The other resistors in the ‘network’ arent the same value as in scat’ either. Unit is original and untouched/serviced. What would you guys do here?
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As it's shown, the input voltages for the two voltage regulators would be 'marginal' or even too high for the regulator specs. - so, I suspect Sony may have increased the 1k resistor values slightly to drop the supply voltages a bit more. I'd probably stay with the 2k4 values, and check the voltages & power dissipation(s) in that whole area, but first - which other resistors in the network are different to the schematic ?
The degree of carbonisation under that area is potentially a concern, e.g., if the board has become conductive at all ... It'll clearly need a good clean up, and preferably elevation of those resistor networks when they're replaced, just to improve thermal dissipation.
 
Agreed. The 2.4k ohm resistors are mounted next to 390 ohm resistors. The outer two resistors each side are 1.2k ohm in parallel.
Of course, I dont have any 2.4k ohm resistors…I do have 2.2k and 2.7k……ok to use?
Picture of trace side…
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I'd probably use the 2k7, then check all the actual values you're getting in circuit.
The trace side seems to have survived, but the top could still be an issue.
 
Ok….I will sub in 2.7k/1w resistors. I will elevate them a bit of the board. The component side looks good, but brown. Should be fine.
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That's how I'd mount them yes, although using larger wattage resistors won't help much with the heat generated (they still dissipate the same power).
I'd probably have elevated all 8 resistors, but two at least helps. All 8 are just for voltage reduction, so all are dissipating heat, exactly how much from each depends on the values of each, that's why I suggested to check what the circuits doing (V^2/R will give you an idea of power dissipation per resistor).
 
I took a quick look at the full schematic this morning, and those 2 voltage regulators only seem to be supplying the phono amp stages, which shouldn't be a particularly heavy current load - so - the extent of the heat damage suggests you may have additional problem(s) downstream of those resistors, e.g., the voltage regulators themselves, or something in the phono stages.
 
the extent of the heat damage suggests you may have additional problem(s)
Click to expand...
Reverse engineer the circuit from the track layout provided (I know you can...) - it's NOT the same as the supplied circuit diagram (the 390R resistor made me do it!). :yikes:

Good Luck!
 
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