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Harman Kardon pm655vxi repair help

lemkoo

By lemkoo,
in Stereo Amplifiers & Pre-Amps

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lemkoo

lemkoo 0

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Hi Guys,

I need a bit of help with my PM655 Vxi unit. I bought it completly dead and I'm trying to fix it, but I've hit the brick wall and can't figure it out. The amp fries resistors R474 and R476, as well as Q430.

So far, I've replaced (as they were confirmed dead) the following, see the replacement too:

Q441 / Q442 - MJL3281AG
Q443 / Q444 - MJL1302
Q437 / Q438 - 2SC4883
Q439 / Q440 - 2SA1859
Q435 / Q436 - KSA1013YTA
Q433 / Q434 - KSC2383OTA
Q430 / Q429 - KSC945CYTA

I've also replaced the trimmers for the right channel and R439/R440 with 220Ohm resistors. It does damage the above 3 componenets, even when I plug a 100W buld into the PSU fuse.

Any feedback is much appreciated.
 
 
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mbz

mbz 179

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Use a lower wattage bulb, eg, 30-50W so as to limit available power to the amp. You've replaced Q430 and "everything" downstream. Since Q430 blows problem is likely to be upstream of Q430. Schematic has R440 as 2.2k, your replacement being 220 ohms, why?

 

If R440 is stressed then maybe Q414e voltage is too low (so high volt drop across R440 and high current).

 

Suggest DBT power up with 30W bulb, check voltage at Q414b or R408, compare with working channel.

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lemkoo

lemkoo 0

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Thanks Mike.

 

P440 swap is one of suggested mods, so that the Q414 runs cooler. I'll keep working up the stream, but so far all of the transistors seem to be fine... 

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