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Harman Kardon 330C blowing fuses

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pcrosewood

Active Member
My 330C worked well and one day developed some distortion in the right channel. In a short time the channel stopped working and no sound in the right speaker. I took a look inside ad spotted a suspect cap, C405 on the main board which tested out of spec. I replaced the cap and still no sound on the right. While I was at it, I decided to replace cap C406 which also tested bad. Fired the unit up and both 4 amp fuses blew. I replaced the output transistors thinking they may be the problem but the same thing happened.Both fuses blew again. I like the 330C and I don't want to give up on it so any suggestions or help would be appreciated.
 
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Did you put a cap in backwards? Did you test the output transistors before replacement? How did you determine they were bad? If they did fail, usually stuff up the line fails in a cascade nature.
 
I checked and double checked to be sure the caps were not in backward. The output transistors I replaced were all good. It turned out they were not the issue.
 
Did you put the cap in exactly as it was originally installed or according to the board markings, sometimes this is oddly incorrect in production.

Otherwise, you likely have other parts down or upstream that need diagnosis and attention. Use a DBT to help prevent catstrophic parts failure.
 
I did use a variac to slowly bring it up to 120V and the fuses did not blow. I then plugged in the wall plug and they blew when I turned it on. I put the caps in the same as the ones I replaced. I must say I appreciate everyone's response.
 
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So, I pulled the two caps I installed and tested them. I cleaned all the old solder and soldered them in. I checked the service manual and the orientation of + and - was right. I then used the dim bulb tester to power it up. The bulb glowed so dim I had to turn the lights of to see it. In 15 or 20 seconds, it was like the light came on but still, a dim glow. Definitely not glaring bright. I'm not sure how bright a bulb must get to indicate a problem but this light glows dim for a 60 watt bulb.
 
Been a few days but back at the 330C. Using my DVM, I tested the 4 white ceramic resistors, R427 R428, R429, R430 near the output resistors. Two of these, R427 and R428, are used when biasing the unit. They are 2w .47ohm resistors and all four tested at 0.8ohms. I would think this indicates they are bad but would this cause the fuses to blow? These can't be sourced locally and so can't easily replace and test the unit.
 
Were those resistors tested in circuit or with one leg unsoldered and lifted from trace or completely off the board?
 
I took the resistors completely off the board when I tested them. I did not touch the meter leads together before testing.
 
petehall347 said:
first touch your meter leads together and record what it reads then deduct this figure from 0.8 .
Click to expand...

Yes, when dealing with measuring sub 2 ohm values, the resistance of the meter leads must be taken into account as they can add 0.3 to almost 1.0 ohm to the reading, depending on meter test wire gauge and clip materials, corrosion or oxidation of the clips, and any resistance in the banana plug / socket interface.
 
Touching the leads gave a reading of 0.5ohms. That subtracted from the 0.8 ohm reading leaves 0.3ohms. That still indicates they need to be replaced but I would think
unlikely the cause of the fuses blowing.
 
I do have an oscilloscope to track these problems down but don't know how to use it. Don't know where to go from here so any advice or suggestions would be appreciated.
 
I have been testing resistors on the main amp PC board comparing resistor readings between the 2 sides of the board. R420 and R419 are 220ohm resistors and both test at 220ohms when removed from the circuit. When tested in circuit, R420 tests at 218ohms and R419 tests at 45ohms. Any suggestions why this would be? The testing was done with the receiver unplugged. When tested in circuit with the power on, R420 tested at 218ohms and R419 tests at .313ohms.
 
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first off dont do ohm readings with power on . it will likely damage your meter .
q419 might be bad . base emitter short . first reverse meter leads to see if ohms changes on r419
 
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