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Old 29th September 2002, 10:00 PM   #11
Jean is offline Jean  United States
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Woohoo. One channel is running with only 16mv DC on the output. Second channel keeps blowing transistors or running w/o a load with 30volts DC on output

I replaced my 2sa/2sc pair with tip2955/tip3055 pair, I should have used tip outputs for testing !

Its now making music

Now I need to find out why second channel isn't working.
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Old 29th September 2002, 10:18 PM   #12
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Please keep us informed of your progress. I'll be starting my p3a soon and would like to know of any relevant information.
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Old 30th September 2002, 10:53 PM   #13
Jean is offline Jean  United States
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KenP, I bought the boards from Rod Elliot online. The layout is pretty straight forward.

I used 1% metal resistors, including the 5 watters on the output transistors and panasonic caps. I measured all resistors one at a time before soldering them to the board. I have not measured the caps or transistors though. I am not still sure why it started blowing fuses. It still does on one channel, I have to take the caps out now because I have already replaced all transistors
twice and its driving me nuts.

The board layout was made to be cut in a half if needed, and thats what I did for ease of installation.

My power supply is pretty simple. I am using 500va 25-0-25 avel lindberg tranny, two 35a bridge rectifiers and 4 10000uF panasonic caps (two per rail). If I remember correctly that gets me about +/- 36volts.

Once I get the second channel working I'll start putting it all together, its not in a chassis just yet and I have not done any listening on my speakers (only a raw 8 ohm driver for testing).

I'll keep you posted, also if you need a part # for the trim pots that fit perfect let me know.
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Old 30th September 2002, 11:17 PM   #14
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Thanks I appreciate that. What capacitor did you use for C4, it's supposed to be a 100pf 500v but all I can find of that type are ceramic disc caps. Would they be ok?
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Old 30th September 2002, 11:30 PM   #15
sam9 is offline sam9  United States
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Default sam9

I've had a couple mysteries like that. It had nothing to do with board either time. Once it was an error with installing the output connectors - something I thought was insulated wasn't. On another occassion it only happened when the enclosure was closed - turned out something was contacting the end of the bolt that attached the trasformer creating a short coil - very major short.

While your situation doesn't sound quite like these, consider looking beyond the PCB just in case.
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Old 1st October 2002, 02:16 AM   #16
Jean is offline Jean  United States
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Quote:
Originally posted by KenP
Thanks I appreciate that. What capacitor did you use for C4, it's supposed to be a 100pf 500v but all I can find of that type are ceramic disc caps. Would they be ok?
I am using 200v Kemet molded radial caps from Mouser.
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Old 1st October 2002, 02:20 AM   #17
Jean is offline Jean  United States
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Default Re: sam9

Quote:
Originally posted by sam9
I've had a couple mysteries like that. It had nothing to do with board either time. Once it was an error with installing the output connectors - something I thought was insulated wasn't. On another occassion it only happened when the enclosure was closed - turned out something was contacting the end of the bolt that attached the trasformer creating a short coil - very major short.

While your situation doesn't sound quite like these, consider looking beyond the PCB just in case.
The board and the power supply is not installed in the case yet, I can't possibly think of anything that could be shorting anything out. I'll start removing and testing caps and then transistors again tonight.
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Old 1st October 2002, 11:02 AM   #18
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Could any of the transistors be grounded on a heat sink? I had a problem with a power amp blowing fuses and it turned out the chips where grounded to the chassis. Just a thought.
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Old 1st October 2002, 02:50 PM   #19
saurus is offline saurus  United States
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Default Regarding those 500v C4's

Triode Electronics (www.triodeel.com) has 100 pf, 600v polystyrene caps for US$0.99 each. They are axial, but they fit OK.
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Old 1st October 2002, 04:56 PM   #20
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Default tips from an unintentional arsenist

Having to replace transistors each time while you are trying to track down the problem is a pain in the neck. What I do is to put a 10-ohm high-power WW resistors in the collectors of the output transistors (or in the power line feeding the output transistors). This will not be good for normal operation but will enable the amp to function adequately to debug it. If there is a short or instability the resistors will get really hot but the transistors will be saved.

Another tip is to power up the amp without the o/p transistors connected at all (no drivers or power transistors). Move the feedback point to the voltage gain stage output (the point before the output drivers) and check all the voltages and currents look sensible. Then add the output stage (with the 10-ohm safety resistors) and test again.

I'm not familiar with the circuit you are using, but generally it is possible to do this.
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