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Old 29th April 2009, 11:38 AM   #621
cyxg is offline cyxg  Australia
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Hi£¬Peter

everything is ok.
the transformer secondary volt is 27.7VAC, rectifier output is 38VDC,after connect amp board the V+/V- down to 35.6V.
no signal input and dummy load,the DC offset is 0.042V.
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Old 29th April 2009, 11:43 AM   #622
cyxg is offline cyxg  Australia
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Hi, Peter

my RCA is direct ground to chassis.see pic.
is there any problem here?

how to make ground arrangement in my chassis, please give me advise .
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Old 29th April 2009, 12:00 PM   #623
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Quote:
my RCA is direct ground to chassis.

Elsewhere in this thread Peter suggests insulated RCA. I am a newbie, but I guess that with your RCAs you completely change the grounding with respect to what suggested by Peter


Renato
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Old 29th April 2009, 02:09 PM   #624
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There is nothing to worry about grounds. Some guidance was given here: http://www.diyaudio.com/forums/showt...20#post1108820

The RCAs need to be isolated from a chassis. You are using what looks like Cardas RCAs, they should come with plastic isolation washers.

I would suggest rotating transformers to minimize wire lengths.

If you are adding source selector and potentiometer, please see this diagram: http://www.diyaudio.com/forums/showt...97#post1516297

The ground does not go through selector, but does through potentiometer: RCAs --> pot --> SG on amp board
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Old 29th April 2009, 02:16 PM   #625
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Peter Daniel
I would suggest rotating transformers to minimize wire lengths.
I would suggest rotating the transformers to minimise hum and buzz injected into the amplifying circuits. Then cut the connecting cables to length.

Remember to twist the flow and return wire pairs. This applies to mains and LV power and audio cables.

If there is a third wire to a pair, (eg. +ve, -ve, Zero Volts) then all three need to be twisted together.
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Old 29th April 2009, 02:22 PM   #626
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Quote:
Originally posted by AndrewT
I would suggest rotating the transformers to minimise hum and buzz injected into the amplifying circuits. Then cut the connecting cables to length.
You may try that too, but in my experience, in a chassis like that, the hum caused by transformers is not an issue.
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Old 29th April 2009, 02:27 PM   #627
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Peter Daniel
You may try that too, but in my experience, in a chassis like that, the hum caused by transformers is not an issue.
Hi Peter,
you are probably right.
It is worth experimenting just in case the thick aluminium is fighting against a strong field pointing in the wrong direction.
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Old 30th April 2009, 02:05 AM   #628
tbrooke is offline tbrooke  United States
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Default troubleshooting PD LM3875 Kit

My amp gets extremely hot and the transformer seems to vibrate a little and of course no sound. I rechecked the wiring and looked for shorts and things seem ok

I measured the voltage on PG+ V+ PG- V- and I think I found my problem since the voltage was about 36 volts and the transformers is 250 VA and is supposed to put out 25 volts.

My problem (I think) is I have 0V -> AC1 and 25V -> AC1 and the same for AC2 on my rectifier board but it should be 0V -> AC1 0V -> AC1

25 V - AC2 25V -> AC2

Is this right I wanted to make sure before I changed it and blew something up

Tom
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Old 30th April 2009, 03:17 AM   #629
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This link shows how to connect dual secondaries transformer to rectifier board: http://www.diyaudio.com/forums/attac...amp=1210694958

On AC1/AC1 and AC2/AC2 you will be able to measure AC voltage only.

If transformer produces 25V AC, you will get approx 36V DC between PG+ and V+
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Old 30th April 2009, 11:54 AM   #630
tbrooke is offline tbrooke  United States
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Quote:
Originally posted by Peter Daniel
This link shows how to connect dual secondaries transformer to rectifier board: http://www.diyaudio.com/forums/attac...amp=1210694958

On AC1/AC1 and AC2/AC2 you will be able to measure AC voltage only.

If transformer produces 25V AC, you will get approx 36V DC between PG+ and V+
OK so I think I am ok coming out of the rectifier. My kit had two rectifier boards but from the forum it seems that one might work better so I am using 1

Another thought I had was that I ran a wire from the CHG hole on each amp board to my star ground. I do not have anything from my rectifier to star ground since I thought it would go through the amp boards.

So my problem must be in the amp boards. I used the continuity check on my multimeter to look for shorts. I thought the chips may have been shorted to the heatsink but I checked the pins to ground and nothing except for the 2 pins with the resister shows a connection to ground.

I have been using an old 8 ohm 1 watt speaker as a load

Any thoughts on where to look next?
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