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Half rail voltage QUAD 303
Half rail voltage QUAD 303
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Old 13th March 2019, 06:33 PM   #1
e00rica is offline e00rica
diyAudio Member
Join Date: Feb 2017
Default Half rail voltage QUAD 303

Prepares me to fix a problem that has got on my QUAD 303 amplifier. it is about the half-rail voltage in the driver boards, both right and left. I measured it (pin 9 and 5) and its the same that the rail voltage (pin 1 and 9) = 67volt. before i start with troubleshooting wanted to ask you about how I should connect a bench DC power supply with current limitation ( 200, 400, 800mA) to this driver boards. should i connect it to pins 1 and 9 or should i use pins 1 and 10? and which current limitation shoud I select?
Thank you,
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Old 14th March 2019, 12:45 AM   #2
Ian Finch is offline Ian Finch  Australia
diyAudio Member
Join Date: Apr 2010
Location: Coffs Harbour, NSW
Pin10 is the signal ground connection. R115 would likely overheat if you powered the amplifier from the front end so don't do this. When you normally power any amplifier that has a a voltage problem, always use a current limiting device such as a DBT (dim bulb tester) which is simply an incandescent lightbulb in series with the AC mains supply.

A DC bench supply can be used but the regulator board that connects to pins 12, 15 must be included so that the DC level at the output capacitor (5) can be set at half supply voltage. The service manual details the operation, setup and test procedure for this model. There's a free download available after signup here: Quad 303 - Manual - Stereo Power Amplifier - HiFi Engine
If only I had listened....

Last edited by Ian Finch; 14th March 2019 at 12:58 AM. Reason: fixed the terminology
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Old 14th March 2019, 08:42 PM   #3
e00rica is offline e00rica
diyAudio Member
Join Date: Feb 2017
Do you mean that I can't isolate the driver board from the PSU board? I will have the C1 ,TR1 and TR2 connected to the driver-board
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