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Old 11th July 2014, 02:27 PM   #51
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Can you do some IC301 pins voltage reading. Read the pins voltage when the power amplifier it is turn on and fail. Do the same test by trying to make the amplifier stay on and read the pins. It will tell you what it is causing the problem. A +5 volt at the Protect pin means that you have a problem with one of your speaker outputs. Please post the reading .

Failed condition
PDET - pin 24 - read = ?
ProtecT - pin 23 - read = ?
pwrly - pin 7 - read = ?

running condition
PDET - pin 24 - read = ?
ProtecT - pin 23 - read = ?
pwrly - pin 7 - read = ?
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Old 11th July 2014, 03:33 PM   #52
sbrads is offline sbrads  United Kingdom
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Location: Kent, UK
Right, time to try something new. Get a permanent connection to Q709 base so you don't have to probe all the time, measure the voltage with a DVM and get used to how long it takes to charge up from -10v to 0.1v with a few turn-ons. Now heat up areas of the board with a hot air gun (hair-drier will do) and repeat. You could also try freezer spray as well if you have any. Look for variations in the charge time, with a bit of luck you'll get a large change in a small area of the board which should help to narrow the range of possible faulty components somewhat.

A quick look at IC702 datasheet shows a quiescent current of typically 60mA, max 120mA. At 52v total supply that's 3-6w dissipation which could be very hot depending on heatsinking.

Last edited by sbrads; 11th July 2014 at 03:44 PM.
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Old 29th July 2014, 12:55 PM   #53
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Join Date: Nov 2010

After convincing my girlfriend that it's perfectly acceptable to be taking her hairdryer out into the shed, I have tried your suggestion and have found that when I heat up the effect power supply (namely C747 and C746), Q709 base voltage on power off drops to ~-4v instead of -9v as would happen at normal temperatures.

So, my thought is that one of them (C746? I'll replace both anyway) is leaky, resulting in the whole protect circuit being pulled low through D712.
The only other cap in this area (where I am directly heating) is C745, so I might as well replace that as well.

What are your thoughts on this?

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Old 29th July 2014, 06:39 PM   #54
sbrads is offline sbrads  United Kingdom
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Join Date: Jun 2005
Location: Kent, UK
It could be - any imbalance between C746/C747 at startup could pull that protect line around. Why is it called PRO on one part of the circuit and PC elsewhere? Daft.
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