Author Topic: SX8350 Powered P.A. Head Rebuild Project  (Read 2630 times)

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Offline Jesse

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SX8350 Powered P.A. Head Rebuild Project
« on: May 25, 2015, 12:16:51 am »
Hi all, new member here.  Well I pulled a SX8350 out of storage and started working on it.  It would play great for about an hour and then slowly lose volume and finally no audio for a good ten minutes and then finally would come back on.  I figured a thermal issue somewhere so I took the covers off the power transistors on the back of the heat sink and didn't realize that the screws were part of the circuit and left them off.  Started to spray freeze spray on each power transistor to no avail.  Went to move the p.a. box and blammo, spark-o, the thing started to cascade in component failure and I quickly pulled the power (the wife just went crazy).  So, $40.00 bucks later and a lot of soldering I now have a green power light again but very low distorted sound from the tape in which when turned up with either the master volume or the aux volume with the gain low on a radio signal causes the limit light to come on with no volume and distortion.  I replaced everything that looked blown and or tested dead.  If the p.a. has no speaker load hooked up then the limit light just comes on for a split second on power up.  With a speaker load of two 8 ohm cabs that make it 4 ohms mono, the limit light stays on for a minute during power up?  I think either the speaker out point might have a bad resister that is affecting the load and then the limit circuit?  I also noticed that the back heat sink is not hot like it use to be when it was cranking.  This is what the first problem would cause to happen when the sound would drop out.  Hoping someone with more experience than I has dealt with these symptoms before and fixed it.  Thanks in advance....I will keep working on this p.a. until i fix it.  I've already learned a lot about electronics from it so far and can tell it's got a lot more to teach me.  Thank God for YouTube...I'm taking all the right safety precautions in case your wondering.  Like discharging the Electrolytic Caps etc.

Offline Jesse

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Re: SX8350 Powered P.A. Head Rebuild Project
« Reply #1 on: May 29, 2015, 10:52:53 am »
Well I made myself a classic light bulb current limiter and plugged the Sunn SX8350 P.A. head into it with the speakers plugged in and without the speakers plugged in and the volume on zero.  When I hit the power switch on the amp the light bulb first came on somewhat kinda dim and slowly faded to a constant dimness(with and without speaker load connected).  I would guess this means there might be some kind of short or a transistor that is full on while the amp is idle.  I would guess the amp is a A/B mono amp?  Does anyone know if the light bulb should or shouldn't light for an idle 350 watt RMS solid state amp that might not have a short?  I plugged a 800 watt Alto powered speaker in to the current limiter with the volume off and had no light what so ever.  The Alto is a Class D Amp which only draws what it needs power wise and was idle with no load so probable wasn't a good comparison.  I also plugged in a 15 watt Fender Champ tube amp and the current limiter light never came on either.  So I guess I'm wondering if the 350 watt A/B Sunn is just drawing more current like a big tube amp would do so it's normal for the current limiter to come on a little and stay on a little or because it's a solid state amp that is idle with the volume on zero than the current limiter should not light up in anyway which means there is a short in the amp etc.  Thanks in advance for your input...
« Last Edit: May 29, 2015, 11:45:29 am by Jesse »