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Old 8th April 2010, 11:03 PM   #1
diban is offline diban  Brazil
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Location: São Paulo
Default Studio 350

Hello,

Recently built my self a pair of studio 350's

My amp is working very good until 40v.

When work in 70v i have noted is that the main transistors are getting hot relatively quickly.

Adjusting VR1 & VR2 i find dc +- 40v in output

I need a help please.
tanks.

(sorry for my modest english)

Silicon Chip Online - Studio 350 Power Amplifier Module

Silicon Chip Online - Studio 350 Power Amplifier Module; Pt.2
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Old 12th April 2010, 12:28 AM   #2
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Join Date: Mar 2009
i have the sch, happy to email andymac1962 AAATTT hotmail DDDOOOTTT com

Recuce power supply back to 40V, and make sure all is working ok.
Are you usign the BC556, 2SA1084, BF469, BF470 as specified, or are you using different transistors. MPSA92 could be used instead of 2SA1084.
With power off, measure ohms C-E of power transistors and drivers to check for shorts.
Is the feedback connected correctly.
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Old 12th April 2010, 02:20 PM   #3
jaycee is offline jaycee  United Kingdom
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Chances are, you got fake MJL21193/4's and they were really 2955/3055 inside. Fine at 40V (just), will fail at 70V.
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Old 12th April 2010, 03:04 PM   #4
diban is offline diban  Brazil
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I'm sending for you.

Ok, with 40v is working better.

i'm using different 2SA1084 changed for 2SA970.
BF469 for BF459.
BF470 for BF472.
MJE15030 for 2sc2238a
MJE15031 for A968

Tank's
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Old 20th April 2010, 10:26 PM   #5
diban is offline diban  Brazil
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Join Date: Sep 2009
Location: São Paulo
Anyone have a suggestion?

I used to think maybe adjustes some Resistors, but i dont know the value.

tanks!
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Old 26th April 2010, 09:46 PM   #6
diban is offline diban  Brazil
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Default help!!!!!

up!!!! hellllp please!!!!!!!!
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Old 27th April 2010, 01:07 AM   #7
jaycee is offline jaycee  United Kingdom
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These transistors should be fine, but if your MJL21193/4 output devices are fake and are really MJ2955/2N3055 inside, then it will blow up at >40V no matter what you do. You could probably use MJ15024/5 as well (these are TO-3 package) - but they must be REAL devices. Fakes are always 2955/3055 which are just not adequate for a high power amp, so if you cannot get good genuine devices, then you are unfortunately wasting your time and money.
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