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Old 26th July 2005, 06:32 AM   #1
Howardc is offline Howardc  Australia
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Default Studio 350 bias problem

I have a friend who has built 2 of the studio 350 amps featured in Jan/Feb 2004 Silicon Chip magazine. They both have the same bias problem, where the output is at rail volts. All components have been double-checked, esp. transistor pinouts. The only way I could get the output voltage close to zero was to reduce the value of resistors in Q4 & Q5 collectors. This causes high currents, & tends to overheat these transistors.

Has anyone had a similar problem with this amp?

Howard
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Old 15th August 2005, 06:58 AM   #2
AllanA is offline AllanA  New Zealand
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I have had the same problem. I also note that another user (Silicon Chip issue 194, Nov 04) encountered this problem. Silicon Chip advised him:
"Our guess is that you have swapped a pair of transistors or you have an open-circuit solder connection somewhere. Replace the fried resistors and power up the board again, with the resistors across the fuseholders, and check each transistor for a base-emitter voltage of about 0.7V. An incorrect reading indicates a fault in the transistor or its associated components".
In my case, I have exhaustively checked component positioning. I have also checked all resistors and most other components by multimeter. I have checked all solder joints. In the end I replaced diodes D1, D2, D3 and transistors Q1- Q9. Didn't make any difference. In my case, the problem is focussed on 3 voltages which are hopelessly out of spec:
- across 6.8k 1W resistor near base of Q6 (123V, should be 60V)
- +70V to collector of Q6 (132V, should be 69V)
- -70V to collector of Q5 (4V, should be 69V)
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Old 15th August 2005, 09:11 AM   #3
rpapps is offline rpapps  Antarctica
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Hi all
Here are some notes and errata from the Silicon Chip web site.

Studio 350 Power Amplifier Module, January 2004: The 470F 100V electrolytic capacitor connected to the -70V rail (adjacent to fuse F2) is shown reversed on the circuit diagram (Fig.7). (02/04)

Studio 350 Power Amplifier Module, February 2004: The 70V power supply wiring colours shown on the overlay diagram (Fig.1) disagrees with the wiring colours shown on the power supply wiring diagram (Fig.6). In all cases, the +70V wiring should be red and the -70V wiring blue. The 70V markings on the PC board and diagrams are correct. The wiring diagram for the Silicon Chip Online website is correct. (04/04)

Hope this is of some help
Cheers
Rob
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Old 23rd June 2011, 01:22 PM   #4
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Hi guy!

Hey I got a new PCB layout for studio 350 power amp and I just wondering if the polarity of the cap close to F2 is correct before I order a couple of boards? Click the image to open in full size.
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Old 23rd June 2011, 01:24 PM   #5
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Sorry I did specify, the 470uF cap it goes that way or the positive pointing up?
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Old 23rd June 2011, 01:28 PM   #6
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damn stupid me ! " I didn't specify" sorry English is my second language
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Old 24th June 2011, 07:23 AM   #7
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Quote:
Originally Posted by vargasmongo3435 View Post
Sorry I did specify, the 470uF cap it goes that way or the positive pointing up?
Hola

The capacitor orientation is correct. The (+) must be in the lower side...
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Jorge
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Old 24th June 2011, 12:04 PM   #8
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muchas gracias seor
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Old 20th November 2011, 12:11 AM   #9
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what this power??
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Old 20th November 2011, 03:16 AM   #10
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check the resistors they are one percent tolerence.
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