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C-Audio SR606 bias problem
C-Audio SR606 bias problem
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Old 14th March 2019, 08:18 PM   #11
neops is offline neops
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Join Date: Feb 2018
Thanks AMV8

I like your idea for separating the AC and DC. sounds like allot of work but very admirable.

whoever put the 27k 1w resistors in my unit also had them raised. I thought it must be for air flow.

If i ever find time to try out some other opamp's i'll post back here.

All the best !
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Old 20th March 2019, 01:22 PM   #12
studio45 is offline studio45
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Join Date: Aug 2010
Now then, thanks for all your input so far - a week has passed, and the damn thing's still on the bench.
There's something really fishy going on with channel B. The main symptom that I can identify, is that the current source for the driver stage is sourcing a lot more current than its brother on channel A.
On ch. A, there is little to no voltage drop measurable across the large 4k7 power resistors in the driver stage. On ch B, they are dropping nearly 30v, indicating quite a lot of current is flowing there.
I have replaced all the transistors in the ch B driver stage with known good units, and still the problem persists (and nothing else has changed either).
Has anyone got a good idea why this might be happening?
TIA,
Matt

PS Still on the lookout for "SR DRIVER 3 ISS 4" schematics, if any has seen them...my boards are some later version than is depicted in the commonly available PDF (which is marked "REVISION 2")
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Old 20th March 2019, 07:07 PM   #13
AMV8 is offline AMV8  United Kingdom
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Join Date: Jan 2005
Location: near london
Hi
The test procedure for the amp is
1 Test the power supply and make sure the correct voltages are being delivered to each channel. If unsure what is happening replace the fuse in the soft start with a 100w 240v light bulb. This drops the voltage to about 60 % of the normal.
2Test the input pair, check current to each transistor and ensure no dc out to the next stage.
3 Test the next stage; again check the driver pair and the current mirror also the standing currents and ensure no dc out of the stage
4 Check the next driver stage and the current mirrors
5 check the output transistors. The easy first check is to see if any source resistors have any dc voltage across them. They should not have.
6 Check step by step frm the power supply omwards.

don
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Old 21st March 2019, 11:26 AM   #14
neops is offline neops
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Join Date: Feb 2018
Studio45 did you replace your fans?, i'm sure you said somewhere you were going to upgrade?
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Old 21st March 2019, 01:10 PM   #15
studio45 is offline studio45
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Join Date: Aug 2010
Hi neops yes replaced those fans. Turns out maybe the old ones weren't so bad anyway, but probably a good idea after 25 years of service!
After checking every single MOSFET on ch B on a transistor tester, I found 3 PMOS (J50) reading gate capacitance of >100uF, where all others read under 100pF. I guess they must be bad?
However, incorrect voltage readings still persist even with them removed, so they weren't the cause of the fault.
Thanks for the test procedure AMV8, I'll get on that today....
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Old 21st March 2019, 02:44 PM   #16
neops is offline neops
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Join Date: Feb 2018
Quote:
Originally Posted by studio45 View Post
Hi neops yes replaced those fans. Turns out maybe the old ones weren't so bad anyway, but probably a good idea after 25 years of service!
After checking every single MOSFET on ch B on a transistor tester, I found 3 PMOS (J50) reading gate capacitance of >100uF, where all others read under 100pF. I guess they must be bad?
However, incorrect voltage readings still persist even with them removed, so they weren't the cause of the fault.
Thanks for the test procedure AMV8, I'll get on that today....



I'm glad you havenít given up! it will be worth it when its all nice and repaired.!



all the best
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