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Old 14th February 2005, 10:18 AM   #1
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Default 30.00 Amp Degraded

Worked one of the SI boards fairly heavily yesterday. Changed the input coupling caps, on board filter cap, and switched to 10 uH chokes from Miller.
Powered up and no smoke. Checked for ac and dc on the output and found 34 mv on on channel as before. But the other channel now reads 9 volts dc offset.
Can anyone direct me to where to start. I yanked it apart again and did not locate any problems with the items subbed. Must have hit one of the surface mount parts with the soldering iron.
I have tried checking it cold with a properly working board. So far have not located the problem. Any ideas?

George
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Old 14th February 2005, 08:06 PM   #2
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I also managed to "degrade" one of my boards two nights ago. Lifted the solder pad on R01 on top (the pad that goes over to L1), and also lifted the trace going to R2. I can probably get it working again, but it's going to be a PITA.

Did you bridge a trace with solder? Can you post some closeups of the board? Also, did you remove the flux on the board with alcohol and a toothbrush after doing your soldering work?

I have a slight problem on the second board that I'm working on. After replacing R4 and R5, two of my resistors don't measure properly. I only read 16.6k across R2 and R4, but get the proper 19.8k on R1 and R5. I know the chips measure properly when desoldered. Maybe someone here will have an idea on that as well?

Good luck!
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Old 15th February 2005, 02:20 AM   #3
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Check C3/C4 and make sure their pads arnt shorted. I did this on purpose and ended up with a nice little DC-DC converter .
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Old 15th February 2005, 03:08 AM   #4
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Ok,

To start figuring out what is wrong with the chanel with offset we need to determine if the problem is on the output side of the chip or the input side. Not much else to go wrong if the other chanel is working.

A couple of things come to mind.

1. There is an open circuit between the output of the amplifier and the speaker output pads. This could be a bad connection or cracked run caused during the replacement of the output chokes. Verify that you have continuity between the snubber diodes and the speaker output pads. The end of the diode closer to the center of the board is the end to measure.

D1 to R+
D2 to R-
D3 to L+
D4 to L-


2. The new input capacitor C2/C4 is leaking DC. What kind of caps did you install? If they are electrolitic capacitors verify that the polairity is correct. A backwards cap will leak DC and the amp will see this as input signal.

The input signal is feeding into an inverting opamp circuit. We have access to all the nessasary nodes to figure out if it is working properly.

Schematic of input circuitry:
Click the image to open in full size.

Pin 16 is the non inverting inputs shared by both channels and biased to 2.4 volts DC internally.

If the input cap C2/C4 is leaking current R1/R2 will be pulled down by the leakage current. This will cause the output of the opamp pins 10/14 to go positive in voltage. The safest thing to check in this case is to measure the DC voltage across R1/R2. In a happy world there should be no DC voltage across either resistor.

If DC voltage is measured on R1/R2 of the bad chanel then there is either a short to ground at the R1/R2 side of C2/C4 or C2/C4 is leaky/backwards.

Check these things out and let us know if things improve.

Gary
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Old 15th February 2005, 05:32 AM   #5
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I'm intentionally shorting C2/4 and replacing it offboard with some 4.7uF Solens =)
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Old 15th February 2005, 10:21 AM   #6
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Quote:
Originally posted by motherone
I'm intentionally shorting C2/4 and replacing it offboard with some 4.7uF Solens =)
From my checking, the input caps are C3/C4. c1,c2 appear to be part of the input filter. If not, this is my problem, I removed C1 and C2. My preamp is transformer coupled, the input sees the 38 ohm dcr and it effectively loads down the input to reduce rfi pickup.
The reason C1 and C2 were removed was that I knocked one off while soldering in a replacement cap for C3/C4. I am having a tough time working with parts this size.
Used a 4.7 ufd ceramic cap from Digikey for the input cap. It checks fine in to the 10K shunt resistor on one end and the 20K inline resistor on the other. It does not appear to be leaking. May be open though.

George
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Old 15th February 2005, 12:57 PM   #7
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Quote:
I'm intentionally shorting C2/4 and replacing it offboard with some 4.7uF Solens =)
That is exactly why I shorted C3/C4 and it was a no go. I put external caps in but even with no input signal connected having them shorted still causes DC on the output. I'm not sure why, but one of you smart fellers will probably tell me .
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Old 15th February 2005, 01:01 PM   #8
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Oh, I also took the elecrolytic caps I tried mounting externaly and managed to solder them to the surface mount pads on the board and the amp worked fine.
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Old 15th February 2005, 01:06 PM   #9
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If you want to short C3/C4 and run a larger input cap off board you have to remove R01/R02. There can be no parts capable of passing DC current to ground connected to R1/R2.

When my amps get here I'm planning on removing R01/R02, C1/C2 and L1/L2. C3/C4 will get replaced with jumpers then off board caps will be used for input duties with direct connection to the input RCA's via shielded cable.

Still thinking about the power supply details. Just found Vinnie's page yesterday. Lots of info there to digest.

Gary

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Old 15th February 2005, 01:23 PM   #10
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Default Voltage pull down

Quote:
Originally posted by theAnonymous1


That is exactly why I shorted C3/C4 and it was a no go. I put external caps in but even with no input signal connected having them shorted still causes DC on the output. I'm not sure why, but one of you smart fellers will probably tell me .

The input of the 2024 is biased up to allow the output to be dc coupled with a single rail supply. Somewhere around 2.5 volts is applied to the input and this is why the c3/c4 is needed where it is at. The 10k shunt resistor is pulling down the 2.5 volt bias and this causes the dc on the output.
I need to dig back in mine, something must be pulling it down to zero to get 9 volts dc. I am back to using a stocker for now.
I have been hesitant to probe around with the chip powered up. All the problems with letting the smoke out of the 2024 chips proves to me that it is easy to kill one if not extra carefull.

George
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