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Old 18th September 2011, 11:38 PM   #11
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I think the only way to check this channel is using an scope. Input a sine wave signal and goes through the signal path.
Both pin should read close. Try putting a signal on both channel and compare the reading. This is the first stage of the amplifier. If this op amp is not working you will not heard anything in the output.
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Old 19th September 2011, 12:47 AM   #12
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Tauro,

I don't have an oscilloscope, however if necessary I will purchase one to diagnose the problem.

I was testing everything near pin 9 and the area where I repaired C153 and I noticed that diodes D108 and D122 are acting strangely. When tested one direction with my diode tester, they read about .6 volts. When tested the other way though, they kept fluctuating between .6 volts and infinite resistance.

When I tested D208 and D222, both read .6 volts in both directions consistantly... which is odd in itself!

Here is the diode part number from the Crown parts directory:

C 3181-2 DIODE, 0.3A 75V 1N4148 AXL

Shouldn't these diodes only conduct one way with a constant voltage?
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Old 19th September 2011, 02:02 AM   #13
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Hi,
The diode when the neg lead is in the cathode and the red in the anode should read .6. When reverse the lead should read infinitive. This is when tested outside the circuit. Did you read the diode in place? To get a good reading you have to lift one side from the circuit board.
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Old 19th September 2011, 02:51 AM   #14
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Hi,
I have some questions. Did you removed the U104? Did you mounted it in a socket? How good you removed it? When you remove an IC you have to very careful because if not you may damage the traces and some time removed the feed thru of the eyelets. Another thing this U104 is critical because it had 4 operational amplifiers. Follow U104a, U104b, U104c and U104d in the schematic and you would see they are used in the preamp and one is used in the low pre driver location F2. So you need to make sure is working good or nothing work in the amplifier
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Old 19th September 2011, 03:00 AM   #15
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Tauro,


U104 actually came mounted in a socket. Otherwise, I definitely would not have swapped it out! I just removed u104 and replaced it with the National LME49xxx opamp which has 4 operational amplifiers like the original chip.


I replaced u204 with a national lme49xxx opamp as well and channel 2 works fine.

To make sure that the national opamp wasn't causing the problem, I swapped in the original MC33xxx opamp and I noticed a difference. Instead of hearing nothing from channel 1, I heard static from channel 1! Obviously, the problem is located somewhere around U104.

Should I reflow the solder on all of the contacts for U104? Could I have damaged something removing the opamp from the socket? I didn't remove the socket.

Thanks

Last edited by techbiker; 19th September 2011 at 03:07 AM.
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Old 19th September 2011, 04:23 AM   #16
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I wish that this forum allowed for edits after a period of time lol.

I checked the DC offset at the channel 1 output connectors and my voltmeter reads 200 mv even though it should read 0. The service manual says that if dc offset is out of spec that this would indicate that u104 is out of tolerance. Note that I saw this with the MC33xxx installed.

Edit: I also was able to get some sound from channel 1. It gave me static that was "in sync" with the music that I was playing.

Last edited by techbiker; 19th September 2011 at 04:35 AM.
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Old 19th September 2011, 07:20 AM   #17
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Since looking back on how the problem started was when you removed the C153 capacitor. It is possible that a traces it is still open. Capacitor C153 is connected to U104.

I have a suggestion. It is a tedious one but it will tell you if everything in the U104 it is OKAY. With the ohmmeter check each pin by touching the IC pin and the other one to a component that the pin is connected. Example U104 pin 8 is connected to R113,R179 and Q113. With one lead of the meter at pin 8 and the other lead to the R113 side connected to pin 8. See if you have conductivity between them. Do the same test with the other pins. This test will tell you if you have a good connection between the U104 and the traces on the board with the other components.
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Old 19th September 2011, 08:34 PM   #18
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Quote:
Originally Posted by tauro0221 View Post
Since looking back on how the problem started was when you removed the C153 capacitor. It is possible that a traces it is still open. Capacitor C153 is connected to U104.

I have a suggestion. It is a tedious one but it will tell you if everything in the U104 it is OKAY. With the ohmmeter check each pin by touching the IC pin and the other one to a component that the pin is connected. Example U104 pin 8 is connected to R113,R179 and Q113. With one lead of the meter at pin 8 and the other lead to the R113 side connected to pin 8. See if you have conductivity between them. Do the same test with the other pins. This test will tell you if you have a good connection between the U104 and the traces on the board with the other components.
Tauro,

I spoke with Crown support today and Kevin looked at the input circuit again. We determined that there might be more components on the input board than are on the schematic because this schematic is from 1990 and my amplifier is from 1996.

Since the IOC lights are not coming on, all of the components are functioning but there is likely some part of the circuit that is open (ie something still isn't connected). Hopefully this new 1996 schematic will reveal something new.

Thanks again for the help.

Last edited by techbiker; 19th September 2011 at 08:44 PM.
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Old 19th September 2011, 08:43 PM   #19
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Hi,
I agreed 100 % that something maybe is missing a connection.
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Old 19th September 2011, 09:28 PM   #20
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I was psyched until I received the correct schematic lol. It doesn't seem like there is any difference between the original 1990 schematic and the 1994-1996 schematic in this area.

Do you see anything that could be related to the problem that I'm experiencing?

Here is the correct schematic:

J0631-2_D.pdf

Thanks
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