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| Macintosh Color Classic No Sound After Recap |
Posted by: AlexTheCat123 on 2019-03-20 06:32:07
I recently pulled my Macintosh Color Classic out of storage and found that the sound was very quiet. I could hear it if I put my ear up to the computer, but it was very faint. I did some research and found that this issue is often caused by bad capacitors, so I recapped and cleaned the board. When I powered it up after the recap, it worked fine, but the sound was still as faint as before. Is there something obvious that I am missing?
Thanks!
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Posted by: AwkwardPotato on 2019-03-20 17:51:18
Have you tried plugging some headphones/powered speakers into the computer and seeing if the sound from them is audible?
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Posted by: AlexTheCat123 on 2019-03-21 02:54:35
When I plug in powered speakers, I can hear it just fine.
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Posted by: AwkwardPotato on 2019-03-21 08:36:14
Probably an issue with the amplifier then, which as far as I can tell is on the analog board. Have you recapped the analog board yet?
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Posted by: AlexTheCat123 on 2019-03-21 08:48:21
I have not recapped the analog board yet, but I plan to do that soon. Isn't the amplifier on the logic board, though? Correct me if I am wrong, but I thought that the amplifier was integrated into the sound chip.
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Posted by: AwkwardPotato on 2019-03-21 09:55:35
I'm not very familiar with the Color Classic, but I'm fairly certain amplification is done on the analog board since there's an amplifier IC (TDA7052A) next to where (I believe) the speaker connects.
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Posted by: AlexTheCat123 on 2019-03-21 10:21:27
Thanks for the information! I will try to recap the analog board soon and see if the problem is fixed.
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Posted by: AlexTheCat123 on 2019-03-22 09:58:57
Do you have any idea which particular capacitors are probably at fault? I would prefer to only replace the ones that pertain to sound if I can because that would be much easier than recapping the entire board. I guess I could trace the signal lines from the amplifier chip to see which capacitors that they connect to if nobody is really sure, but it would be great if someone already knew. Could the TDA7052A sound chip itself be at fault as well or are you sure that it's the caps?
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Posted by: AwkwardPotato on 2019-03-22 11:28:13
Following the traces is probably your best bet since there isn't an analog board schematic to show what's connected where. TDA7052A could possibly be at fault too, it's hard to tell for sure. Just wondering, what are the voltages at pins 1 and 4 of the TDA7052A (not sure what exactly they're supposed to be but it may help)? Also, can you upload a picture of the top and bottom sides of the board around where the TDA7052A is so we can see if anything is obviously damaged?
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Posted by: AlpineRaven on 2019-03-22 15:15:31
Before you fault find the amplifier chip - test with another known working board that has confirmed sound output in that Macintosh. Test another board, ie. LC575 or LC520/550 board etc in that Colour Classic. I've experienced with another Colour Classic that had whisper quiet sound but with speakers its working fine. We did logicboard swap and it had sound which eliminated the A/B that pointed to the logicboard. I tested that logicboard in another Colour Classic and it did the same thing, so the fault lies on the logicboard possibly the sound chip?
Cheers
AP
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Posted by: AlexTheCat123 on 2019-03-22 15:58:17
Before you fault find the amplifier chip - test with another known working board that has confirmed sound output in that Macintosh. Unfortunately, I do not have another logic board that I can test it with, so I am kind of stuck if the logic board is the real problem.
Just wondering, what are the voltages at pins 1 and 4 of the TDA7052A (not sure what exactly they're supposed to be but it may help)? I just took the back off of the Color Classic and I realized that the amplifier chip is hidden in a very inconvenient location. It is directly under the CRT and I don't see any way that I can put a probe on it without touching high voltage components. Would it damage anything if I disconnected the anode cap and yoke connector from the CRT and powered the board like that so that I can slide it out and access the chip? I am afraid that it could damage the computer if I ran it without the CRT connected. I can send pictures of the analog board once I fully remove it from the computer (probably tomorrow).
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Posted by: IlikeTech on 2019-03-22 16:38:26
That is a very bad idea. Flyback would still energize and the possibility of shocking the crap out of yourself is rather high.
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Posted by: AlexTheCat123 on 2019-03-22 16:59:56
That's what I thought. So how should I access the chip then? I can't think of any ways to check voltages with power applied without disconnecting things from the analog board.
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Posted by: IlikeTech on 2019-03-22 17:03:50
You need some of the multimeter probes that are meant for clipping to chip leads.
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Posted by: AlexTheCat123 on 2019-03-22 17:05:51
Maybe I could use my oscilloscope to clip onto the chip and take a voltage measurement since I don't have any of those clip leads for multimeters. I will try this tomorrow and see how it goes.
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Posted by: AlexTheCat123 on 2019-03-23 11:12:14
I just checked the amplifier chip and everything seems to be fine. There is a steady 5 volt signal on both pin 1 and pin 4. I also traced the lines coming from the chip and found that there are 3 capacitors in the amplifier circuit: CS2, CS4, and CS6. I tested all three capacitors and CS6 appears to be bad (it shows no capacitance on a capacitance meter). I looked up the part and it appears to be a 3300 picofarad surface mount capacitor, but I am not completely sure. Can anyone confirm this? I am pretty sure that it is the bad part because the sound behaves exactly the same with it removed. Below are links to pictures of the top and bottom of the amplifier section of the board (with CS6 removed) because it wouldn't let me attach them for some reason.
https://drive.google.com/open?id=1Pqsbh3mUYZUM6trZOk1KzlxZweK8knWQ
https://drive.google.com/open?id=1Huyb6GXhMgnMZt4yBFnKx_WR_NNKaB5P
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Posted by: AlexTheCat123 on 2019-03-25 03:28:01
It turns out that I was completely wrong about the bad part. I tested it again and it checks out just fine. I guess that I didn't make good contact with it the first time I tested it since it is so small. Now that I am certain that all three capacitors are good, I am not sure what is wrong. Any ideas? Some pictures of the amplifier circuit can b found in the above post.
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Posted by: AwkwardPotato on 2019-03-25 10:27:48
It could be the TDA7052A itself that's faulty.
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Posted by: AlexTheCat123 on 2019-03-25 10:33:40
I guess the chip itself is just about the only thing left that could be fried. I will try to order a replacement and get it installed to see if the IC is the problem.
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