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Old 30th November 2007, 08:33 AM   #1
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Default Help with amp??? LM4702+2SK1530/2SJ201

Ok I ordered this amp from Chip-PCB.cn I put it all together according to the schematic. All the parts are labeled on the schematic like they are labeled on the board. I hooked it up to a 22VAC transformer through their PSU module (tested DC out of PSU and its 33.7 VDC). So PSU tested out good so I hooked up the amp module. Immediately when I plug it in the 3W .22 ohm resistors glow red and start smoking on the A side of the board which is channel 1. I didn't notice it glowing on the B side of the board but I quickly unplugged it so I didn't get a chance to test if that side is working.

Here is a link to the amp's page on their website.
http://www.chip-pcb.cn/7300/4702-E.htm

If you like I can upload the schematic PDF. I have a few picture but I'm pretty sure I put the parts in right... although I'm not sure why this would happen if I did so?? Here are a couple pics I took.

Click the image to open in full size.

I know you can't make out anything from this pic really just looking for idea's of what to check? If you need me to I can take a close up pic of the board. Also I figured this was the better place for it as its mosfet outputs with only a chip input.

Edit: I went ahead and attatched the schematic.
Attached Files
File Type: pdf 4702-e_sch.pdf (25.8 KB, 367 views)
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Old 30th November 2007, 09:01 AM   #2
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Why is it you don't mention anything about the bias trim pot ?
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BOXES for crying out loud
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Old 30th November 2007, 09:01 AM   #3
Bone is offline Bone  United Kingdom
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Did you adjust the Bias Pot - Rx - correctly?

Tony
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Old 30th November 2007, 09:35 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi Doug,
assemble a mains light bulb tester.
Always plug in any new or modified project into the tester first before trying direct mains supply.
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Old 30th November 2007, 04:09 PM   #5
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I set it to 987 ohms as it wouldn't go all the way to 1k. And what exactly is a mains light bulb tester?
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Old 30th November 2007, 04:44 PM   #6
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Ok pulled the board off the heatsink and checked the bias setting. It's on 955 on both channels. As soon as I unplugged it before I felt the fets and they weren't even warm? Also like I said this didn't happen on the second channel just the first one.
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Old 30th November 2007, 06:13 PM   #7
AndrewT is offline AndrewT  Scotland
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try "search" on light bulb tester.
35 threads!
Ask for posts with andrewt and it reports 25posts
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Old 30th November 2007, 06:27 PM   #8
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So your saying you think I have the transformer wired wrong and its pulling to many amps? I connected it exactly the way it was connected in the ChipAmp it used to be in in fact its still mounted in that chassis. But anyways the chipamp has been working perfectly fine for a couple months. For this lightbulb tester do you just use a regular 60watt light bulb and socket?
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Old 30th November 2007, 10:07 PM   #9
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I found one problem. I put one end of a lead on the other side of the board (channel B) into the wrong hole.....there is a small hole for testing with a DMM and I stuck it in that hole instead of the proper one. Is it possible that it caused the problem on the other channel? I don't think thats really possible. I've checked the PSU and what not over again I left the PSU powered on for a while checked voltages and everything is fine the transformer isn't even getting warm. Another question are you supposed to use heatsink paste on the isolation pads for the mosfets?
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Old 1st December 2007, 12:41 AM   #10
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Ok well apparently they were supposed to send me an isolator pad for the LM4702 and I only received 4 and thought just the mosfets needed them. So I may have a dead LM4702 ...if I'm lucky hopefully not. Going to replace the 3watt resistors throw in the isolator pad and give it another try see how it goes this time. This will be next week when a package arrives. If it doesn't work then I'll probably try and order a couple LM4702's so I have an extra and try that. The mosfets were all isolated but not the LM4702. And the transistors all have plastic housings.
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