Help with amp??? LM4702+2SK1530/2SJ201

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Dougie085

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Joined 2005
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.

P1000918.jpg


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.
 

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Dougie085

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Joined 2005
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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Dougie085

Member
Joined 2005
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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Dougie085

Member
Joined 2005
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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Dougie085

Member
Joined 2005
Ok well I found an isolation pad. Put it on the LM4702 and what not checked the resistors they measured the same as all the other ones. So I hooked it back up and still smoking so something is damaged. This is what my friend told me

I would tend to think if the problem is in the LM4702 that you would have issues with both channels, not just one.

The amp is a push-pull design. When one mosfet is on, the other should be off, and vice versa. The issue on yours that the mosfets are both conducting at the same time, causing a large amount of current to flow from -B to +B through the mosfets and the two resistors. BTW, the mosfets have a 150 watt dissipation rating while the resistors are only 3 watts, so in the event of a catastrophic failure of this nature, the resistors will sacrifice themselves to protect the rest of the amp.

Check the 2SC5171 and 2SA1930 transistors and the circuitry around them next. If one of these is bad or shorted, it could cause the mosfets to "switch on" at the same time causing the short-out. You should also check (with an ohmmeter) the source and drain pins on the mosfets with the power off. They shouldn't be conducting. If they are, either there is a short in your soldering or the mosfet is bad.

You could disconnect the two 3-watt resistors to break the circuit and then check the gate voltages on the mosfets as well as the voltages around the transistors driving the mosfets. The problem is most likely in that part of the circuit. Compare them with the good channel, you should be able to pinpoint the problem that way. Also, make sure you didn't bass-ackwards anything (putting the mosfets in the wrong places, or the transistors). They are polarity sensitive.
Click to expand...

So I checked the mosfets with the DMM and they aren't conducting anything....so then i checked the transistors and same results. Does this mean that the chip is my problem? Do you think pulling the resistors out so the circuit is broken and then measuring the voltages off the gates would help? To me it sounds like the chip is fried....but I'm really not an expert.
 
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