Atlas Sound CP700 Amplifier Disaster

[CaravanConvoy]

I just bought an Atlas Sound CP700 amp for $60. I'm quite excited considering these are close to $1,000 new. I bought it knowing that both channels just sat in protect mode and the power switch was stuck on. I brought it home, jiggled the power switch until it started working, then removed a bunch of random wires that were attached to the back. It powered on as it should and the protect lights went out!

Thinking I was successful, I hooked up a speaker, my phone as an audio source, and turned up the input level. As soon as I touched the input level knob, a squealing began coming from inside the amplifier and the signal and limit lights came on. Even with nothing connected, moving either channel's input level above zero would make the disturbing squealing noise and the signal and limit lights come on. The whole amp was rather filthy, so I decided the pots were dirty and it needed a good cleaning. I took the cover off, blew out all the dust, and turned it on. The strange input behavior was still there, then I heard a sizzling noise and shut it off. Nothing was visibly wrong, so I turned it on again. That's when a cap on the power supply board exploded.

After that incident, I disassembled the entire amp and plugged pieces in one by one. The power supply board appeared to survive, I think the violently terminated cap was one of three mains filters. I reconnected the power supply's toroidal transformer, and still nothing violent. Then I connected the 25v line to the main board. The fan and indicator lights came on, followed by a sizzling pop and a puff of smoke. Not seeing where it came from, I turned it on again and received a second puff of smoke. Upon closer inspection, an ML7815A voltage regulator and a JRC4580D chip were the source of the smoke. It still powers on with fan and indicators, but those two chips are clearly cooking along with who knows what else.

I have removed the main board, but have no idea where to go from here. There are over half a dozen fuses on top of the electronic protection, and not a single one has blown. As far as I can tell the problem is in the input circuitry and not the high power amplifier section. I'd really like to fix this amplifier as it is high end equipment and appears to all be simple analog components. However, I have no idea what the original problem was or why so many components are being violently fried. Hopefully helpful pictures of internals: imgur.com/a/cyyxAzU

[CJay]

SO, you've got no idea what the fault was, you don't know what basic components are (those 'caps' look like NTC thermistors for surge limiting on power up), you've got no electronics experience and have blundered about inside and blown up a bunch of stuff in addition to the original fault?

This should be easy then... 

The best advice I can give you would be to put it all in a box, pack it securely and find a dumpster to throw it in.

Second best advice would be to find a local electronics expert and ask them to fix it for you but be ready for a *BIG* repair bill.

But, if you want to at least find out a little bit and learn you need to stop messing about blindly and learn how to be safe as that amplifier could easily kill you if you touch the wrong bit because there are high voltages all over the boards inside it.

Now tell us what electronics test gear you have (hopefully you've at least got a multimeter) and find a schematic for that monster amplifier then we might be able to make some suggestions.

[mzacharias]

Atlas Sound does not supply parts or service data. This is the new way for many companies.

I recently fixed an Atlas AA-120 (I think that was the model number - might have been "1200").

Atlas was NO HELP.

[CaravanConvoy]

I've done all my testing with the output stages disconnected and the audio transformers removed. I understand that many hazards exist inside something like this, even with most of the components disabled.

I have a multimeter, but I do not expect to find a schematic. I'd at least like to understand why this failed in such a dramatic fashion. I know I do not know much about the specific circuit designs inside this, which is why I came here.

[CJay]

OK, so first thing we need to do is verify the PSU rails, that's going to be guesswork and not for the faint hearted so you need to be comfortable doing this.

Hopefully you've taken some pics and know where the wires came from?

from the pics I find on the 'net there seem to be three transformers in that device, could you take some top down pics of the chassis and post them here please?

[Gyro]

It looks as if two of the transformers are output transformers for the 100V line (PA) outputs.

[CaravanConvoy]

Gyro is correct that two of the transformers are for PA outputs. These play no role in the amplifier electronics.

The third transformer, the one by itself, is the heart of the PSU. There is one connection off this transformer that goes to the main board, which reads 25v AC. All other transformer connections are to the PSU board. This board has only two outputs, one 6 pin connector for each channel on the main board. There are two bridge rectifiers underneath the PSU board heatsink.

Mains power is supplied to the PSU board via P1 and P2. Pics of PSU: https://imgur.com/a/72yCYhf

[CJay]

OK, so that board just has some surge protecting NTC resistors, fuses and two bridge rectifiers.

I think voltage selection is done there too, can we assume it should be 110V?

I reckon the best plan of attack would be to check the bridge rectifiers on that board and the small bridge rectifier, BD101, on the main board for shorts at first but I think the power for the preamp board will be from the plug with two greens and a black connection, we'll need to sort out the power supply on that board first.

How's your soldering?

[CaravanConvoy]

All those bridge rectifiers appear functional according to a multimeter diode test.

The green-black-green connection reads 25vAC on green-black and 50vAC on green-green.

The PSU board outputs 115vDC and 126vAC on the 6pin connectors. These connect to the 8pin connectors on the main board.

I can solder decently.

[CJay]

Brilliant.

Fasten that PSU board back in so it's safe and cover it with something so it can't be touched or shorted accidentally, that kinda voltage is nasty.

So, given you've got what seems a sensible voltage on the green,black,green connector and the bridge rectifier is working, I would now try and get the +/-15V rails working on the Pre board.

Don't connect any other power to the Pre board apart from that green/black/green connector.

Remove the regulators, all we can do for now is a gross test for shorts so check the +15 and -15 rails for short to ground, remove and replace any obviously faulty chips.

If you find a short you're going to need to track down the faulty part and replace it, good luck, that's going to take patience.

Once you've done that you're going to replace both + and - 15V regulators as well as the faulty capacitor.

That should get the +/-15V rails up.

My personal feeling is that the power transistors on the PA board have shorted and somehow that +/- 58V on the PSU board has found its way into the +/- 15V and caused a *LOT* of damage.

[EHT]

I can only see pics of the transformer and a PCB of fuses (not a PSU). You would need to post pics of the PCB and wiring to get more help.

Actually i suggest that now you know there a multiple serious faults on this amp, it is not something for a beginner to work on. If you proceed, I recommend you trace out the circuit schematic and look up common similar circuits (e.g. linear PSU & 78/79xx voltage regs) so that you understand what it is you have in front of you. Redraw the schematic so that it is in the "standard" form: left to right logical flow, top to bottom potential etc and add on the component part numbers if these are on the PCB. If they arent, take a photo and annotate it with your own. Post this along with the pics. Mark on it measurements taken.

Then you can systematically work through it. You should also be able to identify failed components with the power off using resistance and diode test on your DMM. You can go through and test all the diodes and transistors and investigate any components which appear damaged or near other overheated parts. With this information AND the relevant part of the schematic you stand a chance and will be able to get more help. Do not just blindly power it on after replacing failed components you find.

Good luck