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Having problems with Denon amplifier.

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Arcstarter

Thu Feb 11 2010, 05:18AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

My brother has an old Denon DCA-600 he wants me to fix. It is a mosfet amp, it is 4 channel, left and right for front and back speakers. It is made for 600 watts. It was made in 1992-1994, it's a pretty old amp

.

So first things first, i plugged it into a 12v PSU. I heard a relay click on, so obviously there was no short... I input audio from an MP3 player, and connected a speaker in a few different ways.

When i input audio to the left front input, i get output from the left front output. If i input audio to the rear left input, i get audio from the rear left output.

It works ok for awhile, and the audio signal peaks at the appropriate voltage, and it clips fine. After awhile, maybe 10 seconds or a minute, the relay clicks again, and there is no input. If i turn the supply off and back on, it works again for awhile.

The only thing i can tell that is wrong with the amp is one of the traces blew open circuit. The trace is connected to.... about 4 different dual op amps through resistors (sometimes straight to a pin without a resistor) and connects to the audio inputs.... It is a fairly thick trace, it would take many amps to do the damage that has been done, so i have NO CLUE how it happened. I repaired it with wire

. There is no visible damage to any parts, or any wires. No signs of heat anywhere. Very weird...

Any ideas? I will provide more info as needed, but from the information gathered, i simply don't know where to go from here. It would sure earn me respect and trust from my brother, so even if it is not worth my time, i am still going to try.

Thank ya!

...

Thu Feb 11 2010, 07:29AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Could you take a good picture of the top/bottom of the board?

It sounds to me like the trace is for ground, and probably was exploded due to a ground loop (connecting +12v to the shield of a line in or something), so hopefully it did not take to much out with it.

As to the turning off on its own, I suspect it is complaining about not having all of the outputs connected, try putting speakers or dummy loads and see if it lasts any longer.

radiotech

Thu Feb 11 2010, 08:43AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The amp is a car amp. It had a blown trace which you repaired. Whatever blew the trace was not found. If the fault is still there,is your 12 volt power supply capable of supplying as much current as a car battery? If not, perhaps when the thing starts cycling the relay, the fault has returned, but cant do damage because of limiting of the bench power supply. What are the volt/amp readings on the test supply, and what was the output power to your test speakers?

Bench techs plan approaches to servicing designed to put as much money in their pockets in the shortest time possible.

Sulaiman

Thu Feb 11 2010, 06:56PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Since the amp can provide hundreds of watts from a 12Vdc supply there should be an inverter of some kind in there, and that's probably what is shutting down.
Look for a transformer with one or two big transistors on one side and diodes on the other with electrolytics rated for much more than 16V. Measure/watch the voltage across this/these capacitors.
Near the switching (big) transistor(s) will be an smps ic, find the number and maybe some progress can be made.

Arcstarter

Wed Feb 17 2010, 05:43AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Sorry i have not replied, my computer is dead and simply will not boot, and i have been pretty ill for the past few days, i believe it was the flu, with fever over 100 on and off and migraines out of this world.

I did check the SMPS. It is running fine. It seems to use some sort of a push-pull scheme, each mosfet consists of two paralleled mosfets, with individual gate resistors of 1 ohm... The output is 2x 20v, which provides the 20vac output, 40vp-p, and it never shuts off.

I found the problem was actually with a couple relays that connect the amp to the outputs. The voltage on the coil cuts out sometimes. I need to check the circuit some more and see if there is a faulty transistor which energizes the coil, or see if there is an error triggering a shutdown of the output.

Also, one of the channels has no audio output. I think it may have something like 5v output constantly, which worries me :P. I need to check all the mosfets for it, and especially the op amps. There are like 15 of these dual op amps in SIP

I have alot of op amps to desolder if i get to radioshack to get a desoldering iron, and i need to make a circuit on a breadboard to test them all...

Anyone know if maybe the dead output and the cutting out of the relays is somehow connected? I do not think that is the case, because the time can vary so much. I think it is a dead part or something falsely shutting them off, because the time varies, and because in the first second you turn it on the relay turns off, then back on after about a second, and will work for however long.

If you need a picture of it, i will try to take one, depends on how i feel. Thanks!

radiotech

Wed Feb 17 2010, 05:56AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Can you open the power supply lines to the non-working channel and get the amp back to stable operation on the good channel. This will give you reference voltages and signal levels to use as you repower the bad side stage by stage starting at the input and working to the output.If it is the output, then power down and use the resistance checks on the good side to reference the bad side.

Steve Conner

Wed Feb 17 2010, 10:45AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The relays are protection components to protect the speaker from burnout by a DC offset. So if they keep cutting in and out, your power amp probably has a DC offset that needs fixed.

wrote ...
I think it may have something like 5v output constantly

I rest my case :P Anyway hope you're well soon!

Arcstarter

Wed Feb 17 2010, 10:05PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Thanks guys

After some more researching the amp, it looks like the relays are only switched by the remote input... The remote input goes into a very small transistor, which drives the base of a larger transistor, which then goes to each relay. I think i am going to rip out the transistors, and just jumper the relays to the 12v rail

. I doubt my brother gives a crap about the remote function, i will ask him.

As for the dead channel, i think i will just find what parts are actually used for that channel, and desolder and check them all

Like you said, the DC offset may be a problem. I agree, there is probably some protection, there is this stereo protection IC. Im'a have to take a look at that, see if it is somehow it *is* in fact controlling the relays.

After i am done, i will remember this entire circuit board :P. I already remember the schematic of the power supply just by looking at it, other than the controlling parts of the SMPS IC :P. That does bring up a strange question. Why does the output of this transformer look so odd? Right before it is rectified, and capacitively and inductively smoothed, i checked the waveform on the scope. For each cycle of the output, it looks like one and a half cycles of a sine wave, which then becomes a square, and falls to zero... What causes this? I r cunfuzlled.

EDIT: My brother does need the remote function, i thought he honky-hooked a switch for his amps, but naw. So, if i cannot fix the remote function, i will bypass it anyway, and just use some transistors to switch the whole amp on when the radio turns on... We'll see :P

EDIT: I fixed the cutting out problems :P. I was scoping around on the board. I wanted to test the protector IC input, i was going to put a pot on the input to ground to reduce the voltage, maybe make it turn off. Well, i put my scope probe on the ground of that IC, and it cut back on. It never turned off! I remembered that my power supply (ATX PSU) had a mains grounded ground, so i knew some current was flowing from that pin, through my scope probe, and back to the negative something somewhere :P. So, i just wired that pin to the stereo's output's common, and it works now. Just have to make that channel work, and i am going to wire the fan straight up to 12v (it won't come on :P).

EDIT: I was scoping around, trying to find at what point the audio signal stops, so i know which part to check. Well, i decided to first see what the gates on the other mosfets looked like. So i scoped it, and it seems on the source AND the gate there is a constant 20v!!! I cannot see any audio signal or anything at all other than the 20v, at any timebase setting. But, the output looks like a beautiful audio signal... Damn! Anyone know what is going on here? I am thinking i am no scoping it right...

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