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Old 21st May 2009, 08:17 PM   #1
jol50 is offline jol50  United States
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Default class d turn on thump

Have a fusion FE 300d that turns on with a small thump (can see sub move) then after 3 seconds it has a larger thump then plays music fine. Thump is not huge but you would hear it. When I set my fluke to record max voltage and started it a few times it showed 1.7v for the thump. I put scope on and I see what looks like a faint carrier wave then a jump when it thumps and music gets added in if applied. Power light comes on whole time, no protect light.

Also there is a relay in it, but at rest speaker terminals are connected to outputs and as said carrier wave is always there with remote applied. The relay seems loud, it clicks initially and then clicks louder at thump. It is in input section but not pulled board to verify if outputs go to it like seems typical, but it is out of the way from outputs and speaker terminals/inductor for output. So I wonder if it shuts down the output driver circuit or something like that? Is possibly defective? It really clunks compared to others. Thinking maybe it opens initially then closes after 3 sec delay to play music and it is not working right? Can't find any faults other than this, amp seems to work fine.
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Old 22nd May 2009, 03:52 AM   #2
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Is there any DC across the speaker terminals after the second click?


You could have a leaking relay driver. Sometimes they allow enough current to pull the contacts together but not enough the pull the armature in fully. The second click could be when the armature is fully pulled down.

Can you measure the voltage across the relay coil between the time remote is applied and the second click?
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Old 22nd May 2009, 03:36 PM   #3
jol50 is offline jol50  United States
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There is no offset after it is playing (is single digit mv), it seems to work normally after the second thump. I can see the offset move slightly when it thumps...the base line shifts slightly just enough to see it on 5v/dev. The offset dips negative a little before second thump on scope, but my meter does not show it really. I had a sub connected when I used the scope.

I don't think I can get at the relay at all with the board in I'll pull it next time I work on it. It is the typical small rectangular black box type, on smaller side compared to some others. Will also get a photo.
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Old 23rd May 2009, 06:24 AM   #4
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Many times, there is a diode across the relay coil. If there is one in this amp, it should be near the relay. You can measure the voltage across it.
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Old 12th June 2009, 04:43 AM   #5
jol50 is offline jol50  United States
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Better slow than never?

There is a diode that looks like the one and might say 48 on it. It has 24v across it after the first click and .033v after the second. Also found -5.47v on common to 12v ground before the second click.
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Old 12th June 2009, 06:51 AM   #6
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If it's across the coil, when it has 24v on it, the relay is engaged. When it has 0.033, the relay is not engaged. Probably obvious.

It's likely a 1N4148.


Does the speaker output pass through the relay? I've heard of muting relays that short across the speaker outputs to prevent turn-on/off pop but I've never seen it in a commercially produced car amp.

Was the secondary center-tap shifted 5v from the chassis ground?

If so, was the RCA shield shifted the same?
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Old 12th June 2009, 09:43 AM   #7
jol50 is offline jol50  United States
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I can't tell with the board in, but the relay is out of the way for speaker leads. Only tested the center tap and common on speakers, will check RCA next time but it did not hurt my HU...that is fused. Relay is a 10A 30VDC. Most of the high power traces are on the bottom of the board. To common the diode shows -12.2v on one side all the time, and 12.14v active and -12.15 when amp running on other side.
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Old 19th June 2009, 01:40 AM   #8
jol50 is offline jol50  United States
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The RCA are not affected, stay at .000v the whole time. They are .465K ohms to common. The relay is HR0CR7DC24 found here. It appears to be single throw, isolated.

I pulled the board, the relay does not go to speakers. Best I can see the relay connects common to a pin on the low pass frequency pot.

I read 0 ohms outputs to speaker but it picks up 75v when it is powered on, maybe it sees the carrier. The rails are about +/-42v. But 75v is there before and after second click of relay, that was before I pulled the board.

Suppose I should test the relay in operation somehow? Will try to get you photos of it Perry. I would say maybe the relay has nothing to do with the voltage except it is in time with it, but it is loud for a relay.
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Old 22nd June 2009, 10:40 PM   #9
jol50 is offline jol50  United States
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Not able to look at this again yet, but assuming this relay just grounds the input signal...how would that insert voltage into the common? Maybe it is not grounding it? Will try to test it but will have to do it with the board out of case.
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Old 30th June 2009, 09:32 PM   #10
jol50 is offline jol50  United States
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Got back on this, the relay connects the common to the LP filter pot. But it is a quad pot and half of it is connected to relay. It does change value when turned. Then it goes to a TL074 op amp via some resistors. Maybe it shuts down the input section or something. Going to take photo of bottom of board, going to test and see if the relay is even working since I can see where it goes now. Blacked out board so hard to see traces even with it out.

In previous post IIRC that was 75V AC. It does play a sub fine, or with no visible effect after it is fully on. Even the bump is not that bad but the 12" sub does move a little.

I assume this relay setup must be an odd design since nobody has comment. If I am lucky it is just not working.

14.4 Volt Power Output Specifications
200 WRMS per channel @ 4 ohms and 1% THD+N
350 WRMS per channel @ 2 ohms and 1% THD+N
475 WRMS bridged channels @ 4 ohms and 1% THD+N

475 must be 1 ohm since it only has one channel, but four terminals. Can strap two for 1Kw, the switch is not set to slave I checked.
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