denon receiver going to protection mode

[Anfield]

hi all i'm new here

i have a denon avr1906 7.1 receiver which recently had the o/ps on the Front Right channel busted. I changed a couple of fuse resistors, the darlington pair and a diode and a zener which all went bust. i calibrated the unit (all channels) and all works fine now BUT when I connected all speakers back and cranked up the volume (about 3/4) it goes to protection mode. Nothing gets damaged and you can switch it on again BUT if you crank up the volume it goes to protection again.

To identify if still the problem was from the FR channel I connected all channels with a speaker except the FR and all works fine and you can crank up the volume peacefuly.....so the FR channel is causing the problem. I also swapped some components from the SBR channel to the FR channel on the mainboard such as the bias transistor and capacitors. The problem persists on the FR channel.

Any help or idea please??

Cheers

[mrshow4u]

I connected all speakers back and cranked up the volume (about 3/4) it goes to protection mode.

You might be triggering a properly functioning protection circuit. Try with a single pair of 8 ohm speakers on : Left and Right, then Left surround, Right surround, and then connect a single 8 ohm speaker to Center. If the volume control is at "3/4", that doesn't mean anything about the available power, etc. You can clip an amplifier with the volume set to "1/8" 9:00 o'clock. It depends on the pre-amp/amp gain and the source material level. My guess is, you're running 4 ohm or lower loads and the power your amp is producing has reached it's limit. I think we need a little clarification on the "3/4" output power statement.

[Anfield]

hi mrshow and thanks for youyr help!!

IU've connected an 8 ohm pair as instructed. no problem on SurrBack, Surrchan and centre. Only problem remains with front, when I leave the left channel connected alone it works fine. When I connect right channel alone same problem.

3/4 volume, hope i'm explaining well, the receiver has from -60 to +15 volume. It goes to protection at around -10 and over when a high power sound/effect happens. Both darlingtons 2SD2390 and 2SB1560 are new and I swapped the other transistors on this channel with SBR channel components.

To continue with the mystery, my friend owns the same (works perfectly never had a problem) and he gave me permission to take out his pre-amp board and try it on mine. The same thing happened and my pre-amp board worked happily on his good avr

Any ideas??

Thanks

[mrshow4u]

Okay, a good next step would be to hook up the Right speaker to the Left front output and see if it goes into protection the same way. If it does, your right front speaker may be damaged. It might have a short, or some strange impedance characteristic that makes your Denon go into current limit protection.

If your Left or Right speaker connected to the Right front output makes the Denon go into protect mode, I think the Right front channel of the Denon has the problem.

If it is the Denon and the Denon sounds fine until is shuts down, check the emitter resistors at the output devices. If there are parallel output devices, one emitter resistor could be open and everything will still sound fine. If there is an open emitter resistor AND that happens to be the emitter resistor where output current is sampled (sensed), you'll get premature protection mode activation.

I'm kinda concerned about the speaker, though.

[Anfield]

thanks again for your immediate help.....

swapped the speakers....no luck same problem on FR....so surely the Denon on front right channel.....

Yes, until it shuts down sound is perfect.....I took out the 4 0.22ohm 1watt resitors (2 on each trans) on the emittors of both darlingtons and checked them. all are fine short circuit. Then I also swapped the darlington pair with the ones on the SBR channel, but still same problem. connect a speaker to FR and when volume is cranked up...Click!!

I'm getting mad about this problem. Though, since I know electronics I would like to repair it myself since I repair many of my friends' and family stuff with no problem. But I cannot repair my amp hehe

Any help appreciated...

[mrshow4u]

It might be getting close to obtaining a service manual territory. It sounds like the protection circuit has to be troubleshot. It could be that a filter cap is open where those emitter resistor (0.22 ohm) current is sampled. The cap works to average the voltage sampled across the emitter resistor to make the protection activate during high average values of current.

If the cap is open, it will make the protection circuit more sensitive and trigger prematurely. Please realize that this is just a possible scenario. The actual schematic would be very helpful at this point. You could poke around in the protection circuit area and see if there are caps in the 10uF to 100 uF range. M<aybe a lucky swap would work here? It'd be more satisfying to get a scope on the cap and see if loud movie passages are not being smoothed by the cap. I don't have the schematic, but perhaps someone on the forum does??

[Anfield]

ok and thanks again.

I managed to get the circuit diagram too of all boards except the CPU board. So, as I told before you my friend left me his unit here (I do a lot of odd jobs to my friend so he didn't mind I use his amp to test with). What I did is put my mainboard (circuit attached) and replaced it in my friend's receiver. Same thing with the FR channel. So the fault is in my m/b for sure. When putting my m/b in my friend's unit same problem with FR!!!

Though, I don't think the protection circuit is in the m/b, pls have a look at the attached.

Hope I can help you solve my problem.....

Thanks a million

[mrshow4u]

Hey Anfield, Thanks for posting the schematic. If the speaker is indeed okay (it sounds ike it is) I'd check that C105FR 22nF. It's a cap in the Front Right channel's over current sensing.
Q104FR might be leaky too.

..And double check those R-108, R-109 0.22ohm in parallel. If those are okay. You could disconnect R-115FR 2k2 ohm and see if the problem goes away. If not, reconnect R-115 and disconnect D-102 and see if the problem goes away. If the problem goes away with either of these, you'll almost be there. Troubleshoot between R-108 to either R-115 or D-102

[Anfield]

Mr Show thanks for all your help.

Ok we are always nearer.

I swapped C105FR with SBR but same problem. Disconnected R115FR (2.2k) and Voila!! FULL ON and NO click!! hehe

So, I tested all R108 to R115 and all give correct reading. Then I changed D102FR to a new one....but same problem and then even swapped D102FR with D102SBR....same problem again....Then I also swapped Q104FR with Q104C.....same problem...Click! Could it be ZD102SBR??

I think it is risky to cut out protection on FR (disc. R115FR)....but if I'm getting on your nerves just don't worry cos I feel asking too much help.

Thanks a lot for your help

Cheers :-)

[mrshow4u]

I think it is risky to cut out protection on FR (disc. R115FR)....but if I'm getting on your nerves just don't worry cos I feel asking too much help.

This is absolutely the correct way to think about this. You are right. Do not leave the protection circuit crippled. I appreciate your sensitivity about "getting on my nerves", but don't worry that's not happening. Okay, that is good news we know which path is causing the shutdown, it is through R115FR 2k2. Good going on checking the resistors. My guess is that ZD102FRT is not the culprit. My guess is that Q104FR is leaky or has a breakdown voltage problem. The circuit basically samples the current that is delivered to the speaker through R108FR and R109FR. When you play it loud, the voltage across those resistors increases. That voltage is fed to the base of Q104FR. If that voltage is high enough (>0.6v), Q104 will conduct activating the protection circuit.

ZD102 shouldn't be the problem. ZD102 is what is termed a "clamper". If your not hearing distortion, it is unlikely that ZD102 has failed. If you play it loud and you see the Front Right speaker's woofer cone either "pushing out" or "pulling in", there might be an output or clamper problem. When I say "pushing out" and "pulling in", I don't mean the normal speaker movement. A shorted ZD102 could cause premature clipping on the negative swing of the waveform. This would cause a net DC offset in the speaker cone. ....as well as distortion. I'm thinking Q104FR is bad. Good luck

......Oops, I see that you've replaced Q104FR. I'll be back

Okay, I'm back. If Q104FR was replaced: with a good ohm meter, and the set off, measure the two outside test points at TP105. This is the Front Right's bias test point. With the set off, you'll be measuring the DC resistance of the parallel resistors R-108/R-109. The desired resistance is 1.1 ohms. This will be difficult to maeasure accurately. What you can do is see if there is a resistance difference measuring TP105 (Right Front) relative to TP103 (Center). Make sure they are the same. If TP105 measures higher, replace R-108, or R-109 or both. It's sounding like the protection circuit is working fine, but the voltage feeding it is too large (perhaps due to R-108/09 having changed value?).