Onkyo Amp - Protection Circuit - Page 1
Forum section: electronics / repair

author: matth
author_rank: Contributor
timestamp: July 20, 2015, 06:01:40 am
content: I have an Onkyo TX-NR636 amp that has developed a fault that trips the protection circuit when I power it on.I have tried sourcing the fault but the fact that the CPU powers off the box within seconds means I cant probe for long.Any suggestions on the best approach.M

author: tec5c
author_rank: Frequent Contributor
timestamp: July 20, 2015, 06:55:58 am
content: Tripping mains is a very strong indicator that there's a short circuit on the AC side of the power supply. Start by checking that the fuse hasn't blown, if it is and you have multiple spares, you can put a new one in and see if the amplifier blows the fuse straight away (another indicator of an AC short). Possible causes for blowing fuses/tripping mains, bridge rectifier has a short circuit, mains transformer has sh!t itself (not very common, but I have seen it happen in the past). Start with these and see how you go.

author: mzacharias
author_rank: Frequent Contributor
timestamp: July 20, 2015, 10:45:40 am
content: Quote from: tec5c on July 20, 2015, 06:55:58 amHe's not tripping a mains circuit breaker, rather the receiver's own protection circuit is shutting down. Most likely a blown amp channel; further down the list is a bad voltage regulator; even further down some other fault detected - falsely sensing a thermal issue, for example.So, to the OP, check first for any shorts from emitter to collector of the various output transistors. You may find a couple shorted. If so, a few other small parts are probably also bad.

author: tec5c
author_rank: Frequent Contributor
timestamp: July 20, 2015, 10:48:41 am
content: You're correct. Oh boy, I give up for tonight. I amwayyytoo tired to even think straight.

author: bingo600
author_rank: Super Contributor
timestamp: July 20, 2015, 02:06:41 pm
content: On my Sony that spoadic gave a LOUD buzz , and then went into Overload on the display.I resoldered all the relays on the speaker outputs , it fixed the prob./Bingo

author: Kibi
author_rank: Unknown
timestamp: July 20, 2015, 03:33:06 pm
content: DC content on the output will trip your protection circuit too. Check for any DC voltages on the output. You'll have to probe the output of the amplifier it's self otherwise you'll not measure anything once the relay contacts open up.Check that the + and - voltage rails are reasonably symmetrical. Ripple on these voltage rails could cause a trip too and would suggest bad filter capacitors in the power supply section.As suggested earlier, DC on the output can quite easily be caused by shorted transistors, test the output transistors in the current amplifier stage first (the big ones attached to the heatsink).

author: SeanB
author_rank: Super Contributor
timestamp: July 20, 2015, 04:35:17 pm
content: You will have to replace the faulty output transistors in the faulty channel, along with the emitter resistors. Expensive devices and there are a lot of counterfeit parts as well to contend with for those amplifiers.I also have one with that fault, I am debating whether it is worth repairing or stripping for parts.

author: matth
author_rank: Contributor
timestamp: July 28, 2015, 02:51:38 am
content: Finally got around to taking this thing apart.I checked all the output transistors and all are ok. Same with the driver transistors. Also checked the large wire-wound resistors and these too checked out.I have checked the rectifier as suggested and this was working fine. Also can't see any shorted/blown caps.I need to probe up the protection circuit to see what is triggering it - but I can seem to locate where it is sensing.

author: LA7SJA
author_rank: Regular Contributor
timestamp: July 28, 2015, 08:17:17 am
content: Measure the voltage from ground to where the two large resistors on each of the output transistor pairs are interconnected. If they are more than a few 10s of mV, the protective circuit switch off the relay and break the connection to the loudspeakers. You can also check how much quiescent current that goes in the output transistors by measuring the voltage across one of the large resistors in each channel and compute current. Are these measurement around +-0.1V and +-0.1A then you can suspect the protection circuit, if they are not than you have an error in the output stage that needs to be fixed. The Onkyo TX-NR636 is known for shutting down when somthings not right. Any lights stil on or is the standby light blinking?Johan-Fredrik

author: tibimakai
author_rank: Contributor
timestamp: July 28, 2015, 07:07:18 pm
content: There is any message on the display?

author: matth
author_rank: Contributor
timestamp: August 02, 2015, 05:30:08 am
content: The display comes on only for a second or two and then the protection circuit kicks in.I am going to do some more probing later today.M

author: matth
author_rank: Contributor
timestamp: August 22, 2015, 01:34:16 pm
content: OK,So I stripped down the Onkyo today and started debugging the problems (why the unit switched into "protect" seconds after turning on).I checked all the power transistors (C-E) for shorts and all looks ok. I then checked the driver transistors and they too were ok.I then removed the vertical driver board and powered on the unit. Without the driver board installed the unit did not go into protect mode.So I checked all the transistors on the driver board and they too were ok.Still struggling to get to the root cause.M

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: August 25, 2015, 07:06:54 pm
content: So with it back to fully assembled, check each output channel (before the relay) with your scope for DC offset. Connect the scope with power off. set V/div around 5V. Flip on the power, record your findings for the first channel. Now, do the same for the other channel. With any luck, you have at least isolated the problem to either the LEFT, or RIGHT channel power amp. If the readings are identical, I would suspect possibly the protection ckt, and related components itself. A poorly filtered/weak power supply on the prot. ckt. can also cause this problem.

author: jlmoon
author_rank: Unknown
timestamp: August 25, 2015, 07:39:06 pm
content: Quote from: LA7SJA on July 28, 2015, 08:17:17 amVery similar test to this one is:Put all the parts back as they were, attach your DVM (-) lead to the common node of the power supply (Big filter caps common point) and look around the output stage for the output inductor should be a coil of wire mounted on pcb sometimes coiling up from board . Attach your (+) lead to this point, this is the output node that is fed directly to your speakers through the protection relay contacts. If you have any DC bias at all on either channel this is where you need to start looking. Be sure the input has no signal applied when doing this test.

author: jlmoon
author_rank: Unknown
timestamp: August 25, 2015, 07:42:16 pm
content: Quote from: matth on August 22, 2015, 01:34:16 pmThis could also be a result of a faulty current mirror (diff amp section) in the low level driver / bias generation area of amp. This should be reflected on the output node with a +- DC offset on output stage though.

author: fivefish
author_rank: Frequent Contributor
timestamp: August 25, 2015, 08:57:46 pm
content: There could be too much DC level on the amp outputs. That could also trigger protection circuitry to shut down (to protect your speakers).

author: AdamAzzata
author_rank: Newbie
timestamp: September 25, 2015, 11:30:47 am
content: AC surge is such a serious problem. Mostly it is due to the short circuit. But you can easily resolve this problem by using any surge protection circuit made of transistors. Also opamp based surge protection circuits are available there. You can use any of them.