Turn on after 3 seconds turn off

It could be a cracked solder joint, you have to be careful working on audio amps, sometimes the protection circuit will save them, sometimes they suffer a catastrophic failure and then you're looking at new output transistors and a whole pile of parts they take out when they blow.

I went to my friend he ask for my help if i can repair his kenwood model RX-39 audio system. I turn on the power then it seems the unit have no problem but after 3 seconds the unit turn off and the message appear on the display ( Protect )..I noticed after repeatedly turn on i hear a clicking sound inside the unit before the unit turn off..I plan to open it by next week so that i can first gather informations before i try to repair it.

By the way... My friend told me that sometimes he can used the unit for a couple of months when he will not used the unit for a weeks then after the unit will fail again then he will not used the unit for a weeks..... it seems that the unit is working if had taken a rest.

I'd suspect a speaker load problem.

My guess would be failing capacitors causing DC leakage and upset bias in the audio path or something similar.

It needs attention as using it in this condition and repeatedly trying to power it on could cause expensive damage.

Morse.

I agree. Check the speaker wires for shorts.

Some of the old Kenwoods used to have trouble with the delay cap in the protect circuit, leaking. As the problem is intermittent, might be worth a look, but otherwise, as others have said, good first place to look is at the speakers and their wiring.

Arfa

Thanks guys for your advised il be visiting my friend house this week to check it again...i will post immediately what happen when i check it.. i hope you help me guys till the end to repair my friend equipment ....thanks in advance

I'd go with the delay cap too. Isn't that typically a low value electrolytic in the relay turn on circuitry? Jango.

It is. I've had more than a few over the years.

Arfa

I wonder if the correct impedance speakers are being used? It's not that uncommon for people to think it's OK to just bung any old speakers onto an amp, and end up with 4 Ohm speakers on an 8 Ohm system, causing shutdowns.

The OP said the system works after 'taking a rest', which is why I suggested an amp fault. A lot of amps will shut down when there is DC present on the speaker terminals, even though the sound may appear normal.. I'd expect most people not cursed with cloth ears to pick up on faulty speakers or wiring- the sound would be low, distorted or non-existent. Mind, I've seen situations where people were totally unaware of a dead audio channel so anything's possible!

Morse

A couple of years back, I had a speaker that supposedly caused an amp to shut down intermittently, and particularly on one specific piece of music, which the customer obligingly supplied. I was of course sceptical, but the owner insisted that he had proved beyond doubt, that it was the speaker. When I tried the system out, both speakers sounded fine on the music I was playing, but when I put on his offering, the system immediately started to intermittently shut down. The cause finally turned out to be bass-related, but not for any conventional speaker fault that you would hear on any old passage of music, or even if you turned it up loud. It was actually due to one of the braided flexible tails that go between the cone and the connection tag board. A number of the strands had fractured at one end through metal fatigue, and had come unravelled. This left them flying around in mid air. On most music, and at most drive levels, this was of no noticeable consequence. However, on the offending piece of music, on certain bass passages, some kind of resonance was hit, and the free strands flew about like things possessed, and intermittently touched the other tail, resulting in a momentary short across the amp, which then caused its protection circuit to shut it down for a few seconds.

So whilst I absolutely agree that normally, defective speakers or wiring would be easily heard, it's not always the case ... ;-)

Arfa

Oddly enough, I had a shutdown/incompatiblity issue about 20 years back, with an Akai amp driving Mission 700 speakers. The Akai spec was "minimum 4 ohms".

The 700's sounded fine in the showroom, and could be driven hard by their amp. At home, anything above modest level with certain material caused the amp protection to intervene. Eventually got the impedance curves for the 700's and found there was one sub-1khz area where the impedance dropped to ~2.5 ohm. Tests with a signal source proved the susceptibilty, and the Missions were returned.

Hi Guys...

I just came to my friend place yesterday to repair his audio system with full of knowledge thru the tips I gathered to all of you guys ( Checked for cracked solder joint, speaker wiring, failing capacitors in the protect circuit... I confirmed that when I checked his audio before theres no speakers connected so I plug the cord on the outlet and turn it on again same as before the unit shut off after 2 to 3 sec, I cleaned the connector by tipping using my finger to cleaned the connector outlet before we connect the speakers on the unit then we turn it on the unit does not shut off so we play a cd to test its working! But we noticed that when we turn up the volume the unit shut off( protect) so we try it again to know what level of volume the unit shut off its seems that the unit seems working if only set below 35 ( note. Max volume 60 ). After that we dismantle the unit to check the components of power supply hoping to find fail components by visual check up then and again slap a little the connector pin of the speaker to cleaned hoping to remove the one that causing the shut off if any . After we assemble the unit we connect the speakers and turn it on its working so we decide to turn up the volume above 35 the does not shut so we adjust it to 40,45 and 50 the unit does not shut off. Im happy that I successfully fixed his unit. But Im confused what really the caused why that unit behave like that?????? and How I repaired that ,I only disassemble the unit?????

Don