Sony mini Hi Fi system repair. - Page 1
Forum section: electronics / repair

author: dark_hawk
author_rank: Regular Contributor
timestamp: September 16, 2019, 06:49:22 am
content: Hi,I’m trying to repair an intermittent problem with a Sony mini Hi-Fi system Model MCH-RV600D.The problem is that sometimes, at high sound levels, one of the speakers stops working. I'm connecting the system to my PC, and if there sound coming from the PC when I'm powering on the sytem, even at low sound levels, one of the speakers stops working.Sometimes it's the left speaker and sometimes it's the right speaker.I think the problem might be with the amplifier board.I’ve disassembled the board, and rechecked all the solder joints for fractured joints and I can’t find any, also all the caps looks fine, no bulges.I’ve also changed the fan with a higher airflow fan, that seemed to solve the problem for some time but then the problem came back.Looking at this layout, what should I look for or check to diagnose the problem?

author: benj38
author_rank: Frequent Contributor
timestamp: September 16, 2019, 11:07:15 am
content: What tools do you have available to you?If you have a multimeter, I would start by checking for the supply voltages at the "G" connector and at the pins of IC201, when everything works fine, and when the problem occurs. Any difference between the two sets of readings (or with respect to the schematic) can help pointing you in the right direction.If the tests above reveal nothing, the classic approach is to do some sort of signal tracing through the amplifier --- either by injecting a signal to different points in the circuit and listen to the output through the speakers (for this you will need some signal or noise generator), or go the other way around and look for the signal fed from your computer to the input of the amplifier at different points in the circuit (for this you will need a scope or some other signal detector).

author: coromonadalix
author_rank: Super Contributor
timestamp: September 16, 2019, 02:23:08 pm
content: before going to far, check the speakers connectors and inside the enclosure, i had many bad speakers terminals in a sony home theather system .. i found the problem by mistake, when i dropped a speaker on the table while playing.

author: dark_hawk
author_rank: Regular Contributor
timestamp: September 16, 2019, 10:02:26 pm
content: Quote from: coromonadalix on September 16, 2019, 02:23:08 pmThank you for the reply.I agree, those connectors are very dodgy, I've bypassed them long ago and don't think they are the problem here.Quote from: benj38 on September 16, 2019, 11:07:15 amThank you for the reply.The unit is very compact, several boards sandwiched together in a very tight space, and too many wire and soldered connections to power on the unit, which makes measuring voltages while powered on very hard.I've disassembled IC201 which is STK402-100s Audio Power Amplifier. I've seen some videos which show that these ICs are prone to failures which forces the unit to enter a protect mode. This is not the case here but as this problem is an intermittent failure leads me to think there is a problem with this IC that only manifest at higher volumes.Also the thermal paste on this IC was very dry. Could this be the issue, the thermal paste being almost soild does not transfer enough heat from the IC to the huge heat sink which makes it enter a protect state?As a general question, using the symptoms described above, and without the option to measure voltages, what would be the first guess regarding the failing competent?

author: cheeseit
author_rank: Regular Contributor
timestamp: September 18, 2019, 07:37:14 pm
content: In my experience, and I've encountered this kind of issue on many amps, the problem is very likely oxidized contacts in the relay RY281 making intermittent contact. I'd strongly suggest you replace the relay before going further.

author: dark_hawk
author_rank: Regular Contributor
timestamp: September 19, 2019, 07:28:52 am
content: Quote from: cheeseit on September 18, 2019, 07:37:14 pmThanks for the reply.I desoldered the relay, took apart, and here is some magnified photos of the contacts.Could this be issue? Or the oxidation not that extensive?Could these contacts be cleaned or must the relay be replaced?I ruined IC201 which is STK402-100s Audio Power Amplifier while working on the unit, and I'm waiting for a replacement so I can't further until I get a replacement IC.

author: benj38
author_rank: Frequent Contributor
timestamp: September 19, 2019, 08:05:39 am
content: In general, I would second @cheeseit. Many times intermittent cutoffs in power amplifiers are caused by bad relay contacts.In particular, the contacts I see in your images look far from being satisfactory and can very likely be the cause of the problem (though one can never tell for sure just by looking). The contacts should be clean and shiny, not black like in the images.You can definitely try to clean the relay contacts. Use a fine sandpaper and work gently while mechanically supporting the contacts from behind so they don't bend from the pressure of sanding. I would go with 1000 grit, and then 2000 grit (a fine/extra-fine nail buffing block can be used instead) until you get a clean shiny surface . If you have to remove too much metal to get it completely smooth then a tiny bit of leftover pitting is acceptable in my experience. You may get a few more years out of this relay this way.BTW, the hardened thermal paste is usually not a problem. It is quite common for it to be relatively hard after many years (some pastes do that, others don't), as some of the oil in it evaporates. As long as it is even slightly soft when applied it would fill all the irregularities when the chip is clamped to the heat-sink and do its job quite well.

author: dark_hawk
author_rank: Regular Contributor
timestamp: September 19, 2019, 09:25:22 am
content: Thanks for the reply.I thought that black on the contacts was some kind of carbon conductive material that is black, but you are correct, it's black sot blocking contact, I wonder how the unit used to work at all with the state of those contacts.I cleaned the contacts with 1000grit then 2000grit sand paper like you suggested, here are the result:I think this way the relay can work for a while more, and I'll make sure next electronics parts order to add some of those relays.Now I must for a replacement Power Amplifier to replace the one I ruined.

author: mzacharias
author_rank: Frequent Contributor
timestamp: September 19, 2019, 06:37:43 pm
content: In the future keep the volume down at power on, until the relay has latched.

author: cheeseit
author_rank: Regular Contributor
timestamp: September 19, 2019, 07:13:06 pm
content: Great macro shots and those contacts did look awful. I'd bin that relay immediately and replace it, just to be sure that the relay isn't the problem. Buy a quality gold plated one to replace it.Again, in my experience, cleaning the contacts is, at best, a temporary thing and it may not even work. I think that cleaning plated contacts with sandpaper will likely always ruin them as the plating is so thin, especially if any kind of pitting is to be removed.Quote from: mzacharias on September 19, 2019, 06:37:43 pmAbsolutely. And one should never switch speaker outputs on or off, or between A or B outputs, while powered with anything but very low volume levels. Same goes for switching inputs as this disengages the relay before switching on some amps/preamps too.

author: benj38
author_rank: Frequent Contributor
timestamp: September 19, 2019, 07:38:18 pm
content: Again, I second Cheeseit's advice: it's best to replace the relay.However, I have had a slightly different experience with this cleaning process than Cheeseit had. On more than one occasion, when I have had difficulty sourcing a replacement relay, I have gotten a few more years of life out of the old relay using this cleaning treatment. Even if the plating is removed, it is surprising how long such a repair may last, especially if you treat the contacts with some Deoxit, and keep the volume down on powering on.

author: dark_hawk
author_rank: Regular Contributor
timestamp: September 20, 2019, 01:05:51 pm
content: Cleaning the contacts did seem to work, I wouldn't trust them much but for the time being it works.Problem now is the sound amplifier IC STK402-100S, the one I found locally was the STK402-100 (without the S) and for any one searching in the future, those two models are not the same and not compatible, the unit did not produce sound with the model without the S, at least the power protect message went away.Now since I'm ordering some ICs from China, might as well through in some relays. That relay worked for no less than 15-18 years before failing, I consider that good enough.It's a Panasonic ALA2F24, I also found an Omron G5PA-28 which seems to be compatible with it.I'm now sure that intermittent problem was the relay like cheeseit suggested, when the unit disconnected one of the speakers and I used some percussive maintenance it sometimes came back to work, which suggested either a cold solder joint or something mechanical that is the problem. And since there is no cold solder joints as far as I can see, it must be this relay, also there was a good amount of black sot underneath the unsoldered relay which is not normal dust.