help repairing JVC RC-X740  - Page 1
Forum section: electronics / repair

author: john8893
author_rank: Newbie
timestamp: June 15, 2016, 05:59:15 pm
content: Hello everyoneI have ask repair my father's JVC RC-X740 stereo,around 5 years ago the Cd player stopping reading disc but other wise working fine until last week now when I try to power it up it immediate turn back to standby mode I'm bit unsure where to start.I have including some photo just in case i am missing somethingbottom of cd player boardtopside of cd playermain boardradio boardtop main board

author: tautech
author_rank: Super Contributor
timestamp: June 17, 2016, 10:03:52 am
content: Welcome to the forum.Link to manual:http://elektrotanya.com/jvc_rc-x740.pdf/download.html3.3 MbWait for "processing" to change to "get manual".It may use an SMPS and it's controller is going into auto-protect mode because of failing electrolytic caps and therefore returning to standby mode.Have you any experience repairing equipment?What tools and service aids do you have?

author: john8893
author_rank: Newbie
timestamp: June 17, 2016, 12:15:49 pm
content: Thank you for your replyyes I have experience repairing mobile phones,and couple of tv'stools digital multimeter (vici VC97), soldeing iron station, hot air rework stationThank youJohn

author: raz221
author_rank: Contributor
timestamp: June 17, 2016, 07:27:12 pm
content: I'd suggest you start with testing the resistors & diodes (and if you're getting the wrong values or no reading, try desoldering em and test em out of circuit for verification!) Also, do note that the values of the resistors are listed in the service manual given to you above.Apart from that, also pay very close attention to the electrolyte capacitors (look for ones with a bulged top or leakage).A healthy capacitor should always have a flat top. Do keep in mind however that while there are often physical indications of a bad capacitor, a capacitor can also look physically okay (but be faulty), so in order to fully rule out em all out, you'd need to have a capacitance meter (as your multimeter only supports up to 200 microfared, which isn't much) then you should either remove the capacitor from the circuit board, or desolder one of it's legs before you measure it (just add a bit of fresh solder to the joint, and use a wick, you don't really need to remove the leg from the board, just suck the solder out of it) but before testing any capacitor this way, you must first discharge it to be on the safe side (for low voltage capacitors, you could use a screwdriver or pliers to short both legs, for high voltage ones however, you need to either make your own capacitor discharge tool with the right resistor, wire and heatshrink, or buy a pre-made one) Personally, I use the UNI-T UT601 for testing caps, you can get it from here:http://www.banggood.com/UNI-T-UT601-Professional-Capacitance-Meters-Ohmmeters-Capacitor-Resistor-with-Diode-and-hFE-Test-p-1043052.htmlOr maybe order it from ebay if you want it delivered more quickly.I also suggest you get special testing leads for it:http://www.banggood.com/SMD-Inductor-Test-Meter-Clip-Probe-for-Resistor-Capacitor-Multimeter-p-912618.html

author: john8893
author_rank: Newbie
timestamp: June 18, 2016, 05:52:36 pm
content: Thank youI will check the resistors, diodes and caps.I have ordered tools you suggested

author: janoc
author_rank: Super Contributor
timestamp: June 18, 2016, 07:29:51 pm
content: Quote from: raz221 on June 17, 2016, 07:27:12 pmRaz, that's really not very helpful troubleshooting "method". Starting to check and desolder stuff at random before you even know where the fault is (Is it the power supply? The control board? Is anything obviously fried? Are the power rails correct?) is not helpful and will not lead to actually solving the problem.Also, recommending a capacitance meter like that is pretty much useless. For checking power supply capacitors you need an ESR meter, not a capacitance meter. A cap could be dead as a dodo with high ESR and still measure in-spec capacitance - I have several ancient USSR made ones which are like that. As far as I can see, that cheap meter doesn't measure ESR but measures up to 20mF capacitance (yay, that's is really *super useful* - like those cheap RLC meters that measure inductance to 20H but start at mH range ...). Electrolytic caps have tolerances -20+80%, so measuring the capacitance of filter caps in power supplies is not going to do you much good - it is very rarely critical and in the worst case you get some increased hum somewhere. Increased ESR on the other hand will let significantly increased ripple in the circuit, whichwillcause problems.Moreover, on linear supplies the electrolytic caps are unlikely to be an issue unless the device is ancient, with the caps having dried out, but plenty of 20-30 years old pieces of kits have original elcos and no problem whatsoever - the capacitance is not critical, so even losing some due to aging is not a problem.Buying that meter is a complete waste of money, IMO. Especially for repairing this stereo. And those tweezers leads ..Did you actually check the photos? That boombox is almost completely through hole ...To john8893:You are lucky that there is a schematics - for diagnosing the power problem you will likely need only a multimeter to check the voltages against what is written in the schematics. No need to order anything special.This portable stereo has a linear PSU, there isn't much there except a transformer, rectifier bridge and a few capacitors (left side of the first photo). Then there are several linear regulators for each of the voltage rails derived from the unregulated 12-14V or so that the PSU (or batteries!) produces. So one is unlikely to find leaky or bulged electrolytic caps there. I would say that the PSU is mostly alright, because standby works - and according to the schematics you posted, there is no separate "standby" rail, unlike on many switching PSUs. Basically the PSU is always on, even when the boombox is in standby, only the various loads (motors, amplifiers, etc.) are switched off. So that tells me the transformer, the diodes, the microcontroller actually handling the standby and its voltage regulator are all OK. The capacitors may be out of spec, but most likely they fine - again, it is a linear supply and the caps don't see any significant ripple currents at 100Hz (unlike in switching power supplies).Measure the voltages from the PSU and all regulators, they are indicated on the schematics (p. 23, 24). If they are OK, then I would start checking the stuff that controls the standby line. If I read the schematics correctly, the power button is connected directly to the microcontroller using the PIN line. The MCU then controls the POUT line that arrives to page 24 again and controls the transistors Q914 and Q915 that actually turn the power to the amplifiers on. The power button is fed from the BUP line which is the backup voltage. That's on page 25 up, the transistor Q704. Normally I would expect that transistor to be conductive and thus the power button have one pin grounded. Check the voltage regulator feeding it (IC305), there are voltages indicated in the schematics you should see on the board.If all that checks out, it is possible that the microcontroller is detecting some sort of a fault on power up and refuses to turn the main power on. You did check for loose connectors, broken wires and similar stuff, right? If that turns out nothing, then it will be tricky to find without knowing what the MCU actually checks to turn the power onHowever, to be honest - unless the problem is something trivial like a busted regulator on the PSU board that can be found quickly by checking the voltage rails and comparing with the schematics, I wouldn't bother with troubleshooting this. The stereo is a very old piece of junk and most likely won't be economically repairable after you factor in the time you will have to spend on it. Especially if you discover that some oddball IC on the control board has failed - it may not even be possible to get a replacement for it in the first place. Unless your goal is mostly educational (learning to troubleshoot electronics), then buying a new stereo will likely be a more sensible option.

author: janoc
author_rank: Super Contributor
timestamp: June 18, 2016, 07:51:36 pm
content: Quote from: tautech on June 17, 2016, 10:03:52 amThat's what I thought too at first, but it seemed odd to have an SMPS in a crusty old all through-hole device. Then I have actually checked the manual you have posted. It is a 20+ years old boombox with a completely linear power supply that can also run from 8 D-cells. So no SMPS here.

author: john8893
author_rank: Newbie
timestamp: June 21, 2016, 05:37:48 pm
content: Quote from: janoc on June 18, 2016, 07:29:51 pmThank you very much for your replytoday the regulators and transistors they all check out fine while testing I had to disconnect the cd player part and power up just fine the cd player unpluggedone of the flat felx cable connecting cd player is very looseThanksJohn

author: janoc
author_rank: Super Contributor
timestamp: June 22, 2016, 07:15:17 pm
content: Quote from: john8893 on June 21, 2016, 05:37:48 pmWell, try to reseat the cable properly and check. Also check the connectors - perhaps they have dropped it and the connectors could have partially sheared off the board. That could be very much the reason why it isn't powering up - poor contact because of the loose cable.

author: john8893
author_rank: Newbie
timestamp: June 28, 2016, 06:09:06 pm
content: as soon cable plug in. it turns offso I look up the connector manual (page 25 )it for cd trayI checked the voltage with connector disconnectedcd tray close should read 0v but reads 5,5vopen 4,8v bang on what manual saysresistance checked between the open pin and close one and reads has dead short with the switch different positionsThanksJohn

author: janoc
author_rank: Super Contributor
timestamp: June 28, 2016, 09:09:06 pm
content: Damaged switch? Or you have plugged some cable backwards? The thing turning off could well mean a short circuit tripping the overcurrent protection of the voltage regulators. So if that CD tray assembly triggers the issue, check for shorts on it - bent components touching things they shouldn't, damaged switches, damaged connectors, foreign conductive stuff that landed on the board, wires with broken/chaffed insulation ...Also you could have a dead component that has shorted out - try to use a divide & conquer method. Split the current path in half and check each part separately. Continue with that until you locate the problem.