Repairing a half-working amplifier.  - Page 1
Forum section: electronics / repair

author: Mint.
author_rank: Frequent Contributor
timestamp: July 30, 2012, 06:08:16 am
content: I got a Yamaha HTR-5840 Amplifier which I got off a friend, and its definitely past its use by date and an issue has occurred with it. I am not sure whenever this would be a simple fix or not, but I just want to give it a try because its still working and I am pleased with it otherwise I'll just have to salvage the parts and throw it out.Basically the problem is that it sometimes doesn't want to turn on, this has only started happening recently. I turn the amp off using the mains switch and not the standby button (it still uses electricity in standby mode). Anyways when I turn it on it turns on, but if I were to turn it off at the mains and then turn it on again it wouldn't turn on. It would work after a few hours or so, but not straight after. Sometimes just playing around with the switch works, but thats time consuming.A month back it had the same problem, but I cleaned it on the inside with a vacuum cleaner and then it began working, however then it began to develop the same problem soon afterwards. I am assuming maybe there is a bad capacitor or something inside of it and it would be just a simple fix of replacing that cap and we got a working amp again. Or perhaps there is a cold solder joint there...Any ideas what is going on with this amp?

author: DrGeoff
author_rank: Frequent Contributor
timestamp: July 30, 2012, 06:24:01 am
content: Check around the power supply first for bad caps or cracked solder joints. If there's some kind of turn-on delay relay then there might be a cap that is not charging properly any more.

author: David_AVD
author_rank: Super Contributor
timestamp: July 30, 2012, 06:41:37 am
content: Yes, some of those models develop bad (cracked) joints, especially around the connectors and heavier components.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 30, 2012, 07:01:50 am
content: If you can't find the schematic on the web, it might be a good chance to sketch out the schematic. Not all of it but the mains to the DC power rails.Will the power off and a multimeter you should be able to "bell out" the tracks that are hard to see.Also look for cracked pcbs or connectors falling off, signs of heat or leakage or damage.Check switches to make sure they are working as expected.Check any semi conductors in the area to make sure they are not blown short. Although your intermittant bug doesn't sound like this is the case.Another important thing to do is see if you can reliably reproduce the fault. If you can you have a strong clue as to the cause.If nothing is obvious the next option is probably that you have to work live.Only do this if you are confident you know what you are doing and where potential danger lies.If you are going to work live on the amp you need to have and RCD protected circuit, or an isolation transformer. Always.Also having somebody near you when you are working to turns things off if things go awry, could be handy too.When the unit is powered on, and you have the schematic and you have the unit in it's fault state, you should be able to find the offending components.A quick test is to find the DC power rails. + and -.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 30, 2012, 07:38:44 am
content: Some older switchmode supplies have a high value resistor which links the mains active with the oscillator part of the switchmode.The idea is that at switch on a bit of mains AC appears at enough level to trigger the thing into oscillation.After some years, the resistor goes high in value,& the trigger function is no longer reliable.If you switch on & off several times,you will sometimes "jag it",the oscillator will start,& the switchmode will operate.I'm sorry about the vague description,but it's years since I worked on this type of circuit,which were commonly used in Sanyo TVs.Barco Picture Monitors used the same idea.

author: Psi
author_rank: Super Contributor
timestamp: July 30, 2012, 11:36:47 am
content: I agree with vk6zgo, check the bootstrap resistor (if it has one).If the resistor wattage has been underspec'ed you can usually tell because the PCB will have heat discoloring around it.Also check for ya typical bulging electrolytic caps and stuff.

author: David_AVD
author_rank: Super Contributor
timestamp: July 30, 2012, 12:07:45 pm
content: Hi-Fi amplifiers of that style are almost always a simple linear power supply. Some have an auxiliary 5V rail that powers the CPU in standby mode.If you get stuck, let me know and I'll see if I have the service manual for that exact model.

author: mzacharias
author_rank: Frequent Contributor
timestamp: July 31, 2012, 10:15:43 am
content: Quote from: Mint. on July 30, 2012, 06:08:16 amIt's really OK to just use the Standby-On function on these. The standby supply is designed to be very efficient - on the order of milliwatts consumption.As to the nature of your problem, I would look at the reset and AC power monitoring lines to the microprocessor.The standby power circuit itself is a type of switch-mode I suppose you could say, but does not run at a fixed frequency, rather it charges a large (3300uF) capacitor, then switches off completely until the 10 volt line drains to a certain point, then refreshes the cap again with just small pulses. There is a "Hot-Chassis" aspect here, so I am hesitant to say more until I know your abilities from a repair standpoint, but I do have a training video on these power supplies which I can make available if appropriate.I don't check the forum that often so you may email me at mark_zacharias@sbcglobal.net

author: Mint.
author_rank: Frequent Contributor
timestamp: August 04, 2012, 11:53:55 am
content: The placement of the boards is horrendous and I was unable to remove them from the amp to check the voltages. Not much I can say... Here are a few pics though. (PS. I hate these stupid forum restrictions, can't you just make a code that shrinks the damn images?)

author: David_AVD
author_rank: Super Contributor
timestamp: August 04, 2012, 12:41:18 pm
content: Check the 2k2 resistors and 1u, 2u2, 22n capacitors on the standby power supply board. I can send you the manual if need be.

author: Mint.
author_rank: Frequent Contributor
timestamp: August 04, 2012, 12:52:55 pm
content: Quote from: David_AVD on August 04, 2012, 12:41:18 pm2k2 is fine, as for the caps I got no clue how to measure them. Yes can you please send me the service manual to my email, that would be much appreciated. My email is under my profile.

author: T4P
author_rank: Super Contributor
timestamp: August 04, 2012, 05:10:13 pm
content: You managed to unscrew the screws?I always had problems with that on japanese equipment!They always used #0 phillips screws that are tightened SO TIGHT a torque limited electric screwdriver is required ... or a cordless drill (Worried about overtorquing)But yeah, most of their stuff have horrendous board layout, the equipment i eventually broke apart was so messy,boards everywhere but luckily being japanese, there's no such thing as "botch-jobs" only perfection ... in their eyesHaving boards splayed everywhereGive yourself a pat if you are able to resuscitate it, nice seeing these equipment with OH SO MANY connectors around

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 05, 2012, 01:03:00 am
content: Check the standby voltage rails. Most likely some part of a reset or startup circuit is failing.Also find a switched supply rail and see if it tries to turn on during an unsuccessful startup.QuoteThe newer ones are using switching supplies in order to implement "dynamic supply voltage". At lower volume settings, the supply voltage to the power stages is reduced. That increases efficiency and improves PWM resolution at low volumes.

author: David_AVD
author_rank: Super Contributor
timestamp: August 05, 2012, 01:09:45 am
content: Quote from: Mint. on August 04, 2012, 12:52:55 pmJust replace the capacitors. I've PM'd you a link to the service manual.We had a similar model in earlier this year and I'm 99% sure the fault was similar to yours and those were the parts replaced.