Broken Yamaha RX-A700 - Page 1
Forum section: electronics / repair

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 21, 2019, 04:03:55 pm
content: I just received two broken receivers and on the first one when I plug it in, it just has a red stand by light. I have taken it apart and when the front panel with the display and controls is disconnect and I plug it in the relays go through their power on sequence. When I hook part of the front panel back up I can use the power button to power it up but then it turns right back off. I have checked and I have not seen any visible bad cap. If you need any more info let me know cause I would love to get these working.

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 21, 2019, 04:54:09 pm
content: Another thing to add is when I hook the front panel back up I can only make three attempts to turn it on and then it does nothing but the stand by light. I kinda find this weird, I don't know if this is some sort of protection circuit or if a power rail is messed up and can only power it the three times. If I find any more useful information I'll try to post it.

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 21, 2019, 05:35:38 pm
content: when what I believe to be the display power is disconnected the unit appears to power up with the relay clicks and all and the stand by light goes out for about 30 seconds then it powers off again still very confusing but I'm crossing my fingers it's fixable.

author: bob91343
author_rank: Super Contributor
timestamp: April 21, 2019, 06:38:46 pm
content: I have been through several similar situations. My first idea would be to check the audio output lines from the amplifiers. If any of them is not close to zero volts, the unit won't turn on. Get the diagram and find those output lines. If all the outputs are good, see if they respond to input signals. If you find a substantial voltage on one or more of the amplifiers, the amplifiers are blown and need some attention.

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 21, 2019, 08:27:12 pm
content: Thanks, I'll check that when I get home and let you know what's up. I was looking at the schematics for a bit but I haven't checked that.

author: slbender
author_rank: Regular Contributor
timestamp: April 21, 2019, 10:32:19 pm
content: Those symptoms usually indicate a shorted output transistor. If you have a DMM with lots of ranges and functions, try diode test across the various output transistor leads ( usually three on each ) and see if one pair seems shorted, or maybe all are shorted, good pairs will typically read .667 or so in one direction and infinite when reversed, unless they are Darlington transistors, I have a 5.1 Onkyo HT-R550, it might be similar (or not).Steven

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 22, 2019, 12:08:25 am
content: do you recommend that I remove the transistors to test them??? I dont think its that big of a deal to take them out I would rather not have to tho.

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 23, 2019, 12:26:26 am
content: I removed all of the transistors and all of them read .584 in one direction and nothing in the reverse which i guess is nicer that I dont have to worry about thoes but now thats not the problem soo

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 24, 2019, 06:32:55 pm
content: Just an update, I have checked the resistor packages for the output mosfets and they are also all good, which makes sense cause the mosfets are also good.

author: bob91343
author_rank: Super Contributor
timestamp: April 25, 2019, 05:46:16 pm
content: Measure the voltages on the outputs of each amplifier. They should all be less than one Volt with no signal. If so, the problem is in the protection circuit. If not, you have an amplifier section that needs work.

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 25, 2019, 08:21:14 pm
content: Alright Ill check that thanks

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 25, 2019, 11:10:12 pm
content: Quote from: bob91343 on April 25, 2019, 05:46:16 pmI checked the output by grounding one lead and putting it on the external banana plug on the outside. I got 7.5 volts which don't sound to good. what do you suggest/thing the solution to the high voltage problem is. Im pretty sure the output mosfets are all good because they are all the same and a good value. Thanks

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 26, 2019, 01:01:06 am
content: I just discovered something rather interesting. It appears that the 60v supply rail is 20 ish volts... now what would cause that? I don't see any capacitors that are blown, could it be something pulling it down? I will continue to look for the fault and keep everyone posted if I find something good!

author: bob91343
author_rank: Super Contributor
timestamp: April 26, 2019, 01:05:56 am
content: First of all, you can't measure the amplifier outputs at the rear connectors because they don't go there until the unit is working. There is a relay contact that isn't actuating.So why? Either the protection circuit is doing its job by preventing a failed amplifier from ruining a loudspeaker, or there is something wrong with the protection circuit.So you will need to probe the innards of the unit to find the outputs of the amplifiers. It's often right where you measured the franctional ohm resistors and, in fact, you can usually measure there. The individual channels are independent so you have to measure them all.

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 26, 2019, 01:31:31 am
content: yes the extarnal outputs are the exact same 7 volts as on the mosfet... the center pin on the mosfet is 22 volts which is no bueno

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 26, 2019, 01:32:09 am
content: I should probably add that both sides of the mosfet are 7 volts

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 26, 2019, 01:33:49 am
content: and by removing the front panel it gets forced on I believe cause the relays do the power on sequence

author: bob91343
author_rank: Super Contributor
timestamp: April 26, 2019, 04:10:21 pm
content: Again, you need to understand it and mark up a copy of the schematic with voltages that you expect and voltages that you measure.If a voltage drifts with time it may be that there is an open circuit somewhere and you are watching a capacitor charge via a high impedance. Or it may be a thermal effect. You will never know unless you can trace the paths.Perhaps all you have is an open solder joint somewhere. It does appear that something is intermittent, and that's worth digging to find it.

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 26, 2019, 06:39:31 pm
content: That could be possible that its intermittent, I checked it this morning and it is no longer higher voltages on the output transistor. so there could be a lose connection somewhere.

author: mzacharias
author_rank: Frequent Contributor
timestamp: April 27, 2019, 12:39:21 pm
content: Quote from: slbender on April 21, 2019, 10:32:19 pmIn the case of Yamaha's, this almost certainly does NOT indicate shorted output transistors. This is because the over-current sensing is so fast, the receiver will power off virtually as soon as you have pressed the power button, vs several seconds shut-off for a DC balance issue at one or more amp channel output, or a power supply or thermal sensing issue.I did warranty work for Yamaha for over 20 years, so I have a "little" experience with them.

author: mzacharias
author_rank: Frequent Contributor
timestamp: April 27, 2019, 01:06:27 pm
content: In the service manual, there are instructions for a"protection cancel" function which will allow a technician to troubleshoot at his leisure. This function is not available when the protection issue relates to high current, like shorted outputs or a shorted main bridge rectifier of main filter cap, but your issue is none of these.To start with, while in Protection Cancel mode, you should be able to measure DC offset at any of the individual amp channels, measuring from the bias test points, or the emitters of the output transistors to chassis ground. If only one channel has a DC offset, obviously that channel needs repair. If all channels have an offset - it likely points to a power supply issue - probably a bad voltage regulator or some bias voltage source. There are several test points in every Yamaha which relate to the various protection functions. They are labelled DCPRT, IPRT, PS1PRT, etc. Most of these are connected to a resistor summing network, the output of which (the associated test points) will give definitive indication of which one has been triggering the protection function. Generally the output should be zero volts, or close to it. When there is a failure, the voltage sum is high or low, triggering the protect. In any case the service manual shows the specified voltages in the schematics.In the diagnostics area of the service manual these readings get trickier because the protection history value shown on the display must be multiplied by X/255 where X is either 3.3 or 5 volts (usually 3.3 on newer units) to get the approximate voltage that has triggered the Protect. The manual gives a range of acceptable voltage, anything outside this range triggers the protect.Why they don't just have the microprocessor just report the actual voltage directly, I don't know. Maybe a little extra code required, who knows? Yamaha just does it this way.By the way - they aren't "mosfets" - it uses bipolar transistors. (sheesh)

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 27, 2019, 05:22:59 pm
content: Wow, Thanks so much!!! I'll check that out right now, sound like you know what your talking about

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 27, 2019, 05:47:03 pm
content: so the NPN output transistors have 22 volts on the collector and 7 volts on the base and the emitter. the PNP side has .5 volts on the collector and still the 7 volts on the others. So is this the offset you were referring to? cause it's an offset but i'm not sure what would cause this and yes it is all of the NPNs that have the 22 on the collector. Thanks for your help!

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 27, 2019, 10:44:26 pm
content: and about the mosfets, I never knew exactly what they were just assumed cause that's what i figured most amps used. The 55 volt power rail starts around 42v and quickly drops to 22v if that helps. Edit: the change in voltage was a poor ground it is 55 Volts

author: bob91343
author_rank: Super Contributor
timestamp: April 27, 2019, 11:11:07 pm
content: MOSFETs have been used in amplifiers for a few years but before that, bipolar transistors were the common ones. Still they are used quite a bit.Depending on the age of your unit, you might find either one.MOSFETs have been a long time in development. Surmounting the technical issues that they present has taken a long time but now they have some advantages over bipolar transistors. They have better high frequency response and lower saturation voltage, for example. And require no drive current.I am more comfortable using bipolar transistors, mainly because I have been using them so long that I feel they are a part of my storehouse. For years, when repairing an amplifier, almost the first thing I would look at would be if the output transistors are shorted. If so, replacing them (and maybe their driver transistors) and resetting the bias was most of what needed doing to get the units working again.My experience with FETs has been much less and so my comfort level isn't the same.I have bounced between designing, and repairing electronics. Both have fun and rewarding for all these 50 or more years.

author: mzacharias
author_rank: Frequent Contributor
timestamp: April 29, 2019, 12:13:01 am
content: Quote from: CoopedUp on April 27, 2019, 05:47:03 pmYes, this is a DC offset. It's important to determine whether it exists on all channels, or just one. If on all channels, once again look to power supply issues.Turn the receiver ON and let it shut off again by it's protection function. Then turn on in diagnostic mode and be prepared to make note of the initial display, like PS PRT 022L of something like that. This will tell us which way to go.

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 29, 2019, 12:57:12 am
content: I think I have been on a wild goose chase with the amplifier circuit... after a lot of head banging I think I have ruled it out as fine... I think. So I guess time to look at the protection circuit then.

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 30, 2019, 01:31:14 am
content: So I started on the second receiver and it just needed to be started in the diagnostic mode and it cleared up and works. the other however has a ps2 prt:182 H so I have looked into this and I see that you have seen this exact code on another post. I have replaced the 5v regulator and I have the same code so then I up and replaced the top two of the three stacked boards to see if the problem went with it and it did no so its on the operations board or the bottom amplifier board.

author: Satbeginner
author_rank: Regular Contributor
timestamp: April 30, 2019, 07:31:30 am
content: Hi,I had something similar, one of the tests showed a voltage out of range (don't remember which) , I had to replace a TO-220 12V regulator all the way on the left at the internal heatsink, playing great ever since..Un saludo,Leo

author: mzacharias
author_rank: Frequent Contributor
timestamp: April 30, 2019, 01:50:54 pm
content: Quote from: CoopedUp on April 30, 2019, 01:31:14 amI think if the protection value is HIGH, there may be a missing negative voltage, like a -12V or the like.

author: CoopedUp
author_rank: Regular Contributor
timestamp: April 30, 2019, 11:24:27 pm
content: well I was working on the second receiver, it worked then I was swapping parts with each other. this swapping required soldering and I was way to tired and didn't check for solder bridges and so I shorted 3 ac lines and let out some magic smoke. I then fixed the bridges and found that the transformer was still good. however the receiver then output the same ps2 prt code so I have been checking stuff in the power supplies. the ps2 prt line should be 1.2 volts but it's 5 volts. I was doing something dumb by directly powering the transformer with an auto transformer and then bang on of the output transformers blew a chunk out of its face. so once I fix this channel I need to fix the other problem... any thoughts?

author: mzacharias
author_rank: Frequent Contributor
timestamp: May 01, 2019, 01:33:16 pm
content: Quote from: CoopedUp on April 30, 2019, 11:24:27 pmPut it in Protection Cancel mode and look at all the power supply voltages, and if you have it apart, make sure all the grounds are qualified (connected).

author: plurn
author_rank: Regular Contributor
timestamp: May 10, 2019, 05:41:55 pm
content: same problem/fix as these maybe? it is a common issue with some Yamaha receivers:"replace capacitor C405 (the red one on this video) : 22nF/630V"Yamaha receiver NO POWER : FIX (all models)https://youtu.be/JTeq07wL2aU?t=1EEVBlog #379 - Yamaha RX-V557 Receiver Fixhttps://youtu.be/MwvjAtSr5t8?t=1

author: mzacharias
author_rank: Frequent Contributor
timestamp: May 11, 2019, 02:15:30 am
content: Quote from: CoopedUp on April 27, 2019, 05:47:03 pm7 volts on the emitters is easily enough to put the receiver into protect mode. If it's all the channels, like you say, there's a voltage missing. I'll glance at the schematic and either re-post or edit this post.edit: OK, look at the output of Q1072, a 2SA1708. Should be about -12.6 volts. I'm betting this voltage is missing. It can be measured at the - side of capacitor C1075, since the transistor legs are flush to the board.

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 13, 2019, 03:22:59 am
content: I'll have to check on that, I stopped working on them because I got a little frustrated because I blew the transistor out, but I will have to check that power rail. thanks

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 13, 2019, 10:30:16 pm
content: so, I have been looking at the power rails, and it looks like the +12 and -12 are shorted together and when they are powered they equally get 29 volts so thats an issue

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 13, 2019, 10:32:40 pm
content: maybe I was wrong, this is confusing

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 13, 2019, 10:57:43 pm
content: the 12 and -12 are fine, I had meen measuring it in the incorrect spot

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 17, 2019, 12:09:03 am
content: So after a while of poking around I have found that IGND is around 1.5 volts... and it should be 0. So I think there is a problem on the operation(2) board unless someone thinks its else where. What do you people think about it? any suggestions? thanks

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 17, 2019, 12:44:49 am
content: as well, on the +12 and -12 v lines its 2v on 12 and -22 on the -12 and my ground point is the E line on the board. I dont know why this is but it is so there you go

author: mzacharias
author_rank: Frequent Contributor
timestamp: May 17, 2019, 04:47:02 pm
content: Does seem based on what you said that you have a ground floating.Are all the rear panel screws in place?

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 19, 2019, 02:21:24 am
content: no but I have alligator clips connecting everything and it is still the same

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 20, 2019, 11:48:39 pm
content: I have found that there are some points on the pcbs where the E which should be 0v it is -7.5v. The other voltages around which are +18 +12 and -12 are all reduced by -7.5v

author: bob91343
author_rank: Super Contributor
timestamp: May 21, 2019, 12:18:34 am
content: Yup, missing ground.

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 21, 2019, 08:33:22 pm
content: But I'm pretty sure it's not tho everything is grounded with alligator leads

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 21, 2019, 08:34:22 pm
content: Missing ground would make sense but i have connected all the ground planes

author: mzacharias
author_rank: Frequent Contributor
timestamp: May 21, 2019, 09:09:59 pm
content: I would confirm good continuity between the center point of the two main filter caps, the rear panel, the outer jackets of the RCA connectors, and the main heat sink.

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 21, 2019, 09:54:32 pm
content: I didn't have the power button connected and that was the problem for the offset but the Ignd is 5 volts now and the ps2-prt is 2.5v when it should be around 1.2

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 21, 2019, 09:57:32 pm
content: it wasnt the power button it was the headphone jack

author: CoopedUp
author_rank: Regular Contributor
timestamp: May 25, 2019, 09:08:07 pm
content: I still can't seem to find the problem with these guys... It's a wild goose chase

author: CoopedUp
author_rank: Regular Contributor
timestamp: June 18, 2019, 03:04:12 pm
content: I would really like to figure this one out so if anyone else has input it would be appreciated

author: temperance
author_rank: Super Contributor
timestamp: June 19, 2019, 08:55:31 pm
content: I remember repairing a very similar amplifier four or five years ago.The problem with this amplifiers is the stand-by power supply. Next to the power inlet, you will see a printed circuit board with a CMOS40xx something IC, a relays, some passives,... The circuit in question is fed by a capacitive power supply. The cap has gone bad and the start-up circuit fails to switch on properly because the power supply voltage drops almost to zero.Can you post a top view of the board located next to the power inlet? Then I can tell you which capacitor you have to replace.When ordering a capacitor: be sure to replace the capacitor with an X type capacitor. Otherwise a surge on the mains might destroy the complete circuit.

author: mzacharias
author_rank: Frequent Contributor
timestamp: June 22, 2019, 11:41:31 pm
content: Quote from: temperance on June 19, 2019, 08:55:31 pmThis only applies to the 220/240 volt models.

author: mzacharias
author_rank: Frequent Contributor
timestamp: June 22, 2019, 11:43:40 pm
content: Quote from: temperance on June 19, 2019, 08:55:31 pmAnd by the way, this particular capacitor is in series with the line, not across it. X rating would be fine, but not required. The original part certainly was not so rated.

author: temperance
author_rank: Super Contributor
timestamp: June 23, 2019, 08:28:25 pm
content: X rated capacitors will last longer because the cap still has to withstand surges although with a resistor in series. That's the reason why the original capacitors loose their capacitance.https://www.vishay.com/doc?28153Page 2.An X rated capacitor is a better approach when the full circuit is unknown.

author: CoopedUp
author_rank: Regular Contributor
timestamp: June 24, 2019, 05:16:13 pm
content: This is the board I think you are referring to. The thing will power on in it's protection circut is bypassed and it gives me an error code.

author: CoopedUp
author_rank: Regular Contributor
timestamp: June 24, 2019, 05:53:28 pm
content: I now have a dc prt 255 h. it used to be ps2 prt 255 so I still dont know what to do

author: temperance
author_rank: Super Contributor
timestamp: June 24, 2019, 10:43:52 pm
content: Interesting, this amplifier doesn't have the cap dropper but a fly-back power supply based on a power integrations TOP switch.You might want to take a look at the schematic:https://elektrotanya.com/yamaha_rx-v667_htr-6063_rx-a700.pdf/download.html#dlYou will find a description of what The PS2_PRT means on Page 50. Some more on page 74.I think you should be able to repair the amp with this information. (search the PDF for PS2)Regards

author: CoopedUp
author_rank: Regular Contributor
timestamp: June 24, 2019, 10:45:27 pm
content: I have the manual and all that I have scratched my way through just about everything