(Another) Troubleshooting a Yamaha receiver with DC_PRT (protect shutdown) probl

[cantata.tech]

The previous post is one solution to frustration (with a Class D O/P stage?), I hope we can find a less brutal option...

Yes it's a Yamaha mini system with a YDA-147 Class-D amplifier. I wouldn't say it was a bad system but it didn't turn out to be that reliable and wasn't that user friendly.

[ Attachment Invalid Or Does Not Exist ]

I'm hoping that I can cut out that section and place it along with the power stage into another system that I want to get working.

Using sheet-metal sheers looks dramatic but it made less dust than using a dremel and was very quick. So far so good. The other alternative was to through it all in the bin but I need the D-Class amplifier on another project.

[David_AVD]

When amplifiers like this have offsets on all channels it's wise to check the output of all of the voltage regulators, especially the +/- 12V or 15V ones.

The other thing that will do this is wire links or PCB traces that have corroded out, sometimes causing the ground reference for the amplifier section to be lost.

A grossly unbalanced supply (or missing rail / ground) can tilt everything and make it look like multiple channels are faulty.

[NHcoder]

When amplifiers like this have offsets on all channels it's wise to check the output of all of the voltage regulators, especially the +/- 12V or 15V ones.

The other thing that will do this is wire links or PCB traces that have corroded out, sometimes causing the ground reference for the amplifier section to be lost.

A grossly unbalanced supply (or missing rail / ground) can tilt everything and make it look like multiple channels are faulty.

Thanks for the pointer, I'll check them as soon as I can.  There are a fair number of low voltage (5, 3.3) scattered around but one of the power PCBs has has a few for 12v.

[alanep]

I've got different priorities today, it's my wife's birthday i.e. happy wife, happy life.

Fig 6 (page 20) shows how to disassemble the amplifier but keep all the interconnects. Maybe the rear panel has been removed to gain access to access to main(3) & the other boards.
There's some text as well:

When checking the P.C.B.:
• Put the rubber sheet and the cloth over this unit.
Follow the procedure below to place the amp unit.
(Fig. 6)
• Connect the ground points of the heatsink and subchassis
unit to the chassis with a ground lead or the
like. (Fig. 6)
• When connecting the flexible flat cable, be careful
with polarity.
• Reconnect all cables (connectors) that have been
disconnected.

[NHcoder]

I've got different priorities today, it's my wife's birthday i.e. happy wife, happy life.

Fig 6 (page 20) shows how to disassemble the amplifier but keep all the interconnects. Maybe the rear panel has been removed to gain access to access to main(3) & the other boards.
There's some text as well:

When checking the P.C.B.:
• Put the rubber sheet and the cloth over this unit.
Follow the procedure below to place the amp unit.
(Fig. 6)
• Connect the ground points of the heatsink and subchassis
unit to the chassis with a ground lead or the
like. (Fig. 6)
• When connecting the flexible flat cable, be careful
with polarity.
• Reconnect all cables (connectors) that have been
disconnected.

Hope you have (a good) time!  This diagram won't really help unfortunately.  That top piece labeled "sub-chassis unit" is indeed the front panel, and the two "AMP units" are MAIN(1) and MAIN(2), but everyting else is left in place.  The rubber sheet is sitting on the digital pcb and you can see the other pcbs below it.  The one with the outputs that we're looking at is on the very bottom of that stack.  If I get really bored in the upcoming weeks and this isn't figured out, I might try to build some sort of open air frame to test with, but honestly I don't think I'll last that long before just stripping it for parts.

Right now I'm rectifying a stupid mistake I made in testing with the relay and 3.3v regulator.  The jumper that connects the two digital grounds on the power board is not actually a real part, and the two aren't connected at all, so it's no wonder what I was doing didn't work.  I've now learned that "no_use" on these schematics means "it's not there, dummy."  It seems weird to me that the two aren't connected on the board (DGND on W2505 and DGND on W2001).  Maybe it's some kind of safety feature to prevent it from turning on unexpectedly when you're working on it like this.

EDIT: That was the issue, comes on fine now and I'm getting 83.1vac and 57.7vac out of the transformer.  The schematic calls for 82.6 and 57.2 so I'm calling that good.

[NHcoder]

When amplifiers like this have offsets on all channels it's wise to check the output of all of the voltage regulators, especially the +/- 12V or 15V ones.

The other thing that will do this is wire links or PCB traces that have corroded out, sometimes causing the ground reference for the amplifier section to be lost.

A grossly unbalanced supply (or missing rail / ground) can tilt everything and make it look like multiple channels are faulty.

I really thought this was going to be it after a quick test, but a look at the schematic made me facepalm as I realized I'd been measuring vs. chassis ground.  The +/-18v rectified output and +/-12v regulated output both are fine.  I need to revisit some earlier tests because now I'm not certain that chassis ground was the appropriate thing to be testing against on the amplifier boards either.

[David_AVD]

Something else to keep in mind is that hi-fi amplifiers sometimes use the chassis as the common return (0V) for various signal paths.

You can often spot the important 0V points, such as a metal tab on the RCA sockets, a threaded PCB mounting tab, etc.

If you try and run the unit partly disassembled or with out all of those grounding screws back in place things can get wacky.

[NHcoder]

Something else to keep in mind is that hi-fi amplifiers sometimes use the chassis as the common return (0V) for various signal paths.

You can often spot the important 0V points, such as a metal tab on the RCA sockets, a threaded PCB mounting tab, etc.

If you try and run the unit partly disassembled or with out all of those grounding screws back in place things can get wacky.

Yepyep, I'll be paying closer attention to this as I go from now on.  This is my first time working with a schematic for a power amp or anything similar as well so I didn't realize the significance of the thicker black lines running around the amp sections which represent the ground reference for that particular section.  On the amplifier boards these are separate from the chassis ground, which does exist, but has no electrical connection to the power parts, which makes sense if you're not trying to kill your users I suppose.

[NHcoder]

Last (probably) update for the next 12-14h.

Measuring on MAIN(3), MAIN(2) and Main(1) is revealing the following so far:

1. The DC offset is definitely really there.  There is a thick wire running to each output channel on MAIN(1/2) from MAIN(3), these are wires I cannot disconnect (they are crimped at both ends to either board) and joked early on about splicing spade connectors into.  The offset of about -6.5v is present on all of them except the black one, which is the ground reference for the board.  Schematic indicates that all of these should be 0.0v.

2. MAIN(2) has a small 3 pin connector labeled W1003 that has I_PRT_R, -LB_R, and +LB_R.  These are supposed to be +55.6v, -53.8v, and +55.7v respectively.  The values I get when checking are +57.4, -6.85, and -1.87.  On MAIN(1), the L versions are present @ W1001, and have the values +57.25, -6.86, -1.87.  Close enough to the other side that it's clearly not a coincidence.

3. MAIN(1/2)'s B+ and B- supply come in on a twisted pair of wires and have proper values; +57.4 for B+ and -56.8 for B-.

It seems the problem is probably on MAIN(3) or earlier that is the common denominator between MAIN(1) and MAIN(2).  More tomorrow, hopefully.

[alanep]

1. The DC offset is definitely really there.  There is a thick wire running to each output channel on MAIN(1/2) from MAIN(3), these are wires I cannot disconnect (they are crimped at both ends to either board) and joked early on about splicing spade connectors into.  The offset of about -6.5v is present on all of them except the black one, which is the ground reference for the board.  Schematic indicates that all of these should be 0.0v.

Are these wires the channel outputs (before the protection/muting relays)?

2. MAIN(2) has a small 3 pin connector labeled W1003 that has I_PRT_R, -LB_R, and +LB_R.  These are supposed to be +55.6v, -53.8v, and +55.7v respectively.  The values I get when checking are +57.4, -6.85, and -1.87.

-LB_R,+LB_R (& -LB_L,+LB_L) are power rails to the power amp driver & as noted, they are completely out of spec e.g. -6.85V in lieu of -53.8V, -1.87V in lieu of +55.7V.

With the driver having such out of wack power supply rails, the channel power amplifiers won't be working in any sort of linear operating region.

The DC offset of -6.5V on the channel outputs (assumed from above) is more likely fortuitous voltage getting to the base of the output darlington transistors (which do have the proper supply voltages on their collectors).

-LB_R,+LB_R (& -LB_L,+LB_L) come from the +LB, -LB regulators Q1009, Q1010, Q1096 etc on Main(3). Do you now have better access to this area for a revisit/probing?

If so, could you do a voltage measurement of Q1096's collector? Could you also measure the voltage on Q1009's collector? (see attachment)

I've been harping on a bit (sorry) but there's something really fishy going on i.e. these regulators are turned off/faulty for some reason.

[NHcoder]

1. The DC offset is definitely really there.  There is a thick wire running to each output channel on MAIN(1/2) from MAIN(3), these are wires I cannot disconnect (they are crimped at both ends to either board) and joked early on about splicing spade connectors into.  The offset of about -6.5v is present on all of them except the black one, which is the ground reference for the board.  Schematic indicates that all of these should be 0.0v.

Are these wires the channel outputs (before the protection/muting relays)?

Yes, I believe so.  They are on the schematic on page 138, between MAIN(3) and MAIN(1/2), labeled W1004(A/B), W1005(A/B), etc, and their color is denoted as well.

2. MAIN(2) has a small 3 pin connector labeled W1003 that has I_PRT_R, -LB_R, and +LB_R.  These are supposed to be +55.6v, -53.8v, and +55.7v respectively.  The values I get when checking are +57.4, -6.85, and -1.87.

-LB_R,+LB_R (& -LB_L,+LB_L) are power rails to the power amp driver & as noted, they are completely out of spec e.g. -6.85V in lieu of -53.8V, -1.87V in lieu of +55.7V.

With the driver having such out of wack power supply rails, the channel power amplifiers won't be working in any sort of linear operating region.

The DC offset of -6.5V on the channel outputs (assumed from above) is more likely fortuitous voltage getting to the base of the output darlington transistors (which do have the proper supply voltages on their collectors).

-LB_R,+LB_R (& -LB_L,+LB_L) come from the +LB, -LB regulators Q1009, Q1010, Q1096 etc on Main(3). Do you now have better access to this area for a revisit/probing?

Q1009 is pretty easy to get to, 1096 & 1097 are more of a challenge, but I was able to get most of the readings.  Enough I think.  MAIN(3) is still mounted to the chassis, taking it out requires removing VIDEO(4) because with it in place, MAIN(3) cannot be slid inward far enough for all the output banana jacks to clear the chassis.  VIDEO(4) is one of the power supply boards with a direct connection to the transformer, so positioning it while keeping it plugged in to everything will be tricky. 

On pages 116 you can see what I have access to. 

If so, could you do a voltage measurement of Q1096's collector? Could you also measure the voltage on Q1009's collector? (see attachment)

I took measurements of 8 of the 9 pins for Q1009, Q1096, and Q1097.  Two of them are hardwired together (1096'&1097's emitters, 1009's emitter to 1097's collector) so I treated those as a single measurement.  The traces are pretty fat and look undamaged, but I'll recheck these individually where possible if we have a reason to do so.

I've been harping on a bit (sorry) but there's something really fishy going on i.e. these regulators are turned off/faulty for some reason.

By all means, harp on it.  Rechecking and rerechecking is prudent given my inexperience and potential for mistakes.  Chassis ground was no good for these checks the last time, so rechecking was a requirement anyway.  I did check them all and the results are below, but I think I need to retest.  In the current configuration, AMP_LMT is floating because that board isn't connected.  The schematic indicates there should be 3.1v present there, so perhaps I should retest with that pin tied to the makeshift 3.3v rail I'm using to control the power supply relay?  I think many of these readings may be suspect because of this, and Q1096's gate doesn't have a pulldown or pullup on it to prevent it from floating in this situation.

	Base	Emitter	Collector
Q1009	57.6v	-1.92v	57.6v
Q1096	-0.25v	-0.13v	57.6v
Q1097	---	-0.13v	-1.92v

EDIT:

I went ahead and retested as mentioned above, with AMP_LMT tied to the 3.3v rail, and the results are more sane.  Kind of embarrased neither of us thought of this the last time, oops!

	Base	Emitter	Collector
Q1009	57.5v	0.0v	59.95v
Q1096	3.3v*	1.9v	57.0v

I did not check Q1097 because I can't easily access the base signal anywhere, and the other two are tied directly to already tested pins on the other two.

This seems almost normal.  With the AMP_LMT signal addressed I am going to revisit checking the individual channels on the amplifier boards.

EDIT2:

So the inputs to the transistors in the amplifiers are floating as well.  W1042 and W1043 have signals for each channel coming from the disconnected FUNCTION(1) board which feed into the transistor gates on MAIN(1) and MAIN(2).  I am going to tie these to the shared ground from MAIN(3) before testing the voltages, as the schematic indicates they should all be at 0v.

[NHcoder]

Eureka!  (I think!)

So after tying all those grounds together to make up for the missing boards, and testing with the proper ground, the DC offset has disappeared from all of the channels except one -- back to that troublesome SBL channel on MAIN(1).  This one has -52v on it's output when everyone else is at 0, which is present both on the yellow wire and on both pins of the JST test point connector.

I need to keep poking around to determine which component is bad, but since I have the negative bias, I assume it would most likely be on the positive side?  One of Q1087, Q1091, Q1088, etc.?

Thanks a ton to alanep and everyone else who helped out, I think I'm just about out of the woods with this thing and am really looking forward to putting it back in service in the living room.

Please enjoy this photo of the rats nest of wiring I hacked up to get everything grounded, along with the breadboard powersupply I pressed into service for his AMS1117.

[David_AVD]

If the output of the bad channel is exactly the same as one of the main voltage rails, it will be a shorted output transistor. In that case I'd replaced both the NPN and PNP pair as well as the driver transistors and bias transistor. Also check all resistors around those transistors.

[NHcoder]

If the output of the bad channel is exactly the same as one of the main voltage rails, it will be a shorted output transistor. In that case I'd replaced both the NPN and PNP pair as well as the driver transistors and bias transistor. Also check all resistors around those transistors.

Yeah the pair is definitely bad, which is egg on my face because that was the first thing I checked before coming here to post.  If I'd tested properly to begin with I wouldn't have led everyone down the garden path with me.  Both in the pair are passing current in reverse, from emitter to collector, according to the diode check on my dmm.  If you could glance at the attached image and point out which ones you mean, I would appreciate it.  I assume it's everything on the red path in the schematic, and perhaps even the two that aren't, because what difference will two more make at this point.

Thanks!

EDIT: Shopping list, without the two not on the red path:

Code: [Select]

Q1087N & P -- STD01N & P        -- Transistor pair
Q1088      -- 2SC3906K T146 R,S -- Transistor pair
Q1089      -- KTA1517S GR TP    -- Transistor
Q1091      -- 2SA1208S/T-AE     -- Transistor
Q1092      -- HN4C06J           -- IC
Q1093      -- 2SC2910S/T-AE     -- Transistor

[alanep]

The output transistors are Darlingtons. From the datasheet, they have internal pulldown resistors (see attachment). This may cause the transistor to appear to be "leaking" when diode testing the E-C (in one direction). I tested a known good Darlington (with pulldown resistors) but a different type to the STD01N/STD01P used in this amplifier & it showed up as being leaky.

It might be prudent to diode test another channel's output transistors (with no DC offset), just in case...

My Darlington transistor measured about 9Kohm (E-C). If it's a dead short, that's different i.e. it's stuffed.

[NHcoder]

The output transistors are Darlingtons. From the datasheet, they have internal pulldown resistors (see attachment). This may cause the transistor to appear to be "leaking" when diode testing the E-C (in one direction). I tested a known good Darlington (with pulldown resistors) but a different type to the STD01N/STD01P used in this amplifier & it showed up as being leaky.

It might be prudent to diode test another channel's output transistors (with no DC offset), just in case...

My Darlington transistor measured about 9Kohm (E-C). If it's a dead short, that's different i.e. it's stuffed.

Thanks for that tip.  I was testing in diode mode to show the forward drop rather than resistance.  My understanding is that emitter to collector on a normal BJT or a darlington should be an open circuit, but as I've been reading I've learned that some may have a freewheeling diode between the emitter and collector that might not be on the schematic or datasheet.

I'm trying to understand the possible failure modes that could lead to the B- being present on the output and all I can come up with are either a failure of the P darlington itself, or a failure in the other transistors that put a signal on it's base, for example a dead short between the emitter and collector of Q1093.  Of course it could be both as David pointed out.  Is this correct, or have I missed something?

I did as you suggested and did some simple tests to compare this P darlington (Q1087P) with his P channel neighbor, Q1074P.

Code: [Select]

Collector-Emitter drop: 0.6v.
Emitter-Collector drop: 0.5v.
Base-Collector drop: 0.66v.
Collector-Base drop: 1.5v. 
Base-Emitter drop: Open circuit.
Base-Emitter resistance: Open circuit.
Cathode-Base drop: 1.3v
And resistances

Code: [Select]

Collector-Emitter: 3.5M
Emitter-Collector: 5M
Base-Emitter: Difference here.  8M on the good channel, 14M on the bad one.
Emitter-Base: Difference here too.  4.5M on the good channel, 1.7M on the bad one.
Base-Collector: Difference here too.  5M on the good channel, 10M on the bad one.
Collector-Base: 5M on both.

This seems odd, like maybe this darlington is OK and the problem is elsewhere in the circuit.  This would make sense, since the base is always involved in the weird readings.

R1303 is ok, 10ohm.  Q1093 is ok, 0.6v base to either pin, nothing the other ways.  R1267 is ok, 216 ohms.  Stopped here for the night.  Will continue "back" tomorrow checking Q1091, Q1088, etc, working back until I find something "definitively" broken.

[David_AVD]

I would check everything from Q1088 upwards (towards the output).

If in doubt, remove to test properly. Compare to same parts in a good channel.

[alanep]

I previously did include a post with long winded testing scenario which I've deleted. David_AVD's methodology (above) is much better (& succinct)

[shakalnokturn]

Another of my previous Yamaha repairs (RX-V1500) had a negative output on one channel due to the equivalent of C1128 in your schematic being shorted. (You don't see electrolytics shorted that often so easy to neglect...)
Don't miss R1251 while you're in there testing every component.

[NHcoder]

Just a quick update, been a little busy with other life stuff, but resumed testing a bit today.  Q1093 checks out, tests ok and matches Q1080.  Q1091 though is another story and looks like one of the junctions is blown.  Base-Collector is showing 0.12v drop (did not check resistance), while Q1078 is showing an open circuit as expected.  I don't have any suitable replacements on hand  so I'll be looking at specs and ordering a few.  Before that I'll continue on and at least check Q1088 as well.  If it checks out OK I'll probably stop there, replace Q1091, and see what happens.  The two resistors I checked along the way, 1294 and 1238, are ok.

Thanks again for everyone's assistance.  I'll check in again once the transistor arrives and I've replaced it.

[alanep]

If Q1091 couldn't turn on,  the output transistors' bases would be pulled towards -LB-L (-53.8V) by constant current sinks Q1105, Q1093 i.e. creating the symptoms you've got. Have you checked Q1105 - it might have caused Q1091 to fail?
Fingers crossed (it's only Q1091)...

[NHcoder]

If Q1091 couldn't turn on,  the output transistors' bases would be pulled towards -LB-L (-53.8V) by constant current sinks Q1105, Q1093 i.e. creating the symptoms you've got. Have you checked Q1105 - it might have caused Q1091 to fail?
Fingers crossed...

I haven't checked Q1105 yet, no, but I will do so tomorrow.  Once I found Q1091 was damaged I started hunting for replacements, which weren't easy to come by!  Most sites are sourcing from the same upstream which has a minimum order of over 1000, and on the compatibility list I found, none of the replacements were easy to find either.  I finally found a source in Hong Kong that had a minimum order of 9, so I pulled the trigger on that and am waiting for them to arrive.  I saw some used ones on ebay as well but if I can get them new for a few dollars more, I'd prefer that.

I'll check Q1105 tomorrow along with Q1088.  Fingers crossed here as well.

[alanep]

I had another look at circuit. Maybe check D1080, D1081 as well. They set the voltage on the bases of Q1105, Q1093, Q1095 and their constant current sink value e.g. Q1105 would sink about 2.7mA (0.6V/220ohm). Normally it should be 2 diode drops off -LB_L. If one of the diodes went open, then the current sink would have a much higher current (e.g. possibly frying Q1091).

[NHcoder]

I had another look at circuit. Maybe check D1080, D1081 as well. They set the voltage on the bases of Q1105, Q1093, Q1095 and their constant current sink value e.g. Q1105 would sink about 2.7mA (0.6V/220ohm). Normally it should be 2 diode drops off -LB_L. If one of the diodes went open, then the current sink would have a much higher current (e.g. possibly frying Q1091).

FYI, Q1105 and D1081 test out OK, but with the parts in place I can't determine if it's actually Q1091 or Q1088 as the test path for Q1091-BC and Q1088-CE is the same, so I'll remove Q1091 later today and test it in isolation and Q1088 without Q1091 interfering.

[NHcoder]

I had another look at circuit. Maybe check D1080, D1081 as well. They set the voltage on the bases of Q1105, Q1093, Q1095 and their constant current sink value e.g. Q1105 would sink about 2.7mA (0.6V/220ohm). Normally it should be 2 diode drops off -LB_L. If one of the diodes went open, then the current sink would have a much higher current (e.g. possibly frying Q1091).

FYI, Q1105 and D1081 test out OK, but with the parts in place I can't determine if it's actually Q1091 or Q1088 as the test path for Q1091-BC and Q1088-CE is the same, so I'll remove Q1091 later today and test it in isolation and Q1088 without Q1091 interfering.

Pulled 1091 and Q1088 tests fine in place now, so looks to me like 1091 just failed on its own.  Replacement should be here in 2-3 weeks.