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

[NHcoder]

Apologies in advance for the wall of text!

Let me start by just saying I've looked over many similar threads here and it seems that most of them just die out before a resolution is found, so in spite of the number of similar threads, none of them I was able to find actually resulted in a resolution.

I promise to anyone that decides to help, if I decide I've had enough and give up on the project, I'll say so in the thread rather than just disappearing!

I have a Yamaha RX-A1000 receiver that I've owned since new.  It worked flawlessly for ~10 years before it was put in storage for two years.  When it was pulled out of storage, it still worked mostly ok, but one of the HDMI outputs was putting out garbage.  All of the others worked fine.  After using it for another 18 months or so, it suddenly stopped working.  I came home to find it powered off, and attempting to turn it on results in it going into the power protection mode almost instantly.  This happens regardless of what connections are made on the rear panel, including with no connections at all.

I was able to find the service manual online and boot into the diagnostic mode, which indicated a "DC_PRT 0L" message, which the manual describes as

"Power amplifier DC (DC voltage) output is detected.  The voltage at pin 1 (DC_PRT) of IC902 is displayed.  Normal value: 35-68 (Reference voltage  3.3v = 255)"

Several forum posts I saw recommended testing the output transistors.  I assumed this meant the massive pairs of darlington transistors for each output channel (Sanken STD01N/STD01P), which are a real pain in the neck to get to with this unit due to their position combined with the board to board wire routing, but I gave it a shot anyway.  I took apart the the receiver carefully, board by board, to get to those transistors.  Unfortunately due to their location it's essentially impossible to get to them with the receiver assembled and all the boards in place.

Once all the boards were out of the way, I tested the transistors as best I could determine how in circuit with my multimeter.  I checked for:
  - Dead shorts from the gate pin to any other pin
  - Diode forward drop of the N channel pair between pins 1 and 2.
  - Forward drop of the diodes on the P channel pair between pins 4 and 5.
  - Forward drop of the P channel pair between pins 1 and 3.
 
In retrospect I should have probably checked more, but everything here checked out, and I put it all back together.  Honestly, I don't know if these are even the transistors I was supposed to be checking, but it seemed like the right place to start -- because it was the most difficult, probably.

Since reassembling it, it still shuts off with a DC protection error, but now it shows a high value rather than 0.  On the display for P1-1 it shows "DC: 84" at present, but as I watch it, it jumps around between about 79 to 85.

I took pictures of every board front and back with and without my phone camera flash enabled as I was putting them back in after looking them over for obvious damage, without finding any.

This imgur album has all the photos.  The ones taken of the individual boards were just before reassembly, the ones inside the receiver were taken as I took it apart, so I knew which cables went where.

https://imgur.com/a/KnwSsHe

I do basic circuit design and microcontroller stuff, but it's just a hobby, I'm a developer by trade.  I'm at my wits end trying to figure out what went wrong here, which is a shame, because as old as it is this receiver has been great since the day I bought it, and it seems like this should be an "easy" fix -- I just don't know where to go from here.  Please try a dash of ELI5 if you can, clearly when some post says "check the transistors" I may not know which ones are meant or how I should test them.  Mostly I just build things and when I let the smoke out, I throw it away and build again.  I don't usually have to troubleshoot in this way.

So thanks in advance for any help in figuring this out. 

EDIT:

Oh, there are a few closeups of discolored areas of two of the boards.  Just looked like normal heat discoloration to me, but other opinions are obviously welcome.

EDIT2:

Results of onboard diagnostic pages, normal values from the service manual.

  P1-1 - DC: 100 (normal 35-68)
  P1-2 - 130/120 (normal 84-141 / 81-153)
  P1-3 - TM: 120/122 (normal 116-255 for both)
  P1-4 - OUTLVL: 255 (normal .  255 = 3.3v.  Pg. 62)
  P1-5 - LMTCNT: 140 (normal .  255 = 3.3v.  Pg. 63)

[alanep]

Hello NHcoder,

I'll try to help you out. I'm a newbie when it comes to posting to the EEVblog forum but I have had a fair amount of experience with Yamaha amplifiers (& audio amplifiers in general). The version of the RX-A1000 service manual I downloaded was from Hifi Engine (for the page number references).

Before commencing any fault finding, how competent are you with mains powered equipment e.g. this amplifier? There maybe exposed terminals with mains potential inside which could be lethal. Normal practice is to inspect how the mains wiring enters the amplifier & how it is routed e.g. from the service manual, the "Power Board" (schematic page 139, component overlay page 118) definitely has mains power "bities" to watch out for/keep away from. If you haven't done this before, think very carefully whether you wish to proceed...

It looks like it's a 7 channel amplifier. The output power amplifiers sections (page 138 of the service manual) have plus & minus power supplies. With no signal (volume fully down), the power amplifier output should be around 0V DC (or within +/-0.5V) when measured with a multimeter on the DC range. This has to be measured before the output muting/ protection relay i.e. it's no good measuring it on the rear output terminals as they'll be disconnected. The usual spot to measure the output DC voltage is on the centre tap of the emitter resistors (or close to them) - I explain more about this later.

One or more of the power amplifiers has something other than 0V on it's output (i.e. a DC offset) which is generating the DC_PRT error. A DC offset doesn't necessarily have to be a faulty output transistor, it can be anywhere in the power amplifier stage.

The first step is to identify which power amplifier(s) has the DC offset but there maybe some difficulty keeping the amplifier on long enough to identify it. It sounds like it will stay on in diagnostic mode. Still, it's still best to minimise the time the amplifier is left on just in case otherwise there maybe magic smoke...

The emitter resistors mentioned above for each of the 7 power amplifiers are: R1041 (SBR  CB102), R1071 (SR CB103), R1111 (FR CB104) , R1146 (C CB107), R1183 (FL CB108), R1215 (SL CB110), R1250 (SBL CB111). These have two power resistors in a single ceramic package. They are conveniently brought out on the CBnnn connectors (shown in brackets above) mainly for idling current checking. Note that the idling current CBnnn references on page 23, 24 of the service manual don't match the schematics (frustrating). I'm using the schematics CBnnn.

Although their centre tap isn't brought out to the respective CBnnn connector, measuring the voltage on any part of these resistors is "close/good enough" i.e. it will normally only be a few millivolts different to the centre tap. The idea is to measure the DC on the emitter resistor with respect to the signal ground (or output ground) e.g. connect one meter lead to a signal input ground & the other lead to either the first 2 pins of the CBnnn connector (see attached extract for one of the power amplifiers).

You'll need a steady hand when probing the CBnnn connectors with amplifier powered up as there's +/- 56V (as well as mains voltage) inside!! If you're not comfortable, power it down (wait for the DC power supply caps to discharge) and solder a "flying lead" wire onto the rear of the connector & connect the other flying lead end to the multimeter, then power it up & measure it.

Hopefully, you'll identify a power amplifier with a significant DC offset. If so, we'll dig a little deeper.

Alan

[NHcoder]

Awesome, thanks for that response!  I saw those little test headers on the schematic and was wondering what they were for, it didn't occur to me they were for this sort of testing.  I'm comfortable with mains so no worry there, and while the board diagrams don't mention it, they seem to be a perfect fit for JST XH connectors (page 124 -- I'm using the manualslib PDF in which the page #s printed inside and in the PDF itself match), which I have in abundance, so I'll make up a little pigtail to use for testing with the meter and get back to you later today.

Image attached just so we're talking about the same thing.  I agree with the stupid renaming/renumbering of the connection.

[NHcoder]

Hopefully, you'll identify a power amplifier with a significant DC offset. If so, we'll dig a little deeper.

Decided to forego the JST and just use a female "dupont" jumper to check each pin against chassis ground.  6 of the channels show -1.7v, one showed -2.2v.  I mention the polarity just because it might be important relative to chassis, as I (perhaps naively) expected the bias to be positive.  The 0.5v difference doesn't seem huge, but maybe it is indicative of something.  The -2.2v signal is on CB378 (feeds to CB377, which leads to CB111 on the other board).

[alanep]

-1.7V (or -2.2V) is high. One mistake I made was assuming that CB102 to CB111 were the test points but I can see from your video pcbs attachment that they are connectors to additional pcbs video (4), video (5) where the idling current is set and with the real test points. The schematics for video (4), (5) boards are shown on the manual page 142.

CB372 to CB386 on video (4), (5) are indeed the test points. The idling current setting page 23,24 & the component overlays page 124 & their schematic page 142 now agree.

So, you've measured the DC on CB372 to CB386? If possible, I would like you to do a "sanity check" by measuring the DC directly (with respect to signal ground) on one of the emitter resistors (R1071 to R1250). From the component overlay, I think these resistors may have small wires (x 3) protruding from the top. These are an extension of their connection into the PCB i.e. they act as resistor test points & avoid having to probe in the vicinity of their base/the pcb. If not, then probing around the resistor base might be a little tricky... It is a sanity check so only do one channel.

If the emitter voltage is indeed the same as the test point then it is very unusual for all channels to have a DC offset greater than +/- 0.5V i.e. we're missing something.

I should explain how the power amplifier works in stabilising it's DC conditions. I've attached an extract of one of the power amplifiers with a few markups. At DC, we can ignore all capacitors (i.e. as if they don't exist). Q1022 at the input is called a differential pair & will produce an error signal at the collectors if the base voltages don't match (so it's often called an error amplifier). The voltage on the base of the left hand (LH) Q1022 is supplied a reference ground (0V) signal via R1053 & R1056. The DC voltage on the amplifier output is feed to the base of RH Q1022 via R1045 & R1044. The closed loop gain of the amplifier at DC is one (unity) i.e. assuming the amplifier has no faults, whatever voltage is on the LH Q1022 transistor base will be reflected on the amplifier output & back in the RH Q1022 transistor base. Another way of saying it would be, any error between two Q1022 bases voltage will be amplified by the open loop gain & will force the output back in the right direction so that the base voltages match.

If there is a DC offset on all channels, then it's looking like something is wrong with the reference ground (0V).

See how you go...

[NHcoder]

So, you've measured the DC on CB372 to CB386?

Yes, I measured the voltages to chassis ground from all 14 pins of those Video(4) and Video(5) "For Services" (each one labeled CBxxx and the other pin XHI) connectors on each.  I double checked each one because I thought I might have read 1.7mV on a bunch and actually 2.2V on the one odd man out, but it was volts and not millivolts on each one.

If possible, I would like you to do a "sanity check" by measuring the DC directly (with respect to signal ground) on one of the emitter resistors (R1071 to R1250).

I'm searching for them, unfortunately they must be among those printed vertically, which acrobat reader cannot find when doing a text search, so they only turn up in the parts list but nowhere else when I search for them with ctrl-F.  I should also point out that the boards are mirrored in the PDF for some reason.  For example the "MAIN(2) P.C.B. Side A" has R1056 in the upper left corner, but it's in the upper right corner of the PCB.  The PCB diagrams are laid out as though you're viewing them from "underneath" with X-ray vision.  I've attached a screenshot of the PCB from the manual on page 114, as well as a photograph of what I have to work with inside the receiver without disassembling it.  You can also see the "Video" daughter board there with the test points.  I can't access anything below that, including the pins of the actual power transistors and those test point jumpers, without tearing the thing apart again.

Getting these two amplifier boards out requires unplugging a few of the connectors going to them, so that may affect my ability to test them.

This is just to sanity check *me* really.  You can clearly see the markings in the photo for R1114 and R1056 if you zoom in, along with the parts in between, and they're perfectly mirrored from how the board is actually laid out.  Maybe this is normal.

So I'll get back to hunting for R1071 and R1250 on the PCB but if you see equivalent test points on the PCB diagram in a spot I can access them, that would be great!

[NHcoder]

Ok sorry I didn't realize 1071 etc were the big voltage divider things.  I don't have any access to probe those, they are physically below the video a/videob daughter cards plugged into the main a / main b boards as you can see in the photo.  I might be able to get dodgy access to suitable test points on MAIN(1) if I disconnect at least two of the board to board connectors from it -- FUNCTION (1)_CB706 (bottom left of MAIN(1) schematic on page 138) and MAIN (3)_CB106 (top right of the same board).  I haven't looked at where those go yet though so testing may not be possible with them disconnected, just wanted to get a quick update in.

[alanep]

Ok, no point having to dismantle to gain access to the emitter resistors - don't bother with the sanity check.

In my description of power amplifier DC conditions, I forgot to say that the DC voltage on the amplifier output should reflect the DC voltage feed to LH Q1022 via R1053, R1056 by the reference ground (0V). That's why the amplifier output should be at 0V. If not then there's something wrong with the reference ground (0V).

The next step is to check if the reference ground (0V) is elevated or floating (in voltage) with respect to the input ground on any of the amplifiers. For example, looking at previously attached Amplifier DC condition image for the SBR channel, can you measure the voltage on the side of R1056 or R1054 that's connected to the reference ground (0V) & an input ground? If SBR is inaccessible, you could measure any of the other channels e.g. R1085, R1088 for the SR channel etc.

[alanep]

One additional context, "input ground" is any ground that you can connect to that's on the rear panel inputs etc.

[NHcoder]

Ok, no point having to dismantle to gain access to the emitter resistors - don't bother with the sanity check.

Ok.  I'll attach an image to this post showing how the boards are laid out to give some idea of what I can access and what I can't.

In my description of power amplifier DC conditions, I forgot to say that the DC voltage on the amplifier output should reflect the DC voltage feed to LH Q1022 via R1053, R1056 by the reference ground (0V). That's why the amplifier output should be at 0V. If not then there's something wrong with the reference ground (0V).

The next step is to check if the reference ground (0V) is elevated or floating (in voltage) with respect to the input ground on any of the amplifiers. For example, looking at previously attached Amplifier DC condition image for the SBR channel, can you measure the voltage on the side of R1056 or R1054 that's connected to the reference ground (0V) & an input ground?

Access to R1056 is easy enough

One additional context, "input ground" is any ground that you can connect to that's on the rear panel inputs etc.

I was just going to ask if that's what you meant.  I have a solid and repeatable 2.5mV between the subwoofer input and the pin on R1056 that goes to ground, 0.0mV on the rest.  I also have 0.7ohms between that resistor and chassis ground, which isn't much, but it's not zero, just in case it matters.

Thanks for the quick replies so far, hopefully there is a clue in here somewhere.  I'm wondering if I should check some other channels?  I can get to R1123 and R1114 easy enough on that board, and probably all the other resistors on the other board easily as well.

[NHcoder]

Sorry, that was 2.5mv from subwoofer input tip to the resistor.  It's 0 on the input ground.

[alanep]

I'm not sure about these quick replies/chat. It does cause the topic to be elevated each time (& generate forum "noise").

I was really hoping that R1056 had a DC offset or wasn't (eventually) connected to chassis (2.5mV is very small). 0.7 ohm is pretty good (maybe even multimeter lead resistance).

So, a bit more thought must go into it now...

The idea was to understand where the DC offset was being introduced. It's a little difficult not standing in front of the amplifier as you are i.e. a bit like remote control.

By explaining the principles of how the amplifier works with respect to the DC conditions, it might give you a few ideas.

Another suggestion, can you check the voltage across the R1053 +R1056 (or equivalent in another channel). So what you are measuring is voltage between the base of LH Q1022 and the reference ground (0V). You should use a digital multimeter with a 10Mohm input impedance or higher (normal digital multimeter) so that it doesn't load down R1053 (33K). This measurement is effectively across the two resistors (& NOT to signal ground). You might need "flying leads" to the multimeter if they're inaccessible.

[NHcoder]

The replies will stop for a bit now, I need to hit the sack, got an early morning tomorrow clearing snow, but the replies shouldn't bother anyone who's not subbed to the thread.

I'll need to reread the meat of your posts and do some thinking.  To be honest I still haven't really understood how it knows there's an offset in one of the channels, and why it can't tell which channel it is.  I think it's worth mentioning that the last time I turned it on without doing the protection override and let it shut off on its own, it showed a "DC_PRT: 0L" message on the display, indicating a low voltage detected rather than one that was too high -- the manual says the value should be from 35-68, not the 0 I see right now, and not the high 80s like I also saw.  I wonder if it could be the reference voltage it's comparing against that's bad?  I checked the 3.3v lines going to the MAIN (4) and MAIN (5) boards which look like thermal monitors for the heatsink, and they were solid, but I don't know if the reference voltage is coming from the same source.

I couldn't repeat the 0.7ohm measurement, I probably just didn't poke the metal hard enough where I was probing last time.

If you think of anything else feel free to reply, I'll be looking at this again in 14 or so hours.

> The idea was to understand where the DC offset was being introduced. It's a little difficult not standing in front of the amplifier as you are i.e. a bit like remote control.

As a developer & sysadmin, I understand this struggle completely.  Thanks again for trying to help out.

[shakalnokturn]

I admit I've been skimming over this rather than thoroughly reading so sorry if I've missed something on the way.

Have you been able to confirm lack of DC offset on all outputs (measuring at speaker outputs) with protection override?

If you're sure the DC output is within tolerances your last remark on reference voltage may be the way to go.
I recently repaired a Yamaha HTR something that was throwing random Power Supply and Thermal protection errors on startup although everything was in range.
A tiny 3.3V SMD regulator on the small signal board (CPU / HDMI...) was giving a low voltage that was used for protection references.

[alanep]

A little time to gather my thoughts as well.

As shakalnokturn mentioned, recheck the DC offset. Page 23 of the service manual has a table (RHS) with a column "DC Offset Confirmations" & it shows a DC offset of between 0 to 100mV (i.e. 0.1V). 

You might also need to confirm that protection override enables the output muting/protection relays so that the DC offset can be measured at the speaker outputs. If the relays aren't energised, measuring at the speaker outputs would give erroneous results i.e. use the test points instead.

What happens next depends on whether there's any out of spec DC offsets...

[NHcoder]

OK so I went back to the top just to reverify everything.  First, on power up, it is shutting down after a few seconds, engaging the protection circuit.  Starting it up in diagnostic mode it shows "DC_PRT:  0L" as the reason, and not a value of up around 80.  The 0 value is the "normal" one I've seen for the past few months, I only saw the ~80 once when looking at it yesterday.  It's possible taking it apart and putting it together jiggled something, maybe a broken solder joint.

In diagnostic mode you can see the current live value, and right now it's bouncing around between 0 and 6.  The last 4 recorded shutdown reasons say the same, that it was due to "DC:  0".  If the relationship is linear then the value of 6 would represent 6/255 * 3.3 = 0.778mV, but it may well not be linear.  I assume the datasheet for the protection IC would say but I haven't looked yet.

I went through all the readings at the test points on the VIDEO(4) and VIDEO(5) boards again.  These are the JST connectors shown on page 124 and labeled CB372, CB376, CB374, CB378, CB384, CB386, CB382.  I checked the voltage from each pin of the JST connector to both chassis ground and digital ground.  So over all this was 7 channels x 2 pins x 2 grounds = 28 tests.  The results were:



Board Video(4)

CONN	PIN	DIGIGND	CHASSISGND
CB378	1	-2.021v	-1.976v
CB378	2	-2.014v	-1.976v
CB374	1	-1.565v	-1.535v
CB374	2	-1.563v	-1.536v
CB376	1	-1.568v	-1.542v
CB376	2	-1.568v	-1.542v
CB372	1	-1.553v	-1.531v
CB372	2	-1.553v	-1.531v

Board Video(5)

CB384	1	-1.554v	-1.538v
CB384	2	-1.552v	-1.535v
CB386	1	-1.551v	-1.538v
CB386	2	-1.549v	-1.535v
CB382	1	-1.549v	-1.530v
CB382	2	-1.545v	-1.534v

After I collected these values I shut off the receiver and started transcribing the numbers from the excel sheet I'd initially written them in to the forum post and saw the voltage was reading positive now rather than negative, and was slowly rising.  It reached about +2.4v after 5 min when I stopped watching.  This occured on all test points.  I figure that it just had something to do with p-channel FETs being deenergized while the device cooled off and caps drained, but thought it was worth mentioning just in case.

If I turned the receiver off and watched my meter, it only took 20 or so seconds to go from -2v up to +0.6v before continuing to rise at a steadily higher slower rate.  Shutting the mains connection caused it to drop down to -0.6v and sit there slowly rising towards 0 as caps drained or whatever.

Anway, now sure that I really (really, really, really) had tested each and every one of the test point voltages to ground, I moved on.

As shakalnokturn mentioned, recheck the DC offset. Page 23 of the service manual has a table (RHS) with a column "DC Offset Confirmations" & it shows a DC offset of between 0 to 100mV (i.e. 0.1V). 

I have not been testing the voltage between the two test pins, I have been testing the voltage of each one of them with respect to ground.  I just ran through and tested them as indicated in the service manual and got a solid 1.7mV on each one except CB378, which read 0.0mV, an order of magnitude less than what they should be.  I thought maybe my connection with the dupont jumpers was bad so I went ahead and made the JST pigtail I'd originally planned, and tested with that, and got the same results.  I used the pigtail to go through all the voltages in the above table as well and the results were the same, so I can only assume that the 1.7mV reading is correct, as is the 0.0mV reading on CB378 which is also the one with the higher voltage reading above.

This probably isn't a good sign I take it.

You might also need to confirm that protection override enables the output muting/protection relays so that the DC offset can be measured at the speaker outputs. If the relays aren't energised, measuring at the speaker outputs would give erroneous results i.e. use the test points instead.

I hear a relay click when the protection circuity comes on even in diagnostic mode, but there are other options/modes available that cause relays to energize and denergize as I go through them.  I can't tell which ones are energized and which aren't since they're so buried, but I did discover that if I in the "DSP Margin" mode which is the first mode that is activated when you enter diagnostic mode, I can hear the relay click, while switching to the next mode makes it click again, so it would seem I can activate and deactivate them as needed in diagnostic mode.

After discovering this I decided to check the subwoofer again, as well as looking for DC voltages on the speaker outputs in diagnostic mode.  The output grounds are shared together, but not with chassis ground of course.  This prompted me to recheck for DC voltages at the outputs and I did find voltages on all of them when in the default mode you enter in the diagnostic mode, "DSP Margin."

In this mode there is negative DC voltage between all of the 7 outputs and ground that are all the same except for one.  I have -5.8v on all channels except "surround back / bi-amp L" which is about 400mV off, at -6.25v.

It's worth pointing out that all of these odd readings are tracking to the same channel -- Surround Back L.

Switching to the next test mode causes an audible relay click, and all the voltages test at 0.

[NHcoder]

I admit I've been skimming over this rather than thoroughly reading so sorry if I've missed something on the way.

Have you been able to confirm lack of DC offset on all outputs (measuring at speaker outputs) with protection override?

See my reply above, but I measured a DC voltage on *every* output channel, with one higher than the others.

If you're sure the DC output is within tolerances your last remark on reference voltage may be the way to go.
I recently repaired a Yamaha HTR something that was throwing random Power Supply and Thermal protection errors on startup although everything was in range.
A tiny 3.3V SMD regulator on the small signal board (CPU / HDMI...) was giving a low voltage that was used for protection references.

I have my doubts about it being the regulator now after this last round of tests, but I intend to check anyway.

[alanep]

In this mode there is negative DC voltage between all of the 7 outputs and ground that are all the same except for one.  I have -5.8v on all channels except "surround back / bi-amp L" which is about 400mV off, at -6.25v.

It's surprising that on the speaker outputs it's -5.8V and on the test points it's approx -2.0V. Did you measure a test point & a speaker output at the same time/conditions? It probably doesn't matter i.e. I think it has DC offset problem (which still needs to be investigated).

The DC_PRT protect is a single analogue signal representing the sum of all the channels (via R1152, R1120, R1189, R1047, R1258, R1077, R1221). The controller can't discriminate between channels & will just shutdown the amplifier. I wonder if "DC_PRT:  0L", the "0L" actually means overload i.e. beyond the ADC range. If the DC_PRT is really 0V, you would think it would come out of protection (despite the real dc offsets on the outputs).

I'll have to look into the "DC: 0" error & see what it means.

By the way, I did check the schematic for what shakalnokturn mentioned i.e. 3.3V regulator & reference voltages. This amplifier does have something similar (see attachment). I don't think it's the problem but the information is there if you wish to check.

[NHcoder]

In this mode there is negative DC voltage between all of the 7 outputs and ground that are all the same except for one.  I have -5.8v on all channels except "surround back / bi-amp L" which is about 400mV off, at -6.25v.

It's surprising that on the speaker outputs it's -5.8V and on the test points it's approx -2.0V. Did you measure a test point & a speaker output at the same time/conditions? It probably doesn't matter i.e. I think it has DC offset problem (which still needs to be investigated).

Time was within a few minutes, but I just checked the troublemaker (CB378 / Surround Back L) and his neighbor (CB374 / Front L) one after the other. 
-6.53V between center and gnd for Surround Back L.  -2.11V from the test points to gnd.
-6.00V between output and gnd for Front L -1.6V from the test point to gnd.

The DC_PRT protect is a single analogue signal representing the sum of all the channels (via R1152, R1120, R1189, R1047, R1258, R1077, R1221). The controller can't discriminate between channels & will just shutdown the amplifier. I wonder if "DC_PRT:  0L", the "0L" actually means overload i.e. beyond the ADC range. If the DC_PRT is really 0V, you would think it would come out of protection (despite the real dc offsets on the outputs).

I'll have to look into the "DC: 0" error & see what it means.

The service manual on page 61 says it displays "the voltage at 1 pin of IC902, normal value 35 to 68; 3.3v = 255" and the word "overload" only appears once in the document in an unrelated issue.   0L  doesn't appear in the document that I could find so I assumed it was just the value 0 -- I believe it is a 0 (zero) and not an O (the letter) as the dot matrix VFD can display them differently.

By the way, I did check the schematic for what shakalnokturn mentioned i.e. 3.3V regulator & reference voltages. This amplifier does have something similar (see attachment). I don't think it's the problem but the information is there if you wish to check.

Thanks for that.  I had searched for the term "regulator", logged all of them, and looked up all the part numbers to see what voltage they are meant to produce, but haven't tested any of them yet.  The only accessible ones are going to be on the very top board which holds the HDMI ports, Digital (1) and I doubt one of those being out would have anything to do with this.  Below that is the input board with the RCA jacks called FUNCTION(1), then finally at the bottom is the amplified output board MAIN(3).

I checked the mentioned pin of IC902 which is on the top digital board and is easily accessible and got a reading of -191mV between it and chassis ground.  There is 0 ohms resistance between chassis ground and the ground of that chip. 

[alanep]

Wow, there's a fair discrepancy between the test points & the actual outputs (not sure why). Either way I think the DC offset is the problem to investigate.

I'll reiterate the power amplifier at DC is just one big unity gain amplifier. The voltages (when measured with respect to chassis/input ground) of (1) LH Q1022 base, (2) RH Q1022 base & the (3) output (speaker output) should all be identical.

It may be an advantage to have 2 digital multimeters i.e. one continually looking at the output and the other measuring the points I mentioned above. Have a look at the reattached circuit. You can measure the base voltages indirectly by probing the ends of R1053, R1045 (connected to the respective bases).

Re "0L", the closest I could find was page 32 (Self Diagnosis) with "DC_PRT:xxxH". Also on the same page was "PSx PRT:xxxL". Either way it is just a suffix & as you said, not meaning overload.

[shakalnokturn]

I too doubt that the 3.3V could be the problem here as Vref is common to CPU Vcc you'd be experiencing other troubles if it was bad, besides for DC protection to give false low readings Vref would have to be too high.

I'm back to alanep's first impression:

Very unlikely to have a DC offset related to a fault on every channel (although not impossible), this is more likely a ground / amplifier power supply problem. It's possible you may also have a different problem on surround back L that would need addressing later.

Does this model have a cut-out in the bottom frame that would give access to the relay PCB solder side?

Maybe worth doing a continuity check between all grounds that should be common starting from transformer center point to speaker negative terminals, power amp's then check power stage supply voltages.

I'd have to get more involved and download the service manual, that's not happening on my smartphone...

[alanep]

shakalnokturn, to bring you up to speed, have a look at that previous attachment "Amplifier DC conditions.jpg". I was really interested in what I called reference ground (0V) (about posting 7 - don't know how to link). It provides the 0V DC reference for all channels. The unity gain voltage measurement suggestions were a sanity check.

[shakalnokturn]

Corrected that in my previous post, you were indeed the first to have doubts on the ground.
In my books Yamaha are far better than Denon, still dry solders on interconnect can be a problem.

[alanep]

The reference ground (0V) for SBR, SR, FR is via W1042 &  for C, FL, SL, SBL is via W1041 W1043 i.e. completely different connectors (so if it's missing, then it's further back). I think the voltage sanity check is needed just to "tick the box".

PS I hope I don't sound too abrupt. I am an Engineer (retired) i.e. "on the spectrum".

[NHcoder]

I too doubt that the 3.3V could be the problem here as Vref is common to CPU Vcc you'd be experiencing other troubles if it was bad, besides for DC protection to give false low readings Vref would have to be too high.

That was my initial thought too, but when I catalogued all the voltage regulators in the receiver, there are 22 of them and 5 are 3.3v.  Just occurred to me that it could be using a 3.3v zener for the reference as well I suppose, but I didn't check.  I do agree though that a ground issue somewhere seems more likely.

I'm back to NHcoder's first impression:

My first impression was "help, wtf is wrong with this thing?!"   Saw your correction while posting this.

Very unlikely to have a DC offset related to a fault on every channel (although not impossible), this is more likely a ground / amplifier power supply problem. It's possible you may also have a different problem on surround back L that would need addressing later.

Yeah, this is what made me think of the regulator.  Having virtually the same fault across every channel seems really suspect.  I believe I mentioned that one of the HDMI outputs is "dead" as well, it just puts out discolored garbage since bringing the receiver out of storage.  The circuits don't seem related but maybe something failed even worse here and affected the system on a wider level?  Just spitballing.

Does this model have a cut-out in the bottom frame that would give access to the relay PCB solder side?

Unfortunately no, just tiny vent holes.  When I first saw what a pain in the neck it was going to be to get access to the big power transistors to test them, I looked for something like that to probe through before disassembling the whole thing.

Maybe worth doing a continuity check between all grounds that should be common starting from transformer center point to speaker negative terminals, power amp's then check power stage supply voltages.

I don't think I can test that with it assembled, but I'll go for it if nobody has any other suggestions to try before that.  The big transformer is shrouded and appears to be glued to the chassis as well in spots.  It has about a million wires going into it, give or take, and they are short enough that I don't think I'll be able to get direct access even if I remove the mounting screws and try to lift it up with everything attached.

I'd have to get more involved and download the service manual, that's not happening on my smartphone...

I can't attach the one I have as it's larger than the maximum size allowed by the forum, so I uploaded it to google drive.  Not the easiest thing to navigate on a phone for sure, but at least if you want to look at it on the PC you won't have to find it first:  https://drive.google.com/file/d/1n7gMIAvhxn-x_hrzlnGUg5rOaXkB6AOI/view?usp=share_link

Honestly I find it pretty amazing that it was assembled like this at all by humans.  The larger gauge wires running to the amplifier boards appear to be crimped on both ends; it doesn't look like they can simply be unplugged.  I have a ton of spade connectors here and I'll admit I was sorely tempted to cut them all and put spade connectors in when I had it all apart the last time.

The reference ground (0V) for SBR, SR, FR is via W1042 &  for C, FL, SL, SBL is via W1041 i.e. completely different connectors (so if it's missing, then it's further back). I think the voltage sanity check is needed just to "tick the box".

I think I can probe both of these without difficulty.  We're talking about checking continuity/resistance between chassis ground and the "E" marked pins on those two connectors, correct?  I can see on the schematic for MAIN(2) for example, for W1042 that this ties ground together a bunch of emitters and what appear to be filter caps. 

Thanks again to both of you!

[NHcoder]

PS I hope I don't sound too abrupt. I am an Engineer (retired) i.e. "on the spectrum".

Absolutely no complaints here.

[alanep]

We're talking about checking continuity/resistance between chassis ground and the "E" marked pins on those two connectors, correct?

(learning how to quote, I don't know how to the get the originators name in there)

Yes please, if you can measure the resistance between the "E" pin on 4W1041,2 W1042, W1043 and chassis ground.

[alanep]

Please note incorrect W1041. Should be W1043. Fixed in original posting.

[NHcoder]

We're talking about checking continuity/resistance between chassis ground and the "E" marked pins on those two connectors, correct?

(learning how to quote, I don't know how to the get the originators name in there)

phpBB is a little esoteric, but I've been posting on forums like this for many years, happy to offer what little help I can in the other direction.  If you click the quote button on the message you want to quote, it will quote the whole thing, and the name will show up as well as the clickable link to the quoted post.  You can copy and paste that line over and over to quote different sections, just make sure you close each one as well with

Code: [Select]

[/quote]
You can also type out the name or copy/paste it in a simple tag like this:

Code: [Select]

[quote author=alanep]
Yes please, if you can measure the resistance between the "E" pin on W1041,42 and chassis ground.
[/quote]

Which makes this:

Yes please, if you can measure the resistance between the "E" pin on W1041,42 and chassis ground.

I was able to test W1043 to both pins since that board sticks out past the heatsink, and it's clean, 0 ohms to the chassis.  W1042 (Main(2), goes to CB707 on Function(1), page 114) is 0 ohms as well, but I can't get access to the pins on that one, just the test jumper.  Just to be sure on W1042 I checked from the jumper to both pins at the other end of the connector and it was 0.0 on both of them as well.

Note I checked W1043, not W1041, maybe that was just a typo?  1041 is a bigger connector with the DC_PRT line and other PRTs, an SPI connection, etc.

Please note incorrect W1041. Should be W1043. Fixed in original posting.

Yep, caught that.

[alanep]

Yes, it's W1043 (fixed previous postings)

Finding W1042, W1043 "E" pins measuring 0 ohms to chassis is absolutely bizarre i.e. you'd swear that would have been the problem.

We came to same conclusion/measurement previously (I'll have to look up the post). That was one of the reasons I thought of the unity gain voltage measurements/sanity check (which you may still wish to do).

Thanks for info re quoting.

[alanep]

One more thing just out of left base (and one of the most fundamental things to check first). I'm not sure if the power supply rails were tested (maybe a big oops)....

Could you check/measure +56V, -56V on the power amplifiers somewhere that's convenient.

[alanep]

Continuing on with power amplifier power supplies, there are actually 4 feeds to each power amplifier. There's +/-56V unregulated to the output transistors and also +55.6V and -53.8V (from regulators) to the driver stages.

I've got a hunch, it could be one of the regulators...

Updated below:

I've added the schematics (all extracts from page 138).

A low voltage on the driver part of the amplifier could certainly cause these effects. Just as an aside, it would have shown up on unity gain voltage measurements...

[NHcoder]

Continuing on with power amplifier power supplies, there are actually 4 feeds to each power amplifier. There's +/-56V unregulated to the outputs and also a +55.6V and -53.8V (from regulators) to the driver stages.

I've got a hunch, it could be one of regulators...

Ok.  I've been searching for test points I can access.  I also am pretty sure I can get access to test those resistors directly, I didn't even notice the vertical jumpers you mentioned before, maybe I was too tired.  I believe I can get at them all though as they extend up to where the big inductors are, and I can access that area pretty easily.

[alanep]

I've updated the previous post to include the schematics.

If you want a break for the night (not sure what time it is there, 3:30pm or so here). I'll be away tomorrow (bushwalking).

I think there's something to go on with.

[NHcoder]

I've updated the previous post to include the schematics.

If you want a break for the night (not sure what time it is there, 3:30pm or so here). I'll be away tomorrow (bushwalking).

I think there's something to go on with.

It's almost 1am here but it's Friday night, so I have time to spare.  Sorry for leaving you hanging, getting access to those pins isn't easy, but I think you're on to something.  I was able to trace the paths to test point jumpers for some of them, and all of them I could get to are showing bad values.  I can get to the rest but I'll need to power the receiver off, unplug it, and attach some of my probe clips with it off so I don't risk shorting something trying to maneuver them in.  It's really a no go on all but one of them with the normal meter probe.

Anyway, the result:

W1001 on MAIN(1) only gave me access to -LB_L @ J1333, and I measured -2.47v.

W1003 on MAIN(2) gave me access to both +LB_R @ J1298 and -LB_R @ J1211.  These were -0.6v and -2.5v respectively.

This is in the default diagnostic mode, in which there is an output DC voltage present at the speaker terminals, in case that matters.  I haven't dug in yet to see what's supplying these rails but I imagine it's time to tear the boards back out and test whatever it is directly.  I wanted to get back with an update before proceeding just so you didn't think I'd disappeared.

[alanep]

Ok, it's up to you but mistakes get made when you're tired...

With those voltages (I'll try quoting eventually), there's definitely something wrong with the output of the regulator.

I'll have to look at the layout as to where the regulator actually is (updated: page 116).

The regulator is a discrete design but there are a couple of things:

1. The negative regulator mirrors (almost) whatever the positive regulator is outputting.
2. The positive regulator output voltage can be changed by a signal "AMP_LMT"

Updated:

If the "AMP_LMT" signal goes to 0V, it will turn off Q1096 & subsequently the series pass Q1009 i.e. shutting down the regulator.

Updated:

So if possible, the "AMP_LMT" voltage should be measured. It's on W1041 pin 3. Component overlay - Main(3) (page 116 with page co-ordinates C6)

Updated:

If it's any easier, "AMP_LMT" is also available on IC908 pin 4 or connector CB909 pin 3 (page 132).

[NHcoder]

Ok, it's up to you but mistakes get made when you're tired...

With those voltages (I'll try quoting eventually), there's definitely something wrong with the output of the regulator.

I'll have to look at the layout as where the regulator actually is (updated: page 116).

The regulator is a discrete design but there are a couple of things:

1. The negative regulator mirrors (almost) whatever the positive regulator is outputting.
2. The positive regulator output voltage can be changed by a signal "AMP_LMT"

Updated:

If the "AMP_LMT" signal goes to 0V, it will turn off Q1096 & subsequently the series pass Q1009 i.e. shutting down the regulator.

Updated:

So if possible, the "AMP_LMT" voltage should be measured. It's on W1041 pin 3. Component overlay - Main(3) (page 116 with page co-ordinates C6)

Hmmm.  W1041 isn't something I have access to since MAIN(3) is the guy buried at the bottom of the stack, but fortunately that cable just runs up to DIGITAL(1), the very top board, which I can access easily @ CB909, which shows 1.8v on AMP_LMT.  The manual says that's a 2N5551C-AT and the datasheet for that says it's in saturation at 1v, which I would say points to something wrong on the MAIN(3) board itself.

On the other hand, the schematic says the base of Q1096 should be 3.1v, not 1.8v.

EDIT: Probably sleep time for me soon, but again, thanks again for all the help. 

[alanep]

Yes, I did update the previous post to include CB909. What is the etiquette when a post is modified i.e. do you add a new post saying the previous was modified or should the new post quote the previous post?
EDIT: Ok I get the gist (use EDIT).

1.8V is not good. It comes straight out of IC908 pin 4 (the main controller IC page 132) i.e. the IC is faulty, something is telling it to power limit or:

There is one other small possibility that pin 4 is being driven low (unintentionally) by a fault with Q1096. There's still something strange, even with 1.8V (in lieu of 3.1V), the positive regulator should keep same the ratio i.e. 3.1V reference produces 56V, then 1.8V should produce 37.8V (which it's not doing). A short explanation: whatever voltage is on Q1096 base should also be on Q1097 base (differential pair). The voltage on Q1097 base is determined by the ratio of R1283 and R1284 (in voltage attenuator mode 10K/(200K+10K) = 0.0476). Another way of looking at it is the regulator output should be approx. 21 times the voltage at the Q1096 base.

Anyhow, time for a bit more of a think about it & for you, to get some sleep.

[alanep]

I'll be away for the next 24 hours.

Another thought (for another day): if Q1096 had an open circuit collector, Q1009 wouldn't turn on (positive regulator output voltage would be near 0V) and more importantly, it would load down IC908 pin 4 (AMP_LMT) with R1282 (560ohm) less the Q1096 Vbe (which is a lot). This could explain the lower 1.8V.

Q1096 is probably in the bowels of the amplifier but it might be worth removing and testing. Do you have anything to check transistors? You can use a multimeter (digital on diode mode, analogue on x10 resistance range) to check the base to emitter & the base to collector junctions (they behave like diodes in these tests). I'm hoping the base to collector junction is open circuit. Note, the x10 resistance range on an analogue multimeter is to keep the current down - the x1 resistance range on a 20Kohm/volt meter passes something like 150mA full scale which may blow the junction.

I did a quick calculation on Q1096's power dissipation and it's about 140mW i.e. still within its 625mW rating but it's getting up there as far as dissipation in free air. Maybe, just maybe Q1096 might be faulty...

EDIT:
Possibly another way of testing Q1096 without having to remove it. The idea is to deliberately feed it with 3V. You'll need to disconnect the IC908 pin 4 (ARM_LMT) wire from CB909 & use the wire to feed 3V (maybe 2 x 1.5V batteries) down to Q1096's base. The negative end of the 3V source should be connected to the chassis. If the regulator doesn't turn on then Q1096 is most likely faulty (or something else is faulty in the regulator) & you will then have to go into the bowels of chassis to fix it.

Also if the regulator does turn on, keep it as brief as possible as it might have an actual over current problem (when the driver stage powers up).

[shakalnokturn]

If you go to the trouble of disassembling also check R1008, R1009 (they are fuses), R1283, R1285, R1288 just because high value resistors tend to go open with such voltages.

[alanep]

If you go to the trouble of disassembling also check R1008, R1009 (they are fuses), R1283, R1285, R1288 just because high value resistors tend to go open with such voltages.

Really good point shakalnokturn, with R1009 open, there won't be any voltage on Q1096's collector which will cause loading of the ARM_LMT line (similar to a blown collector junction).

If R1009 was open then finally, the following would also make sense (no ratioing):

There's still something strange, even with 1.8V (in lieu of 3.1V), the positive regulator should keep same the ratio i.e. 3.1V reference produces 56V, then 1.8V should produce 37.8V (which it's not doing).

So maybe bite the bullet, skip the testing of Q1096 and go straight to R1009 - all the dots do seem to be lined up (& while you're in there, check R1008, R1283, R1285, R1288 as per shakalnokturn's suggestion).

EDIT:
If R1009 is blown, it begs the next question: why did it blow? Maybe thermal cycling or is there another fault?

R1008, R1009 are 10ohm 1/4 watt fuse resistors i.e. 158mA through them (dropping 1.58V) will hit the 1/4 watt dissipation. Wait & see?

[NHcoder]

Ok guys, thanks.  I'm going to just tear it apart again and check the resistors & transistors on MAIN(3) and will let you know how it all goes.  More detailed reply follows.

Q1096 is probably in the bowels of the amplifier but it might be worth removing and testing. Do you have anything to check transistors?

Yeah, it is, but I do a lot of hobbyist digital work so I should be able to test the transistor safely in circuit.

Possibly another way of testing Q1096 without having to remove it. The idea is to deliberately feed it with 3V. You'll need to disconnect the IC908 pin 4 (ARM_LMT) wire from CB909 & use the wire to feed 3V (maybe 2 x 1.5V batteries) down to Q1096's base

I thought of this too but the risks seem a bit too high for now, and CB909 is in a package that's at the very edge of my ability to reliably solder individual pins on.

If you go to the trouble of disassembling also check R1008, R1009 (they are fuses), R1283, R1285, R1288 just because high value resistors tend to go open with such voltages.

Will do.  I'll check everything I (or you two) can think of while it's apart before putting it back together.  If that means checking every discrete component, so be it.

So maybe bite the bullet, skip the testing of Q1096 and go straight to R1009 - all the dots do seem to be lined up (& while you're in there, check R1008, R1283, R1285, R1288 as per shakalnokturn's suggestion).

It's all on the same board.  Once it's out, no reason to not test all of it.   Enjoy your day out in the sun, if you can find any!

[NHcoder]

Small update.  Went through some of the testing mentioned already and results are as follows.

• R008 and R009 both seem fine.  Good connections, no discoloration or anything like that.  R1008 reads 10 ohms, R1009 9.9 ohms.
• R1283, R1285, and R1288 are difficult to test in circuit but seem like they're probably ok as well.  The results I got are consistent with an "ok" resistor in parallel with a bunch of other crap, in my experience.  R1283 was very spotty read but seemed to be around 100k, stated value is 200k.  R1285 read 65k, stated value 100k, and R1288 read 118k, stated value 100k.
• Q1096 seems fine as well.  At first this one confused the hell out of me but it turns out it (2n5551c) just has a weird pinout. where the center pin is the collector rather than the gate.  I got 0.7v drops on both base-emitter and base-collector, and nothing on all other combinations.  This satisfied me personally but if you think I need actual functionality, I can do so.

The receiver is in pieces now so can't be tested in a powered on state, but everything is more or less accessible for offline testing.  I'm going to take a break from this for a bit and come back to it later this afternoon or evening.

Once again I appreciate all the assistance. 

[alanep]

This is a slippery one. From my point of view, it is behaving like the regulator is not receiving power hence the focus on R1009. Can you continuity check that one side of R1009 is connected to B+ (same positive supply as output transistors)?

Not sure if B+ was actually tested i.e. the main +56V rail.

I'm away now for 10 hours or so (hiking/bushwalking).

[NHcoder]

This is a slippery one. From my point of view, it is behaving like the regulator is not receiving power hence the focus on R1009. Can you continuity check that one side of R1009 is connected to B+ (same positive supply as output transistors)?

Not sure if B+ was actually tested i.e. the main +56V rail.

I'm away now for 10 hours or so (hiking/bushwalking).

I believe the B+ connection is ok, I tested continuity between R1009 to J1001 which is nearby, and then to a pin on another test port or something on MAIN(3) called CB109.  It's clear on the other side of the board, and is the path R1009 takes to one of the big caps, C1105. 0.0 ohms all the way.

[NHcoder]

I messed around with this a bit more today and with it all apart I thought I'd come at it from another angle, checking for good power at every stage starting from the main board where the mains comes in.  We have been having what I would call "bad luck" trying to trace backwards from the outputs to their supply so I thought it might be worthwhile to try the other direction.  I know that this board (POWER(1)) must be at least mostly good because it runs basically all the digital stuff including the display and power button, but I wanted to check the output of the power transformer since I have access to it now.  This hasn't gone as planned unfortunately so I think I'm missing something on that schematic.

When mains is present, the 5.5v output on POWER(1) comes alive and feeds a currently disconnected output port (W2001).  I used this 5.5v supply to feed a 3.3v linear regulator on a breadboard, and fed the 3.3v back in on the neighboring connector W2505, to the +3.3M, ACPWR_DET, and MCPU_N_RST pins where it should be present if the other control boards were hooked up.  I then tried activating the transistor Q2004 with 3.3v through a 1n4001 on the PRY pin.  This pin says it takes 2.7, which itself comes from a diode at the 3.3v output of an IC on another board.  I figure the 1n4001 would be ok as it has a drop of about 1.1v, but maybe 2.2 isn't quite high enough.  I was hoping to hear an audible click from the relay that Q2004 is controlling, which activates the main power transformer, but got nothing.

Going to get some sleep now, maybe try a diode with a smaller drop tomorrow, or try the 3.3v direct.  I believe the datasheet for Q2004 said up to 5v on the gate is ok, but I'll have to double check.  Attached an image of the power board schematic I'm working from.

[cantata.tech]

I had the same problem with my Yamaha receiver.

Unfortunately my patience was less and I decided to break the problem into two logical parts.



Obviously there is more to this story than just being clumsy. I just gave up on Yamaha amplifiers and switched to a different brand.

I will use the Amplifer and Power section somewhere else.

[alanep]

I'm back from the bushwalk so I'm a bit stuffed but I did think of a few things.

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

Firstly, in relation to turning on the power transformer:

I then tried activating the transistor Q2004 with 3.3v through a 1n4001 on the PRY pin.  This pin says it takes 2.7, which itself comes from a diode at the 3.3v output of an IC on another board.  I figure the 1n4001 would be ok as it has a drop of about 1.1v, but maybe 2.2 isn't quite high enough.  I was hoping to hear an audible click from the relay that Q2004 is controlling, which activates the main power transformer, but got nothing.

You could put 3.3V or more (see below) on the Power (1) PRY pin (W2505) as long as the cable is disconnected from the connector (so it can't back feed to whatever is driving it). Q2004 base current is limited by R2050 (page 139) i.e. it's not critical although it's best not to go lower than the original 2.7V or higher than say 5.5V. The Power (1) board has got mains potential so please be careful.

To check that RL201 is operating, I can think of a couple of options:

1. You could measure Q2004's collector voltage which would also indirectly measure the relay coil voltage.
With PRY 0V, the Q2004 will be off & its collector voltage should be 5.5V (pulled up by the relay coil) & since both RL201's coil connections are at 5.5V, it will be de-energised (no current flowing through the coil). With PRY 3.3V or higher, it's collector voltage should drop to 0V i.e. one connection of RL201's coil will be at 5.5V and the other connection (Q2004's collector) will be 0V i.e. in theory, the relay will be energised.

2. Find a reasonably safe low voltage secondary and check it with a multimeter on the ACV range (must be on AC volts). The Video (2) board (page 142) has transformer secondaries on CB351, CB352 & Video (3) C351 (also on page 142).

Changing subjects, I assume the amplifier is still disassembled?
I had a think about this apparent loss of voltage to Q1096. In addition to R1009 (which frustratingly is OK & you confirmed it is definitely physically connected through to C1105), there's a couple of other possibilities:
1. R1280 - a 5K6 resistor on Q1096's collector, could you check if it's open circuit? If it's open, Q1096 won't get power & it also won't turn on Q1009.
2. Q1009 - check for the base - emitter diode (& might as well check for base - collector diode). If B-E open same story Q1096 won't get enough power.

Hopefully, Murphy will be kind... I'm off to bed.

[NHcoder]

I had the same problem with my Yamaha receiver.

Unfortunately my patience was less and I decided to break the problem into two logical parts.

(Attachment Link)

Obviously there is more to this story than just being clumsy. I just gave up on Yamaha amplifiers and switched to a different brand.

I will use the Amplifer and Power section somewhere else.

Haha ok.  I don't buy them often enough to have any brand loyalty, but I have had this one for ten years or more and it's worked great the entire time until very recently.  One could say I got my money's worth out of it, but I don't think that's a reason to at least not *try* to revive it.  I have a soundbar w/ satellites & a sub filling in now and while it works, it's just "ok" and no match for a real receiver.

I'm back from the bushwalk so I'm a bit stuffed but I did think of a few things.

Great, hope you had a good time.

Firstly, in relation to turning on the power transformer:

I then tried activating the transistor Q2004 with 3.3v through a 1n4001 on the PRY pin.  This pin says it takes 2.7, which itself comes from a diode at the 3.3v output of an IC on another board.  I figure the 1n4001 would be ok as it has a drop of about 1.1v, but maybe 2.2 isn't quite high enough.  I was hoping to hear an audible click from the relay that Q2004 is controlling, which activates the main power transformer, but got nothing.

You could put 3.3V or more (see below) on the Power (1) PRY pin (W2505) as long as the cable is disconnected from the connector (so it can't back feed to whatever is driving it). Q2004 base current is limited by R2050 (page 139) i.e. it's not critical although it's best not to go lower than the original 2.7V or higher than say 5.5V. The Power (1) board has got mains potential so please be careful.

This is exactly what I have been trying to do and it just doesn't seem to come on.  I don't hear a click and am not getting AC voltage on the transformer outputs as measured @ the 6 pin connector as described on page 138 for the MAIN(3) board. 

To check that RL201 is operating, I can think of a couple of options:

1. You could measure Q2004's collector voltage which would also indirectly measure the relay coil voltage.
With PRY 0V, the Q2004 will be off & its collector voltage should be 5.5V (pulled up by the relay coil) & since both RL201's coil connections are at 5.5V, it will be de-energised (no current flowing through the coil). With PRY 3.3V or higher, it's collector voltage should drop to 0V i.e. one connection of RL201's coil will be at 5.5V and the other connection (Q2004's collector) will be 0V i.e. in theory, the relay will be energised.

2. Find a reasonably safe low voltage secondary and check it with a multimeter on the ACV range (must be on AC volts). The Video (2) board (page 142) has transformer secondaries on CB351, CB352 & Video (3) C351 (also on page 142).

The power board is one I have left mounted in the receiver this entire time but with it mounted I don't have access to the area where the relay or transistor are located.  I am pulling that board out now and intend to just test the relay functionality with my meter and bench supply to make sure it's working.  If it is, I'll investigate the board itself more closely, check the transistor, etc.

Changing subjects, I assume the amplifier is still disassembled?

Indeed.  The top four boards in the stack are entirely removed and sitting in a box.  All that remains in the chassis right now are the transformer, the two POWER boards, and  MAIN 1, 2, and 3.  The front panel control/digital boards are still there but entirely disconnected except for a small board I haven't identified that is hard wired into the transformer output.  Pretty much everything that can be disconnected is disconnected, including all the connections to the digital boards you mentioned above.  Those boards are safely in a pile in a box across the room

I had a think about this apparent loss of voltage to Q1096. In addition to R1009 (which frustratingly is OK & you confirmed it is definitely physically connected through to C1105), there's a couple of other possibilities:
1. R1280 - a 5K6 resistor on Q1096's collector, could you check if it's open circuit? If it's open, Q1096 won't get power & it also won't turn on Q1009.
2. Q1009 - check for the base - emitter diode (& might as well check for base - collector diode). If B-E open same story Q1096 won't get enough power.

I'll check all of this as soon as I do my quick tests on the relay and Q2004, good ideas.

Hopefully, Murphy will be kind... I'm off to bed.

All evidence to the contrary so far   Thanks for the reply, maybe you'll awake with a eureka! moment.

[NHcoder]

Ok, the relay is working fine and clicks loudly when I apply 5.5v from my bench supply.  I had my meter's continuity check connected to the other side to make sure there was actually an electrical connection being made as well.

Q2004's diode checks were also fine, 0.7v on BC and BE.  The emitter-collector path also has a 0.7v drop which worried me for a minute, but the schematic shows it's probably fine.  It looks like a path from Q2004's emitter to the ground plane, up through D2015 and then Q2003's BC junction and back into the relay coil and thus the collector via the R2538 shunt resistor.

The protection fuse to the transformer is ok.

I had a think about this apparent loss of voltage to Q1096. In addition to R1009 (which frustratingly is OK & you confirmed it is definitely physically connected through to C1105), there's a couple of other possibilities:
1. R1280 - a 5K6 resistor on Q1096's collector, could you check if it's open circuit? If it's open, Q1096 won't get power & it also won't turn on Q1009.

"Unforutnately" tested fine, 5.54k.

2. Q1009 - check for the base - emitter diode (& might as well check for base - collector diode). If B-E open same story Q1096 won't get enough power.

Base-emitter was ok, 0.6v, but base-collector is strange, showing about 3v.  After checking this I realized that plugging the receiver back in and attempting to get the relay to switch on, I had recharged those massive caps, and they are probably interfering with this reading.  The test points for discharging them are on the bottom of the PCB, let me know if I need to pull it back out to discharge the caps and check this again.

I'm going to take another break from this now and spend time thinking about something else.  After that I'll be back to trying to convince the relay to turn on via the W2001 connector to check the transformer output unless you or someone else has a better idea.  I suspect the transformer is fine, but testing in stages from the source like this without all the other boards connected and in the way seems like it would be beneficial.

[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.

[alanep]

FYI, Q1105 and D1081 test out OK

I'm being a bit pedantic, did that test also include D1080?

Maybe check D1080, D1081 as well.

[NHcoder]

FYI, Q1105 and D1081 test out OK

I'm being a bit pedantic, did that test also include D1080?

Maybe check D1080, D1081 as well.

Yessir, my mistake for being imprecise.  I checked both, both are good.

[NHcoder]

Short update, the replacement transistor arrived today at last.  Installed and checked, weird output channel DC voltage is gone, so I'm starting the reassembly.  Hopefully all goes well, so many wires and screws to put back in now.  Fingers crossed.

EDIT: Bleh.  Still a no go.  I got one "I PRT" (current protection) and since then, "DC 0" again.  So maybe something went overcurrent, blew that transistor.  I'll pull it all apart and start checking things again in the next few days.

EDIT2: A little more testing today, showing -6vdc on all the outputs on MAIN(1) now.  More checks later, been busy lately.