Announcement

Collapse
No announcement yet.

Logitech Z5500 power, but no sound

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    Logitech Z5500 power, but no sound

    Good day folks. This is going to be a long story so please bare with me.

    TLDR: One of the infamous Z5500 sets powers on fine but has no sound AT ALL, not even on the headphone jack. Need a bit of help understanding the schematic and principle of operation of the mute/stby circuit. Attached "original" and edited schematic. Control pod might be bad.

    Long version: replaced a blown TDA7294 in the sub and now the set powers on but has no sound at all. The reason I believe the pod may be bad is because I don't get sound on the headphone jack either ! There's no info on this pod whatsoever, much less so a schematic, other than reports of a CS42526 IC going bad and causing lack of sound among issues. There's no way I can confirm, because I don't have a replacement pod. The pod itself appears to be a more recent revision than most people have because it doesn't have a "power" button, but rather a "headphones" button in its place - it never turns off. It's always on as soon as you connect the power ! I found a schematic of the sub (amp) part and I'm trying to at least understand how the mute and stby pins for the TDAs are supposed to work. It could be the amps are turning on just fine but the console is not sending any audio into them, so I was hoping I could turn on the amps and feed my own audio to at least confirm the TDAs are working before hunting down a replacement pod.

    Looking in the bottom-right, I highlighted in red/green/blue the three wires which trace back to pins 9/10 respectively but can't understand the operation, especially since mine is not functional at this point. From what I see, there's two separate mute circuits: one for one TDA and the other for all the rest - not sure why, but I see they're identical. AFAIK: Q104 and Q100 are identical in terms of setup and operation: when their base is pulled high, the collector goes low (pulled to GND) - this mutes the channels.....trouble is I don't see how the channels UNmute after that :| Even if Q104/Q100 are no longer receiving any base current, there's no pull-up resistor to feed voltage in the mute pins to get them to UNmute. Either that, or I'm missing something. It could be they're referenced to -Vs and not GND, but I'm not sure how to measure this. I tried it with my negative lead on both -Vs and GND and I get 22v on the mute pins when using -Vs for my meter's black probe and around 2-3v when using GND as the reference.

    Same story for Q101: pull its base high and the STBY pins get pulled to GND via the C-E path of Q101. It's not clear how it turns back ON then....even after the fact, it would mean that all 3 transistor should be kept OFF at all times to get the system to be "ON", however, it seems everybody goes on about feeding 5v into pin 8 of the DB15 port, which corresponds to the base of Q104, which would mean that mute pin gets pulled to GND, which if my theory is correct would have the opposite effect and mute the TDA....what am I missing ?

    I did make a discovery: Q100 is actually shorted B to C...not sure how that affects the functionality, but I'll replace it and see what happens. My money is still on the pod....
    Attached Files
    Wattevah...

    #2
    Re: Logitech Z5500 power, but no sound

    UPDATE: replaced Q100 and it thankfully didn't blow again, but had no effect either - still no sound and rightfully so, because the STBY pins manage around 3.1v which should be just about enough (based on the graph on page 12 of the TDA7294 datasheet), but the mute pins are still stuck at 1.5v...
    Wattevah...

    Comment


      #3
      Re: Logitech Z5500 power, but no sound

      UPDATE 2: I think I got it all figured out now. All mute pins are connected through some jumper wires to the "bus" (the collector of Q100). This allowed me to very easily snip those jumpers right in the middle to separate pin 10 of every TDA from the collector of Q100 to see if the voltage goes up as it should with no load at all and sure enough it did ! Without any of the amps connected to Q100 through pin 10, I now got around 5v on on the collector of Q100 ! This brought me to the conclusion that for some reason one or more TDAs are drawing too much current on pin 10 (MUTE), causing the entire rail to sag.

      To confirm it's indeed one of the amps to blame and not the rail itself not delivering enough current, I used my meter probe to briefly bring the ends of each bridge that I just cut back together, which allowed me to monitor the voltage at the same time and this is what happens: two of the TDAs driving the rear channels are fine - they cause no drop in voltage on the mute rail at all. I even played some music through them at this point and it works, so the control pod is also functional, voiding my original diagnosis about it being faulty. The center channel is also fine: it's got its own separate mute circuit (Q104) but I broke its bridge to pin 10 too just to confirm. Same story: voltage stays at a constant 4v which is more than enough to unmute it and get it to play - you just have to select a different "effect" on the console to get it to unmute.

      The remaining 4 amps for the front channels and the subwoofer are what's causing issues, because when I connect any of them to the mute rail, the voltage drops down to around 2.5v - never mind actually connecting all at the same time, which further lowers it all the way down to the original 1.5v I was initially getting.

      Ironically, measuring the resistance between GND and the mute pins gives me a higher reading on the supposedly faulty ICs than on the 3 that work: it's somewhere in the mega-ohm range for the former and in the Kohm range for the latter :| I was expecting it to be lower on the failed ones.

      Guess I have no choice but to get some replacements and see if my diagnosis is correct....no point swapping the functional ones with the busted ones because the connections are identical and would prove nothing...
      Last edited by Dannyx; 09-30-2021, 05:34 AM.
      Wattevah...

      Comment


        #4
        Re: Logitech Z5500 power, but no sound

        Before replacing the ICs that appear faulty / to be pulling the MUTE/VSTB lines low, perhaps try pulling up these lines to a positive voltage via a resistor to see what happens. I'm guessing something like 1-10k resistor and 5V source should work here. TDA7294 datasheet lists 3.5V as the minimum needed to bring the IC out of mute and STBY operation. Also, the sample circuit in the datasheet shows 10k and 22k resistors on the MUTE and STBY pins, respectively. But on the Logitech Z5500 schematics, I see a more complicated network with 100 Ohm resistors, diodes, and other components. So perhaps try disconnecting each "bad" TDA7294 IC's MUTE and STBY pins from the rest of the circuit, then feed those pins directly with 5V via 10k or 22k resistor, as specified by the datasheet to see if the ICs actually work. If they do, I think the problem may be somewhere in the MUTE/STBY circuit. I didn't bother tracing the circuit around Q100 yet (uploaded schematics image is way too tiny), but I can see some electrolytic caps around it (4.7 uF to be exact). Check the leakage and ESR of those electrolytic caps. If they are bad, that could be why the circuit is struggling.

        Comment


          #5
          Re: Logitech Z5500 power, but no sound

          Thanks for the time to reply first of all

          Yeah, for some reason the schematic shrunk here. I just noticed this myself, despite being larger and perfectly readable when I first downloaded it. Here's the direct link to it - hope it opens fine on your end because it's the only schematic of this system I could find and funnily enough it's from a local electronics forum where I'm logged in, possibly giving me access to stuff which may not be visible as a guest.

          I too was thinking of the trick of supplying the mute voltage externally but I was afraid something might pop and ruin my whole progress so far. I guess I should

          If you can see the schematic better now, it's a pretty weird setup indeed and it goes a bit over my head, that's why I was hoping someone with more experience could set me straight. D105 in the lower-right where the 3 transistors are is what I don't get: for one it doesn't have a part number in the schematic - presumably who drew it up couldn't figure out what it is and I didn't feel like pulling it off my board to inspect it up close because it's covered in hot melt. That aside, buzzing it with my meter, it's indeed reverse-biased, with the cathode connected to the +18v rail and the anode feeding the 3 control pins (2 mutes and 1 STBY). I was expecting this to be the other way around so 18v would go into the control pins when the transistors are "off", thereby pulling up the MUTE/STBY pins respectively. Somehow, in practice, this setup does indeed results in 5v appearing on the anode part - the key word being SOMEHOW....could this be like the breakdown voltage of the diode or something ??? How does this even work ??? It's this voltage that is then switched on/off by the 3 transistors to control the MUTE/STBY pins, though in the case of this particular control pod which doesn't have a soft power button (has the headphone button instead), the middle transistor, Q101 never actually turns on to pull the STBY pins low, so the TDA ICs are always on because the pin driving the base of Q101 is never asserted high by the pod...no problems there.

          Inserting a jack into the headphone jack on the side of the pod does indeed mute all the amps by asserting the bases of Q100 and Q104 high and pulling MUTE low.....but then again so does pushing that headphone button AND the dedicated mute button directly above, so it's like having 2 mute buttons and no power button...WTH Logitech ???? WHY ??? :|
          Wattevah...

          Comment


            #6
            Re: Logitech Z5500 power, but no sound

            Another idea is that D105 is a 5v zener diode but it's not marked as such....but then again, the anode is not connected to GND, so how would it regulate ? The only way it would be able to regulate anything would be if its anode was more negative than the cathode (if I know my zener basics), which in this case is not really possible, other than maybe through the pod itself when it pulls one of those 3 base control pins low. Still, the resistor values are pretty high for a zener regulator IMO, though I haven't done the math.

            If C140 was a piece of wire, then yes, it would make perfect sense: GND is on the other side and R116 would be the series resistor, but that cap means DC can't pass through, so I'm not entirely sure how the circuit operates and where those 5v come from. 13v are being dropped somewhere....
            Wattevah...

            Comment


              #7
              Re: Logitech Z5500 power, but no sound

              Alright, I finally took a look at the circuit. Man, what a cringe that was to look at. I mean, whoever drew this diagram... props for the effort and all, considering it was drawn from looking at the boards. But the way the software drew some of those lines, it was really hard to trace them. Still helpful, though. Anyways... now we here.

              So first thing's first:
              Originally posted by Dannyx View Post
              Another idea is that D105 is a 5v zener diode but it's not marked as such....but then again, the anode is not connected to GND, so how would it regulate ?
              Yes, D105 has to be a Zener diode.
              And since it's supplied by the 18V rail on the Cathode and you measured 5V on the Anode side, that means it must be a 13V Zener diode.
              No, a Zener diode does not need to have it's Anode connected to ground to regulate. Rather, just think of it as a device with fixed voltage drop: in the forward direction, it acts like a regular diode and drops approximately 0.6-0.7V, while in the reverse direction, it drops whatever "fixed" Zener voltage the Zener is rated for. Of course, still keep in mind that it's a Zener diode and not a voltage source of any kind. So for example, if it's a 9V Zener and you connect the Cathode to a 5V source, the Anode won't necessarily measure -4V. It can... if there is something connected to the Anode to pull the voltage below 0V. But in a circuit with only a 5V source and ground, then the Zener diode won't do anything.

              In the case of the Z5500 circuit where D105 is...
              Cathode is connected to the 18V supply while the Anode is kind of "floating" about... until resistors R116, R119, and R121 draw current and pull it lower. If they try to pull it below 5V, they won't be able to, because D105 is a 13V Zener, therefore dropping that 18V supply by 13V and "producing" 5V on its Anode.

              That being said, let's focus on resistors R116, R119, and R121 and NPN transistors Q100, Q101, and Q104, and the other components surrounding these. Resistors R116, R119, and R121 pull the Collectors of Q100, Q101, and Q104, respectively, up to 5V (supposedly) from the 5V rail produced by Zener D105. However, in the event that DB15 connector is disconnected... or if the control pod is keeping pins 7, 6, and 8 at a high impedance.... then the Bases of Q100, Q101, and Q104 are then left connected with only two biasing resistors to the 5V rail produced by Zener D105. Take Q100 for example: 5V goes through a 200k resistor (R124), then 10k resistor (R116), and into the Base of Q100. Because the Emitter of Q100 has a relatively low resistance to ground (only 100 Ohms through R132), that means some Base current will flow through Q100. More precisely, 5V/210k = 23.8 nA. Even with a minimum gain (hFE) of 20, that will still result in at least 0.4 mA of current to flow into the Collector. In reality, the gain will be higher, so the Base resistors of Q100 will try to push even more current through the Collector. Meanwhile, R116 being 10k will allow a maximum of 0.5 mA from the 5V rail. Thus, more than likely, all of the current that can be supplied by R116 will be "taken back" by the Collector of Q100 and sent to ground, resulting in nearly 0V to be present on the Collector of Q100. And with 0V on Q100 Collector, Mute won't be in either the On of Off states. Same goes for Q101 and Q104... suggesting all of the amplifiers should be OFF.

              Of course, all of the above is assuming the drawn circuit you attached is correct. If it is, I'm honestly a little worried and unsure about how the whole Mute/Stby circuit works. It seems that then the control pod would just have to vary the resistance/impedance on pins 7, 6, and 8 of the DB15 connector in order to control the Off/Stby/Mute/On states... which would be an extremely sketchy way to do it, because the above circuit's operation of the transistors will greatly depend on their DC current gain... which in itself can vary quite a lot with temperature and between batches of transistors.

              But I think you are correct in assuming that when the Bases of transistors Q100, Q101, and Q104 are pulled to ground, then that is the only time these transistors will be fully turned off and the Mute/Stby circuits getting pulled up to 5V via resistors R116, R119, and R121.

              ... which moves us to the next part: resistor R17 in the Stby circuit. It's given to be 100k - same as R20, also 100k. Now I think that has to be a mistake on the schematic... because if 5V is being sent to one side of R17 (assuming Q101 is being turned completely OFF), then only 2.5V will come out between R20 and R17, since they form an equal voltage divider between ground. And with 2.5V, how exactly would amplifiers U1-U7 come out of Stby? If anything, 2.5V will make them sit exactly in Stby mode, since TDA7294 data sheet says Stby pin will activate the Standby function of the amplifier if it falls between 1.5V and 3.5V. So something isn't quite right here in the schematics.

              Therefore, I think it might be a safer bet to go with my original suggestion of pulling each amplifier's Mute and Stby pins "manually" with 5V directly at the IC's pins with a 1-10k resistance, if possible.

              To (hopefully) prevent any major damage on any of the amplifier ICs, consider using an incandescent light bulb in series with the PSU's input. Obviously the charged caps on the output of the PSU could still damage some parts if something is wrong, but the damage will be much more limited, since at least there won't be an "unlimited" power from the wall.

              And oh yeah, while talking about corrections to the schematic...diode D10 near U10 (LM217 regulator IC) is most definitely not drawn correctly. The schematic shows it dumping power/current flow from the output of the LM217 regulator right into ground. I think D10 is more likely connected between the input and the output pins of the LM217 regulator, with the Cathode connected to the output pin. A lot of linear regulators will have a diode connected like this for protection.
              Last edited by momaka; 10-08-2021, 10:40 PM.

              Comment


                #8
                Re: Logitech Z5500 power, but no sound

                I can only thank you for going through all that trouble This may very well be the only analysis of this circuit out there, as I couldn't find a similar one so far !

                Yes, there may be some errors in the schematic (see that diode on LM317, which I spotted myself a mile away), but still heaps better than none at all - saved me a lot of headaches trying to track down what is what's what.

                I'll try using a 5v power adapter to supply those mute pins. Maybe place a high-ish resistor (200k ?) in series with it to limit any possible excess current - sick of repairing blown up tracks on the amp board

                Assuming the "faulty" TDAs magically start working with this external supply, would that make the zener diode bad ? What would cause that 5v supply to "sag" as if overloaded ? Although the datasheet for a TDA7294 doesn't offer too much info on the MUTE pins, I assume they're very low current and shouldn't sink too much.....unless the IC is internally faulty somehow :| It might also mean the mute circuit is somehow blown, despite the amp part being fine ??? Who knows....
                Wattevah...

                Comment


                  #9
                  Re: Logitech Z5500 power, but no sound

                  Fortunately my theory was right: the ICs are faulty somehow. I just replaced two more today and the mute voltage is at a stable 3.8v overall. I had two of those TDA7294 evaluation kits lying around so I just pulled the ICs off of those and installed them in the Logitech, since they weren't seeing any use whatsoever. I just need two more to get the sub going too and hopefully it doesn't f-up at the last hurdle and ruin my progress so far
                  Wattevah...

                  Comment


                    #10
                    Re: Logitech Z5500 power, but no sound

                    Originally posted by Dannyx View Post
                    Fortunately my theory was right: the ICs are faulty somehow. I just replaced two more today and the mute voltage is at a stable 3.8v overall. I had two of those TDA7294 evaluation kits lying around so I just pulled the ICs off of those and installed them in the Logitech, since they weren't seeing any use whatsoever. I just need two more to get the sub going too and hopefully it doesn't f-up at the last hurdle and ruin my progress so far
                    Sucks you live in Romania, if My 5500's ever go out I wanted to send it to you but that shipping is hell
                    Cap Datasheet Depot: http://www.paullinebarger.net/DS/
                    ^If you have datasheets not listed PM me

                    Comment


                      #11
                      Re: Logitech Z5500 power, but no sound

                      Originally posted by Uranium-235 View Post
                      Sucks you live in Romania, if My 5500's ever go out I wanted to send it to you but that shipping is hell
                      Why send them to me in the first place ?
                      Wattevah...

                      Comment


                        #12
                        Re: Logitech Z5500 power, but no sound

                        Originally posted by Dannyx View Post
                        Why send them to me in the first place ?
                        Cause they're expensive and it seem you have experience fixing t hem
                        Cap Datasheet Depot: http://www.paullinebarger.net/DS/
                        ^If you have datasheets not listed PM me

                        Comment


                          #13
                          Re: Logitech Z5500 power, but no sound

                          Actually this is the first set I dig into
                          Wattevah...

                          Comment


                            #14
                            Re: Logitech Z5500 power, but no sound

                            more than I have
                            Cap Datasheet Depot: http://www.paullinebarger.net/DS/
                            ^If you have datasheets not listed PM me

                            Comment


                              #15
                              Re: Logitech Z5500 power, but no sound

                              I just replaced the last 2 remaining ICs (for the sub) and it all seems to be working. The amps seemed to get VERY hot when I was testing them, even when sitting idle, but maybe that's normal since they had no heatsink on them whatsoever at that stage. The 8v regulator also gets hot...seems this thing likes to run hot overall. I was thinking of adding a fan, even though the ICs are inside the box and adding a fan on the inside would not be a bright idea given there's also a speaker there

                              Also, rather than facing the horrors of more thermal paste again, but still needing to get the ICs to contact the heatsink, I just used thermal pads alone, without any paste....not sure if this yields better, similar, or worse results than the original setup, which had mica pads and was also absolutely COVERED in thermal grease - even on the front of the ICs ! WTF ?!

                              Looking past that, I ran some very short tests with only one speaker attached to one channel at a time to ensure I am actually getting sound in all channels and I thought I still had some problems left, because only 1 channel seemed to sound decent and loud enough - the others were tinny and VERY quiet, so I had to turn up the volume almost all the way to even hear anything at which point I also started getting noticeable hum ! I then tried a different male-male audio jack and ironically enough now all my channels suddenly started working loud and proud....hope that's it.


                              I haven't packed it up yet to give it a proper test because I need to come up with a way to connect the wires from the transformer back to the board. Not wanting to take the transformer out of the box and because 5 of the wires have no removable connectors (counting the wires for the speaker), I chopped them in the middle and used some screw terminal blocks to tie them back together and make them removable later on if I need to take the board off again. I thought I'd just leave it like this, trouble is, I'm worried the screws may come loose overtime due to the vibrations inside the box, so I'm thinking of using Wago terminals which I don't have yet, so it's on hold for a while until I grab some....
                              Wattevah...

                              Comment

                              Working...
                              X