Technics SU-v505 amp problem/discussion

[momaka]

Re: Technics SU-v505 amp problem/discussion

No short circuit does *not* mean a transistor is okay. In particular, BJTs can go open-circuit too.

So when you're doing these measurements, I think it would be more helpful for both us and you to post resistance/diode readings in the following way
Q___: B-C xxx mV, B-E yyy mV, E-C zzz mV (though ideally o/c)

And again, I suggest pulling most of the components mentioned in post #28 above and checking them out of circuit rather than comparing each channel to the other. That's just not a reliable way to check things. Some circuit designs just don't lend themselves to checking certain components in circuit, no matter how many identical working and non-working units you have.

Otherwise, that's all I have for you. Without the above measurements, I'm pretty much all in the dark as to what's good and what's bad. It's not that I don't trust your word/knowledge, but to be 100% honest, I really dislike it when people tell me some component is reading as "good" without posting any measurement values. So for the sake of being on the same page, I always think it's helpful when people post measurements of their components.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

This was one mother of a job, but here we go, let's do this. Indeed, doing it piece by piece did reveal something which may be of interest. There's an elephant in the room, so to say, in that list. I'm sure you can see it even at a superficial glance.

Q301 B-C 595 B-E 664 E-C open
Q302 B-C 658 B-E 662 E-C open
Q303 B-C 620 B-E 628 E-C open
Q304 B-C 585 B-E open E-C open
Q305 B-C 601 B-E 624 E-C open
Q306 B-C 589 B-E 623 E-C open
Q307 B-C 627 B-E 653 E-C open
Q308 B-C 603 B-E 636 E-C open
Q309 B-C 529 B-E 546 E-C open
Q310 B-C 541 B-E 551 E-C open
Q311 B-C 514 B-E 530 E-C open
Q312 B-C 552 B-E 563 E-C open
Q313 B-C 507 B-E 509 E-C open
Q314 B-C 506 B-E 508 E-C open
Q315 B-C 472 B-E 484 E-C open
Q316 B-C 472 B-E 486 E-C open

[momaka]

Re: Technics SU-v505 amp problem/discussion

Yup, pretty easy to see like that (assuming, of course, this is not a measurement error - i.e. trying to measure a P-N junction backwards by mistake when flipping between measuring PNP and NPN transistors - happens to me quite often until I do a second measurement to check myself).

So looks like Q304 has an open P-N junction.
As for the E-C reading on Q301... I'm guessing probably a protection diode there or something similar, but I'll check the schematic a little later just in case... unless that reading is of the transistor out of circuit - in which case, I would say that transistor is suspect too.

**EDIT**
Looks like there are no paths in the circuit that can make Q301 show a P-N junction reading between E-C pins, and since Q301 is the same transistor as Q302, and Q302 does not show that reading, I would say Q301 is either bad or leaky.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Q304 was double-checked. In fact I double or even triple-checked every transistor in that list, both by taking multiple readings and by swapping the probes around to check for "leaks".

About Q301: that is indeed an an error which I forgot to correct after posting. It's open as well, so it's OK. It was the first one I measured and since I had nothing to compare it to up till that point, I just assumed it's normal to read something, that was until I compiled the list and had a look at the other ones and decided to take my my measurement again and sure enough it was open too.

Everything was measured partly out of circuit, now what does that mean ? It means instead of actually removing the part from the board completely, I desoldered just two of its pins while leaving one attached. That way I was able to isolate the pins from the pads around them and measure that way, since the pin stood in the middle of the hole without making contact with the solder around it, so it was technically out of circuit, though still on the board. Definitely handy if you've got heatsinks in the way.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

While being able to identify Q304 as faulty was good news, the bad news now is that even after replacing it there is no sign of life: same exact symptom: flashing standby LED indicating an error...

What I'm noticing is that the output stage seems to be "stuck on", meaning that just about every transistor has -50V going through it (measured with respect to chassis GND). The absence of Q71 and Q72 could be keeping the output full on, like cranking the volume to max. Don't know what to make of that, but to me, seeing 50v at the base of Q305 for example means the rest of the transistor after that are going to be "on" and go straight to the 50v rail.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Amp's still not working even with all parts put back in, so my idea was incorrect...I think I'll start removing components until it DOES turn on, starting with the 4 main transistors so they don't blow if anything DOES happen...

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

More digging around: I THINK I managed to isolate the problem to the right channel, consisting of output transistors Q314 and Q316, the reason I say this is because I probed around IC702's pins and found pin 13 to stay low all throughout, which contradicts what the 5.0v label says and also the other channel (pin 11 of IC702, which DOES go high to 5v). I tracked down the signal through the inverter IC701 and down to Q702. Comparing the readings to the other channel, I got wildly different results, and we're talking about voltages, not readings of the transistors, because I removed those to check them and they seem OK - the left channel stays LOW, while the right one is HIGH at 35v, what I mean by that: D705 reads nothing at the cathode, D706 on the other hand reads 0.7v, which is apparently enough to cause the inverter to "flip" and pull pin 13 low, which I SUPPOSE is interpreted as an error. I'm now back to the earlier situation where the R channel seems to be cranked all the way up. L seems to be sorted, at least with my brief knowledge of how to measure amps....

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Made a bit more progress: removed Q306 and Q308, as well as all 4 main drivers and the amp now turns on. Q306 and Q308 measure fine, but as soon as I solder them back in, the amp goes into protection, even with the main output transistors missing.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

This is just not working, and I clearly suck at this transistor business and have no idea how it works.....Poking around the amp, removing and testing components, more smoke came out unfortunately...at least I THINK it did....what does this mean?
Since the L channel appeared normal, I decided to track down why the R channel seemed "stuck on", with -40v going to just about every transistor and component there, which resulted in around 4v on the middle of the large current sense resistor which kept the thing from powering on since the MCU interpreted that as an error. By comparison, the voltages on the L channel were more normal and also the output was much smaller, at only a couple of milivolts.

No matter what I removed, it seemed I always had those 4v there...all was going well, I thought I was on to something and getting close, until I tried powering it up without D314 connected. For some reason, Q306 decided to smoke up, along with the series lightbulb indicating something's not right, as it got brighter. I powered down and inspected the damage. Ironically, Q306 appears to have survived: I get "normal" readings B-C and B-E, with C-E open...I put it back in and this time reinstalled D314 and tried it again to at least bring it back to where I left off. Now, Q72 smoked up (possibly taking out IC71 again since it now gets slightly warmer that I'd like it to), but same story: removed Q72, measured it, all OK....I doubt IC71 was the culprit all along because it should've started without it from the very beginning, when I was still waiting for it to arrive...
I'm not sure what to do next, since now the amp is back to not starting again: I get -6v on the middle of the current shunts and it just stays like this...when it DID work, this would swing positive to a couple of milivolts, but now it's stuck in this "preheat" stage at -6v. Nothing's smoking anymore, with Q72 and Q71 removed, but not working either. I'm sure that if I DO reinstall Q72, it'd burn again.

Q309 thru Q316 were removed from the amp all throughout this ordeal, that's worth mentioning......

[momaka]

Re: Technics SU-v505 amp problem/discussion

Made a bit more progress: removed Q306 and Q308, as well as all 4 main drivers and the amp now turns on. Q306 and Q308 measure fine, but as soon as I solder them back in, the amp goes into protection, even with the main output transistors missing.

Hmmm... maybe there is something wrong with the "computer-controlled" biasing. Looking deeper into the service manual, on page 18 it appears that the computer-controlled biasing is buffered by PNP transistors Q712 and Q713.

Check these transistors out of circuit, and even try powering the amp without them if they appear good (as perhaps they may be leaky). However, do this with all of the output-stage transistors and their drivers installed first (that is, Q305 through Q316) - if they are all good, of course. Perhaps this could be what was wrong with the amp the whole time and we missed it?

Let's see where this gets us.
Worst case, maybe we can just bypass the whole computer nonsense and have a plain-ol' class AB amp.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Shortly after writing my last post, I poked around the interwebs a bit myself and found another forum (not sure whether I'm allowed to name it here, so I won't for now) where a chap was having trouble with his own amp. It was also Technics and a very similar model to mine, a su-v707 if I remember correctly and the guys there actually helped him get it going again. Sure enough, having a look at the service manual for that one, albeit slightly more complex with more components, the setup is virtually identical and it helped me at least understand what some of the components do, since not having any real experience with amps, I do all my troubleshooting by ear, not exactly knowing what some of them do. I see a transistor/diode, I know what they are and how to measure them, but not what they do in that particular combination.

Anyway, there was this one guy there in particular who really seemed like he knew what he was talking about and really knew his way around these Matsushita amps, so he was the guy I wanted to hear from. So I join the forum myself, describe my issue starting from where I left off here to save repetition, and sure enough, in less than an hour, the chap in question replies and very kindly begins walking me through what I should go about checking.

He left the computer drive alone for now and instead prompted me to replace Q306 and Q308. Sure enough, I removed Q308 and it was shorted E-C, so it went in the bin. It was also here that he suggested upsizing them and adding +1 at the model of each one, so A1123 becomes an A1124, which is what I got myself next. I want to replace all 4 "pre-drivers", so Q305 thru Q308, but I could only find like 1-2 pcs at my local shop, so I'm waiting for more. He also suggested pulling the collector of Q303 and 304 to gnd via a 3.9Kohm resistor which should cause the amp to start and read 0v at the output.

[momaka]

Re: Technics SU-v505 amp problem/discussion

Any time anything smokes or you think it does, you always have to check every major transistor, diode, and resistor again. If you did that, you would probably have discovered Q306 and Q308 were bad by yourself.

And yes, you can post other forums names here. No filter on BCN, unless the content is highly objectionable. Read the forum guidelines if unsure. Or use your best judgment if you prefer. But in any case, as long as you're being reasonable in what you do/post, I am pretty sure no one is going to ban you.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Yes, but those two smoked AS I was "diagnosing", since apparently removing D314 was a bad idea. It's on Audiokarma, here.

[momaka]

Re: Technics SU-v505 amp problem/discussion

Cool.
I don't have an AK account, but I often find threads from that place when looking up audio gear, so I think it's a pretty decent forum.

Keep us updated how it goes. I'll probably even try to follow the thread myself on there, if time allows and/or I don't forget to. I just hope you get your amp going.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Still waiting for my mate to go pick up my trannies...

[momaka]

Re: Technics SU-v505 amp problem/discussion

Still waiting for my mate to go pick up my trannies...

Oh boy! I'm glad this post is not in the VIP section of the forums. Otherwise, this could have swung the conversation in a very different direction.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Yes I know now that you mention it, but TBH it didn't even cross my mind when I first wrote it like that