Technics SU-v505 amp problem/discussion

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Thanks for the reply. Fortunately no other components seem to be damaged, but failure is still an option after replacing the resistors and attempting another power-on Amazingly Q71 is fine, as I compared it to the one on the opposite channel, Q72, and they both give the same readings on the meter. Q305 is also OK, again, by performing a "mirror" test, since everything is symmetrical. Good thing I didn't hook BOTH headphones up, as I imagine I would've fried something on both sides, leaving me stumped as to what's good and what died. Not sure whether I should give it another go with the current IC71 or wait for my replacement to arrive. Then again, I could very well blow that up too and regret it, so I'll probably try it again anyway now.

My failed attempt also means that now I'm back to not knowing exactly why the right channel sounds awful...there's probably a capacitor somewhere that "withered" away over the years, since I imagine that if it were some other major component like a transistor or diode, it wouldn't have worked at all, as those instantly fail shorted and not gradually like capacitors to the point where the device sort-of works, but not perfectly.

Use a pair of 1 to 2.2 uF electrolytic capacitors, connect them on one side to a to a 5-10 kOhm potentiometer (the other side of the pot goes to ground), and connect the variable resistance output to the sound card of your computer. Before connection to your computer, though, first turn down the potentiometer all the way down (for minimum signal going into the PC) and also turn On the amp before connecting to the PC, so that any large voltage spikes do not get coupled the audio input. If you have any 3.3V Zener diodes, use those to protect your computer's audio inputs.

So would this setup go on the input of the amp where the sound SOURCE comes in (between the source and the amp's input), or on my "stethoscope" headphones ?

[momaka]

Re: Technics SU-v505 amp problem/discussion

My failed attempt also means that now I'm back to not knowing exactly why the right channel sounds awful...there's probably a capacitor somewhere that "withered" away over the years, since I imagine that if it were some other major component like a transistor or diode, it wouldn't have worked at all, as those instantly fail shorted and not gradually like capacitors to the point where the device sort-of works, but not perfectly.

Maybe. Maybe not.

I too agree with other earlier suggestions in the thread to check any mechanical switches. I've personally seen bad/marginal switches cause all kinds of distortion to the sound - though in most cases, loss or distortion in bass. Of course, it could also be some IC or transistor on the pre-amp/input stages.

I'll have a look at the schematic attached on the previous page when I get time and see if I can find anything. Meanwhile, see if you can look at it yourself as well and tell us how similar (or dissimilar) it is to your amp.

So would this setup go on the input of the amp where the sound SOURCE comes in (between the source and the amp's input), or on my "stethoscope" headphones ?

You can use this setup pretty much *anywhere* you want to measure a signal. Because the setup is AC-coupled, you won't disturb any DC biasing, be it on an amp input or output side. Also, the 5-10 KOhm potentiometer being AC-coupled should present a fairly high impedance (i.e. AC resistance) to ground, so again, that shouldn't disturb anything. In fact, for better safety, start off with even smaller coupling capacitors - maybe 0.1 to 0.47 uF, and go up in value if you notice that the low-range in the signal (i.e. the bass) doesn't come through or is severely attenuated. I imagine the inputs of most sound cards have an impedance of around 50 KOhms or more, so the 5-10 KOhm pot shouldn't attenuate the signal too much.


**EDIT**
By the way, the first picture you uploaded in post #1 shows a loose screw on the main transistor heatsink. Make sure that is tight, along with all of the other screws.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Maybe. Maybe not.
Meanwhile, see if you can look at it yourself as well and tell us how similar (or dissimilar) it is to your amp.

What do you mean ? It's this same exact amp

By the way, the first picture you uploaded in post #1 shows a loose screw on the main transistor heatsink. Make sure that is tight, along with all of the other screws.

Don't worry about that - that hole had a missing screw when I got it (someone had been in there before me) and I found one that closely matched the other two next to it and tried jamming it in there, but because the face plate was in the way I couldn't get a good grip on the screwdriver and the angle was also awkward, so I just left it half-way in like that. Now with the amp in bits I managed to torque it down really good, so it's solved.

[momaka]

Re: Technics SU-v505 amp problem/discussion

What do you mean ? It's this same exact amp

Ah woops, I was reading another audio thread in parallel to this one, where the service manual posted was of a similar model to the one posted in the thread. Hence the suggestion.
But yeah, NVM about that.

Don't worry about that - that hole had a missing screw when I got it (someone had been in there before me) and I found one that closely matched the other two next to it and tried jamming it in there, but because the face plate was in the way I couldn't get a good grip on the screwdriver and the angle was also awkward, so I just left it half-way in like that. Now with the amp in bits I managed to torque it down really good, so it's solved.

Well, make sure all output and biasing transistors have good thermal contact to the heatsink. If any of them don't, that could possibly cause the distortion too - particularly if any biasing/temperature compensation transistors are not coupled well to the heatsink, which can cause a decrease in the idling current across the output transistors, and hence the distortion. Each channel has it's own temperature compensation transistor(s), so that's why it's possible to get this over one channel and not the other.

Good thing you have it torqued down. That's how it should be.

Anyways, since you do have the correct service manual, just check the V+ and V- voltages going to the output stage, as well as check for DC offset. Run amp through an incandescent bulb test and see if it turns on and works at least as good (or bad) as before. Then we can try to troubleshoot the problem from there.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Run amp through an incandescent bulb test and see if it turns on and works at least as good (or bad) as before. Then we can try to troubleshoot the problem from there.

What would the bulb indicate ? Do I just drop it on the mains between the amp and the plug ? Is this to test it after replacing the burnt components ? If so, I was planning on going full-out TBH, straight into the mains

[momaka]

Re: Technics SU-v505 amp problem/discussion

What would the bulb indicate ?

Nothing. It's just done to prevent damage to the output transistors should something go really bad. Instead of getting blown output transistors, you'll just get a bright light bulb. Don't you think that's better?

And yes, light bulb goes in series with the amplifier mains, just the same way as when testing an SMPS.

If so, I was planning on going full-out TBH, straight into the mains

Why? Most amps can run and even be tested up to 1 Watt of output power with a 60-100 Watt series incandescent bulb, if not more. That should be plenty to reveal any distortion while also protecting the amp.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Great news...amp's no longer turning on >_> I replaced the burnt resistors and tried both with and without the bulb and the relay is supposed to come on but it doesn't...something else got busted unfortunately ...I'm not even sure where to start with this....

EDIT: power LED blinks at about 1 second intervals then after a while (corresponding to the time where the relay usually clicked), it starts flashing rapidly at about 500ms, so there's definitely something dead somewhere and the "computer" picks it up. I believe I found it: Q71 is shorted...until I get my replacement, should I attempt a power-up without it ?

[momaka]

Re: Technics SU-v505 amp problem/discussion

Time to desolder stuff and check them out of circuit. Doing the in-circuit "tests" and comparing to the components on the opposite channel may not give you enough info on what's good and bad.

At this point, replacing IC71 is pretty much necessary, since R79 and R80 are directly attached to transistors Q71 and Q72.

As R_J suggested, also check Q305, Q307, R313, D301, D303,, Q301, R305, D313, R309, Q303, R311,, D71, and D72. It is suggested to remove all these from the circuit to get more accurate readings/test results. But if you want to make it easier for yourself, then just remove D313, R309, R311, R303,, Q305, and Q307. This will allow you to check the rest of the parts in circuit without getting erroneous readings.

Again, don't try to "match" the readings of the opposite channel. For all transistors, you should get a voltage drop between the P-N / N-P junctions when forward biased and nothing when reversed biased. Smaller transistors are prone to going open-circuit, so that's why it's best to remove them from the circuit and verify them.

There may be more things to check, but run through these right now and tell us what you get. Also measure resistance to ground from the emitters of Q313/Q315 output transistors. While at it, also check the two 0.22 Ohm resistors connected to their emitters.

Finally, if you don't have a replacement for IC71, then just remove it altogether, along with Q71 and Q72. Since the biasing of the output stage is controlled by an "onboard computer", I think the amp may be able to turn On without IC71 - if everything else is okay on the output stage, of course.

That said, I will repeat this again: keep the series incandescent light bulb on the input at all times until the amp is fixed. It really can prevent more parts from blowing up, so there's really no reason to not use it. Moreover, this amp uses an oldschool linear transformer and not an SMPS, so this shouldn't affect its operation at all, as long as you are using an adequately-sized bulb (100 Watts should be okay, or two 50-60W bulbs in parallel).

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

I made a few more discoveries. In no particular order:

I went a bit deeper into the control side and found that pin 7 of IC702, the main computer IC, remains high at 5v all throughout. Pages 11-12 of the manual tell us that if this pin is high it's interpreted as an error and the amp stays off. That's telling me that Q303 and Q304 the "DC detectors" turn on for some reason and they pull pin 4 of inverter IC701 low, which in turn causes pin 7 to "flip" at 5v...so there's that. I believe this is pretty valuable info, since I can now track the signal line for those two transistors.

I double checked just about every transistor and diode, although through the method of "comparison", by checking with its neighbor on the opposite channel and the values were pretty consistent nonetheless, but apparently this is not ideal as you pointed out. I also checked most of the resistors and all of them are in spec with the schematic, of course with the exception of those in parallel with thermistors or other components which gave a random reading...

The two large current sense resistors Z301 and Z301 are OK. They are 3-pin resistors in a ceramic package, instead of 4 individual resistors.

I tried the amp both with and without the bulb (YOLO ), but nothing happened like I said...no explosions, no fire, but no stratup either. I'll carry on WITH the bulb nonetheless just to be sure it doesn't jump-scare me. It's a 45w bulb I believe, but like I said, it's not too relevant at this point since it doesn't work. Q309 through Q312 get warm to the touch after a while, with our without it.\

On removing IC71: I'm a bit concerned about what would happen to the bases of Q305 and Q306 if Q71 and Q72 are no longer there and they're flapping in the breeze like that...

[momaka]

Re: Technics SU-v505 amp problem/discussion

YOLO

I can understand doing that with new crap that is built not to last. But older amps like this deserve a little better treatment, IMO.

It's a 45w bulb I believe, but like I said, it's not too relevant at this point since it doesn't work.

It IS relevant.

You don't want your power rails / PSU output voltages to be too low. It can create new problems (at least temporarily) that we may not know about - especially with this amp, which cranks up the biasing on the output stage so that it heats up faster so that the amp comes up to normal running temperature quicker. So you definitely want to make sure that you have a more powerful bulb. Most old amps like this can pull anywhere from 10-40 Watts while idle. With a 45W bulb, the voltages in the power supply may fall too low.

On removing IC71: I'm a bit concerned about what would happen to the bases of Q305 and Q306 if Q71 and Q72 are no longer there and they're flapping in the breeze like that...

Nothing.
Q305 is DC-biased by Q301/Q303 circuit and is AC-coupled to ground (well, actually the +/-50V rails), so even without Q71, the output stage should still bias normally. Same applies to the other channel. There will just be no sound output (or maybe a slight 50/60 Hz hum, depending on your country's line frequency.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

I can understand doing that with new crap that is built not to last. But older amps like this deserve a little better treatment, IMO.

I was actually under the opposite impression I wasn't sure how "great" this amp is anyway, being that it's all old and stuff (my workmate surely laughed at me for messing with a class A amp ), so I thought what the hey, let's do this ! Does it fall into the "vintage" category which ought to be venerated and praised ? Just askin'...I got it second hand off an auction site, so if that's the case I may have scored a good deal

Actually let me answer my own question from a semi-professional point of view, ignoring the outdated technology: the construction is unbelievably strong, it's all friggin' metal in this thing, it's all bolted together, no plastic, no nonsense. Drop it on your foot, you're done for I also see it's all "proper" genuine parts in there - no generic stuff. Understandably, this IS Matsushita (Panasonic), so it's only normal they use their own parts and don't resort to el cheapo third party stuff (at least back in the day when this was made, as nowadays even Panny can stoop pretty low and use a Vestel chassis for instance...have seen it).

I'll begin by pulling out IC71 and giving it another go. Something is telling me IC71 is faulty and feeding so much noise into the input it's like cranking the volume to max, causing the amp to go into shutdown immediately after the "warm-up" procedure, hence the 5v on pin 7 I mentioned there...

[momaka]

Re: Technics SU-v505 amp problem/discussion

I was actually under the opposite impression I wasn't sure how "great" this amp is anyway, being that it's all old and stuff (my workmate surely laughed at me for messing with a class A amp ), so I thought what the hey, let's do this ! Does it fall into the "vintage" category which ought to be venerated and praised ?

It's a pretty decent mid-80's amp with a clean-looking design, so I would consider it good "vintage" gear. It may not be the most powerful thing (only 60 Watts per channel into 8 Ohms). The damping factor is so-so (meaning you probably don't want to drive huge modern 15" deep subwoofers) and SnR is also a bit wimpy at 91 dB at best. But it can take 4 to 16 Ohm speakers and even has an automatic detection circuit for that - nifty! Also, audiophiles will probably appreciate this amp even more, since its rated to have a THD of 0.004% - that really is pretty damn good, actually! Even better that it stays in the 0.02-0.03% from 5Hz to 50/60 KHz. Not to mention you have the option on this amp to turn off the sub-sonic filter, meaning you can probably go down to almost direct DC coupling between input and output signals. I personally hate newer stereo amps that have a sub-sonic / low-frequency filter, as that can cut-out some of the low-range output. And, as you noticed, the build quality is pretty solid throughout. So again, IMO gear like this definitely deserves more respect.

My dad's Sony TA-F70 amp is over 30 years old (closer to 40, actually) and still providing us with service today. It's been through a lot of weddings and parties when my dad was younger and even managed to survive through my music collection (mostly drum & bass), so I think that says a lot.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Sounds good (pun SO intended ), but I need to fix it first Weekend time, then it's back on track on Monday - pull out IC71 and try it out. If it still won't turn on (which I have mixed hopes about), then...it's on to checking transistors I guess....

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Back on topic: removing IC71 unfortunately did NADA...amp still doesn't turn on or make a click or anything to indicate that it's working, so the problem's somewhere else. I'll be replacing IC71 anyway since I already ordered two from Ali, so that just may solve the noise issue but until then, I need to start it up, which is not happening unfortunately.

The service manual tells me to check TP303 through TP306 (instructions on page 6) to see if I get 2mV on those current shunts. I did this and upon plugging in the amp, I get absolutely nothing on the R channel resistor (Z302), so TP304 and TP306 are dead, while TP303 and TP305 on Z301 (left channel) do give me a reading, but it's more like 8-9mV rather than the 2mV the manual says. The little graph on page 6 next to the measuring procedure tells me there's supposed to be a relay turning on in there somewhere as well, but it doesn't happen, so I'm not sure if my readings are accurate. The fact that I get nothing in the right channel now might also correlate to a degree with the issue I was initially having: lots of noise when the balance is set to the right channel only.

[momaka]

Re: Technics SU-v505 amp problem/discussion

Forget about the SM troubleshooting procedure for a moment and just check the above components I mentioned. Obviously something is messed up with the amp, so that's a more sure way of finding out what's wrong.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

I forgot what those were TBH (too lazy to scroll down ), but we're talking about all the transistors basically. Should I attempt to power on without some of them ? The "computer" might detect they're missing and add even more variables to the mix...

[momaka]

Re: Technics SU-v505 amp problem/discussion

No, just pull those parts out, check them, and put them back in if they are good. If they are all good, pull more parts near those and keep checking. No point in powering anything until most of the power amp stage is checked out.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Pulling out those main transistors is going to be a pain with all that bulk of a heatsink around them, but I'll manage...I'll actually leave those for last and start with ones which are easier to remove.

[momaka]

Re: Technics SU-v505 amp problem/discussion

Well, if you don't want to remove the main transistors, then just try to remove as many other components connected to their Emitter, Base, and Collector, so that they become sort of "isolated" on the board by themselves (i.e. connected to nothing else). Then you can test them on the board without removing them.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Ok, today I had a little spare time to try this amp again. I stared with the main output transistors Q313 thru Q316. Since I didn't want to go through the trouble of removing that mother of a heatsink, I removed the solder from their legs so they no longer contacted the pads, thereby isolating them from the circuit. As expected, they checked OK on my meter - no low readings anywhere, which is what I predicted initially, because in diode mode my meter picks up shorts even in circuit, so I would've spotted anything "off", so that's that.

I attempted a startup without the transistors connected, but of course nothing happened. I'm not entirely sure the amp would start without the output transistors even under normal working conditions.