Technics SU-v505 amp problem/discussion

[R_J]

Re: Technics SU-v505 amp problem/discussion

You should check transistors like Q305, Q307 and R313, for R79 to burn up I suspect current flowed through Q71 and through your headphones, and that maybe protected the power amp section. remember this is a Dc coupled amp and it don't take much to be upset.
I don't see why the sound car was effected unless you had IT connected directly to the circuit somewhere.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

The reason I thought this might be a good idea is because I did it with a CRT TV once which had a buzzing noise on its speakers and I tapped into the signal between the main IC and the amp IC and it worked with no trippy side effects, but looking back, I can now see the setup wasn't quite the same here: I used headphones instead of a desktop speaker here (lower impedance perhaps) and also here I tapped into what appeared to be an amplified line, whereas with the TV it was line level going into the amp IC...I think the trick MAY have worked BEFORE IC71, but not after it where the signal is already "hot".

Not sure whether a scope would pay off...it's been a couple of instances where I cursed not having one, but I'm looking to quit my current job anyway and if the new one is not going to be in the same field, it would be even more of a waste, so I shall sit it out for now. I've ordered a new IC and shall try firing it up normally again after replacing just that resistor...hopefully nothing goes haywire again and I can get it back to at least where we were. I didn't find any shorts, but TBH I'm worried about frying another output device...THAT'S my main concern.

[CapLeaker]

Re: Technics SU-v505 amp problem/discussion

yeah... I was afraid its going to end like this... maybe next time you should invest into a O'scope. ;-)

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Ok, well this was a bad idea that went wrong on so many levels: like I said, I thought I could tap into the signal line just before it enters the amplifier stage. I soldered two RCA jacks, one to pin 14 and one to pin 2 of the riser board (page 17 of the manual) and their ground to ground (pin 7). These were supposed to plug into a working amplifier or, ideally, a set of crappy computer speakers. I didn't have the latter and didn't want to risk my home theater system, so I THOUGHT I could get away with using my headphones and gently increase the volume until I got an idea of what it sounded like...I believe this is what caused the actual failure and audio gurus may spot this from a mile away upon reading.

I took an RCA to jack adapter and I only hooked up one channel of the headphones to the "taps" I made - the left one. The amp started normally and upon plugging in the headphones to one of the RCA connectors I soldered, I heard a pop in the left channel - ok, normal, no big deal I though. After a few seconds though with the volume all the way down and nothing going through the amp yet mind you, I hear another pop and then see something catching fire on the amp....literally: something was glowing orange and was on fire with smoke ! After immediately pulling the plug and waiting for the smoke to clear, I assessed the damage, which is NOT minimal unfortunately and happened on all 3 devices: the PC (albeit a junky one) no longer outputs sound on its left channel, the left channel of my headphones is now dead and there's of course the damage on the amp itself :| After taking a closer look, it's R79 (page 17) that got charred. D71 and Q71 don't appear to be shorted, which are directly linked to that resistor. What I THINK happened and this is just a theory, so please correct me, is the headphones have a low enough impedance to act as a short, pulling the collector of Q71 to GND, Q71 and sending 50v into the headphones.

I can't quite figure out why the sound card died though. I suspect IC71 also died because to travel back into the sound source, the current must've somehow reached pin 10 of the riser and from what I can see it can only do that through IC71, though I didn't read any shorts there either. In the second picture, you can just about make out a bit of solder sputter on pin 17 of the IC which wasn't there before - must've got hot.

This really was a disaster and I'm very disappointed, especially since it killed the sound card... :| I'll replace the resistor, but I'm not sure whether I should go for it straight away, except this time with an amp or desktop speakers or something, or would the same thing happen again ? A little help please ?

EDIT: theory no.2 is that the earphone could not take the juice the preamp pumped into it and shorted out, eventually leading into the same situation: collector of Q71 shorted to GND...

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Since there IS sound coming out of the speakers, just distorted (worse on the right channel), I don't think doing that screwdriver job would be of significant relevance since I wouldn't be able to tell what's "buzzing" and what should be "normal" sound.

I'll try something else though which I did with a TV once: I'll "intercept" the signal just before it enters the amp stage (at pins 2 and 14 of the riser board which holds IC71) and send that to another amplified speaker which takes a "line in" and see if it sounds good. If it does, the problem is in the amp stage and not the "voltage amplifier" stage, as the schematic calls it. If it still sounds bad, the problem is IC71 or something around it.

[R_J]

Re: Technics SU-v505 amp problem/discussion

Since you don't have a scope, try injecting signal, Use something metal, (small screwdriver) and while touching the shaft of the screwdriver with your finger, touch the screwdriver to Q71 or Q72 collector, or some place similar on that audio path, and see if you get any hum out of the speaker and if it sounds distorted

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Had a closer look at the schematic today and I noticed something that's going to complicate matters: the headphone output is also powered by the amp. This opens up a whole new dimension of troubleshooting, since now the issue can also be in the amplifier stage....

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Sadly replacing the caps did not yield any improvement, so I still have my money on that IC, but feel free to come up with suggestions and feedback.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

And a timely warning: be careful when adjusting the bias current, the output stage of the power amp tends to oszillate!

Not gonna mess with adjusting any of those trimmers if that's what you mean...it's a bit over my head and I don't suppose it's necessary. If anything, not having a scope and not being entirely sure of how to do it, instead of getting better it could get worse...

[Raymond.G]

Re: Technics SU-v505 amp problem/discussion

The IC AN7062 outputs to the base of PNP transistors Q71, Q72. It might be helpful to replace these, taking a device with somewhat higher power rating: 2SA1124, 2SA1145, 2SA1285, 2SA1208 will all do the job. And a timely warning: be careful when adjusting the bias current, the output stage of the power amp tends to oszillate!
Good luck for your repair!

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

I replaced C404 and C405 (page 17, middle-top). Indeed they looked pretty bulgy at the bottom to me...plus even if they somehow were still "OK", it doesn't hurt to replace them. There aren't that many electrolyitcs in this thing anyway, so might as well replace as many as possible.

My next two targets are C71 and C72 which feed the inputs L and R to that IC. I don't have 3.3uF caps on hand at the moment, so we wait until Monday

[R_J]

Re: Technics SU-v505 amp problem/discussion

Have you checked the two supply voltages to that ic? +46 & -17, That -17 is what I would suspect going bad C407 could be bad . I have my doubts the ic would be bad, check the the 2 input caps, I would just bridge them with another 10µf and see the sound improves.

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

The first thing I would do is to clean the controls and switches, they can cause all sorts of problems including distortion. At least try and work the switches back and forth to see if anything changes.

Did that as best as I could with some contact cleaner. It's likely not a physical issue, since the distortion corresponds to the sound to a degree...it's hard to explain and I was thinking of capturing some samples for you guys to hear. Imagine the bass line of a song and every time it hits instead of hearing the bass you hear like a fart....that should give you an idea. Again, only the right channel does this, though the left channel is too far off either - sounds pretty muddy...I have my money on the AN7062 I mentioned.

[R_J]

Re: Technics SU-v505 amp problem/discussion

The first thing I would do is to clean the controls and switches, they can cause all sorts of problems including distortion. At least try and work the switches back and forth to see if anything changes.

[CapLeaker]

Re: Technics SU-v505 amp problem/discussion

check your pre diver and driver on the PA. Compare and see if those are one of the known noisy or prone to fail transistors. You can also check each stage with your oscilloscope. Caps could have an effect too...

[Dannyx]

Re: Technics SU-v505 amp problem/discussion

Forgot the other pic, the one of the riser board holding IC71...

[Dannyx]

LONG STORY WARNING !

Good day folks. A very complicated and at the same time interesting story here: I ended up with a HiFi amp which looks decent enough to keep and also has good documentation to help us troubleshoot. It's a fairly old thing, I think it's a class A amp, making it outdated now, but any amp is better than no amp at all in the shop to blast some tunes, plus this is all about learning, since I must admit I'm not very vexed in this field and all I have is some general knowledge about amps. Here goes:

The amp works overall: you plug it in and it makes sound, but it has a quirk I'd like to sort out to make it 100%: distortion...I know there are virtually endless reasons for this and without a scope, they're hard to pinpoint, but perhaps we can come close and narrow it down to something like a cap or something that ages naturally, since there are no signs of visual damage otherwise. The problem is as follows: when the balance control is in the middle, it works relatively well, I mean there is a bit of noise, but unless you have the sharpest of ears, you can't really hear it, especially at low volumes, so that's issue no.1

Issue no 2 is that when the balance is adjusted to the RIGHT channel, all that changes and all you get is a distorted mess in the right speaker. It's worth mentioning that the distortion is present on the headphones output as well, so it's not a problem with the amp stage, since if it receives trash it simply amplifies trash with no control over it.

What I noticed so far: the amp has a "speaker selection" knob which toggles between 2 sets of speakers: "Remote" and "main". One interesting feature, which I absolutely adore TBH is that the headphones jack does not disable the speakers like "conventional" amps, so you can listen like a DJ to both the output and the headphones. To listen to JUST the headphones, you throw this switch to the "speakers off" position and you get the sound through just the heaphones. When set to "off" (speakers disabled), the distortion becomes significantly worse on the headphones. By contrast, when set to "all on" ("main AND remote"), the sound is nearly perfect ! My limited knowledge tells me the difference in load ("impedance" ?) causes it to spring back to life. It should normally do this by itself, but a component is dead somewhere and not doing its job.

More to come: Page 17 of the schematic shows that the inputs eventually converge (bypassing all "enhancements" and sound effects and setting everything to "flat) at block F, governed by IC71 AN7062 which looks like an op-amp. The signal passes through two electrolytics C71 and C72, one for each channel, on its way to pins 16 and 2 respectively...I tried snapping a pic of the riser board this IC lies on. Now purely on a whim, I touched my finger onto this IC to check its temperature and while not burning hot, I did notice one thing: the cooling effect of my finger caused the right channel to drop out in volume slightly, along with the noise of course...interesting. I then remembered I encountered this exact issue when I fixed a Tascam USB sound card (some of you chaps helped me there too), which also had an op-amp which got hot and upon cooling it the sound would return to normal. I took it to the next level by spraying some alcohol on it: the right channel cut out entirely this time. As the alcohol evaporated, the sound along with the noise came back in the right channel...replacing this guy shall be at top of the list I reckon.

The manual is VERY comprehensive so perhaps some audio guru out there who immediately recognizes the topology and overall design can point me in the right direction. Here's a photo of the interior....gotta love that heatsink The volume gets very loud with minimal movement of the volume pot, so there is some grunt behind it, but it's a shame it doesn't sound good.....I'm sure we can work it out eventually. Thanks for any thoughts and support