Technics SU-v505 amp problem/discussion
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 It's a fairly old thing, I think it's a class A amp, making it outdated now, but any amp is better than no amp at all in the shop to blast some tunes, plus this is all about learning, since I must admit I'm not very vexed in this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
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
Attached Files
by the screen name of Th3_Un1Qu3 mentioned it many years ago, and I've been using that software ever since (so big Thank You goes to him for that).
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