 Posted: Sat Dec 17, 2011 9:58 pm Post subject: |
|
| Ok , that clearly explains the hum in the video. I've never been a fan of just replacing parts as a shotgun approach and very much prefer the "see-through-the-circuit" methode but I can certainly understand the situation here. Not sure if you want to go thru the trouble of swapping the caps though? More than likely, you are not gonna have the value/physical shape ones. In general, in the world of troubleshooting caps, you can always jumper the suspected cap with one with same or higher value as a quick means of testing.The only thing you must observe is polarity and be careful around voltage rails. You mentioned having a scope at school and seem to have an unusual electronic knowledge/interest in fixing a turntable of all things! All I can say ...KUDOS AMIGO  |
|
| Posted: Sat Dec 17, 2011 8:25 am Post subject: |
|||
Ah, you're right, when you listen carefully there's indeed some hum as long as the unit is turned on, even with the volume turned all the way down. That hum is not what's causing real trouble, though - it's barely audible to human ear (my condenser mic seemed to pick it up pretty loud). However, the hum I was originally referring to - the really loud one that's masking the actual audio - is what you hear only after the volume is turned up. I have access to a scope at my school, but I don't know how to use one yet. Of course I could swap all electrolytic capacitors. I've got plenty of used ones extracted from working appliances. I've got a feeling it won't fix the turn-volume-up--get-lots-of-additional-humming problem, though. Getting a replacement pickup is unfortunately out of the question. I don't have one lying around and buying another is probably an overkill, considering the turntable itself cost five bucks. |
|||
| Posted: Sat Dec 17, 2011 3:54 am Post subject: |
|
| Perhaps I am mistaking but it just does not seem to react as it should(red control). There should be no hum whatsoever when you turn counter- clockwise all the way ( right before turnoff click). ---hard to tell for sure. Any way to get a replacement pickup? Or do you have access to a scope to rule out the power supply/capacitor problem? |
|
| Posted: Thu Dec 15, 2011 5:48 am Post subject: |
|||
Hmmm...isn't this exactly what happens in my video? In the beginning (0:00-0:05) hum fades in as I turn the volume all the way up, 0:40-0:46 I turn the volume down and back up - hum dies and builds up again, 1:02-1:04 I turn the volume down for the last time and away goes humming again. Also, telling by the fact that humming dies the moment I short ground and signal wires coming from the stylus (tried this with a metal screwdriver shaft), the problem should be related to an open connection in stylus, right? |
|||
| Posted: Thu Dec 15, 2011 2:06 am Post subject: |
|
| A bit puzzling here.... if it has a magnetic pickup(very likely), the resistance reading is definitely way too high. It should be in the kilo ohms range which points to an open winding? But if it has a piezo type pickup, then the resistance could be around a meg or so? However, after paying close attention to your video, I wonder if you may have an entirely different problem all together? You see, If/when there is a ground issue related to anything in the front end(such as wiring/pickup/connection..etc), when you turn UP the volume control, it should get louder(much louder) and as you turn it DOWN, it will effectively short the same signal line to the ground effectively killing any hum noise getting picked up along the way. It does not appear to be much difference as you turn the vol. control. You may infact have dried up electrolytics capacitor(s) causing the hum. Do you have access to a scope or how about some spare electolytic capacitors?  |
|
| Posted: Wed Dec 14, 2011 11:06 pm Post subject: |
|
| Hmm...I've measured resistance for both the signal wire and ground wire all the way from the stylus terminals on the back side of the tonearm to the circuit board (at the spot where you instructed me to desolder the connection). Both wires show continuity all the way their length. However, resistance across stylus terminals is more than 2Mohm which is the highest my DMM will show. Correct me if I'm wrong but doesn't this mean that, as there's an open connection somewhere in the stylus, wiring will function as an antenna, picking up whatever interference is present, which is the very hum I'm hearing? See http://img23.imageshack.us/img23/900/img0523mow.jpg for a photo of internal electronics. |
|
| Posted: Tue Dec 13, 2011 1:57 am Post subject: |
|
| Ok then, it must be the ground wire that goes to the pickup. Somehow the ground side of the pick up is not making it to the amplifier ground. Not quite sure of the piece shown? Is that some sort of a plug/connection? Either way, need to check the ground return line from pickup to amplifier. While there is always a chance of a pickup opening internally, it is very rare. |
|
| Posted: Mon Dec 12, 2011 5:47 pm Post subject: |
|
| It did. Well, at least hum was attenuated to a fraction of what it was when the input wire to the tone pot terminal was connected. The way the tone pot is actually connected differs slightly from the schema image. Corrected version below, with the desoldering spot marked red.  |
|
| Posted: Mon Dec 12, 2011 3:09 am Post subject: |
|
| That is only an indication that the thermal fuse is either part of the transformer or is actually mounted ion the transformer. Don't see how that would have anything to do with the problem. Ok, desoldering from the p/u had no effect, how about desoldering the connection at red arrow. Does the hum go away?  |
|
| Posted: Sun Dec 11, 2011 10:06 pm Post subject: |
|
| I desoldered wires coming from the pickup. No difference. As I began measuring ground points, I noticed something. Correct me if I'm wrong but, based on the schematic, doesn't the point at the end of the dashed line coming from the thermal fuse indicate a common ground point which every other ground point in the system should connect to? According to an ohmmeter, there's infinite resistance between that spot and other ground points. |
|