electronics, repairs, service manuals and datasheets discussion community for the repairs professionals and technicians
FAQSearchMembersGroupsRegisterProfilePM'sLogin/Out
Philips turntable humming problem
Goto page 1, 2  Next
 
Post new topic   Reply to topic    Electronic equipment repair and service manual search discussion Forum Index -> Equipment Repairs
View previous topic :: View next topic  
Author Message
Kidney



Joined: 08 Dec 2011
Posts: 7

PostPosted: Thu Dec 08, 2011 8:41 pm    Post subject: Philips turntable humming problem Reply with quote

Hi there,

A friend of mine bought an old Philips turntable and asked me to check it. The speaker output has mostly hum and very little actual audio playback signal. The problem could be related to a grounding issue somewhere, because humming gets louder when I touch any metal parts of the unit. Having not been tampering with turntables at all before now, I could use any help/suggestions etc on how to pinpoint the culprit.

Here is the schematic:
http://img225.imageshack.us/img225/1796/img0521mz.jpg

A video clip where humming can be heard:
http://www.youtube.com/watch?v=AHOTND8pV4Q
Back to top
View user's profile Send private message
vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Fri Dec 09, 2011 2:58 am    Post subject: Reply with quote

That is an old one;
Definitely sounds like a grounding issue. Majority of the problems related to your description lie in the front end. Meaning that it could be broken wiring or even the actual pickup or connection issue.
Back to top
View user's profile Send private message
Kidney



Joined: 08 Dec 2011
Posts: 7

PostPosted: Sat Dec 10, 2011 10:05 am    Post subject: Reply with quote

Thanks! Maybe I should go through every grounding point, as shown in the schematic, with an ohmmeter.
Back to top
View user's profile Send private message
vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Sat Dec 10, 2011 9:46 pm    Post subject: Reply with quote

Good idea. Pay extra attention in the tone arm/pickup.
Even better see if you can unplug/disconnect the pickup and see if the problem still exist?
Back to top
View user's profile Send private message
Kidney



Joined: 08 Dec 2011
Posts: 7

PostPosted: Sun Dec 11, 2011 10:06 pm    Post subject: Reply with quote

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.
Back to top
View user's profile Send private message
vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Mon Dec 12, 2011 3:09 am    Post subject: Reply with quote

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?
Back to top
View user's profile Send private message
Kidney



Joined: 08 Dec 2011
Posts: 7

PostPosted: Mon Dec 12, 2011 5:47 pm    Post subject: Reply with quote

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.

Back to top
View user's profile Send private message
vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Tue Dec 13, 2011 1:57 am    Post subject: Reply with quote

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.
Back to top
View user's profile Send private message
Kidney



Joined: 08 Dec 2011
Posts: 7

PostPosted: Wed Dec 14, 2011 11:06 pm    Post subject: Reply with quote

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.
Back to top
View user's profile Send private message
vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Thu Dec 15, 2011 2:06 am    Post subject: Reply with quote

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?

Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    Electronic equipment repair and service manual search discussion Forum Index -> Equipment Repairs All times are GMT
Goto page 1, 2  Next
Page 1 of 2

 
Jump to:  
You can post new topics in this forum
You can reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Електроника София - Service Manuals Free Download - Prikachi - Service Diagrams

Free TV Schematics Download - HARDW.net - Pinout Master - Jabse Search - Jabse.InK  - Pulsator

Connector Pinout Forum - Новини - SnimkiOt.com - безплатен DNS - Whois разузнаване - Poblizo.com : пътешествия