Technics SL-1950 hum problem

N8Nagel

AK Subscriber
Subscriber
So there's a SL-1950 sitting on my kitchen table that was given to me by another AKer to fix, to "pay" him for a bunch of stuff he's given me over the past year or so. Name withheld so he doesn't get crap for trusting a decent turntable to a complete hack :) He said that it played fine but only had one channel, but playing with the interconnect would sometimes get the left channel back. I found the left channel shorted to shield, and it was the interconnect not the tonearm wires because it persisted after I pulled the tonearm and lifted the L channel shield from the little circuit board. I also noticed a monster hum, but one thing at a time right?

So I whipped up some new ICs out of Mogami stock and Switchcraft RCA ends, got it in, kind of half put the table back together to test. Sounds great! Went to take a pic of what I did before I put it together for real, and soon as I touched the Switchcraft ends to line them up for the camera, the hum is back.

So I metered everything out, fortunately the headshell I installed for testing was the kind where you can probe the connections, everything tests good - all four connections test good and also all are isolated from each other. However, the only time it does not hum is when the RCAs are touching. If you separate them - as you would if connected directly to a receiver and not using dangly extensions like I am in the kitchen for testing TTs - you get a monster hum. The chassis ground seems to make no difference as it is completely isolated from the cartridge and tonearm grounds, in fact it is quiet without the chassis ground connected - SO LONG AS THE RCAS ARE TOUCHING.

I did notice when soldering on the new ICs onto the circuit board, there were two black wires connected to right shield (tonearm ground?) while ONLY the left ground was connected to left shield, and the two had no connection with each other. This doesn't look like it was ever disturbed/changed from the factory.

The receiver I'm using to test is a Sansui 2000A. Now I would think that the shields would be tied together at the receiver end but since I am getting this hum that must not be the case. The reason the RCAs could touch is I'm using a 6' interconnect and some F-F RCA connectors to provide an extension so I can test a turntable sitting on the kitchen table when working on it. I know that that may roll off treble, but it is great for basic functionality check. If I hooked the table directly to the receiver, the RCAs would not be touching obviously.

Does anyone have any ideas how to make this work? I know I've had other tables hooked to this receiver before and never had this problem.
 
Hmmm... scat for 2000A shows L channel Phono 1 shield isolated from all the others, I assumed that was an error, but...?
 
Well hell never mind. Decided to unrack 2000A and found R channel RCA not all the way plugged in. @##$%ERT@%^
 
Would it be a good idea to recap the electrolytic caps to fix the hum problem?
My sansui amp had problem with humming once and it was sorted out by recaping old polarised caps.
 
Nah, it sounds brilliant once I gave the RCAs a firm shove. Apparently one had become dislodged at some point since the last time I had a turntable hooked up for testing. Just goes to show you, check the obvious stuff first.
 
Back
Top Bottom