Technics SA-500 with a hum

Rsclj

Active Member
Picked up a Technics SA-500 for $25.00...it had a known hum...the hum is in all selection locations AM, FM, phono, and so on...the hum is also in both channels. ..I haven't really broken into it other than to give it a cursory glance in the guts (nice feature that slide off top) but any ideas...a recap is slated...but just throwing this out there to see if any of you guys have had this or know where I should start looking...
 
It should be noted that the hum occurs with the volumn all the way down...but it can be overcome by the volumn of the music
 
Have the service manual...printed it before I picked up the reciever...I will start with the PS first...you thinking cap or transistor?
 
It sounds like power supply hum. What kind of test equipment do you have?
 
Most probably caps! But you have to check with a dvm and compare the res. with the service manual!
 
I guess its time to start testing voltages and pulling and testing components...the schematic has pretty good voltage lables and the board is better labled than Pioneers...ecb locations are printed on the board like Sherwoods...
 
Cool...looks like I have a direction and some instruction to learn... thanks guys...again this place rocks...I will report back when I can dig into this...hope to start in ernest tomorrow. ..
 
High energy doesn't mean high voltage necessarily. 50v with a bank of caps behind it can whack you pretty good.
 
OK I got a bit of digging in yesterday.
First despite being a "single board" layout...crap is fairly well buried in there...particularly the PS sections...
I started pulling caps...replacing those I had replacements for and test any others...fortunately the ones that tested the worst were also one I had replacements for...although almost all were within the 20% tolerance...

I have not replaced (or re-soldered) the main filter caps yet but they are spot on for uF tests (no way to test ESR)...

I will likely temporarily return them to their home on the board to continue the testing and replace them when I replace the other caps...

MANY...and I mean MANY of the chassis screws are missing...including EVERY one of the screws that hold the heatsinks down...

the next step is to find some replacements and re attach things (that are not in the way for further testing) and see if the hum was a ground issue within the receiver itself...

then we can move on to testing voltages and the transistors in the PS circuit...as the caps seem fine
 
Ok..although the volume does not effect the hum...the bass control does change it...as does the low filter switch
 
total recap parts ordered...all nichicon KL low leakage caps(should be good on the AM/FM board no?)
With the replacement of the few caps that I had for the PS section...the hum seems reduced but still present...just waiting for parts to come in now...
 
It would be good to continue to troubleshoot the problem while waiting for parts. Better to know what actually failed rather than throwing a bunch of parts at the problem with the hope that it goes away. That's called 'shotgunning' a problem, has a 50/50 chance of success and is usually a last resort. I'm not saying don't replace the caps, just find the smoking gun first.

Could be a cap, could be a diode/rectifier, could be a cold solder joint. You also mentioned missing screws, so could be a missing/poor ground.

Good chance to learn how to use that scope. Look for a DC voltage coming out of the power supply that has a large amount of 60 or 120 Hz AC noise/ripple riding on top of it. Wish I could be more specific about what a 'large amount' of ripple is. But considering you have an audible hum, start by consider anything over 20 - 50mV suspect, and you'll more than likely find 100mV or more AC ripple causing your noise.

http://en.wikipedia.org/wiki/Ripple_(electrical)
 
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most voltages I checked were close to spec (+- a volt or 2) I'll put the scope on the pins and see
 
the Sherwood I just finished had a weak channel...in troubleshooting that I found a bad cap was the cause...read zero instead of 47uF...during that repair I was told if I intend to recap anyway to do it before going to far into the troubleshooting...I have pulled every cap on this (save those on the AM/FM board and they all seem to be close to spec...but again no ESR test ability...yet...At this point I think the scope would be good (once I figure out where to connect it) then start pulling transistors and diodes and testing them one at a time
 
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