Pioneer SX-880, FM Hum

[markthefixer]

in the interim, you need to learn how to read the road map for these units.
look back to your last picture, that next post has it added now.

my conclusion was reached that the AC voltages as reported as being on these DC power supplies,
if the readings are accurate, are indicative of a serious fault.

In examining this thread, I find you have ducked making and posting fresh DC voltage readings for the parts you have replaced - only reporting that the hum was unchanged.

That is unacceptable.

I need fresh DC ground referenced readings of Q26 & Q30, emitter base and collector from each.

i have been unraveling the poorly drawn dc power supply schematics...

 

[louye2]

Mark, I didn't realize you wanted DC measurements for Q26 and Q30. I thought the priority was AC measurements with the 1uF capacitor in place. That is why I pursued borrowing a higher quality DMM from a friend. I have never "ducked" anything in my life. That is not who I am as a person. I'll post the DC measurements soon.

 

[markthefixer]

I shouldn't have said "ducked" - which implies non cooperation, I should have said "skipped" which implies misunderstanding. I am sorry.

the ac measurements were to be the icing on the cake, instead they practically shout for closer examination of how they are taken

NO FAULT OF YOURS!!!!

roughly said - the more oddball and unexpected the numbers are, the more important it is to trust them.

and this thing is kind of driving me nuts, with regards to potential hum and how the jfet current sources may be stomping out a serious problem.

all the supplies should be humming like the dickens, yet only one is. and it is intertwined and dependent upon the other supplies.

Your ground reference for all these readings should be a bare metal patch of the chassis far away from any other wire connections to it.

If you can make the DC measurements with the borrowed DMM, and include all the digits displayed that would be great.

 

[louye2]

Guys, I may have found the root cause of the FM hum problem. While examining the bottom of the board trying to identify the location of D24-D27, I noticed that one of the solder joints on C407 was missing some of its solder. So, I re-soldered the joint, crossed my fingers, and turned on the receiver. Voila, the loud FM hum was gone. The only negative is a huge amount of static and noise between stations as I turn the tuning knob. Perhaps this is just a matter of cleaning the fins on the tuner with compressed air. I remember reading that spraying Deoxit on the fins is a not recommended.

Here are the current DC volt measurements for Q26, Q30, #7, and #60:

Q26 Q30​

B 13.1v 40.1v
C 25.8v 55.2v
E 12.5v 39.5v

#7 55.2v
#60 25.8v

Questions

1. Given this new data, should I still replace C407, C408, and D24-D27?
2. How does one get rid of the loud static between FM stations?

Thank you for your advice.

Lou

 

[markthefixer]

1. no, not worth the risk right now.

2. static while moving fins or static hands off the receiver?

 

[louye2]

There is loud static between stations while my hand is on the tuning knob or if I take my hand off the knob. The static goes away when I fine tune onto a station signal,

Thanks for your help. Going to bed now as it is 11:15 PM on the east coast!

 

[louye2]

I moved the receiver from my basement workshop to the first floor and attached a better antenna. The static between FM stations greatly diminished to an acceptable level. So, the next steps are to fix the stereo light and the FM tuning meter that does not move. I've confirmed that the red LED bulb works, so the problem is likely on the circuit board. I'll search Audiokarma for threads on these topics and will only open a new thread if I am stumped.

Thank you to everyone who helped me solve the FM hum problem.

 

[markthefixer]

fm tuning meter is switched - so bad/dirty contacts can apply... or just plain low signal strength...

low signal strength would affect stereo light.

 

[louye2]

Guy, I'm going to close out this thread and move on to other projects. The FM hum is gone and the unit works fine when receiving input from the phono, tape, and aux ports. Fixing the FM stereo light and tuning meter probably involves replacing a 16 pin chip on the tuner board, something I don't want to attempt given the possibility of creating more problems if I mess up the soldering process. Since I have other working receivers, I'm going to sell the SX-880 to someone who doesn't care about FM stereo performance or wants to fix the stereo issue themselves. Thank you to everyone who responded to this thread. It's been a great learning experience.