Recapped and restored sx-750 50hz hum and white noise problem

[Cichoslav]

Hello, after a 3 day fight I gave up. My sx-750 started to generate a very quiet but audible hum mixed with some white noise on both L and R speakers. Not volume dependant. Unit is cleaned and fully recapped, worked for some time with no issues at all.

I unplugged L and R signal posts on amp pcb to check if it's still there - it is, so it's not a preamp, vol or tone control issue.

I replaced 2sc945 transistors on power supply board, tested all 3 to-220 transistors, replaced 330uF 50V caps with 470uF 63V, tested 3 zener diodes, (13,7V, 24,6V drops on negative line, 13,8V on positive). Added 4700uF caps in parallel to main filter caps - nothing better.

Voltage on pins 16 and 18 seem to be uneven.
Pin 16: -41V
Pin 18: 39,6V

I'm using dbt and my power line is 220V at the moment, but due to PV installations allaround I switched the unit to 240V as my power line exceeds 240V on sunny days.

I scoped the speaker outputs, both seem to have 50hz with some white noise distortion that's audible, it's about 15-20mV. It's new to me toy, so if I have to make any oscillograms for further analysis, I need some instructions.


Any help will be greatly appreciated.

 

[zebulon1]

Can you take some AC readings on those uneven DC voltages?

 

[Cichoslav]

Can you take some AC readings on those uneven DC voltages?

I sure can as soon as I come home back from work. Thanks for reply, we've already met here when I was battling with another pioneer gear

I'll measure all supply pins for DC and AC readings and list them below.

 

[zebulon1]

Help with hum in a Pioneer SX-750

I bought a Pioneer SX-750 receiver on ebay and it has a hum in both channels A, B and in the headphones output. Everything else seems to work fine except for a few scratchy pots. I should say that even with the hum it sounds pretty awesome at high volume! So, to fix the hum, I know first I...

audiokarma.org

 

[Cichoslav]

Help with hum in a Pioneer SX-750

I bought a Pioneer SX-750 receiver on ebay and it has a hum in both channels A, B and in the headphones output. Everything else seems to work fine except for a few scratchy pots. I should say that even with the hum it sounds pretty awesome at high volume! So, to fix the hum, I know first I...

audiokarma.org

Unfortunately c17 is not the case as I replaced it already at first recap with the same parameters, and on saturday I replaced them again with 470uF 63V. No difference. But I seriously think of swapping all 50V rated caps with 63V ones in the supply.

 

[Cichoslav]

Ok, so after checking voltages on the power supply pins we have:



I used my dmm with DC blocking 100nF cap in series with positive lead.

Pin 18 oscillogram with C17 470uF cap attached. With original value C17 330uF cap AC on dmm rises up to 1V.




Edit: i tried to narrow it down as well as I could, tested both rectifiers. All diodes tested well, 570mV drop for small ones, 530mV for big ones (in circuit, power off)

Also tested both 2sd313 transistors and seems like Q8 (replaced earlier with mje15030g) has straight line voltage on its collector, but Q10 (original 2sd313) has AC ripples on collector. Maybe this will help with looking for a culprit.

Edit2: I'm thinking of replacing C9 and C13 with higher value capacitors just to make sure, as the negative 37V voltage line (pin 17) seems to have some AC. I'll try to confirm it with scope later.

 

[bzenic]

I've had about the same problem on my SX-780. Unfortunately even after complete recapping and replacing output STKs with discrete modules the hum is still there on both channels, about 20-30 mV.
I am not convinced that the problem is some grounding loop, everything seems too repeatable and consistent but would not exclude it. To make things even crazier, I have another SX-780 without this issue. From my experience the hum is by far the hardest issue to solve.
Good luck with your effort.

 

[zebulon1]

Q8 is clean and Q10 has the 1v ripple?
I think if you want to check if a coupling cap is causing a signal loss you can short across with a good cap. But otherwise you remove test or replace the cap.
You might be working around a bad part? C17

 

[Cichoslav]

Q8 is clean and Q10 has the 1v ripple?

Yeah, I found out why it had that ripple later when I checked the schematic... Somehow I suspect bad ground somewhere. You know these units, the wires they used are fragile and don't like too much bending. That's why you can see solder blobs on few pins. I'll try looking for broken grounds as I didn't think of it earlier. What a mysterious and demanding piece of gear... These and unexperienced amateurs like me don't seem to get along.


Also note the output voltages - positive lines seem to have no ac ripple, but -37V on pin 17 has 0,2V AC and this voltage goes to amps differential pairs, and through a resistor it's lowered to -19V and goes to fm, tone control and phono section. Most likely rippled.

 

[Cichoslav]

@zebulon1 I found it! Murphys law strikes again. I Removed d313 transistors and checked them outside the circuit, made some measurements and tested b507 transistor in circuit. Measurements and voltage drops looked pretty fine. I don't know what happened but today I checked all grounds and decided to remove that b507 transistor. Guess what! Two resistors. Shorted. Replaced it with b633, hum gone, just some hiss but so quiet you can't tell it's there until you put your ear next to the speaker.


the culprit


-37V ac ripple on pin 17



-37V after replacing b507 transistor.

Problem gone, finally. Week of chasing my own tail, yet the issue was so simple.

 

[zebulon1]

You didn't need any help.
You made it look easy.
Persistence for sure. I hate to think what I would of done knowing the -48v was okay.

 

[Cichoslav]

@zebulon1 honestly I can't figure out what emotions are standing behind words you said. Translator didn't help either.


I sincerely hope that folks who will be battling with hum in their sx-750's find this thread helpful, as this time it wasn't the caps that needed replacing.

 

[analogwisdom]

@zebulon1 honestly I can't figure out what emotions are standing behind words you said. Translator didn't help either.


I sincerely hope that folks who will be battling with hum in their sx-750's find this thread helpful, as this time it wasn't the caps that needed replacing.

I think he was trying to congratulate you on a job well done. At the same time he was lamenting that he wouldn't have been able to figure it out so easily. Good emotions

 

[zebulon1]

Very good work!

 

[Cichoslav]

Final tests oh how happy I am.

Next in line for full resto work - Diora Merkury DSH-303A, Philips 22ah799, Teac AG 5700, Sony STR 7055A. Tons of work, but I love bringing old stuff back to its former glory.

 

[Cichoslav]

Well, seems like I started to celebrate too soon.

I noticed that hum still exists, but only when it's plugged at home. At my workbench with receiver connected to small 10W speakers it's all fine even at full volume. I tried rotating the plug, 60hz hum still present but it's totally volume dependant. Starts at like 10 o'clock on volume pot and it's getting louder. Acts and sounds like rca's hanging. Speakers sensitivity is 93dB. Jamo 704

Aux, phono, doesn't matter. Both plugged in, TV connected to aux via rca-jack cable, turned on, off, no difference. Phono turned on, off, no difference, grounded to 750 chassis. But! I changed the receiver and plugged in my sx-780. Same thing happens and it's also fully serviced and recapped with good grade Gold nichicons. Can this hum be caused by tangled wires behind my cabinet and other electrical devices like fast phone chargers? Something transfers 50hz hum. And by ear I can tell it's more like triangularish, not sawtooth.

 

[Cichoslav]

Plot twist! What an unbelievable coincidence.

Today I plugged it in again. No hum for few minutes, then hum. Then few minutes of silence and again few minutes of hum. I was wondering what's going on. Found the culprit. It's caused by an oven in the kitchen. When heater is turned on, it causes hum. When it's off - it's all good. My hair almost turned gray because of that hum hunting. Phew... The unit is fine. It's some electrical problem with my oven.