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Sansui 661 HUM problem

Greetings Audiophiles,

Here´s my progress report! Recapped the entire Driver Board, and also changed the "trannies" on the Sansui 661 Driver Board, which according to many people are prone to problems, the 2SC1124 and 2SA706 (TR06,07) and (TR11,12) respectively. Also changed the 2 (C21,C22) (0.1uF 50V MC) green caps. Cleaned the board and performed the DC Offset according to the instructions in the Service Manual. The trim pots, I have to say, are very unstable, just looking at them makes the DMM go crazy but I managed to keep it within specs, +-10mV. Sounds EXACTLY THE SAME as before. That fricking hum is still there, on both channels, independent of volume. I don´t know what else to do on the Driver board. Should I change TR09,TR10? On the Service manual they figure as 2SC1124´s but on the board, as you can see, they are "Sony 01124 DW12", and they are somehow "joined" with the TR07,TR08 transistors. If I take one out, they other will be left hanging.

Here´s the PSU, completely recapped, and the Driver Board along with the F-1499A from the Service Manual. Any suggestions would be appreciated!
Sansui 661 Trannies_10.jpg
Sansui 661 PSU Board Recapped.jpg
Sansui 661 Driver Board what else to change.jpg

In the meantime, should I go ahead and start recapping the Tone Board?

Cheers!!
 
Hum in both channels would point to power supply issues eg, tired voltage regulator.
Constant hum independent of volume suggests hum introduced after volume control.
Is the hum audible when seated in normal listening position or only when standing within within 1-2meters
of spkrs, common for vintage units to hum a little (they're old, forgotton the words?...)
 
Hi mbz,

Thanks for your tips! It´s not a "vintage hum". It´s a hum that is an "annoying problematic hum". When you crank up the volume, obviously it is inaudible and not a problem. If I turn the "Speaker Selector" knob to "Off" with the unit powered on, there is NO hum. All the caps on the Power Supply board are brand new Nichicon caps. Don´t know what you mean by "tired voltage regulator"?

Cheers!!:beerchug:
 
Last edited:
Hello again,

Forgot to mention.
What if I place a little "bipolar" small cap underneath the PSU? In place of that little "resistor" I could place a ".01uf film cap" or a Carbon capacitor like those one in the pic? I saw another 661 on youtube that instead of my NEC? resistor it had a ceramic one underneath. Would that help filter out the DC that might be leaking?
Here´s what I have on mine:

index.php

index.php

index.php



Saw this on another Sansui 661. Don´t know the value of it. Here´s the link of the youtube video. You can see it clearly at minute 0:43.

index.php
 
Last edited:
Zeus12 said:
Don´t know what you mean by "tired voltage regulator"?
Click to expand...
A voltage regulator will "smooth out" ripple, a tired regulator will let some ripple through. A failed regulator (eg, c-e short)
will let "all" the ripple (120Hz) through affecting both channels.

Zeus12 said:
Would that help filter out the DC that might be leaking?
Click to expand...
Hum is AC. Do you have another (DC) problem?
 
Hi mbz,

Thanks again for your nice explanation. Where could I find the "voltage regulator"? Is it on the PSU board?

Cheers!!
 
Many causes of hums, failed/tired voltage regulator is just one of them.

Power supply has +23Vdc to preamp and +/-27Vdc to power amp.
+13.2V to tuner.

You could try removing power supply TR05(2SC875, regulator, preamp supply) and check
if the hum is still present. This may confirm that hum is in pre or power amp stage or...

You should check GND connections to both preamp and driver boards. Reflow if necessary.
Measure resistance between these GND points and chassis, 1-2ohms will cause some hum.
 
Last edited:
Zeus12 said:
So basically it was that little microscopic resistor (NEC RO244 -43 on the bottom) that was put back upside down?
Click to expand...
This looks a lot like a zenerdiode. In the schematic is one located next to c03-04, rd24a, page 23. That's why the polarity matters. This should be connected to + of c03 and ground. R03 3k3 is between the + of c03-04. But according schematic they have there place on the PCB.
Good luck
 
Hi mbz and GrisPato,

Thank you for ENLIGHTENING ME!! :):)

Figured out that polarity matters for the little NEC zenerdiode!! Was crying for 2 days because I put it back in reverse! I have seen so many variations online of the layout of the 661 that I´m more confused then ever. By the way, can that zenerdiode have something to do with it? Can it go bad? Should I change it?

Plan of action:
1. remove power supply TR05(2SC875, regulator, preamp supply) and check if the hum is still present.
2. if hum is STILL PRESENT that means something is wrong on the PSU board, correct?
3. Leave the "zenerdiode" between + of C03 and ground.
4. Measure resistance between the GND points and chassis. Put the DMM on OHM, connect the black lead to chassis and red lead to ground point. Should be LESS than 1-2ohms.

Will check GND connections to both preamp and driver boards again and triple check the solder connections.

Cheers!!!
 
Zener diodes can perform a smoothing action, they only conduct when the applied
voltage exceeds the zener voltage, in this case, +24Vdc. The banded end is "north"
in the fig below, connecting to the + of C04. Reversing the orientation changes it
into a -24Vdc zener. Zeners can certainly fail and possibly go intermittent. Candidates
are D05,06 (rectifier), C02, 03, 04, R02, 03, ZD02 and TR05. Since caps have been
replaced start with TR05 then ZD02.


2. hum still present points to power supply issue to power amp stage, maybe rectifier
since main caps have been replaced.

upload_2021-2-17_19-25-57.png
 
Hi mbz,

Thank you once again for your nice and easy to understand explanation. As you can see, I´m a complete newbie at this!!

The D05, D06, are "bridge rectifiers", from what I learned from my Sansui Au-2200, and I have a few N.O.S. replacements for those. Just to double check, I have the "tombstone" ones, but again, in my 661 they are different. Would imagine, makes no difference, correct?
https://www.silicon-ark.co.uk/10dc-1-dual-diode-by-international-rectifier

Been "ILLUMINATED" AGAIN, with the ZD02. Thanks for the highlight. It was right in from of my nose. I was using a magnifying glass, squinting and trying to figure out the letters on it,:rflmao::rflmao:.

Cheers!!!
 
Hi mbz,

OK! Will do and keep you posted! Thanks for all your help!!:):)

Hope you guys are doing OK in Australia with the so called pandemic. Saw horrible stories on the news.

Here in Spain, we are on a permanent curfew. Not much to do so we have plenty of free time!!

Cheers!!!
 
  • Like
Reactions: mbz
Zeus12 said:
Hope you guys are doing OK in Australia with the so called pandemic. Saw horrible stories on the news.

Here in Spain, we are on a permanent curfew. Not much to do so we have plenty of free time!!
Click to expand...
Doing ok, there was 1 or 2 outbreaks and one very long lock down, maybe lost 1000 country men. UK and USA
are doing it tough. Southern europe during the first phase was bad. Hopefully you get your 661 working, nice unit.
 
Zeus12 said:
Greetings Audiophiles,

Here´s my progress report! Recapped the entire Driver Board, and also changed the "trannies" on the Sansui 661 Driver Board, which according to many people are prone to problems, the 2SC1124 and 2SA706 (TR06,07) and (TR11,12) respectively. Also changed the 2 (C21,C22) (0.1uF 50V MC) green caps. Cleaned the board and performed the DC Offset according to the instructions in the Service Manual. The trim pots, I have to say, are very unstable, just looking at them makes the DMM go crazy but I managed to keep it within specs, +-10mV. Sounds EXACTLY THE SAME as before. That fricking hum is still there, on both channels, independent of volume. I don´t know what else to do on the Driver board. Should I change TR09,TR10? On the Service manual they figure as 2SC1124´s but on the board, as you can see, they are "Sony 01124 DW12", and they are somehow "joined" with the TR07,TR08 transistors. If I take one out, they other will be left hanging.

Here´s the PSU, completely recapped, and the Driver Board along with the F-1499A from the Service Manual. Any suggestions would be appreciated!
View attachment 2134191
View attachment 2134190
View attachment 2134189

In the meantime, should I go ahead and start recapping the Tone Board?

Cheers!!
Click to expand...

Don't change TR09-TR10 at the driver board. If these not shorted, you don't need to change. You can change D01-02-03-04 with 1N4148.
I would change old trimpots with multiturn ones for future reliability.

You should recap tone board. Also change high value carbon resistors with metal films. Not all of them, only high value ones.
 
Greeting Audiophiles,

Thanks Starbender! :):) Here´s what I have done so far.

1. I recapped the Tone board (all E.C.). What about the little Tantalum caps (C01,C02)? They are 1.5 uf 25V according to manual. Can I replace them with WIMA Film?
Sansui 661 Tone Board_13.jpg
Sansui 661 Tone Board_11.jpg
Sansui 661 Tone Board_12.jpg


2. I removed TR05 Transistor 2SC875 on PSU, powered it up and HUM is STILL there.
Sansui 661 TR05 PSU Board2.jpg
Sansui 661 TR05 PSU Board.jpg

Should I put it back and continue with the transistors (problematic ones) on the Tone Board?
I have "KSC1845FTA" for the "2SC1313" and "KSC1815Y" for the "2SC100". I would imagine they are equivalents and they should be inserted the same was as the originals, correct?

You mention "You can change D01-02-03-04 with 1N4148". I think I also have a part for that. The "1N4148TR" DIODE GEN PURP 100V 200MA DO35 on the Driver Board, correct? Don´t know which ones you are referring to the "Carbon Resistors", high value ones? On what board?

Pending to change the trim pots, ABSOLUTELY RIGHT Starbender!! The go crazy just BY LOOKING AT THEM.

Cheers!!!:beerchug:
 
You can change c01-02 with wima film caps. These are looks like solid alu caps.
Put c875e back.
If tone board transistors are all ok don't replace any of them. If you found shorted transistors replace all of them with new substitute.. Check pin compatibility.
change D01-02-03-04 on the driver board > 1N4148.
All resistors on the tone board are carbon. Check schematic and change high value ones with metal film (1mohm or 820kohm etc.)
 
Zeus12 said:
@Starbender...By the way, the 2SC1313 and 2SC100 are on the Top Ten Worst Transistors - noisy, failure-prone,...
I really don´t want to change them if I don´t have to. All I want is to get rid of that fricking HUM!!
Click to expand...

Did you mean 2SC1000? If so, the BC550C is a reasonable replacement.
 
@Watthour

Yes, meant 2Sc1000. I have the KSC1815Y.

Another thing I tried is to put a little film cap (Panasonic 0.047uf) underneath the PSU board, exactly where this is (red wire and neighbor underneath): Got the idea from a youtube video restoration. Unfortunately, nothing changed. HUM is still there.
Sansui Carbon Capacitor from 661 underneath PSU.jpg

Could it be a bad ground somewhere? Any other suggestions?

Cheers!!!
 
Where is 2sc1000's located?

You don't need to replace this 0.01uf cap. Reflow all ground pads and tighten the ground screws.
 
Make sure the edge connector for the driver board is clean, also the socket it goes into - make sure all the 'fingers' of the socket are making good contact.
 
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