tracking down scratchiness - not pots

[larryderouin]

Which protection board do you have in your 727??? The 011 or the 027 board?

 

[rmunson]

Thanks Larry - I have the AWM-027 - my measurements below:
https://image.ibb.co/ksi9zw/awm_027_notes.jpg

 

[larryderouin]

The pic won't show due to errors in the file.

 

[larryderouin]

Replace all the caps, and transistors listed below. The AWM-025 board is the one in the 737 and is the same except for Q1 & Q2. Q3 thru Q7 are the same. Q1-Q2-Q6-Q7 are replaced as a matter of course. The DIODE goes across pins 9 & 10, with the Cathode @ pin 10 (striped end).


AWM-027 Q1 2SC857K-A .........512-KSC1845FTA
AWM-027 Q2 2SC857K-A .........512-KSC1845FTA
AWM025 q3 2sc945 only if bad 512-KSC1845FTA
AWM025 q4 2sc945 only if bad 512-KSC1845FTA
AWM025 q5 2sa733 only if bad 512-KSA992FBU
AWM025 q6 2sc945 512-KSC2383YTA
AWM025 q7 2sc1384 512-KSC2690AYS
chassis prot relay 653-LY4F-DC24

 

[rmunson]

Thanks Larry -
OK - plan is to start with either the power supply board or the protection board -- and continue working from there --

Would you recommend that I start with the PS or the protection?

Thanks!
Reed

 

[Oldsansui441]

Did you happen to work out if the relay contacts were stuck together or do you think the protection circuit is closing them much too quickly after power on without you being able to hear it click?

 

[rmunson]

Thanks OS441 - yes - for some reason the relays were sticking - very odd -
I opened the stereo up last weekend and examined everything - I observed the problem and confirmed that the relays were not releasing -

here's where it gets weird.... on my bench, I was using a household switch and socket wired into a little box - this was how I normally powered on/off the receiver because I was nervous about the selector switch on the receiver wearing out -- I hadn't been using it since I did my lamp replacement project - so with it on the bench, I was using it because it's convenient to power on and off the unit --

with the pioneer plugged into this little switched outlet, the relay went back to opening and closing normally - all I can figure is, because the power off is a forced, kill switch, the power off is so sudden that it releases the magnets in the relay - then on power up, the magnets are fully released and the relay receives the voltage it is expecting and it operates normally --

Because the switched outlet is working, I haven't gone back to test my hypothesis by using the selector switch to power on/off --

Isn't that an odd one? If my hypothesis is correct, there might be something amiss in the selector switch -- or the relay might just be getting old and not functioning normally --

The direct blast of deoxit probably did not help things --

Did you happen to work out if the relay contacts were stuck together or do you think the protection circuit is closing them much too quickly after power on without you being able to hear it click?

 

[larryderouin]

The Selector (Rotary Switch gang) on the 727 is tougher than you think. I've never heard of a x2x series power switch going bad. But that doesn't mean it hasn't happened. There is a .01uf cap (ceramic....C-8 ) across the switch. Turn the main switch off and plug it straight into the wall. Then probe the switch and past the cap on the downstream side for AC voltage. Also check the Line to Neutral "Death cap" (C-4) for leakage. It might be slightly leaking just enough to not allow the relay to disengage totally. Nah, that's not right as you'd have a dead short if the cap started leaking or failed short (which standard caps can do.) Best to put a X-Y2 rated Safety cap(fail open) in C-4. C-8 is optional replacement with regular film cap unless it's leaking.

With the unit Plugged in directly to the Wall, and the power switch in the OFF position, put your DMM on pins 13 and 14 of the RELAY (DC VOLTAGE..Black probe to chassis ground) and check for voltage. There shouldn't be any voltage with the switch off. If there is I'd look seriously at the power switch for carbon tracking inside the case, (clean the switch inside and out, along with burnishing the contacts and make sure they actually separate completely when in OFF position. ) You shouldn't need an external switch/plug box to completely kill the unit.



RELAY is the 653-LY4F-DC24 $10.41 @ Mouser. 12 in stock as of 5 minutes ago

 

[Oldsansui441]

with the pioneer plugged into this little switched outlet, the relay went back to opening and closing normally - all I can figure is, because the power off is a forced, kill switch, the power off is so sudden that it releases the magnets in the relay - then on power up, the magnets are fully released and the relay receives the voltage it is expecting and it operates normally --

Yes very odd indeed, some good points by Larry esp about checking voltage at the relay with the unit plugged in with power switch off. Will be interesting to see what you find. Obviously if no voltage present relay is bad.

 

[larryderouin]

I think you meant if Voltage IS present at the relay, there is a problem in the Protection board.

 

[Oldsansui441]

I think you meant if Voltage IS present at the relay, there is a problem in the Protection board.

Hmm, no l don't think so but maybe l was tired . I was agreeing with you, if there is voltage there with the power switch in the OFF position then there is a fault in the switch. If there is no voltage there and the relay is still closed then the contacts must be stuck together.

 

[rmunson]

Thanks guys -
I did use deoxit on the power switch/speaker selector -- which is when the troubles started -- maybe I damaged the on/off rotary switch...
I'll get the unit on the bench soon to do some checking....
This week the 727 is behaving itself - it has not been doing any snap crackle or popping....
Reed

 

[Oldsansui441]

I did use deoxit on the power switch/speaker selector -- which is when the troubles started

Ahh yes, this might well be the problem. It is not recommended to use any type of cleaner on the power switch, you will prob have to pull the switch apart and clean the contacts and all internals manually. Good news on the crackling/popping noise, what did you end up doing there?

 

[rmunson]

Thanks for staying with me on this one guys -
I measured pins 13 and 14 of the relay -
with power on, relay pin 13 measures 8.2 and pin 14 measures 33.7 (measured at DC 200)
powering off and watching the dc current fall, I observed:
pin 13 and pin 14 show trace DC: -.26 when bumping the DMM down one level of sensitivity (from 200 to 20)
does that indicate presence of DC voltage at the relay that shouldn't be there?

regarding snap crackle and pop, the issue is still there - so I've probably got a faulty transistor as originally suggested -- I'm working up the courage to go into the board and recap/replace transistors --

 

[rmunson]

OK - so thinking about this snap crackle and pop -
from looking at other threads, it sounds like the probably suspect is a transistor - I'm going to start with the power amp board because I do not get the snap crackle pop when sending audio out to another unit via PRE OUT - so my hypothesis is the issue is downstream from the preamp side -
http://audiokarma.org/forums/index.php?members/oldsansui441.226028/
Oldsansui441 recommended starting with main amplifier input differential pair, -- on the 727 that is 2 pairs of 2SA572 -- highlighted here: https://image.ibb.co/dwDMbb/poweramp.png

should I use the same transistors - 2SA572 -- or is there a recommended replacement?

Thanks!
Reedhttp://audiokarma.org/forums/index.php?members/oldsansui441.226028/
http://audiokarma.org/forums/index.php?members/oldsansui441.226028/

 

[Oldsansui441]

does that indicate presence of DC voltage at the relay that shouldn't be there?

This is fine, the DC voltage will not fall straight to zero immediately, some mV will be normal.

should I use the same transistors - 2SA572 -- or is there a recommended replacement?

As l mentioned in post #9, a suitable and reliable replacement is KSA992, if you try and purchase 2SA572's they will most likely be fakes.

 

[rmunson]

Thank you Oldsansui -
zeroing in on the 2 pairs of 2SA572s on the main amp -
I have identified the right channel as the snap crackle pop (scp) culprit -

this far upstream from the speaker outputs, is it possible to identify which of the pairs or which series of transistors are right channel (and are making the SCP)? Would blasting the right channel pair of the input differentials with compressed air when the SCP is occurring help me confirm that one or both of these two transistors is the source of the SCP?

Or is it not that simple? one side or chain of transistors on the schematic is not right side/left side ---

I would like to try giving certain transistors a quick blast of compressed air to see if it effects the SCP -- as PeteHall recommended in post #26

 

[petehall347]

freeze spray would be better .

 

[rmunson]

cool -
I've never heard of freeze spray -

am I on the right track in thinking one pair of the input differential transistors is right and the other pair is left?

Thanks!
Reed

 

[petehall347]

yes if only one diff pair per channel .