Project: SX-727

[Super Noob]

The sickness continues... picked a SX 727 last week for not much for much fun.

My goal is to gift to a friend - pass on the love.

The offset was a touch off in one channel. drifting around. So I started with the power supply board... full recap & all transistors.

Before:

After:

Parts:

Also changed the plastic hardware and paste on heatsinks...

 

[Super Noob]

Up next was the power amp board...

Before:

Found first differential pair had one transistor changed and another one on the drifting channel. Elected to change every transistor on the board... variable trimmers on order.

After:

Parts:

Repasted Output Transistors and New Micas:

 

[Super Noob]

Offset was now stable and less than 2mV on each channel... up next was the head amp.

Changed all transistors and caps...

After:

All transistors were changed as well.

 

[ivandezande]

Lookin good! Only thing I'd say is bump that 330 ohm resistor on the power supply up to 3W and get it off the board. I've seen those get really hot and leave cold solder joints and a very burnt board.

 

[Super Noob]

Protection Board:

Before

After

 

[Super Noob]

Tone Control Board:
All transistors and caps.

Before:

After:

DeOxit’d the control pots and flushed with Faderlube.

 

[Super Noob]

Tuner Board:

Before:

After:

Impressive tuner...

 

[Super Noob]

Under Caps... and full LEDs.

Rest of parts should be here tomorrow.

Shaping Up...

 

[EastPoint]

I usually redo the power supply diodes in these as well. Not sure how mandatory that is.

 

[Super Noob]

I usually redo the power supply diodes in these as well. Not sure how mandatory that is.

Diode / rectifiers (in this case) might be something I start looking at... hell I am already in there.

Usually focus on the caps and selected
transistors.

Picked up anther SX today. I cannot refuse a screaming deal.

Need of a good cleaning and full treatment.

 

[EastPoint]

Diodes are pretty cheap, and it's a pain when they go bad.

 

[gbroot]

Love those 727's. I've got one sitting out in the garage with issues. Please send me your address!

 

[Super Noob]

Love those 727's. I've got one sitting out in the garage with issues. Please send me your address!

It is not too bad to work on... my final filter caps and relay did not show today. Anyways I will finish it next weekend. Look forward to the test... I expect this thing to sound amazing with new transistors (only outputs retained). Got about 20-25 hours in it.

Test Bed:

SX-727
$60 purchase + $80 parts = $140. Bet it will sound like $1000 when I am done.

My Sansui 800 opened my mind early low wattage SS equipment:

 

[whatadish]

thanks Super Noob for unknowingly answering my post....your picture of the underside of the main filter caps is what I needed. Can you please verify the resistor under the 220uf cap is indeed a 47 ohm? It appears to be yellow, violet, black. Mine is burnt up black, but I do have the amp up and running with a 33 ohm 2 watt in there. I was just guessing at the resistor value based on where it was...it is not in any schematic I have seen, nor is the 220uf cap. If you look at my post you will see a pick of another unit with no cap or resistor or even a terminal strip. Thanks in advance.

 

[whatadish]

Oh yeah I also noticed that there is no reference to the polarity of the main filters in the schematic and if you did not know the functionality of each one in the circuit you could put one in wrong...which is exactly what I think the previous tinkerer did...the top was blown open on one of them and the little resistor was smoked. Guess he unplugged it fast enoughas the only other issue is head amp transistors on one channel not passing signal. The aux port sounds very good though and even with the temp repairs I have made was able to dial in bias and offset dead on.

 

[Super Noob]

Oh yeah I also noticed that there is no reference to the polarity of the main filters in the schematic and if you did not know the functionality of each one in the circuit you could put one in wrong...which is exactly what I think the previous tinkerer did...the top was blown open on one of them and the little resistor was smoked. Guess he unplugged it fast enoughas the only other issue is head amp transistors on one channel not passing signal. The aux port sounds very good though and even with the temp repairs I have made was able to dial in bias and offset dead on.

Am out of town till Thursday night... will get you the value then.

 

[Super Noob]

Finishing Up:

Main Caps Went With 8200 uF:

2.2 uF bypass Caps:

 

[Super Noob]

Top View - Covers Back On

New Relay - Installed.

Screw Those Dopey Plugs.

Relay - 1
Caps- 63
Transistors - 30
Trimmers - 4
Lamps -13 (to LEDs)

 

[Super Noob]

Listening To Now:

Clear and Tight.

Merry Christmas!

 

[kydog]

Beautiful Receiver!! Love mine but it doesn’t get much use. How does it compare too the SX 737 ?