Sears Professional Series model 28747 receiver

[newclassA]

I bought this one about a month ago, cleaned it, tried it all good. That's what I thought. After running it for couple weeks, each time an hour or so in low volume it lost the left channel sound but a hummm. Then when I removed the top cover, can see a resistor (green, blue, grey) is toasted next to a PNP 2SA815, but it was ok when I took picture earlier (see photo). I changed the resistor and the 2SA815 with no luck. I don't have any equipment except a DMM, solder and bunch of screwdriver (going to make a dim bulb soon). I have checked AK for Sears stuff and found the following:
http://audiokarma.org/forums/index.php?threads/sears-professional-series-28747-receiver.238318/
http://audiokarma.org/forums/index.php?threads/sears-receiver-that-looks-like-a-yamaha.279088/
etc. and this
https://get.google.com/albumarchive.../AF1QipOZsHewnR2L8q69TjIYF0dOLW7pFcOfn2yua8xE

also found a SM for Sears RE1201 author claims it as a clone.
I was thinking to fix it slowly as a hobby project with AK's help, what do you think, doable?

 

[eizner23]

I don't see 2SA815 in the schematic. It's a TO-220 device. I would guess it drives the output transistor like the 2sa794 in schematic. Maybe 2sb558 output driver shorted. Pull it and test with your vmm .

Guide here http://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/

 

[newclassA]

I don't see 2SA815 in the schematic

I think the schematic is similar but not the same, A815 (green) is near R34 (toasted one). Where do you see TO-220 (A815 is a TO-220 package??) in the schematic. Thanks.

 

[newclassA]

 

[eizner23]

A815 is a TO-220 package. See the black wires going to the big transistor on heatsink, I would remove that transistor and test it out of circuit.

Did you test 2SA815 and that toasted resistor?

You could also take reading of 2SA815 on the good channel and compare with bad channel and post measurements here.

 

[newclassA]

A815 is a TO-220 package. See the black wires going to the big transistor on heatsink, I would remove that transistor and test it out of circuit.
Did you test 2SA815 and that toasted resistor?.

Among four one showed (one that you suggested) is short (beeps on diod setting), so I removed it and tested 0 resistance and beeps on all b,c,e. So I have ordered a 2SCA1619 (NPN) which might show up end of this week. In the mean time, do you think I should look for other component down in the circuit of that transistor.
Got some New 2SA815 and resistor (560ohm, green, blue, grey band).

You could also take reading of 2SA815 on the good channel and compare with bad channel and post measurements here.

Should remove it first and then measure or just the resistance while it is on the board.

 

[eizner23]

What are you replacing a 2SC1619 with? Fyi It's best to replace bad devices on both channels with like device, but shouldn't hinder getting it back in operation mode right now.

Since you found a shorted output, try to find modern equivilants and replace all 4 large TO-3 output transistors with complimentary pairs.


Build a dim bulb tester and use it before powering it back on. But don't do so until all bad devices are replaced. Or remove all 4 TO-3 transistors (one of which seems to be causing catastrophic short), take them out of the circuit then you can power up and test the rest of the circuit downstream without blowing things up.

 

[newclassA]

What are you replacing a 2SC1619

These were Sanken, I found same on the bay and it's coming but I will replace all of them (2+2) with NTE equivalent (nte130 and nte180) if everything come out ok.

I am also replacing the 5.7ohm carbon composite with 5.6ohm 1W Metal Film Resistor, I like to know your thought on that. Thanks.

 

[eizner23]

These were Sanken, I found same on the bay and it's coming but I will replace all of them (2+2) with NTE equivalent (nte130 and nte180) if everything come out ok.

I am also replacing the 5.7ohm carbon composite with 5.6ohm 1W Metal Film Resistor, I like to know your thought on that. Thanks.

You probably want to avoid using NTE parts. We can help you find better replacement, just ask.

The replacement resistor should be fine.

 

[newclassA]

You probably want to avoid using NTE parts. We can help you find better replacement, just ask.

Thank you eizner23. I have no idea about NTE, Google search led to the NTE equivalent. These Sanken 2SC1619 and Sanken 2SA808 are the originals, what are the acceptable replacement you suggest. Also I got the 2SA815 from China through Bay, hope these are ok.

 

[eizner23]

I doubt the 2sa815 from china is counterfeited you should be ok.

Are you located in the US?
If so a company called Consolidated Electronics often has replacement Japanese semiconductors:

(2SA808) http://www.ceitron.com/cart/advanced_search_result.php?keywords=2SA808+&search_in_description=1
(2SC1619) http://www.ceitron.com/cart/advanced_search_result.php?keywords=2SC1619+&search_in_description=1
(2SA815) http://www.ceitron.com/cart/advanced_search_result.php?keywords=2SA815+&search_in_description=1

They also have same parts listed on their ebay store if you want to avoid the minimum order/high shipping fee
https://www.ebay.com/itm/2SA808-Ori...836879&hash=item4634585e3a:g:21QAAOSwPhdU28jm
https://www.ebay.com/itm/2SC1619-Or...238615&hash=item43f7e337e4:g:guMAAOSwx2dYDm0A
https://www.ebay.com/itm/2SA815-Ori...014963&hash=item461c3c11bc:g:uHcAAOxykMpTH1JF

As for modern sub, I couldnt find a suitable sub on mouser.com with high enough fT (10 MHz), at least for the 2sa808 replacement. Maybe post a new thread requesting help on modern, less expensive sub for 2SA808/2SC1619 pair.