Sony receiver blowing fuses, replaced blown output transistors

Thread Starter

Eversr007

Joined Apr 10, 2016
40
So I took this receiver apart and found two of the output transistors where blown, the main fuse, and two smaller 6.3 amp fuses. I replaced all and plugged it back in and fuse didn't blow standby power came on. So far so good, but when I pressed the on button the main fuse blew along with the two 6.3 amp fuses. So I took back apart and checked most everything. Is there a logical way to narrow this down. I will circle what fuses and transistors I replaced in the pictures. Thanks for any help I am learning alot!!IMG_5717.jpgIMG_5719.jpgIMG_5722.jpgIMG_5718.jpg
 

Hymie

Joined Mar 30, 2018
877
Are the transistors you replaced OK?

It might be that the driver transistor to these output transistors has failed – switching at least one of them on (all the time).

The above was a common fault in the days when I used to fix power amps with discrete components.
 

Thread Starter

Eversr007

Joined Apr 10, 2016
40
Ok thanks. Where can I download the service manual? Also which ones are the driver transistor? And just out of curiosity I attached another photo. Can any one tell me what this component is. Under a microscope there are no markings except one has an N on it.. Thanks for taking the time to help.IMG_5724.jpg
 

eyesee

Joined Oct 19, 2013
60
And just out of curiosity I attached another photo. Can any one tell me what this component is. Under a microscope there are no markings except one has an N on it.. Thanks for taking the time to help.View attachment 159123
Q855 is a small surface mount transistor. There should be other letters/numbers on it if you look closely at different angles.
 

abrsvc

Joined Jun 16, 2018
41
Check the emitter resistors (large white rectangular parts near the output transistors). The value should be marked on the part. I suspect that one is open. You can also check voltages by not connecting the center pin on the transistors. Check the voltages on the 3 pads and report the values here.

Dan
 

Thread Starter

Eversr007

Joined Apr 10, 2016
40
Check the emitter resistors (large white rectangular parts near the output transistors). The value should be marked on the part. I suspect that one is open. You can also check voltages by not connecting the center pin on the transistors. Check the voltages on the 3 pads and report the values here.

Dan
I have the board out of the unit to check things. I check the ohms of the white emitter resistors and they all had about .4 to .6 ohms. I don't think I can check voltage cause it blows main fuse and two on the big board. But the other two fuses on the big board are untouched. Could there be a bad capacitor that would short that much power. It has standby power and is all good until the second I press the on button.
 

abrsvc

Joined Jun 16, 2018
41
Leave the center pin of the transistors disconnected and power up the unit. You should be able to trace voltages that way. Measure the voltages at the 3 transistor pads (be careful not to connect the center pin when doing this) and report the values here. I am suspecting that the stage prior to the outputs has a problem, most likely shorted transistors there too.

Also, what is the model number so I can look up the service manual?
 

Thread Starter

Eversr007

Joined Apr 10, 2016
40
Sorry for not getting back sooner. I got really busy and kept pushing this off to the side. So the Model # is STR-DH750
I desoldered all center pins and measured voltages. All of them but the 2 that I replaced. So here goes.

The ones that measure different than the rest are

MN2488 Left Pin- .653, Center- .000 Right pin- .000
Mp1602 Left Pin- neg.654, Center- .000 Right- .000

All the rest of them measure on average=

MN2488 Left Pin- .853 / Center - .178 / Right -.000
MP1602 Left Pin- neg.795v / Center- neg .172/ Right - 000

Thanks for the help, not sure if that narrows anything down.
 

Ylli

Joined Nov 13, 2015
984
So you have the collectors of all the output stages (except the one you fixed) lifted. Does it power up without blowing any fuses? Can you get audio through the one intact channel? Are the rails (where the collectors connect) +60 and -60 volts? Voltage across each side of R603 (each side to center)? Voltage at center pin of R603 to chassis ground?
 
Last edited:

danadak

Joined Mar 10, 2018
4,057
Any caps bulging at top, a little or a lot ? Any stains around polarized caps
on the PCB where they might have leaked ?

Regards, Dana.
 

Thread Starter

Eversr007

Joined Apr 10, 2016
40
I have all collectors desoldered even the ones I replaced. Unit does come on without blowing any fuses. The rails that the collectors go to have -60 volts on every other one but can’t get to the other ones except from the bottom side of the board which would be hard to get to with it plugged in. Voltage at center pin of R603 to chassis ground is .003. Voltage from center pin to outside pins on R603 is .000. Didn’t see any stains around caps and all look good to me visually. I did notice something different when measuring transistors. The ones that i replaced measure different than the other pairs. Checking with multimeter in diode function.


mp1620= +probe on collector, - on emitter measures .508v then reverse leads and .493

On all the rest of the mp1620 I get nothing going from neg on collector and positive on emitter


Mn2488 same on this one except the others do nothing when i put positive on collector and negative on emitter.


So there must still be something wrong on that chanel right. I’m pretty sure I tested the new ones before installing. Should I pull those out of circuit and test again. Or is there something back feeding causing this. Thanks
 

Ylli

Joined Nov 13, 2015
984
With power off, and after allowing the rails to drop to zero, measure the resistance of each collector lead to chassis ground. If you can get a good connection on the heat sink itself, make the same measurement there (the heat sink may not be connected to chassis ground). You should see opens, we do not want any of the collectors shorted to ground.

Still with power off, connect the neg lead of the ohmmeter to the center TP of the one of the emitter resistor blocks (such as R603). Pos lead to the collector of each of the associated transistors. Here you are looking for any transistor with a collector-emitter short.

If we don't find any collector to ground faults, or any collector - emitter shorts, then the next step would be to reconnect the collectors on *one* channel and see if that channel works without blowing any fuses. You might want to have extra fuses on hand for this part.
 

Thread Starter

Eversr007

Joined Apr 10, 2016
40
Ok. Thanks. With power off collector pins on the 2488 and 1620 that I replaced are shorted to ground. But I just noticed that all the rest of them have a piece of plastic separating the transistors from the heat sink? Is that what is causing my short. I must have not transferred them with the new ones. I thought it was supposed to be metal on metal behind them? If I need the plastic what kind could I use? Cause I'm sure I don't have them anymore. Also all the 1620's have 356 ohms on collector and the 2488 are OL. I'm thinking I messed up with the plastic pieces......
 

djsfantasi

Joined Apr 11, 2010
6,950
You can buy replacement pads on Amazon. Assuming your transistors are in a TO220 case. Here’s the link:

https://www.amazon.com/DealMux-Sili...l+heatsink&dpPl=1&dpID=41lGy8jAwSL&ref=plSrch

It’s probably more than you need. But it solves your problem. You can pack up the remaining 995 or so and offer them for sale. You might recover your $10 or so.

Note: I don’t think those are plastic pieces. Plastic would melt. The link posted is for silicon thermal insulators.
 

Ylli

Joined Nov 13, 2015
984
They are TO-3P packages. Bigger than TO220. The mica insulators can be found at https://www.amazon.com/Palomar-MIC-3P-440-10PK-Transistor-Mounting-Hardware/dp/B075363MD1 . Also should have some thermal paste. As long as we are at Amazon.. https://www.amazon.com/White-Thermal-Grease-Compound-Silicone/dp/B00VLQX7VQ .

In the mean time, check the rest of the power transistors for collector to ground and collector to emitter (emitter resistors test point) shorts. Maybe we can get the other 6 channels working while you wait for the insulators to arrive.
 

Thread Starter

Eversr007

Joined Apr 10, 2016
40
Sorry to take so much time between post. I installed the transistor insulators and tested everything and all seemed good. Powered it up and every thing works. Test every channel and all is good. Thanks for all the help and patience. Defiantly still learning. Can't believe I overlooked that when I changed them. Now I will never forget that one. Thanks
 
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