Onkyo TX SV727: Found bad transistors, what else?

[Zeppelin1007]

Hey Guys!

So thanks to this forum, i successfully repaired a Toshiba LCD TV power supply. Component level repairs I've always been pretty bad at, but, simply because I didn't fully understand the process. Now the last repair has me inspired to move on to more and more importantly, learn more.

Anyways. off my soap box. I have sitting here an Onkyo TX SV727 receiver. The second you plug it in, the fuse goes poof and the relay toggles rapidly before eventually giving up. So some voltage does flow through this thing as I do get an amber light. However i get no VFD display or anything else.

I started checking with my meter and found 2 of the output transistors being shorted. ok sweet!

But, i also found, somebody else has been in here. Looks like the output transistors have been replaced at one time, and a pretty rough solder job on the bottom. Even more troubling, something in here looks to have caught fire! Or at the very least, heated up enough to disrupt the traces on the board. Somebody did a solder slob job + jumper wire.

The guy who gave me this claimed it was working and went out one day. I think he blamed lighting, but, i checked the small board coming in and nothing seems shorted there.

I'm sure there's the topic of "Is this worth fixing?" Well, to me, if its a few bucks, some soldering and some learning, yes.

But i come here in case one of you guys looks at this thing and goes "Well theres a good chance the microprocessor is shot" then to me? not worth it.

So here's a few pics of the insides:



Starting from the left, transistor 3 and 4 are shorted. (The smaller ones)



And here's the ugly solder fix job somebody did on the front:



It does seem to tone out ok here though.

So what else should i be looking for when these transistors go south?

Thanks in advance guys!

[budm]

Re: Onkyo TX SV727: Found bad transistors, what else?

Start with service manual from:

https://www.hifiengine.com/manual_li...tx-sv727.shtml

[tibimakai]

Re: Onkyo TX SV727: Found bad transistors, what else?

Check all the components, at that channel. The ceramic resistor, may be open as well.

[Zeppelin1007]

Re: Onkyo TX SV727: Found bad transistors, what else?

Ceramic resistors so far just checkout as not shorted. I'll pull em and ohm them out. Grabbed the service manual. Schematics are hard to read a bit but will probably do just fine

[petehall347]

Re: Onkyo TX SV727: Found bad transistors, what else?

resistors dont short out they go higher resistance ..
when changing outputs because they shorted its best to renew the emitter resistors as a matter of course .i have had ones that test ok go bad a while after sending unit back to customer . they dont cost much .

[Zeppelin1007]

Re: Onkyo TX SV727: Found bad transistors, what else?

See thats the kind of good advice I'm glad to find here.

Surprisingly the resistors tested good in circuit, but, I'll follow your advice and change em out too.

Now lemme ask this: can I fire this thing up with the transistors out in order to see if it alll powers up? Or will i take out more components?

The transistors dead i found are a toshiba B1558 and D2387. Ive been reading these are out of production so I'll dig for a cross reference tomorrow. Is it wise to switchout more? Will i have different tone between channels?

Also: i keep hearing about biasing. Is it like biasing a tube amp? Is it needed here? And i do have myself a variac isolation transformer, and an older techtronix scope.

Thanks guys!

[rievax_60]

Re: Onkyo TX SV727: Found bad transistors, what else?

As mentioned by petehall347 in another thread, you can do tests without the output transistors fitted. The feedback loop needs to be closed so that it can actively control the output at zero volts.
Fit 1K resistors or something close across the Base Emitter pads of the missing output transistors.

[jiroy]

Re: Onkyo TX SV727: Found bad transistors, what else?

See thats the kind of good advice I'm glad to find here.

Surprisingly the resistors tested good in circuit, but, I'll follow your advice and change em out too.

Now lemme ask this: can I fire this thing up with the transistors out in order to see if it alll powers up? Or will i take out more components?

The transistors dead i found are a toshiba B1558 and D2387. Ive been reading these are out of production so I'll dig for a cross reference tomorrow. Is it wise to switchout more? Will i have different tone between channels?

Also: i keep hearing about biasing. Is it like biasing a tube amp? Is it needed here? And i do have myself a variac isolation transformer, and an older techtronix scope.

Thanks guys!

B1558 and D2387 are 2SB1558 and 2SD2387 . Always complement to each other as the first is PNP Darlington planar transistor and 2SD2387 is NPN Darlington planar transistor ..

Available on Aliexpress and Alibaba and very cheap .

[madan1]

Re: Onkyo TX SV727: Found bad transistors, what else?

Page 21 ... there are the amp schematics.
Once you have removed the blown transistors, the amp should start. Tr 3 4 5 6 most likely are for the back/surr channels... I cant read the markings on the pcb .
Check out my topic for the amp which keeps blowing.. there's some nice theory there and might help you save few fuses and sets of transistors . Also the design of your amp is similar to the one discussed there.

[Zeppelin1007]

Re: Onkyo TX SV727: Found bad transistors, what else?

Madan1, lot to take in there. Nice little dispute half way through the thread lol.

I like the idea of testing with a 1k resistor. I'll see if we have some here at work.

So whats the purpose of powering up with a variac? will that let the shorted component build up enough heat without popping the fuse and now blowing up other components?

Not having much luck on Aliexpress for the transistors, and im pretty familliar with aliexpress. I do see it listed in other languages though. Never purchased through them that way.

thanks again guys!

[jiroy]

Re: Onkyo TX SV727: Found bad transistors, what else?

Here are the links :
https://www.alibaba.com/trade/search...chText=2sb1558

https://www.alibaba.com/trade/search...chText=2sd2387

[tibimakai]

Re: Onkyo TX SV727: Found bad transistors, what else?

I wouldn't trust Chinese transistors, they could be fake.

[tibimakai]

Re: Onkyo TX SV727: Found bad transistors, what else?

Maybe, you can find NTE ones.

[petehall347]

Re: Onkyo TX SV727: Found bad transistors, what else?

Maybe, you can find NTE ones.

i would never use nte ..just select a proper sub instead .

[jiroy]

Re: Onkyo TX SV727: Found bad transistors, what else?

So whats the purpose of powering up with a variac? will that let the shorted component build up enough heat without popping the fuse and now blowing up other components?

There are hundreds of ways to deal with shorted components , but the primary target is not protecting Fuses , expendable they are . The primary target is finding the culprit , so you need to see some little smoke , or a trained sniffing dog , lol (only kidding) , while raising the Voltage input . Another way is to shunt the fuse with a crocodile bridge while clamping a Multimeter and noticing any suspicious Amperage rising , so , in such case , fuse doesn't exist virtually and the shorted component would be exposed . Once you located the shorted component the rest is obviously easy .

Other ways of course include Freezing , heating , concentrating a high speed fan on a suspicious area , etc ..

Those ways are usually instant creative , and you can implement any idea according to the raising situation .

Fuses are Martyrs

[Zeppelin1007]

Re: Onkyo TX SV727: Found bad transistors, what else?

I was surprised Alibaba had em, but was leory of them being knock offs. Actually I'm moreso surprised some sellers take visa on there. I've only dealt with aliexpress.

Not big fans of NTE brands on here i'm seeing.

Grabbed some 1k resistors from work. Nice big 5W ones (all we had)
Got home, got the iron fired up and!.....i forgot all my solder back at my family's house by my CNC mills I'm working on. So hopefully tonight i can do a power on test.

jiroy, makes sense! I do remember way back in my past while working in a tv shop (i was just a dumb board jockey, no real elec skills) the one guy would freeze IC's on RPTV's to see if that was his problem.

Actually, a guy at my work now has a thermal camera on his iphone. Could be incredibly useful if i keep doing this stuff.

[tibimakai]

Re: Onkyo TX SV727: Found bad transistors, what else?

What is wrong with NTE? The ones that I have used, were made in Japan.

[petehall347]

Re: Onkyo TX SV727: Found bad transistors, what else?

What is wrong with NTE? The ones that I have used, were made in Japan.

NTE use any transistors they can find then put NTE numbers on them .

[jiroy]

Re: Onkyo TX SV727: Found bad transistors, what else?

I was surprised Alibaba had em, but was leory of them being knock offs. Actually I'm moreso surprised some sellers take visa on there. I've only dealt with aliexpress.

Not big fans of NTE brands on here i'm seeing.

Grabbed some 1k resistors from work. Nice big 5W ones (all we had)
Got home, got the iron fired up and!.....i forgot all my solder back at my family's house by my CNC mills I'm working on. So hopefully tonight i can do a power on test.

jiroy, makes sense! I do remember way back in my past while working in a tv shop (i was just a dumb board jockey, no real elec skills) the one guy would freeze IC's on RPTV's to see if that was his problem.

Actually, a guy at my work now has a thermal camera on his iphone. Could be incredibly useful if i keep doing this stuff.

Chinese crap , i know .. Who trust those thieves anyway ? . But necessity apply , you'll have to do some risk to get the Onkyo working once again . Cheap isn't always the answer as it is the trap they use .
I had similar problems acquiring original transistors for an Onkyo , and as i don't trust those chinese crap , i told one customer that he should help me searching Lebanese markets .. To my surprise , he returned with original ones from some unknown local seller , lol .. Maybe you should try the same .

Thermal camera is one hell of an invention , i'll be looking for it , so thanks for the tip ...And good luck

[petehall347]

Re: Onkyo TX SV727: Found bad transistors, what else?

2SB1560
2SD2390

http://uk.farnell.com/webapp/wcs/sto...rue&st=2SD2390