Pioneer SA-7800 - Help replacing transistors!

groenhen

Active Member
Hi,
Just got a SA7800 from ebay...
Problem is : one channel is not working,,, probably was fried.
Primary investigation reveals this on board GWH-118:
- power transistors both dead. 2SA1075 and 2SC2525. What should I buy as replacement from mouser?
- 2SA913 and 2SC1913 (Q23 and Q26 I think - hard to read the marks). What should I buy as replacement from mouser?
- R33 (560 ohm) is scorched
- R57 (150 ohm) 0.5Watt is scorched
- R59? (150 ohm) 0.5watt is scorched.

What could have happened?...
And more important: if I replace all these transistors with new ones from mouser, should I change some other components too? I mean the resistors values will remain the same as in original schematics?

Please help, I really want to bring this amp back to life!
Bare with me, I'm a sw programmer but I like electronics too :)

TIA,
Stanley.
 
Caution, tread slowly and carefully - you are tackling a NSA amplifier - a very non-forgiving and complex circuit. You really jumped in the deep end here.

IF you follow my directions carefully and thoroughly AND NOT JUMP AHEAD, it MIGHT not blow up again when you turn it back on.

The major challenge may be physical - those output transistors don't come in that case anymore. And while I don't remember directly (pictures!!) the area around where they are mounted might be rather crowded.

When I say 100% component testing, I MEAN IT!! The damage is widespread, and the cause is still lurking within.

Sigh, this isn't going to be easy. Especially for me. pages and pages of typing.

Check the area around Q5 first, look for HIDDEN solder joint cracks. Thermal, circular ones.

Eventually every transistor will at least be tested, if not pre-emptively replaced, along with the diodes (except the stv-2h) and every resistor tested with one leg lifted, for correct value.

Read some 3800, 3900, sx-d7000 amplifier threads, it is the same type of amp circuit.

IF you find parts lists for the sa-7800 and they are from 2012 or earlier, and / or they have ksa910 or ksc2310 transistors in it - DON'T USE IT. Those lists BADLY need to be upgraded and NOT just because you can't GET the 910/2310 transistors...

plus you will want to work on the upside down phono amp board, that's where I had concentrated on in the past, and have a bunch of pictures from. It (the phono amp) COOKS itself to death.
 
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one of my suppliers has these http://www.nikko-electronics.co.uk/products.php
SmallImage_23485.bmp

they are larger than the originals if they are same as i bought a long time ago .
they are expensive too .
 
Hi Mark!
Ok, step by step, as I have plenty of time :)
I really wanna do a good job... I wanted this amp since I was 10... Now I'm 44 :(

I'm an expert in AVR microcontrollers. I made a super multi tester, so I can test all components: transistors, caps (ESR included), resistors, inductors, etc.
(project is from internet, not my idea. I just built one for myself).

I already mentioned (see first post) a list of damaged parts.
Took your advice and tested Q5 and surrounding area... It seems to be ok according to multi-tester. Q5 has a logo similar to Mitsubishi, but different somehow... Could it be former logo of Panasonic? I mention this as it seems the GWH-118 board had some intervention(s) on it... not sure yet... dust doesn't seem equal on all components ;)

Honestly, I don't trust old list of transistor replacements. That's why my initial question still stands: what should I buy for finals (2sa1075 / 2sc2525) and for those ("drivers" I think they are) 2SA913 and 2SC1913 ?
I prefer mouser as I will order e-caps from them...

Mark sorry my ignorance (bare with the SW guy) but what is it got to do with the phono stage?!

My uncle, he is about 78, an old school guy. He saw the amp and he smiled. We are in a serious competition. He is a guru of all kind of analog circuits, I am good at digital.
He said he can fix it in 1 hour but he won't do it as he wants to see me sweating :(

So, Mark I promise to follow you advices carefully just to show him I can make this amp work...

BTW, I tried to search the forum... Results were from 2 years ago, something like:
2SA1930 pnp AND 2SC5171 npn for "drivers" ... but not sure it's the best choice...?

Regards,
Stanley.
 
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The major challenge may be physical - those output transistors don't come in that case anymore. And while I don't remember directly (pictures!!) the area around where they are mounted might be rather crowded.

Now THAT is really not a problem! My second name is Mr.Bricolage (DIY in French). And I have some colleagues at the office that can help a lot (they are from HW / service department).
 
Ok, here's what I did so far... :

- removed scorched resistors (R33, R57, R59)
- removed damaged power transistors (2SA1075 & 2SC252)
- removed damaged "driver"? transistors (2SA913 and 2SC1913 )

- put new 150 ohm/0,5Watt for R57 and R59.
I don't know if it's ok to put or WHAT to put for R33 (original was 560 ohm). I'm not expert, but I have the feeling the value of R33 should be "adapted" to new transistors...
I'm not sure... Maybe you can help me with this issue!?!

- the new transistors (power & drivers mentioned above)... nothing yet. Not that I don't have them. But even more, I do not know what should I buy as replacements!?!

Thanks for your time!
Stanley.
 
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But even more, I do not know what should I buy as replacements!?!

I will GET to THAT.
It will be on the LIST!!!

You say you have plenty of time and want to do a good job. Then the next words posted contradict that completely, pushing me on individual details that I have to figure out.

Right now the list I have needs work, and that means a few hours of my time to reverse engineer things, reexamine my old transistor choices in light of learned knowledge applied to similar but not identical amps.

AND SUNDAY is the day I have company over. Period.

If you have not seen MY lists from the past, to understand what I have to do, you need to start looking and reading.
The LIST is a turnkey solution. EVERYTHING is covered, including a cut and paste text block that mouser's parts list importer slurps up like candy.

YOU on the other hand CANNOT just go on appearance of components. When I said 100% testing, I damn well meant it. EVERY resistor in the thing must have a leg lifterd and tested. Diodes except the stv will be replaced, as will the transistors.

Now FIND that solder joint crack I mentioned or we are going to spend a lot of time figuring out WHY it blew, so it damn well doesn't do it again.

And the phono amp -
(while I was doing my profession of embedded software engineer,
I did NOT let bad code lie, because it wasn't in the area I was currently working on.
I fixed it, because more sooner than later it was going to bite me in the ass)

it is run too hot in class A mode of the output stage and for the transistors they spec'd under the heat dissipation conditions they built. When it goes, it most likely will take out the regulated supply as well. Going off the top of my head the 2sa912 to-92L transistor gets replaced with a to-126 transistor and a heat sink. Serious heat producing resistors are moved away from the board, onto the other side where they will not bake components to death.
 
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If you have not seen MY lists from the past, to understand what I have to do, you need to start looking and reading.
Searching audiokarma, I could only find a thread started by leesonic with a problem quite similar to mine... same amp, same burnt resistors, same transistors, etc BUT few years ago.

I ruined the chances to find the solder joint crack, as I already measured about all components by pulling them off circuit...
 
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Hmmm, a little bit OFF topic, but anyway, who knows...
I was afraid to break/crack the FL display while buzzing around the amps circuits.
So I decided to remove it... [board# AWV-001]
BTW display is fully operational... but I thought better put it away while restoration,,, and visually inspect it... clean it (dust), etc.
Two things caught my attention:
1) Q2 (2sd712) metal part is kinda bluish... it probably runs hot?!? Should dissipate about 0.8 - 0.9watt according to my calculation.
2) resistor R13 (2.7k) is a little bit smoked. I measured it and it's ok (2.7k precisely).
Ok, there is a third one:
3) R15 (1.8k / 1watt) had a solder joint crack!

- I cleaned & washed the board,
- removed old pins (for wrapping) and put some decent connectors instead
- soldered wrapping around R14 (270/2W) resistor pins and ... shortened them as they were unbelievably long
- mounted heat sink on Q2 - 2sd712
- reinforced all dubious solder joints and... removed some tiny solder balls from circuit!

Now it looks like new, unfortunately pics taken with mobile phone (camera ran out of battery)

EDIT: I'll be damn... those two little (white) semi-adj resistors seem to be (made by) Alps ?!?
Except 2 that are NCC, all capacitors on this board are Nichicon. Hmmm, did they (PIONEER) use Nichicons at that time [1980] ?!?
 

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It CAME from the FACTORY with Nichicons.

Big grin means they "woke up" on caps.
 
working on sa-7800 power amplifier transistor / diode / cap list including power outputs.
 
working on sa-7800 power amplifier transistor / diode / cap list including power outputs.
Thanks for help! No need to rush, I have plenty of time and I want to do a good job :)

Off topic a little bit... but again, who knows, maybe not: I saw that orange power-on lamp sometimes it works, sometimes it doesn't. Bad contact(s) I suppose...what else can it be? My colleagues suggested a LED-conversion would be optimal solution ?!... but ok, that's my last problem :(

Ummm, I may seem naive but I really need to ask: Those power-transistors 2SA1075 they sell on eBay... are they genuine Fujitsu? It's weird that on ebay they are green and AFAIK (I also had a Pioneer SA708) they were all black (or kind of dark grey)!!! My uncle says those from ebay are chinese crap and are only good as a "trinket" on a key-ring... after cutting off their legs :scratch2:
 
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Listen to UNCLE!!! Fujitsu hasn't made those transistors in at least a decade. The replacements that MTF will specify in his BOM will most likely be either ONSEMI's or Fairchilds from Mouser. STICK WITH A MAJOR DISTRIBUTOR for output transistors, PERIOD! NO E-PREY CRAP! Do exactly as he says, don't get ahead of him, and you'll end up with an amp that'll probably run good for your Grandkids!(let them recap it again. )

Larry
(One of Mark's students...........with the lumps on the back of the head to prove it too) :D .
 
I would think that a standard list of components, will do the job
1) VAS, Fairchild ksc3503/ksa1381
2) Drivers, Toshiba 2sc4793/2sa1837
3) OPS, OnSemi mjl3281/mjl1302 or mjl4281/mjl4302
All available at Mouser
 
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My uncle visited me today...
He already knows my plans, but he insisted just to see the amp on inside.
I agreed. After a close examination, He said:
1) "This task is too complex for you. This is not SA708, this is NSA! Very complex circuit!"
2) "You should replace all e-caps..."
Me: "ELNAs too?!?"
UNCLE: "Yup!... All means ALL!"
3) "If you upgrade one channel, you must upgrade the other channel too! They MUST be symmetrical!..."
4) "The pots and switches should be cleaned and lubed"
5) "Put heat-sinks on these transistors..." - he indicated some TO-220.

Other things he said, but I don't remember all... Not bad for an old-school-er... I think :(
(unfortunately he is too old and shaky... I mean he has some health problems).
BR,
Stanley.
 
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I've walked others through NSA repairs. Successfully.

The slow down is relating this one to already done work on similar circuits in other models. And not repeating old mistakes.

And had he and I talked, he would have backed me on the phono fixes, based on what he did say.
 
Brian; UNCLE sounds like he's his sh** wired up in one bag! He might be "old and shaky" but his knowledge is something you can cherry-pick to increase yours.

My dad is 82 now, a former Radarman/ET in the NAVY. He knows tube circuits like crazy, but he gets lost before I do with Solid State. But still there are similarities that he can help me with. And watching him playing around with 60-70 year old military radio/radar gear is just plain awesome. I've learned more from him in the last 5 years on tube gear than the previous 54 years. But I've still got a lot to learn.

Use UNCLE as a sounding board. Listen to what he does say, and even to what he doesn't. He can teach you some things that may be too hard to teach via text. All you have to do is ask. The worst he can say is no. But if you approach it in a way that shows you are willing to learn from him, he may very well go for it.

Between Mark The Fixer and UNCLE backing up, you'll get the 7800 running very soon. DON'T rush it. Even Brownies take time to make.

Good Luck

Larry
 
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