Pioneer SX-990 receiver problems/questions

djmtascam

Active Member
Hoping someone can help me out with this....
I've been looking into finding the problem with a 2amp fuse blowing on one side of the receiver's amp channel board.
I found one 1/2ohm 2W resistor with a broken lead and the white casing crumbled when I went to measure it and lift it out.
The output transistor connected to that resistor shows a shorted C-E (#2A).
I removed the other 3 output transistors to check them and it looks like the rest of them are OK per my table below?
Do I need to get a matched pair for that channel and replace both?

2SC793
+ - 1A 1B 2A 2B
B - E .325 .521 .395 .512
E - B OL OL OL OL
B - C .457 .526 .393 .488
C - B OL OL OL OL
E - C OL OL Short OL
C - E OL OL Short OL

I found NTE130 or NTE130MP replacements available at Mouser.
$4 for one or $9 for a matched pair....is this a good deal?
And I need to find a new resistor...
Is there any reason to replace all of output transistors and recap the amp board?
I'm fixing this for my buddy as part of his Xmas present, so I'm hoping I can get it all done by then....:banana:
Any suggestions or comments would be greatly appreciated!
 
Do I need to get a matched pair for that channel and replace both?
Replace the complementary pair..Yes (2SC and 2SA)
Matching..Nope..

I found NTE130 or NTE130MP replacements available at Mouser.
NTE...:thumbsdn:
:no:...Onsemi..:thmbsp:
MJ21193G and MJ21194G..:thmbsp:
 
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Is there any reason to replace all of output transistors and recap the amp board?

I'd recap it,as long as its pulled and acessable. Replacing all outputs at least on the buggered channel. Onsemis are only $4 each wouldn't hurt to replace'm all. There some very robust transistors compared to the original OEM devices.
 
Search AK Google.. SX-990 recap list,parts list
Markthefixer probably has a pre selected parts list in there somewhere.
 
I might do both channels, all outputs.forget the re capping until you hear it work and sound clear.
If you pull ALL the outputs, is the way you can work on the board, just make sure you don't fuss, breath or bend the leads for the two parts screwed to the heat sink, loosen those FIRST !!!!

I would order new drivers and all the outputs, and both channel input trasistor pairs all at once.

And once you change the outputs , you really have to do the bias adjustment.
 
I might do both channels, all outputs.forget the re capping until you hear it work and sound clear.
If you pull ALL the outputs, is the way you can work on the board, just make sure you don't fuss, breath or bend the leads for the two parts screwed to the heat sink, loosen those FIRST !!!!

I would order new drivers and all the outputs, and both channel input trasistor pairs all at once.

And once you change the outputs , you really have to do the bias adjustment.

Search AK Google.. SX-990 recap list,parts list
Markthefixer probably has a pre selected parts list in there somewhere.

I'd recap it,as long as its pulled and acessable. Replacing all outputs at least on the buggered channel. Onsemis are only $4 each wouldn't hurt to replace'm all. There some very robust transistors compared to the original OEM devices.

Replace the complementary pair..Yes (2SC and 2SA)
Matching..Nope..


NTE...:thumbsdn:
:no:...Onsemi..:thmbsp:
MJ21193G and MJ21194G..:thmbsp:

Thanks for the comments and help but I'm still a little confused...
I found a post from Markthefixer listing MJ21194G to replace the 2SC793Y output transistors. So I only need to buy 4 of those???...no 2 matched pairs like the NTE130MP???
So I assume NTE130 are the same thing but the OnSemi parts are better? They are the same price at Mouser.
I have the board removed from the chassis and already have all 4 output transistors removed....so I might as well replace them now...should I also get new mica insulators??

Markthefixer listed the other transistors:
512-KSA1013YBU (2SA497)
512-KSC2383YBU (2SC497)
512-KSC1845FTA (2SC732)


The 512-KSC2383YBU is no longer available and the alternate is 512-KSC2383YTA not sure if that is OK....I assume that is the same part just a different code at the end??
Can I just buy 4 of each of those as well???...no matched stuff?

Those triple diode thermal "varistors" are mounted onto and the leads go through the heatsink soldered directly to the board...don't know if you can see them in my photos. I haven't touched those.
I checked them and they seem to read like a good diode. 0.6V one direction and OL the other.

I found these on Mouser for replacement power resistors, 588-WHCR50FET
Are these a good replacement part?
I suppose it would be best to replace all those as well. They have some corrosion (green crust) on some of the leads...so they may be ready to let go like the end one did. Are there any other parts like that I should replace for good measure?

I have never biased or adjusted a board. So I may need some guidance or is there a tutorial/instruction on how to adjust this??
I hope the existing trim pots etc...will be OK with the new transistor replacements?

As far as a recap is concerned maybe I should do it now. I'm sort of leaning that way...might as well. My friend probably won't notice a difference but it would be cool to give him a really good sounding receiver he can enjoy for years to come...
Any suggestions for this amp as to types to get.
I have bought audio grade Nichicon's (KZ, KW etc.) before for some of my own stuff...but if there is something better I wouldn't mind trying them out if there were better parts...not too high priced but reasonable.

I'm still not sure why this channel went out so I am still looking at the cause for that. I had pulled the preamp/amp jumper and measured the preamp side and the bad channel was not the same as the other channel.
I still need to see if there is something else wrong upstream.
There is something maybe out of balance there....
And finally when I get this back up and running I think there are some lamps out.
I can't seem to figure out how to get to the dial lamps to test those.
So any advice on what to take apart to get to those would be appreciated.
Those bayonet types on top of the dial face are easy and of course those are seem OK....LOL
Thanks for all your help!
 

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OK my mistake, my eyes wanted to see SX-"950"

Go with the "on" brand outputs.

If you removed the mica and didn't bend or crack them, use the old ones.

That looks like both channels too me,

I highly recommend changing the drivers and all the outputs for both channels. that would make you much farther ahead than even thinking about capacitors at this point.

I would also measure the tiny glass diodes(one end pulled) with an "ohm" meter at room temperature, and just compare a few readings of those.f they all come out fairly close then solder the one end back in.

Also I think the bias is done with a scope, I can't remeber, you ned to find someone who worked on these recently, or find the repair manual.
 
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NTE is a Distributor, not a manufacturer. They take and mark their stuff so that they will xref with more than a couple of dozen items. (Not good!) Their QC is marginal at best and horrible at worst! Only as a last resort in the Solar System will most of us use NTE transistors.

ONSEMI is the generational progeny of the old MOTOROLA Transistor Division. THEY MAKE THEIR TRANSISTORS! The specs are sufficiently robust and tight so that the transistor in question will not be overloaded by the 990. They are proven in any number of amplifiers/receivers/other gear on AK. You'll need 4 ea. of the MJ21194G. Get some Heat Sink Compound. DO NOT Use Artic Silver or any Silver bearing compound. They are electrically conducive and will short out the output. Mouser has non conductive compound. Also Ratshack has what you need.

As for your questions above BU= BULK TA= TAPE mainly different packaging. The TA is fine.

If the ratings on the resistors match what your are replacing they should work.

Schematic HERE

Manual HERE
 
NTE is a Distributor, not a manufacturer. They take and mark their stuff so that they will xref with more than a couple of dozen items. (Not good!) Their QC is marginal at best and horrible at worst! Only as a last resort in the Solar System will most of us use NTE transistors.

ONSEMI is the generational progeny of the old MOTOROLA Transistor Division. THEY MAKE THEIR TRANSISTORS! The specs are sufficiently robust and tight so that the transistor in question will not be overloaded by the 990. They are proven in any number of amplifiers/receivers/other gear on AK. You'll need 4 ea. of the MJ21194G. Get some Heat Sink Compound. DO NOT Use Artic Silver or any Silver bearing compound. They are electrically conducive and will short out the output. Mouser has non conductive compound. Also Ratshack has what you need.

As for your questions above BU= BULK TA= TAPE mainly different packaging. The TA is fine.

If the ratings on the resistors match what your are replacing they should work.

Schematic HERE

Manual HERE

OK my mistake, my eyes wanted to see SX-"950"

Go with the "on" brand outputs.

If you removed the mica and didn't bend or crack them, use the old ones.

That looks like both channels too me,

I highly recommend changing the drivers and all the outputs for both channels. that would make you much farther ahead than even thinking about capacitors at this point.

I would also measure the tiny glass diodes(one end pulled) with an "ohm" meter at room temperature, and just compare a few readings of those.f they all come out fairly close then solder the one end back in.

Also I think the bias is done with a scope, I can't remeber, you ned to find someone who worked on these recently, or find the repair manual.

Thanks for the comments and instructions qsaudio and larryderouin...
Here is what I was going to order as far as the transistors go.
I've read on here a lot of members hate NTE....so I'll definitely go with the ONSemi and Fairchild at Mouser.

Amp Board
Q1 2SC732 512-KSC1845FTA
Q2 2SC732 512-KSC1845FTA
Q3 2SC497-O 512-KSC2383YTA
Q4 2SC497-O 512-KSC2383YTA
Q5 2SA497 512-KSA1013YBU
Q6 2SA497 512-KSA1013YBU
Q7 2SC497-O 512-KSC2383YTA
Q8 2SC497-O 512-KSC2383YTA
Q9 2SC793-Y 863-MJ21194G
Q10 2SC793-Y 863-MJ21194G
Q11 2SC793-Y 863-MJ21194G
Q12 2SC793-Y 863-MJ21194G

R31 0.5 588-WHCR50FET 2W Wirewound
R32 0.5 588-WHCR50FET 2W Wirewound
R33 0.5 588-WHCR50FET 2W Wirewound
R34 0.5 588-WHCR50FET 2W Wirewound

I have picked some of the caps and the 4 new .5ohm 2W power resistors. There are some 3.3uF and 100uF caps that I can't seem to find the right voltages.
So I need some suggestions on the voltages if these will be OK...before I send this off...

C1 3.3uF 10V 35V? Electrolytic
C2 3.3uF 10V 35V? Electrolytic
C3 100uF 50V Electrolytic
C4 100uF 50V Electrolytic
C5 3.3uF 25V 35V? Electrolytic
C6 3.3uF 25V 35V? Electrolytic
C7 100uF 50V Electrolytic
C8 100uF 50V Electrolytic
C9 100pF 50V Ceramic
C10 100pF 50V Ceramic
C11 100uF 3V 6.3V? Electrolytic
C12 100uF 3V 6.3V? Electrolytic
C13 1000uF 35V Electrolytic
C14 1000uF 35V Electrolytic
C15 47pF 50V Ceramic
C16 47pF 50V Ceramic
C17 0.002uF 50V Mylar
C18 0.002uF 50V Mylar
C19 100pF 50V Ceramic
C20 100pF 50V Ceramic

So another $10 for those and the whole board would be like new...so I think it makes sense to do it all now while I have it open and accessible.
As far as the bias thing goes is it possible I won't need to change the bias pots (they have some sort of paint over the knobs)???....
If need be I will have to research doing that and learn....without blowing anything up.
I'm sure it's a fun thing and good stuff to know how to do anyway!!
I have a scope, DMM, signal gen...the basics.
It would be nice to have the test equipment needed to do AM/FM alignments but...maybe someday in the future....
This is lots of fun and excitement for me and it makes me happy working on these things and figuring out all the parts, soldering etc....very therapeutic....I know....some would say you need a life....but this is the best right now!....so thank you all for your assistance and encouragement....:thmbsp:
 
Interesting new development while looking at the main amp board output capacitors and it's causing a bit of confusion....
There are two 1000uF 35V (C13/C14) soldered onto the main amp board and they are each wired in parallel with two chassis mounted 1000uF 50V.
The on board caps measured 1700uF and the chassis mounts measured 1200uF. Which I believe makes 2900uF total for each output channel.
Not entirely sure what happens to the 35V/50V differences...?

Now I'm having a difficult time de-soldering the two circuit board caps as they are small can style caps with 4 tabs. Two tabs across from each other, go to two (-) traces and one tab goes to a (+) trace.
The 4th tab opposite the + tab appears to be just for support. Is there some trick or easy way of removing these caps? They're really in there!!

So...I want to replace the output caps and I'm wondering if I could install just two better quality 2000uF (or even 3300uf) 50V caps and then mount those in place of the chassis mount straps which will be hard wired over to the board traces. I could then discard the two 4 tab cap cans on the board entirely!!
There's plenty of room for bigger caps on the chassis!
I've attached some pictures to show the issue in case my description is confusing...:scratch2:
Also do you think this was a factory revision of the design for 2000uF??
The schematic only shows 1000uF cap for each channel output??
 

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When you complete you BOM from Mouser, can you post it please! I'm adding this to my Pioneer Recap list!

This is what I have ordered so far...
I'm not sure if the 1000uF/35 (KA) were correct or necessary but they were the new audio grade caps from Nichicon.
I'm debating on changing C13/14 to 2200uF/35V instead of having the two 1000uF/35V in parallel and then just mount those on the board.

If you see a second voltage or different value listed next to the original part number that was the available value at the time.

Main Amp Board (W15-060)

C1 3.3uF 647-UPW1H3R3MDD 10V 50V Electrolytic
C2 3.3uF 647-UPW1H3R3MDD 10V 50V Electrolytic
C3 100uF 647-UPW1H101MPD 50V Electrolytic
C4 100uF 647-UPW1H101MPD 50V Electrolytic
C5 3.3uF 647-UPW1H3R3MDD 25V 50V Electrolytic
C6 3.3uF 647-UPW1H3R3MDD 25V 50V Electrolytic
C7 100uF 647-UPW1H101MPD 50V Electrolytic
C8 100uF 647-UPW1H101MPD 50V Electrolytic
C9 100pF 140-50N5-101J-RC 50V Ceramic
C10 100pF 140-50N5-101J-RC 50V Ceramic
C11 100uF 647-UPW1H101MPD 3V 50V Electrolytic
C12 100uF 647-UPW1H101MPD 3V 50V Electrolytic
C13 1000uF 647-UKA1V102MHD1TO 35V Electrolytic
C14 1000uF 647-UKA1V102MHD1TO 35V Electrolytic
C15 47pF 140-50N5-470J-RC 50V Ceramic
C16 47pF 140-50N5-470J-RC 50V Ceramic
C17 0.02uF 667-ECQ-V1H223JL 50V Mylar
C18 0.02uF 667-ECQ-V1H223JL 50V Mylar
C19 100pF 140-50N5-101J-RC 50V Ceramic
C20 100pF 140-50N5-101J-RC 50V Ceramic

R31 0.5Ω 588-WHCR50FET 2W Wirewound
R32 0.5Ω 588-WHCR50FET 2W Wirewound
R33 0.5Ω 588-WHCR50FET 2W Wirewound
R34 0.5Ω 588-WHCR50FET 2W Wirewound

VR1 30KΩ 652-3386H-1-503LF 50K Trim Pot
VR2 30KΩ 652-3386H-1-503LF 50K Trim Pot
VR3 50Ω 652-3386H-1-500LF Trim Pot
VR4 50Ω 652-3386H-1-500LF Trim Pot

Q1 2SC732 512-KSC1845FTA NPN
Q2 2SC732 512-KSC1845FTA NPN
Q3 2SC497-O 512-KSC2383YTA NPN
Q4 2SC497-O 512-KSC2383YTA NPN
Q5 2SA497 512-KSA1013YBU PNP
Q6 2SA497 512-KSA1013YBU PNP
Q7 2SC497-O 512-KSC2383YTA NPN
Q8 2SC497-O 512-KSC2383YTA NPN
Q9 2SC793-Y 863-MJ21194G NPN
Q10 2SC793-Y 863-MJ21194G NPN
Q11 2SC793-Y 863-MJ21194G NPN
Q12 2SC793-Y 863-MJ21194G NPN

Some miscellaneous parts:

Indicator Lamps 8V 0.25A 560-46 (6.3V 0.25A)
Dial Fuse Lamps 8V 0.3A GF680 (8V 0.30A)
TO-3 Mica washers 534-4667
Thermal grease (pack of 5) NTE303

Patiently waiting for parts to arrive!
 
Sorry to bring up an old post. I have a 990 here that I want to bring up to spec, was wondering how did your project end up?

Thanks!
 
Sorry to bring up an old post. I have a 990 here that I want to bring up to spec, was wondering how did your project end up?

Thanks!

It turned out great and my friend is still using it and very happy with it!
Is your 990 working OK or does it have problems?
If you need any info let me know.
I think the capacitor list in here is still a good reference...being pretty old unit those would be the first thing I would look at....it's a nice easy job...
The only tricky part in there are those 2 thermal diode modules mounted on the heatsink.
 
I'm working on one too. It's frustrating the begeezus out of me. I continue to troubleshoot the cause of distortion on one channel of this receiver. I had replaced two power transistors on the bad channel with no change. I noticed after removing the board again that I missed seeing a bad resistor. It was R25. I replaced that along with the four .47 ohm 2 watt resistors. After powering up now both channels seem nasty. I checked some voltages and the first one I check is the B+ which now reads high at 76 volts. I checked the voltages at the PS board and AC-1 is 54 v ,and AC-2 is 17.6. B+ is 76. So I guess I have an issue with the PS board ? What could be the issue ,and could this be causing the nasty distortion now on both channels ?
So I just noticed the voltage selector is set at 110 v ,and my voltage runs at 123v . should I change the selector to 117 ? 130 would be too high ?
Another thing I just found. I now have very low bias current on both sides ,and it will only move about 3-4 ma in either direction. I'm seeing only 14ma.
 
I'm working on one too. It's frustrating the begeezus out of me. I continue to troubleshoot the cause of distortion on one channel of this receiver. I had replaced two power transistors on the bad channel with no change. I noticed after removing the board again that I missed seeing a bad resistor. It was R25. I replaced that along with the four .47 ohm 2 watt resistors. After powering up now both channels seem nasty. I checked some voltages and the first one I check is the B+ which now reads high at 76 volts. I checked the voltages at the PS board and AC-1 is 54 v ,and AC-2 is 17.6. B+ is 76. So I guess I have an issue with the PS board ? What could be the issue ,and could this be causing the nasty distortion now on both channels ?
So I just noticed the voltage selector is set at 110 v ,and my voltage runs at 123v . should I change the selector to 117 ? 130 would be too high ?
Another thing I just found. I now have very low bias current on both sides ,and it will only move about 3-4 ma in either direction. I'm seeing only 14ma.

I essentially rebuilt the entire main amp board with all four new power transistors and all the electrolytic caps. Plus I checked all resistors and replaced the trimmers! One thing you should do is remove the pre-amp/main amp jumpers on the back and then test the main amp section separately. I found I had some distortion and bad mylar caps in the tone control section!!!
So try to isolate where your distortion is coming from. Use a function generator and scope, clean all the controls and switches....with something this old there are just too many things that can go bad...even resistors!!!

If this is something you want to keep for a long time, it's worth the small amount of money in parts to go through each board, replace at least the electrolytics and then test each section. I don't think I did much to the tuner section. The power supply needs to be checked and the filter caps should be replaced so you're getting good clean power to all the boards. So that is a standard thing to thoroughly check anyway. That's really where you should start, make sure all the PSU rails are good first!!
Like I mentioned before those bias trim pots should be replaced with a better solid trimmer. Don't forget testing the driver transistors either. They can be bad, especially if you had an output transistor meltdown or failure...
I'm sure there are people on here who have better tech skills at troubleshooting but my approach worked really well taking one section at a time and going through the checks and tests of each....replacing all the ecaps and checking all the transistors and resistors....
I don't think moving the AC input to 117V will hurt, I think I did that on mine!
I assume you have all the service manuals and schematics?
Be careful with those SVT-3 diode packages on the heatsink. Those are impossible to fix or replace. You might test those to make sure they're good! There are some good articles on replacing those with series diodes (1N4148 I think) by Markthefixer who is a great source of knowledge and help here in case they are bad!!
Let me know if you need more info! Good luck! :music:
 
Are the preamp/main amp jumpers the plugs on the back of the amp ? Thanks for all the info. I'll probably just rebuild the two boards. What I don't get is I had the correct voltages and bias before I replaced the bad resistor and the 4 .5 ohm resistors.
 
Yes, removing those jumpers will allow you to check the signal path...."pre out" and then "amp in". Very helpful in splitting the receiver front end and troubleshooting something in the preamp causing problems in the amp and vice versa....
Not real sure what would cause things to change on your voltages or bias, maybe someone else here has a suggestion. I would double check everything you did just to make sure....
Also not sure if you found this thread but it has more info on my rebuild and that might be helpful....:music:

http://www.audiokarma.org/forums/showthread.php?t=485488
 
If you have 123V on the wall, setting the voltage selector to even 117 is overloading the primary windings on the Main Transformer. The settings are the maximum voltage that is allowable for that setting. SET the selector to 130V. This will allow you to run the unit at any voltage UP TO 130V. In addition it will allow cooler operation of the transformer windings, and better voltages on the boards, which will be closer to what is shown on the schematic and usually will be a couple volts below to even with the schematic readings.
 
Yes, removing those jumpers will allow you to check the signal path...."pre out" and then "amp in". Very helpful in splitting the receiver front end and troubleshooting something in the preamp causing problems in the amp and vice versa....
Not real sure what would cause things to change on your voltages or bias, maybe someone else here has a suggestion. I would double check everything you did just to make sure....
Also not sure if you found this thread but it has more info on my rebuild and that might be helpful....:music:

http://www.audiokarma.org/forums/showthread.php?t=485488

Did you have a solution for the extra 1000uf 35v cap ? Did you just use a 2000uf cap ,or replace them both ?
 
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