Pioneer SX-950 No Speaker Output

[lennyfishman]

Hi All,

First time posting here since one of my goals next year is fixing up this old Pioneer SX-950 amp that I received for free a couple months ago. Searching this site I was able to find the repair manual in addition to several other interesting documents and threads. Here is what is working

-The amp currently powers on and is able to receive a signal from all inputs. All lights, knobs, and switches appear to be in good shape and the amp is in fine aesthetic condition.

-If I connect powered speakers to the adapter out RCAs in back I am able to play audio however this output is not affected by the amps volume control.

-If I connect powered speakers to the pre-amp out I am also able to receive an audio output, however this time I am able to adjust volume and filter controls.

-By comparing the signal output from a device to my powered speakers with and without the amp it appears as though the Pioneer SX-950 is boosting the signal even when using the pre-amp out.

Here is what doesn't work and what I am wondering:

-I get no output through any 4ohm/8ohm speakers using the speaker outputs on the back of the amp. I have tried every combination of A/B/C with various settings and no results.

-Opening the amp up I see that some of the components on the power amplifier board are blown. I'm guessing that this may be the source of the problem and probably the place to start. My current plan is to order replacement components for any item on the power amplifier assembly that appears damaged. I have the repair manual so this should not be too difficult.

-I have been offered $100 for this amp in its current condition so I'm trying to weight the level of effort that this fix may entail. I have soldering skills and the following video shows the power amplifier disassembly, however I am wondering if my diagnosis makes sense with what I know about the amplifier?

-Besides a soldering iron, basic tools, replacement components, and a multimeter are there any additional tools that will be necessary for this repair?

-Finally, I have seen many threads on this forum related to this specific amp and have seen people post about re-capping. Should I just go ahead and re-cap the whole thing while I'm at it using the capacitor list from the following thread? What is the level of additional effort for this procedure, is it worth it, and about how much will all of those caps cost?

http://audiokarma.org/forums/index.php?threads/pioneer-sx-950-recap-issue.722951/

Thanks!

 

[tsd71]

First of all what do you have as your STV diodes with the green wires running to the heatsink with a glob of something at the end? Also all the wires coming from the outputs have been soldered on, are they all making connection to the outputs? Lots of bad looking joints. Looks like a very unhappy amp. I would rebuild that entire amp, proper bias diodes, all new caps and transistors, also check every resistor also, looks like some of the emitter resistors took some major heat.

 

[lennyfishman]

I'll post a closer picture, but I think the glob is where the board caught on fire at some point near where the charring is. One of the insulating tubes melted a bit and the glob might just be plastic residue.

 

[tsd71]

What was replaced for the original STV-4 it that is not original.

 

[lennyfishman]

No idea, this is how I picked it up. Gotta say though it doesn't look like too many maintenance operations have been performed on it yet. Here is a better pic though.

 

[petehall347]

is that emitter resistor on the far right really blown apart or is it just the picture ?
edit ...you beat me to it .. likely a few blown transistors etc to find there .

 

[tsd71]

Hi All,

First time posting here since one of my goals next year is fixing up this old Pioneer SX-950 amp that I received for free a couple months ago. Searching this site I was able to find the repair manual in addition to several other interesting documents and threads. Here is what is working

-The amp currently powers on and is able to receive a signal from all inputs. All lights, knobs, and switches appear to be in good shape and the amp is in fine aesthetic condition.

-If I connect powered speakers to the adapter out RCAs in back I am able to play audio however this output is not affected by the amps volume control.

-If I connect powered speakers to the pre-amp out I am also able to receive an audio output, however this time I am able to adjust volume and filter controls.

-By comparing the signal output from a device to my powered speakers with and without the amp it appears as though the Pioneer SX-950 is boosting the signal even when using the pre-amp out.

Here is what doesn't work and what I am wondering:

-I get no output through any 4ohm/8ohm speakers using the speaker outputs on the back of the amp. I have tried every combination of A/B/C with various settings and no results.

-Opening the amp up I see that some of the components on the power amplifier board are blown. I'm guessing that this may be the source of the problem and probably the place to start. My current plan is to order replacement components for any item on the power amplifier assembly that appears damaged. I have the repair manual so this should not be too difficult.

These pictures are not the same amp in the video?

-I have been offered $100 for this amp in its current condition so I'm trying to weight the level of effort that this fix may entail. I have soldering skills and the following video shows the power amplifier disassembly, however I am wondering if my diagnosis makes sense with what I know about the amplifier?

-Besides a soldering iron, basic tools, replacement components, and a multimeter are there any additional tools that will be necessary for this repair?

-Finally, I have seen many threads on this forum related to this specific amp and have seen people post about re-capping. Should I just go ahead and re-cap the whole thing while I'm at it using the capacitor list from the following thread? What is the level of additional effort for this procedure, is it worth it, and about how much will all of those caps cost?

http://audiokarma.org/forums/index.php?threads/pioneer-sx-950-recap-issue.722951/

Thanks!

These pictures do not look like the amp in the video. I'm an old confused man right now, which is your 950, the one in the pictures or the one in the video?
Are you hearing the relay click after 4-5 seconds after power up?

 

[lennyfishman]

No, I was posting the video for reference since it shows the power amp disassembly for this model. Totally different problem.

My receiver is in the pictures I posted. The issue looks to be tied to that busted resistor on the power amp.

So it looks like I'm gonna have to replace that emitter resistor and some of the other parts in that general area of the board. Does that make sense with the issues I described?

How worth it is recapping the entire unit?

 

[petehall347]

you need to be checking every component in that side thats burned up .. for sure there are shorted transistors and possibly more resistors etc . replacing obvious looking burned parts will only lead to more burned burned parts . they all need testing or replacing ...testing is the way to go unless you are very good good at replacing everything correctly first time .. i have been doing this stuff for many years and still get things mixed up a little bit . i get distracted very easily .
what i do generally do is forget the bad side and get the good side working for reference plus its builds moral that way .
good luck with the repair .i will jump in when its needed .

 

[lennyfishman]

Note that my recapping question is separate, I'm just curious about the benefits/difficulty of a full recap on an sx-950.

 

[lennyfishman]

Thanks for the quick reply by the way. For testing some of these components will I need anything more than a multi-meter? Is there a good beginner repair thread somewhere that details the general process testing various components on vintage receivers (resistors, transistors, capacitors, etc)? Most of my electronics experience involves fixing old guitar amps so this is a bit of a different ball game.

 

[zebulon1]

The 950 will sound much better with a recap. The tone board seems to deteriorate more than most.
I'm finishing up a 950 that had the right channel damaged similar to yours.
This is an example of what you may run into:
R40, 150 ohm 1/2 watt toast
R38, 150 ohm 1/2 watt toast
R62, 100 ohm 1/4 watt toast
R68, 100 ohm 1/4 watt toast
Transistors Q10 and Q12 shorted
Three of the four right channel outputs were shorted.

Ended up rebuilding the entire right and left channel with new transistors, diodes, and caps.
Replaced the burned out resistors.

 

[lennyfishman]

Yup, good eye, indeed R40 is the toasted component on my board as well although I'm probably going to have to replace quite a few components. I created a spreadsheet from the parts list in the repair manual (see attached). Once I get the board off and start testing components I will order what I need.

Any retailer suggestions apart from Mouser?

Looks like I have quite a few old forum threads to guide my component decisions:

http://www.audiokarma.org/forums/index.php?threads/need-new-transistors-for-sx-950.614453/
http://www.audiokarma.org/forums/index.php?threads/recapping-sx-950.364598/
http://www.audiokarma.org/forums/index.php?threads/pioneer-recap-lists-sticky-suggestion.457663/

The user from the first link appears to be doing a very similar thing to myself (fixing a free pile sx-950). They ended up rebuilding the entire power supply board which actually seems like a good idea since I wouldn't necessarily need to test all of the components (and I have no idea how to do that). I guess the only con there would be the additional expense of the components.

 

[zebulon1]

Not as hard as you think
Google

 

[rcs16]

Of course there is addition expense on components. You can change them all or you can sort through what is working and what is not.
Really up to you if you want to unload it as is or invest time and money into fixing it up. That depends if you want to keep it or not and want to invest time and $ into it, only you can answer that.

 

[jeffpaletz]

As far as keeping or selling it, if you want to have a receiver that you can listen to music through that will sound great, then keep this and fix it up. I have an SX-1050 I bought new in the late 1970s and love the strong deep bass it puts out and the tone overall. Playing CDs, records or radio it just sounds great. Parts are cheap, especially resistors and capacitors. There is a way to import the parts lists into the order form at Mouser. That's where most here get there parts.

 

[lennyfishman]

Good advice all, I have found some great testing tutorials online and have disassembled the board prior to testing. My parts list is staged with Mouser PNs. Now I have some actual te questions:

-I have found two types of capacitor testing tutorials online: 1) using a simple multi-meter read for uF rating, 2) using an ESR meter with a hot/cold test. Which is the correct test for this type of work?

-It looks like I'm kind of screwed if either of the STV-4H varistors are busted since replacements are hard to come by. However, I found one thread that mentioned the possibility of using four UF1004s. Is there any consensus on AK for replacing these parts?

-For the Q7 and Q8 transistor replacements KSC3503 is available on mouser with the PN suffixes: DS and DSTU. I have chosen my other PN variants based on availability. What is the difference here? I can't tell from the Mouser technical jargon although it doesn't really seem to matter.

 

[rcs16]

1) ESR(Equivalent series resistance) is yet another parameter to test for a caps performance, lots of information on the net to find out more. The lower the number the better. There are others like DF(dissipation factor), DA(dielectric absorption). kemet is a big cap mfg, their site has lots of info for you to read if what to learn more.
2) Figure out how to measure a STV-4H, it is 4 diodes in series, so if you pass a constant 1mA through it you will get 4 diode drops which is around 0.6VX4~2.4V. EW has a good wrrite up on possible substitutes if you yours are toast. Most times it is a broken wire at the base. I always suggest to epoxy that area to make it more secure.
3) "TU" is just the shipping packaging, I think bulk vs in a tube, "D" is the hfe grade, both are the exact same part.