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Pioneer QX-949 repair trouble...

pgunn

pgunn

New Member
Hello all! This is my first post as a registered user, but I have been browsing this forum for several years seeking solutions to various DIY home stereo setups. Everyone is very knowlegable and helpful so I finally joined as I am facing an issue I don't know how to resolve...

Let me start by saying that I am a database administrator not an electrical engineer, but I have tinkered with electronics some in the past. Much of what will be said here may go over my head, however I am resourceful and know how to use Google quite effectively, so I shouldn't be too hopeless with your explanations.

Now for my issue... Several years ago I was handed down a QX-949 from my uncle who purchased overseas back in the 70's. Almost none of the lights work, the 4 channel feature has never worked (i prefer 2 channel for the power boost anyways), but it has played music beautifully and more powerfully than any other receiver I have ever owned! A few months ago the receiver started producing a light static noise through the speakers, which over the course of a few minutes grows to overpower the music, and then the receiver goes into protect mode for a few seconds until it reboots. Upon reboot the static is as loud as ever and causes another protect loop shortly after.

After some quick plug and play troubleshooting I was able to narrow it down to channel 1, both A and B speakers. Channel 2 works great, and it doesn't seem to protect fault often when only outputting to channel 2. Channel 1 also comes through very quiet sometimes, but not always.

After reading dozens of related forum posts I think it might be either the electrolytic capacitors or one of the transistors, but before I order anything or risk subjecting myself to a significant electrical shock, I was wondering if anyone here could offer some insight into a possible solution for my issue. None of the capacitors that I can easily see appear to be "bulging", and there are no other tell-tale signs of malfunction other than the static and protect fault loop.

Any thoughts? Should I purchase and replace the electrolytic capacitors or transistors, or is it something else?

Thanks in advance for any help.
 
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Not to discourage you, but you are in over your head.

The QX-949 is a complicated unit to work on.

I suspect amp problems and just to access the amps to work on them is a trick best left to someone with some seriously good vintage repair/restore skill.

I paid my dues on one of those. It took a few trips through the amps to get everything sorted out. I thought I had checked and rechecked and replaced all bad parts and still missed something.

My suspicion is that it's in the driver part of the amp circuit.

The Hail Mary act, for someone without the tools and skills, would be to replace all the small transisters in the driver section of the amps, replace all electrolytic caps while you have access to the amps and replace all the trimmers while there. If I remember right, the trimmers are known to go bad on those.

But that is a Hail Mary and it's based on a guess that the problem is in the amp so the chance of success is low.

Accessing the amps themselves is a pain because they did not make that one easy to service. All the amp wires must be removed on at least one amp to get in there. Of course, they all need to be reconnected. They are wire wrapped and we prefer not to unwrap those but rather pull the pins on the board. That is a fair trick in itself.
 
I would tend to agree with BLH. This is probably not a good unit for a novice repair. If you have electronic repair experience in other areas (than audio) we can probably walk you through the diagnosis.

By your description of its operation there are several common possibilities and numerous more obscure possibilities. The unit contains several types of transistors which are know to fail and exhibit these characteristics. Namely, 2SA725/726 devices (static and shot noise generators), 2SC1451s (static and open-circuit prone), 2SK30s (unavailable), very fragile thermal compensation diodes (STV-4s), and the aforementioned trimmer pots and potentially failing electrolytics and tantalum caps. And those are just the devices which need to be replaced for reliability - That may not solve the core issues.

Search the board archives for some of those part numbers and find deeper explanations, then search for repair threads on this model and you may get an idea of what you are up against.

Make no mistake - The unit is certainly worth repairing and restoring, but it's not one of the easier designs by all accounts.
 
pg,
Good advise. If you're not sure where to start looking, leave it to someone who can.
Where are you located. Perhaps someone close to you can be of help.
 
Its going to take a few sets to get the hang of repairing (Until you can start on the Quad). No doubt They are packed tight.
 
Thanks to everyone who responded! I am fully aware I'm in over my head but I'm also confident that this unit is worth the attempt.

@blhagstrom: Thanks for the diagnosis and the sheer honesty. I appreciate both. I will look into the cost for the hail mary attempt and may take a shot at it for kicks. If nothing else it's worth tinkering around a little bit. This is a place to start and that's what I need.

@Watthour: I have some basic electronic repair experience with computers, as I have built/repaired quite a few, but nothing more than soldering some loose connections and replacing parts. The unit has not been given any repairs in the 40 years it's been alive I'm sure it could use a routine reliability renovation as far as swapping error-prone parts. While I have it open I might as well do what I can for the inside. I feel it's worth the trouble and I'm certainly up for the challenge. Thanks for reaffirming that for me!

@jblmar: I'm certainly not an expert (barely a novice really), so you are probably right in that I should have someone experienced take a look. I am located in Bellingham, WA, but I have family in Everett so I frequently visit.

@zebulon1: I'm not ready to give up as I've barely even started. I plan to see this project through and I will gain valuable experience through it. I'm confident that I can, with enough effort, fix the receiver and then I will be better prepared to help with future issues. And yes, it is packed very tight haha.

If anyone is familiar with these units and is near my area I would be willing to compensate for some assistance in the matter, if it comes down to that.
 
I would say your in the same shape and experience level I was at when I began this crazy hobby seven years ago. I had found a Luxman R-1120 that needed repair. It was last year that I finally finished it and the two sets I bought for parts. I could of finished it sooner but the frustration was harder to get over. Of course I didn't have AK and a set of experts helping me.
If it was a 950 or even a sx-1010 I would say go for it but Pioneer Quad. :dunno:
I like your tenacity.
Where is this QX-949 located?
 
@zebulon1: It's located in my living room in Bellingham, WA. I'm sure there are experts in my area but I'd feel defeated to pass the problem off to someone else so quickly without putting in my own hard work and fair share of dead end fixes :D.

I have always tended to dive headlong into projects well above my expertise level, but I also have turned into somewhat of a jack of all trades, so the experience has suited me well. From what I have read Pioneer Quads are one of the most difficult units to diagnose and repair, but if I can do this one I can do anything i guess.
 
At a minimum, you will need desoldering equipment including a moderate wattage solder iron, solder sucker and/or solder wick, paste flux, and 60/40 tin-lead solder - The type for electronics, NOT plumbing or general repairs. You will also require a multimeter capable of reading voltage drop at diode junctions, some common hand tools for disassembly and assemble, forceps or hemostats for holding parts, long-reach mini grabber test leads, a clear and clean copy of the full service manual for the unit, and a place to work where the project can be left in various states of disassembly for days at a time while parts or supplies are gathered/delivered.

It would also behoove you to assemble a Dim Bulb Tester (DBT) and equip it with various wattage incandescent lamps from 40W to 150W.
 
pgunn said:
Thanks to everyone who responded! I am fully aware I'm in over my head but I'm also confident that this unit is worth the attempt.

@blhagstrom: Thanks for the diagnosis and the sheer honesty. I appreciate both. I will look into the cost for the hail mary attempt and may take a shot at it for kicks. If nothing else it's worth tinkering around a little bit. This is a place to start and that's what I need.

@Watthour: I have some basic electronic repair experience with computers, as I have built/repaired quite a few, but nothing more than soldering some loose connections and replacing parts. The unit has not been given any repairs in the 40 years it's been alive I'm sure it could use a routine reliability renovation as far as swapping error-prone parts. While I have it open I might as well do what I can for the inside. I feel it's worth the trouble and I'm certainly up for the challenge. Thanks for reaffirming that for me!

@jblmar: I'm certainly not an expert (barely a novice really), so you are probably right in that I should have someone experienced take a look. I am located in Bellingham, WA, but I have family in Everett so I frequently visit.

@zebulon1: I'm not ready to give up as I've barely even started. I plan to see this project through and I will gain valuable experience through it. I'm confident that I can, with enough effort, fix the receiver and then I will be better prepared to help with future issues. And yes, it is packed very tight haha.

If anyone is familiar with these units and is near my area I would be willing to compensate for some assistance in the matter, if it comes down to that.
Click to expand...


I am near Duvall.
It may be worth a trip over just to have me remove the amps and point you in the right direction.
 
blhagstrom said:
I am near Duvall.
It may be worth a trip over just to have me remove the amps and point you in the right direction.
Click to expand...
Duvall is very close to my folks place in Silver Lake. If you would be willing to help me out I would be greatly indebted to you. I am planning on heading down south this weekend if you would have some available time. Probably Sunday, but maybe Saturday. Let me know if it's a possibility!

I'll gladly compensate you for your time and help if necessary.
 
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