Pioneer qx-8000a broiled resistors

[ivo]

Hi everybody! First time i own a quad receiver - Pioneer qx-8000a. I got this beauty from a friend of mine with a note that right front channel is down.Ok. I opened it home and it is looking very, very clean inside no one leaking cap pretty clean surface, obviously low mileage device.After careful investigation, i got three resistors broiled bad in the channel 1 board and one bigger green (resistor??) with digits on it: 2BYD O.5K obviously kiloohms. The green one has bad insulation (probably over heated) but it is not broiled.
I have a question and favor to ask of you for help - the question is: What may cause this fire there and what is the chance to burn out the new resistors after installing them? Right now the receiver is working very good as two channel stereo with the rear amp, or with channel 3 on the front and channel 4 on the rear amp.Thank you for any answer, I really, really appreciated any help!!

 

[llwhtt]

Resistors for the most part don't "broil" themselves. You probably have some shorted transistors in the circuit somewhere. We need to know what resistors are "broiled".

Craig

 

[Binkman]

Greetings;
https://www.hifiengine.com/manual_library/pioneer/qx-8000a.shtml

will require some studying.. good it's playing on off but might check clean the selector switches.. canned reply but could still be a switch. assure inputs wires are good, too. Sometimes these quad amps were rewired at the outputs to parallel or serial speaker by hackers.

still reading will hopefully gain a few posters to help.

 

[Goldie99]

Your best starting point for any problems like this is the service manual - in this case, the hifiengine site has it for free. You need to join, but it's free, and it's a very useful & clean resource, free of 'junk':

https://www.hifiengine.com/manual_library/pioneer/qx-8000a.shtml
https://www.hifiengine.com/manual_library/pioneer/qx-8000a.shtml
With the limited info you provided, it looks to me as though the large green resistors in your photos are actually the main emitter resistors on the outputs - the spec is shown in the parts list, but is basically 0.5R 2W wire-wound. The 'K' almost certainly means 10% tolerance, not 'kilo'.

The usual reason for one or more of these "broiling" would be failure of one or more of the output transistors - the other 3 "broiled" resistors are likely to indicate the route the over-current took when the output(s) failed. Simply replacing the resistors unfortunately will not fix the problem - you've almost certainly 'lost' one or more transistors, as well as ??.... so it'll need proper fault-finding & repair. If you can give the exact references of the burnt resistors, someone may be able to be more specific, and point you in the right direction.

Edit: Bink beat me to it on the manual - great minds....

 

[Binkman]

Golde33 from UK.. cheers back to you. have one on me. I miss a nice pub w/draft ale.
------ chips instead of popcorn.

 

[petehall347]

i see a 1/4 watt sized one thats been barbecued . broiled might be not so bad

 

[petehall347]

quick guess here . output went open and drivers were left to hold the fort .

 

[ivo]

Hi guys!! Thank you very, very much for all of the answers and help! Yes i download the service manual from hifiengine, that was the first thing I did.The color code of the damaged resistors is : 1. brown, black, black,gold 2. red, green, red, gold 3. brown, green, brown.
For me it is mean: 10, 150,150. But maybe im wrong. I took very carefully view over this circuit board and didn't get any damages except the described resistors. This is my question - if somebody plug the very wrong speaker in to any amp and lauds the amp on max , what is going to happend with the resistors on the ouput?Because this receiver has a protection fuses on every channel!And the fuses are ok.There is another fuse which is protecting the transistors on the main amp board and it is ok too....For me this is very strange!Thank you, guys, for all posts, was real pleasure to read it . Im waiting with impatience your shаre opinions

 

[Tom B]

Echo on what others have sad. Channel 1 and Channel 2 probably have shorted outputs. You also need to check the drivers as they could be shorted or open, or leaky.

 

[Goldie99]

As several of us have suggested, you almost certainly have at least one blown output transistor, and quite possibly damaged driver transistor(s), etc. - it's not unusual for a blown output to take other components with it. It'll need proper fault-finding to determine exactly what's gone. The damage will not all be visible, so unfortunately just visually inspecting the boards is not sufficient.

There's various scenario's for why an output may have failed, but one of the unfortunate characteristics of fuses is that they are rarely fast enough to save transistors, so you'll often lose transistors before the fuse has time to react.... the fuses may also have been replaced since it happened ?

 

[cademan]

That green resistor may be ok. It just had some of its protective outer layer bumped off somehow. Other fried resistors are certainly reminiscent of a shorted transistor up stream.

 

[ivo]

Thank you, goldie, thank you cademan, thank you bideman thank you for all of you! First I'm, not radio-tech this is my hobby i love old analog gears. It looks like I am going to use my Pioneer as two channel stereo receiver unless ill try to do something my self - i know the hi price of professional repair is reasonable but i just can afford it. Ill try to get a litle bit more time of yours with this question: Can i check the transistors with my Knight k-360tr multimeter? It is has a HFE section which i believe is for this purpose? And what is the way to do this - do i need to plug some source of signal ( tuner or TT) and speakers when the checking start?To heck the signal between base and emitter - receiver? I know that probably my questions sound like silly kid, but this is wht it is.Thank you again !!

 

[petehall347]

use diode test mode on your meter if it has it . or ohms if not . with the unit unplugged from the mains supply .

 

[ivo]

tHanks, Patehall347 ! I did it the same way i check out the fuses.So obviously both of output BIG transistors (Toshiba 2SC793) on channel 1 are bad - the meter is giving me signal all other transistors are silent. I checked it with negative probe on the base pin.

 

[ivo]

How to check the driver transistors? Are they located on the same main amp board or in the Head board?Or control board? What is the other name of the drivers?

 

[petehall347]

q5 q6 q7 q8
odd numbers are left channel .even numbers are right channel .
it will be obvious finding a short the way you are doing things but other faults can be there .
best way to test them with your meter is take them out of circuit first .
if you can say which channel is obviously blown its possible the other channel might be fine . so i would first get the good side working as a reference point .

 

[Binkman]

this may help with pics for testing the transistors. Courtesy member avionic. (one of my favs)
http://audiokarma.org/forums/index.php?threads/testing-transistors.201303/#post-2362176

 

[ivo]

Thanks petehall! Yes, the other three channels are ok - they are working properly and sound very good and yes ill take out the board.So there are two three legs transistors on the same main amp board just before the outputs, ill start with them first. Thank you Binkman!! Ill go to this link, let me first find where are the drivers and ill measure them in the way it is shown

 

[petehall347]

also check the 0.5 ohm resistors and the 150 ohm and the 22 ohm ones .

 

[slimecity]

Yes, yes, yes.....

But how do they TASTE?