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Pioneer SX-828 Please Help with the elusive Stereo Light

Johnny_Law

Johnny_Law

o.O
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EDIT: SOLVED! May have been the diode or the flying cap or both.

When I got this, the STEREO bulb was constantly lit and very bright.

Suspecting a transistor issue, I replaced Q19 (2SC968 with KSC2383) and Q20 (2SC711 with KSC1845) on the AF board. Thanks @markthefixer for your help selecting replacements here.

With the new transistors, the bulb was no longer lit. However, I discovered it had somehow become open in the process.

I put in a new bulb. The new bulb does not light either, ever.

There is a 470uF capacitor and a diode underneath. I replaced the capacitor which didn't fix anything, so I put the original cap back in. The diode tests good.

To recap, both Q19 and Q20 on the AF board (AWE-016) are newly replaced and correctly installed. The STEREO bulb is new and good. The cap and diode underneath appear to be good. The light was constantly lit BEFORE I replaced Q19 and Q20. Now it does not light at all.

Thanks in advance for any guidance!

-Ryan

Note that I reviewed various previous threads in my research including:
http://audiokarma.org/forums/index.php?threads/pioneer-sx-727-stereo-light-is-always-on.135744/
http://audiokarma.org/forums/index.php?threads/picking-up-a-pioneer-sx-727-tomorrow.321187/
 
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What are the base, emitter and collector voltages on Q-19 & Q-20 and other voltages directly upwind of them. The lamp runs off a differential voltage in that when in MONO The collector voltage on Q19 is 12v and in stereo is approx .07v. With the other side of the lamp getting a steady 8v or so this means that the lamp is effectively OFF when in Mono and ON in Stereo. Triple check the leg orientation of the new transistors against the old ones. Not all transistors are created equal.
 
Thanks! Everything seems OK except for Q19 collector when in stereo. Instead of close to 0, it drops to 4V. Would this imply there's an issue with the underside flying capacitor / diode after all?

My readings are (in VDC): (Not stereo reception, likely stereo reception)

Q20:
Base 0, 2.1
Collector: 3.8, 2.4
Emitter: 0, 1.4

Q19:
Base 0, 0.6
Collector 8.2, 4
Emitter 0, 0

And I triple checked orientation which is correct to my eyes. Both are E-C-B from the front.

http://www.mouser.com/ds/2/308/KSC1845-1120759.pdf
http://www.mouser.com/ds/2/308/KSC2383-1120764.pdf
 
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The voltage checks above are kind of ambiguous. Which mode? (Stereo or Mono). I should have been more specific when I asked for them.

Voltages
Schematic, Mono, Stereo (Note Schematic values, MONO has no parenthesis, Stereo is in Para's) List them as shown, then list yours next to them in both modes.

Are your power supply voltages per spec? If not, anything down stream will be goofy.
 
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john stumpf said:
humor me,check and make sure the function switch is in STEREO not in MONO! the light doesnt work in mono! DONT ASK ME HOW I KNOW!!!
Click to expand...

Haha, double checked! Switch definitely in STEREO!

larryderouin said:
The voltage checks above are kind of ambiguous. Which mode? (Stereo or Mono). I should have been more specific when I asked for them.

Voltages
Schematic, Mono, Stereo (Note Schematic values, MONO has no parenthesis, Stereo is in Para's) List them as shown, then list yours next to them in both modes.

Are your power supply voltages per spec? If not, anything down stream will be goofy.
Click to expand...

Does the STEREO light come on all the time when in STEREO mode, or only when getting a stereo signal? The voltages above are all in STEREO mode, but one set is while receiving a strong FM stereo signal and the other while receiving a weak, non-stereo signal.

I will check again tonight in Mono mode to compare, and will check out power supply voltages. The power supply has new caps and transistors.
 
the reason i asked was i've been following your thread. it also prompted me to fix my 828 which had exactly the same issue, stereo ligjht "on" all the time. i just rebuilt the the thing and finished about a month ago. i had tuner issue's which caused me to replace the tuner board, but all has been fine until about a week ago. so... it was game on yesterday. as i have a lot of unused transistors on hand i decided to go ahead and change q 19 2sc968 and its driver q 20 2sc711,which on my card was actually a 2sc458. in all the flailing about i managed to put it in mono without knowing it. and was quite disturbed to find out that q18 was 'on' when its supposed to be off. q 18 ENABLES q20. by removing the ground from q20's base when it is TURNED OFF. it is turned off in the "auto" mode by the function switch grounding pin 6 which shorts the base voltage q18 to grnd. q 18 is a 2sc711. so i replaced all three with a ksc2383 and two ksc1845's. and IT STILL didnt work so it was time to start measuring base and collector voltages. and low and behold no ground on q18's base as i had the switch in MONO!!!! putting the switch in auto revealed a functioning stereo on light! (when its supposed to). the really odd thing was on my peak dca 55 all measured as good. but i believe the real culprit is 2sc 711 in q18. but it could be in reality any of the other's as testers may show 'ok' but under load is another story. i had the same issue with "on all the time" stereo light in my sx1500td last week which also used 2sc711's replaced them and all was fine and all tested ok on my peak dca55. maybe it will help. looking at your voltages it would appear that your light driver q 19 is operating as it should and the bulb should operate im a bit concerned about the 4 volts on the collector when its turned on, it seems a bit high. do you have the board at this point screwed down and therefor "grounded"?
 
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Does the STEREO light come on all the time when in STEREO mode, or only when getting a stereo signal? The voltages above are all in STEREO mode, but one set is while receiving a strong FM stereo signal and the other while receiving a weak, non-stereo signal.
I will check again tonight in Mono mode to compare, and will check out power supply voltages. The power supply has new caps and transistors.
Click to expand...
Stereo light is supposed to come on ONLY when the circuit detects a STEREO SIGNAL. Which is when the "CENTERING METER" needle is centered in the "FAT" part of the arc in the center(if it's correctly setup or aligned). This denotes center of the frequency (say for instance 101.1mHz). with 100kHz margin on each side. Once it's inside that margin the light should come on and when the pin goes out of the margin it goes off. You'll notice a decrease in signal strength with noise as you go out of the margin, and into the next station. This is interstation noise/hiss and is normal. The 828 should have a FM MUTE button which negates this noise between stations. It will also block some weaker stations.
 
Just to clarify; in the first post the op stated that the stereo light was lit constantly. Then he replaced two transistors which, and correct me if I'm wrong, took out the filament in the stereo bulb and now even with a new known good bulb, it does not light. I suspect he now has an 'induced' failure which when corrected will get him back to the original complaint of 'constantly lit'. Ft...Ft.
 
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Thanks both - I just haven't had a chance to play with it this week. The original bulb going bad, I will admit I manhandled it a little and may have physically broken the filament doing so. The new bulb certainly hasn't blown with the new Q19 and Q20 in place. So if there is a new induced failure causing no stereo light that is separate from the original failure causing a constant stereo light can't be known, unless I put the original transistors back in, which I've thought about doing in order to explore that hypothesis. But where we are now, it looks like that 4V Q19 collector voltage may be the issue.
 
'Manhandling' a 40 year old incandescent lamp could also mean 'breathing on it'. The filaments are fragile, to say the least so don't feel bad. ' unless I put the original transistors back in'.....this would prove a few things but be advised, both are known bad actors. If putting in the originals gives you the original complaint, try different subs. At 4 volts on the collector of Q19, it is able to drop only 4 of the 8 volt lamp supply. Possibilities? Noise on base bias voltage influencing it's ability to conduct properly. Or, the KSC2383 has different switching characteristics than the 2SC968.
 
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Update after a month of being so mad at myself I couldn't touch the unit. In the process of checking the floating cap and diode underneath, in my rush and with my fat fingers, I bridged something bad with my multimeter lead and was rewarded with a popped lighting fuse - easy to replace - but also a newly dead right channel (not totally dead, but very very weak as if muted). UGH. I have double checked my power supply and protection boards - both are still fine. Now I suppose I need to dig into the main amp board (freshly rebuilt before my screw up) and see if I can figure out if something went bad on that board.

Edit: Fixed - transistors on the AF board's low pass filter were bad.
 
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test the bjts and diodes with them out of circuit. if you leave them in circuit, there are other comps that they are connected to that could throw off the measurements.
the circuit uses the signal from q20e to drive Q19 base in order to ground the lamp.
it could be a comp or an alignment issue.
 
Ok, I put in a new 470uF cap and diode (1N4004).

Edit: TA-DA - STEREO light! Only comes on in FM mode when getting a Stereo signal. I hope that's correct.

IMG_20180107_210451.jpg
 
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Congratulations! While you've got it on the bench, CAREFULLY check the idle current and offset. adjust as needed. Button it up and enjoy it.
 
Tacking on to this thread from three years ago. The stereo light on my SX-828 was on all the time, even when the selector was in AM, FM Mono, Phono, Aux, Mic mode -- that thing was on. When in FM Stereo mode, it got brighter when an FM Stereo station was actually tuned in, but went back to the slightly dimmer on all the time mode on when not receiving a stereo station. Based on reading the above thread, and related threads regarding an SX-727, I replaced Q19 with a KSC2383, which I had on hand - carefully bending the leads to retain proper EBC sequence, and BINGO, the stereo light works properly. Off, except when actually receiving an FM Stereo broadcast.

AK comes to the rescue again.

The old 2SC968 tested as a pair of diodes on my Peak Atlas DCA55, rather than as the NPN transistor it is, but I could not detect a short with the multimeter. So the old one was definitely the culprit, but not exactly sure how it failed. Not over thinking this one - it's fixed.
 
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