Pioneer SX850 left channel out

[pjf610]

Hi all, I am in the process of recapping a SX850 all was going good until the left channel stopped working, I unplugged the jumpers from the main in-out and plugged my phone into the in side and both channels work so is it safe to assume that the fault is in the tone board??? Or could there be some place else to look??? There was a lot of cracking and popping going on. Prier to the left channel going out so I recapped and changed the semi's on the power amp board and that took care of that problem, the unit is all recapped and all semi's was replaced, but no left channel any suggestions??
Pat

 

[Watthour]

Anything upstream of the Pre-Out jacks is suspect. Does the unit pass audio in any mode, with input to any jacks? You can try an external source (CD, tape player, phone, PC audio, etc.) in any of the TAP or AUX inputs. The Phono input is pre-amplified so if you use any external source there other than a turntable, the level will need to be turned WAY down.

All the mode switches (FM, AM, PHONO, AUX, DUPLICATE, ADAPTER, and TAPE MON) are suspects.

 

[zebulon1]

Amps are good.
Another thought:
On the pre amp sections:
Double check the replaced transistors for proper placement.
Missing solder connections? Easy to miss.
Look for broken wire-wrap connections (By tugging on the connected pin wires).
The signal wires are the ones that are twin lead shielded (Pink and white) or (Pink and Black).
The signal can be traced using a poor mans tracer. A, DMM set to AC mV's. While playing music through some input, with the black lead chassis grounded, probe the + side signal wires to and from the boards. The meter will move up and down in millivolts ranging from 70 to 370mV, to the beat of the music. Much like a Vu meter. See if you can find where the signal stops from jack to board. Don't forget about the volume control. The signal will go silent if there is no volume after the pot.
I suspect you may of made an easy mistake all of us make, from time to time.

 

[pjf610]

Thanks I will give both recommendations a go

 

[pjf610]

When I started to service this unit I test all my work with the use of a DBT and proceed to the next board and so on and so on this way if I come across a problem I can narrow it down figuring it had to be something I just did, when I did the tone board that is when I noticed that the left channel was out. I doubled and tripled check all the cap's and semi's pulled and tugged on all the wires and still no left channel, I would have stopped at this point but there was so much crackling and popping that I decided to service the power supply, after I did that all the snap crackling and popping stopped and still no left channel but at least that solved the popping. I did as Watthour suggest using my phone and plugged into the AUX. Phono, but still no left channel. I then unplugged the jumpers and plugged directly into the Main Input and both channels work fine, I did some probing like zebulon1 suggest and used my DMM on the protection board pin 3,pin7,pin8, got the reading like a VU meter this is were I am at now, no left channel.

 

[zebulon1]

Trace the signals in the pre-amp section. Begin at the left AUX, RCA jack. With the set selected to AUX. Trace the leads from the jack, watching the DMM. Following the signal in the schematics.
You've done well so far. You can jump ahead from one board to another if the access to the signal is difficult to reach. Your looking for a point where it stops. Don't go probing the board circuits. Look for a point where it goes in but not out - isolating it to a board or switch.
This is a good exercise and leave the jumpers out for the time being.

 

[pjf610]

OK i'll get back to you

 

[pjf610]

OK I pulled the jumpers out and plugged my phone into the aux. jack but got nothing from either channel

 

[Watthour]

With the jumpers removed (between the pre-out/main-in jacks) there will be no audio. You can now begin tracing signal through the preamp stages to try to determine where it is failing.

 

[pjf610]

So I hook up my phone to the aux. jack and start probing from the front end correct till the signal stops am I just looking at the left channel since that is out ??

 

[zebulon1]

Correct. This procedure is loaded with pit falls - you have to remember when the music stops it doesn't necessarily mean you found the blockage. Signal past the volume is effected by the volume control. Following the other channel can help in tracing the signal through the pre section and seeing what to expect.

 

[pjf610]

Hi zebulon1, I think I found were the problem may be all indications are going to the tone board I checked this board before I guess I will give it another go around

 

[pjf610]

Hi zebulon1, The method of using the DMM as a VU meter worked great I started to trace the signal from the tuner board and followed the path of the L-R channels and found were the L channel stopped it was on the Flat board not the tone board, it was a faulty transistor,changed it out and both channels are singing. That method worked great and thanks for all the input I received on doing this project. This one can be put in the book. On to the next one. Again thanks for all your help

Pat