Perplexing SX-838 Light Issue

[john stumpf]

well lemme see. it seems to me if you pull the pre/main jumpers on the back. and replace with rca jack cable's you can cross them and see if you have output on the missing channel. IF YOU SWITCH THE MISSING channel, then you know its NOT the main amp and further upstream. you could also use a meter set on mv scale and with the set running on one channel see if you have a difference on each channel on the pre amp jumpers to ground,each in turn, this would tell you the same thing. its crude but it should work.lots of switch contacts in that thing, it very well could be its a deoxit fix. i use d100 initially either with a nail polish brush or a syringe leave it sit overnight cycle the hell out of each switch and flush with d5. if its a success i lube with fader lube and the SHAFTS with singer sewing machine oil. repeat as necessary i just recapped re silica'd an 828.

 

[ringading]

Okay, switched the pre out/main in jacks and the dead channel did move to the left side so it seems it is not the main amplifier.

I already recapped the AF (tone amp) board and replaced the 2SA725 and 2SC1312 transistors and none of the old ones tested bad with a DMM.

Should I check the FETs? They are 2SK30-Y. Not sure how to test them - I will need to look it up. From what I read these are not typically prone to failure.

Looking at the SM, Could the head amp be the issue? If it isn't the AF board I don't see anything in front of the main amp other than the AF unit and the head amp.

 

[john stumpf]

did it have both channels before you went into the af board? did you check the pinouts on ALL the replaced transistors you changed? id go back and recheck ALL your work and recheck the polarity on all your caps too DONT FOOL AROUND WITH THE FET"S

 

[ringading]

I could have sworn it had both channels. Checked all capacitors and transistors on the AF board and all are oriented correctly. I need to check the SM and see if I can take some measurements to see if something is off.

 

[ringading]

Hopefully someone is still following this thread and can chime in.

So I checked the AF board and nothing was looking wrong from an installation standpoint. The only thing prior to the AF board is the Head Amp Unit. I measured voltage on the output pins and I get 23mv on one channel but only 5mv on the other. Could this point to a potential problem in the low channel?

 

[ringading]

Well scratch that. I found a missed solder on the leg of one of the replaced capacitors. Both channels measure 23mv on the output pins but still only get sound in the left channel. Perplexed. I assume since I get equal measurements on both pins then the problem is not on the HEad Amp Board.

Going to take some measurements on the AF board.

 

[john stumpf]

thats a reasonable work theory . on the af board what do you have on 15 and 17? theres a whole lot of switch contacts between the head amp and the af board. you do have the balance centered, YES?

 

[ringading]

Yeah, I traced from the output pins on the head amp board and they do run through the input switch to the AF board. Measured the point where the left channel reaches the AF board at pin 1 and it measures 23mv as well so it appears that signal is reaching the AF board as expected.

Balance is centered.

15 and 17 on the AF board both measure around 21-22mv.

What to check next? Maybe compare the emitter on Q2/Q1 - Q4/Q3 and so on to see if I find any that aren't similar?

 

[john stumpf]

how much trouble did you have removing parts from it? its double sided with plated through holes.. if you ruin the plated through portion you wont make connection to the component side. this is where a signal generator and an o scope are useful. i imagine your tired of hearing this, but i say again, recheck the af board and its transistor pinouts.

 

[ringading]

You were right. I had one transistor installed wrong - got mixed up with the ECB indicators on the bottom of the board .

Good sound now from both channels. Volume pot is still touchy so I think I will order up some D100 and let it soak overnight as you suggested. I have worked the hell out of it with the D5 which improved it but it is still touchy.

In the meantime it is time to replace the big cans and figure out how to get them to fit since the originals are huge.

 

[john stumpf]

thats good !!!! you need to read the" IDIOTS GUIDE TO DEOXIT" just use this forums search . i use a lot of d100 i flood the switch and leave it sit, on push buttons and i syringe the gang switches. reapply as necessary but its best to apply and leave it sit . i would even retry that lighting switch as you've nothing to lose. remember on push buttons the contacts are in the bottom so you want to have the switch upright parrallel to the earth those resistor pots MUST be relubed with fader lube after you got them clean. de oxit is pricey but its well worth it. i always use d100 first and the flush with d5. NOW you know why we test after each board to make sure its not worse than when we started, i mean to say. if it was stereo both channels then it should be after each board. you would have never went to the af board next, as you can see the errors can compound and then no body knows what the f..k is going on. its also another reason why its best to get working first before upgrades are done. any way im glad you are outta the weeds. TAKE YOUR TIME BE CAREFUL AND DRINK LOTS OF COFFEE. CHECK AND RECHECK AFTER YOU install a part AND DO IT AGAIN BEFORE YOU POWER UP. as for the filter caps i gutted them and installed new one inside with closed cell urethane foam. but arent you forgetting the main amp board?

 

[ringading]

Thanks John. I have probably replaced over 1000 caps and a few hundred transistors so I guess we all screw up sometimes. I usually check after making upgrades or changes to any board. I was able to get the new filter caps in with some rubber matting used for gym floors - worked out pretty well.

It is tempting to power through the capacitor replacement but have trained myself to fix the problems first and the proceed with other caps, lights, etc.

I have the DeOxit on order and will give that a shot and am stripping and refinishing the cabinet while I wait.

Thanks again for all of the assistance.

Cheers.

 

[john stumpf]

ive also installed my fair share of transistors the wrong way

 

[larryderouin]

Haven't we all! BTDT, got too many "T" Shirts for my own good. The biggest problem I see with transistor placement is 1.) lack of using the Data sheets,(1a, assuming the old pin-out is the same as the new transistor) and 2.) depending on the PIONEER board drawings assuming they are from the component side. They aren't from the component side, they are the solder side, which screws up a lot of folks. And PIONEER isn't the only ones that did it this way.

 

[ringading]

Thanks Larry. I always check the pinout of the old and new transistor to make sure they are the same. In this case the PNP/NPN diagram was printed on the top on the board (pretty fuzzy) and the E-C-B indications were on the bottom of the board which doesn't make much sense. I think I got two of them mixed up flipping the board upside down and back again.

 

[larryderouin]

Yep that's the big problem with PIONEER S.M.'s the board layouts are from the solder side and you have to remember to reverse the pins on the component side. I'll take and print one out and then scan them in reverse (print on the top vs. the bottom) and then adjust the gamma, brightness and contrast on the jpg so I can read it, rather than trying to remember which pin is which.