SX-1000td Balance Issue

[Mr_Mustrd]

Good Evening from Ohio!

Been working on restoring this beautiful receiver for some time now and I've replaced all caps and transistors except for the tuner boards, and the 4 main output transistors. The 4 big filter caps have been replaced, installed new 7 0.68 wirewound cement resistors on the bottom that have been recommended to swap out, and also replaced the diodes on the power supply. My dc offset is at steady -14 mv for both left and right channel, and on the main amp W15-027 i'm getting 75.2v for pin 6 on both left and right channel, and pin 3 is at 39.3v for both left and right.

But, when I turn the balance pot to the right channel the right channel cuts out, also when switching the low filter on and off I get a loud pop and when switching the high filter on and off I still get a pop but not as loud. This occurs no matter what the receiver is set too, fm, am, aux, etc, currently it's on aux, vol down to zero, bass and treble set to middle.

I know enough to know that I don't know anything but I do have a fluke multimeter, dim bulb tester, variac, scope tektronics 2235, and a signal generator leader lag 120a. I’m still super new to electronics and I bought the scope awhile ago with hopes of learning how to use it properly, I’ve read the manual and have learned how to set a baseline, but beyond that I haven’t had much experience, so this looks like the perfect time!

Yesterday after reading other threads on audiokarma and some youtube videos I was able to run a 1k signal through the aux input on the receiver and was able to pick up a signal using the scope. I started at the Vol and did get a clean signal, then at the balance knob the signal was jittery on the right but clean on the left but I’m not sure if that’s because of the probe I’m using (It’s a cheap knockoff) or the scope.

On the control amp W15-031 at pin 10 for the right channel which according to the schematics for W15-031 is the input I was able to get a clean wave and at pin 2 which is output for the right channel the signal is very jittery, as in there are two waves and they keep crossing each other, I hope I’m making sense.

I tried following the schematics and working back from Pin 2 and comparing it to the working channel but it’s still very confusing, but I feel that I was able to find that the signal comes back clean after C622 which is 1uf 10v. I’ve tested the following by lifting one leg from the circuit for the right channel, R632 1k, R636 47k, and C622 1uf 10v and they’ve all tested within spec. I have checked and triple checked the orentation of the caps and transistors, i've attached the two transistors that are in the path for the right channel and I believe they are in correctly but i've been wrong before, therefore i've posted the photos of them below. I've also tested the balance pot as well by desoldering the wires except for two the black wires and the inner ring tested at 64k and the outer tested at 74k. As far as i can tell i'm not finding any lifted pads or solder bridges, also for good measure gave everything a dexoit bath again but no change (can't be too easy eh?)

At this point I’m not sure what to check on next, these boards are very old and i'm trying to not stress them too much by testing stuff uneedlessly plus i'm not even sure if i'm heading in the right direction and feel like a dog chasing it's own tail so I'm turning to the experts at audiokarma with hopes of being pointed in the right direction! Thank you for reading, I know it's a lot!

Below are the measurements I’ve taken connected directly to variac at 120v, no speakers connected, vol down to zero, bass treble and bal set to middle, on aux. Also photos of the boards in question

Control Amp W15-031
Left Pin 1 39.27v Right Pin 1 39.27v
Left Pin 2 1.34v Right Pin 2 1mV
Left Pin 3 0.01v Right Pin 3 0.01v
Left Pin 4 1.93v Right Pin 4 1.95v
Left Pin 5 1.94v Right Pin 5 1.95v
Left Pin 6 1.93v Right Pin 6 1.95v
Left Pin 7 1.93v Right Pin 7 1.95v
Left Pin 8 1.93v Right Pin 8 1.95v
Left Pin 9 1.93v Right Pin 9 1.95v
Left Pin 10 0.01v Right Pin 10 0.01v
Left Pin 11 0.01v Right Pin 11 0.01v

Q601 Left Channel
E: 1.5v
B: 2.0v
C: 7.7v

Q603 Left Channel
E: 1.5v
B: 2.0v
C: 6.8v

Q602 Right Channel
E: 1.4v
B: 2.0v
C: 7.9v

Q603 Right Channel
E: 1.5v
B: 2.1v
C: 6.8v

W15-010 Filter Unit
From Left to right
White: 0.01v
White: 0.01v
Black: 0.01v
Orange: 1.3v
Orange: 5.4v

 

[Mr_Mustrd]

Had some time today to work on the receiver, followed the schemtics and not following the board I traced the path from pin 10 to pin 2 to make sure each cap and transistor is in the board correctly as per the schematic and they are. Hooked back up to the scope, switched to AC instead of DC this time, and at R614 8.2k the signal goes from clean to jittery, so pulled that resistor and no luck it tested a lil higher than 8.2k but otherwise within 10%, I then proceeded to test everything dotted in red below by pulling on leg to bring it out of circuit, they all tested within spec, the mylar caps were dead on. While I was in there I reflowed everything and even got out my maginfying glass to make sure there were no solder bridges. Could it be the balance pot? there is one on ebay that I could buy. Any thoughts?

 

[john stumpf]

you could try some contact cleaner to flush the control out but you MUST relube it with fader lube

 

[john stumpf]

delete

 

[john stumpf]

but the way i read the schematic your issue starts at the beginning. the balance pot is on the output of the board if it were a pot issue as the volume is on the input side i'd start there just to be sure and then the bass and treble controls.if they resemble the pots in my 1500td and i believe they do. they arent as well enclosed as those used in later pioneer sets. it might be wisest just to service them all with fader lube to be sure.. i just went around with a volume control in my 1500td this past week. the wiper is 3 tiny fingers that were solid black from oxides. while whats coating the non contact parts are irrelevent the actual wiper area on each finger is incredibly tiny. i am not saying that is your problem but it might be wiser than opening the wallet on ebay at this early stage and if the deoxit gear is on hand why not? in the end it its probably a plus whether its the problem or not

 

[Mr_Mustrd]

Thanks john! you bring up some really good points, if it was the balance pot then the outputs on the control amp would match, which is why I was hopeful in tracing the signal using the scope but I fear that my lack of experience reading schematics and working with the scope is holding me back from locating the problem. I am getting a clean signal at the volume pot and at the inputs on the control amp at both pin 10 for right and left so thats good news that the signal is getting to the board fine, it's just not leaving fine? But I've got a cart filled out on mouser to replace all of the resistors if that is the direction that this may be headed.

I did give it a deoxit bath followed up with faderlube which was the very first thing I did to it. I like to try to clean the receivers when I first get them because you have no idea were they could have been plus cleaning the pots and switches could fix whatever problem they may have had before it came to me.

To just make sure, this morning I've went ahead and deoxit all the pots and switches with deoxit 5 followed up by deoxit faderlube, this time around I made sure to work each pot or switch at least 50 times in both directions at minimum. For the balance pot and switches I went double that, and then once the faderlube was applied only worked each pot and switch ten times. Currently it has a fan pointed to it help speed up the drying, once dried I’ll fire it up and see what happens which should hopefully be later today and give an update, i'll also test the treble and bass to see if there is a clean signal there as well.

 

[zebulon1]

I'd like to help but the scat I have for the TD version shows W15-047 as the control amp.(?)
Popping when switching anything in the pre amp section to me, means something is in backwards or shorted. Like a transistor or cap.
Also look for the voltage on the output pin of the control board when making the channel cut out and see what your up against.
I'm thinking there is some stray untamed voltage in there.
Keep it simple.

If it wasn't a cap coupled amp, it would provably go into in protection.

 

[Mr_Mustrd]

Progress, but not fixed yet. Deoxit unfortunately didn't fix it but following what zebulon said I checked the other boards and found that I put in KSC945 in wrong on the main amp W15-027 which are replacements for Q811 and Q812 2SC538A. According to the datasheet the C suffix the collector and base are swapped, so it's E C B and I had it in as E B C, great! I even remember reading markthefixer warning about that!!

Corrected the two transistors, but still getting balance issues, and the low filter and high filter are still having pops. Could the KSC945 have damaged something? I'll have to check after the kid goes to bed and will work on this later today. Thank you so far for the help! Below are the readings I got from Q811 and Q812

Q811
E 21.03v
C 39.14v
B 21.41v

Q812
E 20.53v
C 39.13v
B 20.90v

 

[john stumpf]

yes its possible it was damaged, ive managed to pull that off myself a few times. i learned just to throw them away as they can initially work for a while but then later die. i put a clamping diode in backwards on a protection relay after powering up and catching my mistake it worked 4 times and then shorted, no sense in preaching about it as its really easy to do

 

[zebulon1]

You better check everything. There's not that many components in that amp circuit. It shouldn't take long if organized.
The diodes can be checked using the diode check on a decent meter. The resistors should be checked. Cheating is comparing the good amp resistors to the suspect amp.
Replace all the transistors except the outputs if you find checking them a hassle.
Those diodes are 1N60's which are geranium. They have a higher forward voltage than the silicon. They are slightly bigger if original.

 

[john stumpf]

maybe its time to just fall back and regroup with a cup of coffee and a flashlite and recheck the pinouts on all the caps and transistors in the entire amplifier chain and check for errors rather than courting disaster by continuing in the powered up mode. theres a lot of truth in the theory about first fixing whats wrong in a set before a wholesale rebuild is in play. the idea being to able to rebuild a board then check for proper operation before proceeding to the next, im not saying thats what you did im just speculating. some times a lot of gripes can be worked off and detected easier with your eyeballs and its a whole lot safer. it also follows zebs reasoning. by the way,did you notice that c807 in the main amp is.bipolar? if you used a film cap to replace that then disregard this. why it is bipolar i'd really like to know.

 

[Mr_Mustrd]

yes its possible it was damaged, ive managed to pull that off myself a few times. i learned just to throw them away as they can initially work for a while but then later die. i put a clamping diode in backwards on a protection relay after powering up and catching my mistake it worked 4 times and then shorted, no sense in preaching about it as its really easy to do

Not gonna lie, it's very frustrating but it's still a learning experience and regardless I had a blast using the scope for the first time even though I wasn't even in the right area. Something that I struggle with is reading the flow of the signal through the schematic and to visually see that on the scope was very helpful and made a couple of things click for me which I know is going to come handy in the other receivers that are waiting for me to work on. So regardless, this has been a really fun project, just some set backs that hopefully can be resolve.

maybe its time to just fall back and regroup with a cup of coffee and a flashlite and recheck the pinouts on all the caps and transistors in the entire amplifier chain and check for errors rather than courting disaster by continuing in the powered up mode. theres a lot of truth in the theory about first fixing whats wrong in a set before a wholesale rebuild is in play. the idea being to able to rebuild a board then check for proper operation before proceeding to the next, im not saying thats what you did im just speculating. some times a lot of gripes can be worked off and detected easier with your eyeballs and its a whole lot safer. it also follows zebs reasoning. by the way,did you notice that c807 in the main amp is.bipolar? if you used a film cap to replace that then disregard this. why it is bipolar i'd really like to know.

Got coffee in hand and flashlight ready togo! Today I'm going to go over the power amp board and make sure each cap and transistor is in correctly, and also test everything as zeb recommended, which should be fun since for Christmas my wife bought me a peak transistor tester .

I'm not sure if I’m going to go the route of replacing all the transistors, maybe just the smaller ones and leave the drivers one alone, but I will be making sure each cap and transistor is in correctly before powering up next time, also will go over the power supply board as well just to make sure a diode isn't in backwards or anything. but I’m thinking that if I was able to get a good idle and dc offset, then that should indicate the driver transistors are okay? idk I’ll find out soon enough.

And I agree, I didn't start this rebuild with the intention of fixing something due to the receiver was working prior to the rebuild, I enjoy working on these receivers and restoring them, it's how I de-stress from the day. But, it's apparent that my testing isn't thorough enough inbetween working on boards and this is something that i'll review and find a better system for, i'm still super new to this, this is my fourth? receiver I've restored so I still have tons more to learn.

As for C807 and C808 I have a bipolar cap installed, 647-UES1E4R7MDM, do you think i should swap this out with film? I do have some that I can put in there.

 

[Mr_Mustrd]

You better check everything. There's not that many components in that amp circuit. It shouldn't take long if organized.
The diodes can be checked using the diode check on a decent meter. The resistors should be checked. Cheating is comparing the good amp resistors to the suspect amp.
Replace all the transistors except the outputs if you find checking them a hassle.
Those diodes are 1N60's which are geranium. They have a higher forward voltage than the silicon. They are slightly bigger if original.

heard! when you say in that amp circuit, are you refering to everything in the entire receiver? or just on the main amp board? You're right, once I get rolling it'll be done before I know it.

I may not go the route of replacing all the transistors, the KSC945C will def be replaced and the smaller ones like the 1845 but i'll leave the bigger driver ones, but two legs will be pulled and tested with my peak transistor tester.

I think I’m going to go ahead and just order all the resistors for the main amp board because I want to start building a stock of resistors, anything I should know before getting resistors? I know I’ll be looking for Carbon Film 1/4w unless otherwise stated in the service manual.

For the 1N60 geranium diode, in case I fried this, would the below be a suitable place to purchase a replacement?

https://www.mikeselectronicparts.com/product/1n60/

 

[gadget73]

metal film are generally quieter

honestly I've not had problems with significantly off-value resistors in my Pioneer of this vintage. Pops when messing with things off the preamp make me wonder if you have DC leakage to the switches. Not sure what the circuit around the low filter looks like without the full thing but I'm guessing its just a small value cap that is bridged when the filter is off, probably sitting between output of the control amp and input to the volume and balance controls? If there is DC leaving the control amp for whatever reason it will make all of this act funny. Easy enough to check for, just measure from ground to the two 'out' terminals on the control amp. It should be 0vdc if the 1uf cap is doing it's job.


as for the balance thing, just ohm check the balance pot. If its got drop-outs you should see it on the meter. Probably won't measure quite right in circuit but you'll see if its got opens. It should read the same as the left channel, just in the opposite direction. If the pot is the same as the one used in the SX990 and 1500td, on-center it should be very close to zero ohms from center to one end with the pot in the middle, and I think 500k from middle to the other end. Which end is which depends if its the front or rear section, and I do not recall which is what. The ohm meter will tell you though. If you find its right in the middle and basically open circuit on the right side with it off-center, the pot is probably shot.

 

[john stumpf]

1n60's are germanium and i would stick with them in the absence of knowing what else might work.you can get a fist full on ebay for peanuts,but why not just do what you are planning on doing. and see how things pan out?

 

[Mr_Mustrd]

Had some time today to go over the receiver and was able to fix it! Inspected every cap and transistor that I replaced on the main amp board and found C802 was in backwards!

Replaced both C802 and C801 with a new cap, replaced Q802 and Q801 with a new 2sc1845, and also replaced Q811 and Q812 with new KSC945C. I tested everything that was directly connected to Q811 and Q812 and they tested good, which is a relief.

After warm up of 10 mins, Pin 6 is reading 77.4v for both channels and adjusted pin 3 to 38.7v, Dc offset is at a solid -14mV on both left and right. Been playing music for the past several hours with no problems. Balance pot is super quiet, not more pops in the low and high filter. Added some photos all setup in the living room, wife says it looks cute

This was a great project and I learned a ton, I feel that I'm just a little bit better in reading the schematics or at least can figure out the direction of were the signal flows. Thank you @john stumpf @zebulon1 and @gadget73 for your help!!

 

[john stumpf]

hindi malaki bagay. i'm glad our suggestion paid off, as yogi berra so eloquently remarked decades ago,"you can observe a lot by just watching"

 

[john stumpf]

riddle me this,i'm getting a really trashed tw tomorrow. (you dont get much for 30$) i didnt notice when scoping out s.s hesperus but did after the sale looking at others on ebay that while they have a wooden cabinet they appear to have an inner steel cover and if so mine is missing. it seems in the pics online THEY MIGHT have come in two flavors with the wood box or without and the parts listing isnt much good as it listed both but doesnt indicate if the wood case is optional or if it has a wood case the steel cover is not used or its supposed to have both.

 

[Mr_Mustrd]

Mine does have the steel cover and the wood case. But it does fit will with the wood case only so it looks like the steel case isn't mandatory for the wood case to fit.

Good luck with the 1000tw! Let me know if I can send you pics of anything from the 1000td if you need some reference.

 

[Electrodyne]

Yes,my SX1000TD has both the wood case and steel cover under...the 2 helped keep alot of ..farm shop ..dust out when i had it there. BTW,Thanks John for the diode #.