Rebuilding Pioneer SX-1280 with humming noise

markthefixer said:
The other trim pots (excluding two per channel for idle current and dc offset adjust) on the 1280 are on the tuner for stereo decoder adjustments (leave them alone unless you have the correct gear to set them) and the meter calibrations (leave them in, recal isn't difficult with a good ac dmm). None are critical (as being able to cause damage upon failure), nor are they under stress.
Click to expand...
Mark,
My Power Supply Board is AWR-157-B. And it does not have C24 (10/10) and it looks as it was never installed there at all. There is a blank space for it with 2 holes and capacitor sign on the board. Should I leave that empty or install upw1v100mdd cap per your list?
Thanks,
 
Is it possible that it was missed at the factory? As a defect? Holes for missing cap were soldered shot on the film side.
Look to the left of Q5 (bottom transistor with heat sink)

R13 & R14 820 Ohm resistors were obviously getting hot. They tested pretty close at 770 Ohm. So I cleaned leads, pre-tinned them and soldered back in.
All E-caps, Transistors, Diodes and Zenners are replaced. I'd love to hear from someone with knowledge what should I do about that "missing" cap before I fire it up.
Thanks

AWR-157B.JPG
 
Grind, not a factory defect. My Power Supply board is the same way. And my 1280 was running just fine prior to my recap (currently in progress). Oh, I also ran into another unused cap spot, but on the Flat Amp, I believe.

- Jeff
 
Thanks Jeff,
Rechecked everything and turned it on through DBT. No shorts. Connected to outlet. Faint click and no illumination lights. Tested voltages and nothing on pins 16,17,18.
Traced to bad contact at the fuse holder. Moved fuse and receiver powers on with following results:

1. Wire Resistor (1.2Ohm) on the fuse board is extremely hot within minute.
2. All 3 transistor heat sinks on AWR 157 are considerably hot but not burning the finger.
3. Both R13 and R14 are super hot. Can't touch hot.
4. Hum is gone! Plays clear through headphones.

Voltages:
Pin #8 +75v (+68v spec)
Pins #11,12,13 +35v (+32v spec)
Pins # 16,17,18 +13.3v (+13v spec)
Pins #20,21,22 -33.5v (-32v spec)
Pin #25 -72.5v (-68 spec)

What I see on schematic is pretty obvious that R13 and R14 are working harder. Higher voltages of pins 8 and 25 driving 11,12,13 and 20,21,22 respectively higher too.
Why are those voltages off?
Any suggestions?
 
Running a bit high. R13 and R14 are 2W resistors, they are supposed to get hot.
Did you change D7,8, they show a 32V zener, did you use a 33V zener?
 
The list was from 2015 Markthefixer's post from Mouser. See my original post.
Pin #8 +75v is not within 5% of +68v
 
I assume that you did not measure what the supply V's were before you did any work on it?
Pioneer does not list tolerances, I assume 5% is a good number to work from but in reality you have 5% tolerance zener and two 5% resistors, so worst cases can exceed 5%. In those days, 1% resistors were a premium, now they are the norm and about the same $ as 5%. Using a 33V zener just makes the variation all the worse.
the 1250 design uses a pot to adj these voltages, for the 1280 they started to cut down on costs and removed the pots.
I guess you could put the old zener back in and measure what it was to begin with. PITA.
Could also ask the lists owner, what he thinks about your situation with the recommended replacement parts?
Problem with these BOM lists is they are not under any revision control, so any changes, are very hard to follow. Like how am I to make a correction to the list that everyone goes to.
It is a fault of the way the forum runs their business = hack n wack engineering.
 
Here is what I did.
Replaced R13 & R14 820 oHm with 5w and R23 & R24 10 oHm with 1/2w to control heat first.
Voltage dropped about .5v. Board runs at much lower temp. Possibly close to half of what it was before.
Then I replaced D7 & D8 with 12v zeners from Mouser (771-b2x79-c30133) and got much better results.
Now voltages are 68.5, 32-33, 12.4

I have next question. Some caps that listed in the manual as electrolytic are bright blue dipped plastic type. Are those e-caps?
Do they need replaced? Mark's list suggests replacing those cap positions with regular style e-caps (except one I think).
Filter board AWM-119 does not have any traditional looking e-caps on it at all and replacement in Mark's list does.
Can someone recommend?
Thanks,
 
They (the originals) ARE electrolytic caps. Tantalum they are called, and are frowned upon for passing audio. Power bypassing's ok, until they turn into solid short circuits, which is their preferred mode of failure. CSZA type in the Pioneer BOM's.

The other type is CSSA which were solid caps LIKE tantalum, only with aluminum. These have a guaranteed failure mode of increasing leakage current, to the point that Fred Longworth uses the term "capsistors" which is a term for a cap that turns into a resistor - eventually becoming a short circuit.


edit - film caps have no polarity sensitivity, either way is fine,
 
Do you have recommendations to make bias and idle current adjustments after recap and trim pot replacement?
Would it be different from procedure in the manual?
Where should I set trim pots before I turn power on?
 
grindfix said:
Then I replaced D7 & D8 with 12v zeners from Mouser (771-b2x79-c30133) and got much better results.Now voltages are 68.5, 32-33, 12.4
Click to expand...
Good to hear, so that is 32V zeners :)
Well you usually turn the bias down to start with, adjust offset, adj bias to spec ?mA or ?mV across the emitter R, let it heat up, check/adj offset again and then bias again.

I do have a suggestion as far as BOM rev control goes. Anyone with an account can create a BOM in Mouser and give everyone reference to it. It can be maintained that way. It is a fair bit of work however.
 
Last edited:
Latest update.
Recapped all boards except the tuner so far. Turned it on through 150watt bulb. Adjusted DC and Idle current. Fired up and sounds great in Aux. No hum except some hum in Phono but I think its due to missing covers and open Phono input (didn't try turntable yet). Readjusted DC and Idle Current and re-tested after 10-15 minutes. Slight drift but still within specs.

Then, after about an hour of playing out of nowhere comes crackly feedback noise. Best to describe it as if microphone input is pulled at high gain setting. Left channel only. That is the channel that was humming before Power Supply board was rebuilt. Pulling the jumper stopped it so it is not the power amp circuit. So, hum caused by AC bleed in Power Supply got some transistor tired somewhere in pre-amp circuit. How do I find that? Noise comes and goes. If power is turned off and back on noise is not there anymore. I did not try to leave it on and see how long it keeps going as it is quite loud and does not respond to volume but goes away with balance turned to Right.

I know I'm not pro, but it does not sound like this condition is caused by recap or anything I did to the unit. It is very intermittent and cannot be re-duplicated with any controls or wiggle test. Random occurrence and high level of the noise suggests actual pinpoint troubleshooting to be very difficult. What do you guys think?
 
So it is in the L channel of pre-amp then, there is only a jfet (2sk117)and a bjt( 2sa726) in the pre-amp and some passives.
grindfix said:
but goes away with balance turned to Right.
Click to expand...
It acts up with the volume at minimum?
That is suspicious, as if it was a bad contact issue. If was the pre-amp gain ckt, I would think that it would act up regardless of the balance control position.
It could be a bad connection or solder joint, just have to do some more checking. Make sure you put those polarized ecaps in the correct orientation.
I do not think it would be the 2sa726. Does not hurt to change it with a ksa992 as they are on the to be changed list.
 
Found bad solder joint on tone amp board. Loud intermittent noise seems to be gone but, I can tell the difference in sound (without any input, just background sound) when tone defeat switch is activated. Much cleaner in defeat. Will replace tone board transistors and will see if that helps.

Next question. Several of caps listed for replacement on the tuner board are obsolete. Can you guys recommend suitable replacement/

sx-1280 awe-092 c27 0.1 35 csza 667-ECQ-V1H104JL
sx-1280 awe-092 c28 0.47 50 cea 667-ECQ-V1H474JL
sx-1280 awe-092 c44 0.56 35 csza 667-ECQ-V1H564JL

Thanks,
 
Good news,
Search Mouser for any polyester film cap that has 5mm lead spacing, Kemet is a good source.
Stock up on ksc1845 and ksa992.
 
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