Pioneer SX-1280 Protection Relay Cycling On And Off

distortion13

New Member
I recently finished re-capping a portion of my SX-1280 receiver, including the replacement of other components, in an effort to increase the life cycle and performance of this receiver. Before moving on to re-capping the main caps and those in the power amps, I fired it up to check things out. Upon power up the relay on the protection board keeps cycling on and off with approx. 3 seconds between changes in state (on vs. off vs. on again, etc.). This receiver was working before the re-capping efforts. Any help would be, as always, greatly appreciated.

After reading a post on a similar issue, I started to adjust the DC balance of the power amplifiers per the manual. With VR2 fully clockwise and VR1 centered, per chapter 10.3, I get readings at pin 14 of each amp board way out of the acceptable limits as follows. As I understand it, these figures should be 0V +/- 20mV.

Left Channel: 4.51V Pin 14 to Grd.
Right Channel: 4.75V Pin 14 to Grd.

The boards re-capped to this point are as follows.
AWE-092 Tuner
AWF-031 EQ Amp
AWG-060 Flat Amp
AWG-059 Tone Amp
AWM-119 Filter Amp
AWM-123 Protection Board
AWR-157 Power Supply

The electrolytic cap replacement is pretty straight forward, but I am including a list of the transistors (and Misc) changed on the power supply and protection boards below in the event that I may have crossed something incorrectly.

AWR-157 Power Supply (Q6 and Q7 are especially concerning)
Q1 - 2SD712 D or C part used is Mouser 512-KSC2073TU (Replaced heat sink compound as well)
Q2 - 2SB682 D or C part used is Mouser 512-KSA940TU (Replaced heat sink compound as well)
Q3 - Component tested and re-used - didn't have a replacement at the time.
Q4 - 2SA904A-G or F part used is Mouser 512-KSA1013YBU
Q5 - 2SB682 D or C part used is Mouser 512-KSA940TU (Replaced heat sink compound as well)
Q6 - 2SC945A-Q or R part used is Mouser 512-KSC945CYTA
Q7 - 2SC945A-Q or R part used is Mouser 512-KSC945CYTA
D1-D4, D11-D12 - Radio Shack 1N4004 used.

AWM-123 Protection Board
Caps only. Q1 and Q2 were left alone.
 
check datasheets for transistors pin outs then check you have them the right way around .
if ok check caps are correct way round .
 
check your power supply voltages, the pin 14 voltages are way unbalanced, if they are volts and not millivolts:

Left Channel: 4.51V Pin 14 to Grd.
Right Channel: 4.75V Pin 14 to Grd.

and who told you to use : 512-KSC945CYTA ?????????????

I don't use that because it can have TWO pinouts, ebc and ecb.... causes too da** many problems...
 
Last edited:
Thank you for the replies. I will double check polarities and pinouts.

markthefixer, I will also swap the 512-KSC945CYTA parts for 512-KSC2383YTA in the power supply. The use of the 512-KSC945CYTA was not a recommendation from anyone. It was something I had laying around that seemed like it would work. Thanks for the heads up on the potential pinout differences. Besides the difference in pinout, would this have worked in place of the original in terms of the specifications. Just trying to gain more knowledge on the art of transistor substitution.

Seeing as though the 512-KSC2383YTA is a replacement for 2SC945A, is it also the recommended replacement for 2SC945 (no "A" ar the end)?

Some of the other part numbers for transistor swaps that were supplied in a copy of a parts sheet i was working off of were not available at Mouser at the time, but Mouser gave a substitute for these as follows. Does this look right. Reversed pinouts were identified on some of these, at least in the 2SC945A and 2SC 1312 replacements and possibly the 2SA726.

Flat Amp AWG-060 Q3-Q4 2SA726-G or F, Part Used 512-KSA992FBU (Pinout Reversed ECB/BCE)
Pwr Amp AWH-074L Q12 2SA899A, Part to be Used 512-KSA1381ESTU
Pwr Amp AWH-074L Q14 2SC869, Part to be Used 512-KSC2383YTA
Pwr Amp AWH-074L Q5, Q10 2SC1904, Part to be Used 512-KSC3503DSTU
 
You have to read the FINE PRINT!! Case in point. KSC945YTA vs KSC945CYTA.
Also one of the 733's (2sa or 2sc, I can't remember which right now) has the same thing, a "C" version. I got hit with that one a couple years back.

The "C" here means "Center Collector, and is stated so in the data sheet (FINE PRINT). The KSC945YTA is a E-B-C orientation. The "Y" is the gain level or hFe. TA is Ammo tape packaging, BU is Bulk packaging.

99.999994% of todays transistors will have a different pinout than the ORIGINALS. So you MUST check, Doublecheck, Triplecheck on each transistor you install or you WILL have problems.

As for the KSC2383YTA being a replacement for the 2sc945 and 2sc945A, YES. In some cases a heavier duty transistor is mandated, when the 945 is run at the edge of the "SAFE OPERATING AREA" or SOA. Aviators call it pushing the edge of the envelope. Not all 945's will get replaced with 2383's. Sometimes in the case of lower requirements (in the middle of the SOA) a 945 is sufficient. Which is why you'll see different replacement transistors for the same original number on the same board. Or the same transistor used for different originals, as it has characteristics which will work well in that particular spot.

Larry
 
Got a chance to make some changes as follows.

Power Supply AWR-157
Replaced Q6 & Q7 with KSC2383YTA, ECB - NPN Transistors paying special attention to pinout change.
Verified orientation of Q4 (previously replaced with KSA1013YBU, ECB - PNP transistor)
Replaced Q3 with KSC2383YTA, ECB - NPN Transistors paying special attention to pinout change.
Verified orientation of Q2 & Q5 (previously replaced with KSA940TU, BCE - PNP transistor)
Verified orientation of Q1 (previously replaced with KSC2073TU, BCE - NPN transistor)
Verified polarity of all capacitors and diodes that were previously replaced.

Protection Board AWM-123
Verified polarity of all capacitors that were previously replaced.
Neither of the (2) transistors were replaced on this board.

Protection relay keeps cycling on and off.
The New Pin 14 voltages I get on the power amps are:
Left Channel: 7.47V Pin 14 to Grd.
Right Channel: 7.13V Pin 14 to Grd.
 
Is the idle current turned all the way down? WHAT is the DBT wattage?

You BADLY need to read the regulated voltages as I have already asked.

If you do not know where to measure them, please ask.

If they are different from the original, that could explain the 7v and it is SEVEN volts (plus a bit) referenced to ground?
 
Markthefixer, please help on where to take the readings. The ones I supplied were taken with the black lead clipped to the chassis (bracket holding the power supply board).
 
I finally got a chance to look a little closer to clarify where the readings were taken from and the initial setting on the potentiometers of the power amps for the attepted DC Balance and idle current settings.

The attached picture is of the power amp on the right side of the receiver. I have also attached pictures of the schematic for the power amp and the procedure in the manual for the DC Balance and idle current settings. The top blue pot is VR1 and white pot toward the center of the board is VR2.

VR1 is a 47k ohm pot that measures 42.9 Kohms fully clockwise and 17.4 ohms fully counter clockwise.

VR2 is a 100 ohm pot that measures 100.6 ohms fully clockwise and 0.3 ohms fully counter clockwise.

The manual states to set VR2 fully around counter clockwise and VR1 to the center as a starting point, then measure voltage from Pin 14 to ground, adjusting VR1 to get as close to 0 as possible. This is where I am measuring 7+ volts and listen to the protection relay click on and off. I connected the ground lead to the far end of the wire connected to pin 9 (earth grd) for the measurement. Adjusting either pot has no affect on the reading.

The manual's guidance for measuring the idle current states to measure between pins 17 and 11 which should be around 15mV. I get 0V with no change when adjusting VR1 or VR2.

Mark, you asked for measurements of DBT wattage and the regulated voltages. Your help on where to take those measurements would be more than greatly appreciated.
 

Attachments

  • DSC00078.JPG
    DSC00078.JPG
    83.1 KB · Views: 54
  • DSC00079.JPG
    DSC00079.JPG
    208.2 KB · Views: 42
  • DSC00080.JPG
    DSC00080.JPG
    203.5 KB · Views: 32
  • DSC00081.JPG
    DSC00081.JPG
    180.1 KB · Views: 32
I was able to get some voltage measurements last night. I have attached pictures of my notes with the voltages at each of the transistors in the power supply board as outlined in the schematic given in the service manual as well a picture of additional voltages . The actual measured voltages are circled and the values of what they should be are supplied (not circled). All measurements were take with respect to ground with the black lead of the DMM clipped to pin 23 (grd) of the power supply board. I performed a quick continuity check between this pin and the chassis to verify it as ground.

The voltages at the base of Q2 and all points of Q4 are way off. The voltage at the base of Q2 reads -1.79V (should be 68.6V). For Q4, the base reads -0.56V (should be 32.6V) the emitter reads -2.8mV (should be 32V) and the collector reads -2.0V (should be 68.6V).

I rechecked the pin orientations of all transistors and the polarity of all capacitors and diodes in the power supply. All are good.

For kicks, I replaced Q2, Q4 and R8 with no change in the low voltage condition of Q2/Q4.

Please help as I am at a loss and the idea of this beauty sitting on the bench and not pumping out jams is killing me.
 

Attachments

  • IMG_1127.jpg
    IMG_1127.jpg
    47.3 KB · Views: 26
  • IMG_1128.jpg
    IMG_1128.jpg
    51.2 KB · Views: 20
  • IMG_1130.jpg
    IMG_1130.jpg
    65.3 KB · Views: 23
Check the zener (I think D5?) on the negative regulated side. I bet it's open. Good job so far.
Dont forget to reset the Idle trimmers back to full CCW in preparation for the amplifier settings. Use the DBT and check for good VR response before you go to full house power for the final Idle setting.
 
Last edited:
Thanks for the reply zebulon1. (2) quick questions.

Do both of the idle trimmers, VR1 & VR2 at each of the power amp boards, start at fully CCW for subsequent adjustment?

I have been out of electronics testing for a good number of years. What is DBT and VR response?
 
VR1 center for the DC neutral setting.
VR2 fully CCW
On DBT,
First, You are setting the plus (+), minus (-) sides of the amps to zero (0) volts with VR1.
Second you check and see if the idle VR2 can be adjusted and that it responds to trimmer movement. You won't be able to set the idle completely due to the DBT limiting the current. If it checks out good, then return the trimmer (VR2) back fully CCW and go to full house current/voltage.
On house current, set the idle (VR2) to the manual spec. The DC Balance (VR1) should be fine, but it wouldn't hurt to check it.
Take your time and enjoy this part of the repair. This is my favorite part!

Note:
Be careful and turn the set off before moving your DMM probes to check the other setting! Many a fine work went up in smoke when the probes shorted out against the wrong post.

The Pioneer members call this procedure "Tickling the Dragons tail" or "Tickling the Dragon". You can search the phrase's and find a more detailed explanation and the reasons for using the procedure.
 
Last edited:
Problem Solved!!!! It wound up being a burnt up D8 and R6 in the Power Supply board. The resisitor didn't fully open, reading around 3M ohms instead of 4.7 ohms. All the voltage was being dropped across this resistor.

After component replacement the receiver came to life perfectly. Tickled the dragon to values well within the acceptable ranges, within 4 mV for DC Balance (Very touchy adjustment) and 1 mV for Idle Current.

Now its time for main cap replacement and Power Amp re-capping.

Thanks to all that supplied help in fixing this issue. Cheers.
 
Congratulations!! :D :thmbsp:

Sorry, I got pulled away from online troubleshooting for a while...

re: touchy adjustment, during recap - now you see why we use multi-turn pots...

if q2 and q4 are original, and d8, r6 were trashed, q2 and q4 were stressed when whatever killed D8 happened. I would replace them in the future.
 
Will do, Mark. Again, thanks for the help.

I am semi new to the forum and hope I am not going outside of proper etiquette with the following.

Quick question in regards to the power amp boards. I am currently re-capping the boards and reapplying new thermal compound to the main output transistors, (the insulators will be reinstalled per past threads I've read). Are there any transistors in these boards that are known to fail? I want to swap them out while i'm in there if needed.
 
nope, just the 4 power filter electrolytic caps, the input 1uf cap (you can go to a non-electrolytic cap on that one for a slight increase in sound quality) the c16 in the over current protect sensing - also a non-polarized replacement and finally the two trimpots.
 
The caps are done minus the non polarized one. Both of the 100 uF 80v caps were leaking on both boards. What should I use for the trim pots? Thanks again Mark.
 
Never mind. Just found this in another post.

the bias pot (3386H-101LF). The offset adjust will be a 3386H-104LF (100K).

Cheers
 
Back
Top Bottom