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Pioneer sx1010 bad protection board voltages

NickG69

NickG69

New Member
Hi all, I have a Pioneer sx 1010 that went intermittently in and out protection; the unit is in a good shape and Just the big filter caps have been substituted some years ago; now the relay is stuck in protection so I opened the unit and made some measurement; the PS seems ok, the values are in the range, the protection board values are bad in pin 5 and 6 showing different values from 2 to -40 (average -12) pin 5 and from -2 to -40 (average - 9) on pin 6; I already bought the needed components for rebuilding the board but before starting I’d like to understand what’s causing this. I read many threads here but I haven’t found a similar problem (to this point).
All test have been performed with a couple of 10w 10 ohm resistors connected instead of speakers and with speakers, with no differences and fm selected.
Any advice on this?
Many thanks,
Nick
 
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Pins 5 ,6 are the input side. Voltages will fluctuate with signal. Put the unit in AUX (NO SOURCE), center all tone controls, Loudness OFF, Volume to Minimum. NO RESISTORS OR SPEAKERS Required. Pioneer put in the resistor thing as a factory adjustment item for initial power up and adjustment. RETEST.

Also check the IDLE OFFSET and BIAS per the manual. As for the manual value for BIAS, Pioneer screwed the pooch, and promulgated too high a bias(they used a variant of common core math ie: 2+2=5 :confused::confused:). It should be 25mv, not 50mv.

Rebuild the Protection circuit. Make sure you install a 1n4004 diode between pins 9 and 10 (cathode (striped end) to pin 10). When the unit is shut off, the field in the relay collapses and the voltage backs up normally thru pin 10. In a stock unit this will backflow into Q7 and eventually damage it or take it out completely. The diode prevents this voltage backflow. PIONEER recognized the problem and made it a line item in later models. D8 also gets changed to a 1n4004 IIRC.
 
Larry,
Thank you for your reply; I tested in the condition you suggested and pin 6 and 5 of protection board are now -4v and -4,5v (avg) respectively; but left amp board between pin 19 and ground shows -4.3v avg with spikes up to -12 v and above!
Right board is ok ~1mv between pin 19 and ground, bias is about 25mv for both.
I’ll rebuild the protection board but do you think there’s some other component that failed?
Nick
 
The left amp is wonky. Exactly where is beyond my level. Best practice is to just rebuild both. Check your emitter resistors, and output transistors for shorts/opens. But before that, rebuild the POWER SUPPLY. They get very hot (due to location, orientation, and lack of air flow!) which is NOT good.

Build yourself a DBT (if you haven't already) and use a 100W incandescent. This will limit current to the unit, saving it if you should develop a short in one of the boards.
 
I have already all the stuff for rebuilding the PS and the protection board, I’ll buy the components for the amp boards too; looks like it will be a long project...
many thanks for your help, I’ll update you soon.
Nick
 
What's on Pins 1 and 3 of the Protection board? These are the outputs of each channel and should be 0VDC or close to it.

Craig
 
Hi Craig,
Pin 1 is about 4/5 v avg after a minute or so; it does not change with time. pin 3 seems ok (about 1 mv). All with the conditions Larry suggested.
I inspected the amp boards looking for signs of burned/broken components but they seems (visually) ok. Will check again tomorrow. Power board seems quite simple, same for protection board; But I’d prefer to identify and substitute the failed component(s) without rebuilding both amp boards, as it will take a lot of time (and skill).

So every hint is precious!
Thank you,
Nick
 
OK, you have 4V of offset on that channel, the protection circuit is doing it's job. There is no DC offset adjustment on this amp, try replacing Q1 and Q2 with a matched pair.
 
tsd71 said:
There are both idle current and DC offset adjustments on the 1010.
Click to expand...
Yes, idle is ok (about 25mv) offset is out of range (and the little pot seems do nothing). I’d order Q1 and Q2 plus the pot, just in case. Then will check again. It seems that, to reach the foil side of amp board, I’ll have to remove the big heatsink, correct?
Cheers,
Nick
 
Yes. IIRC the Heatsink is separate for each amp but it's easier to just undo both and get them out of the way. You'll need to undo the STV-3 or 4 from the heatsink 1st. Be EXTREMELY CAREFUL when moving them, the leads like to snap off at the housing, so don't let them bend there. Hen's teeth are less rare. If you have to bend them, bend the leads down by the amp board itself!!!!

Then remove each output transistor, placing them in ORDER. Good time for new mica's and heatsink grease. PIONEER used a clear adhesive type insulator spray (you'll see it when you remove the transistors). Clean it off with some acetone (outside!), and use new Mica insulators and heatsink grease.
 
I looked at the schematic a couple of times to make sure what I said was correct and still missed the DC offset pot, oops! The voltage on the base of Q1 doesn't change with the pot?
 
llwhtt said:
I looked at the schematic a couple of times to make sure what I said was correct and still missed the DC offset pot, oops! The voltage on the base of Q1 doesn't change with the pot?
Click to expand...

Hi, checked right now; no, it doesn’t; today after 10 mins of warmup the relay started clicking out of protection so I could check Q1 base within a range of -1,5v -0,5v but it is very unstable, and turning the pot does not change much the values;
 
This morning I checked the voltage at the emitter of Q3, and it’s more than 550mv (the same on the other board is 580mv), it’s 4-5 v avg up to 8-12; under 800mv the relay goes out of protection; base and collector are respectively 57,45v and 58,20. Can Q3 be the one that failed?
Cheers,
Nick
 
Q3 is the Constant Current Source for the input diff pair, Q1 and Q2. As the name implies it "should" be constant. Emitter should be 55.5V, the base 54.9V and the collector is .55V. If the base and emitter are good/constant I would replace all three, Q1, Q2, and Q3.
 
Last edited:
hi,
Latest:
I changed every cap on the power board and on the protection board, some of the transistor on these boards, then I changed every transistor and Electrolytic cap (except the 1,5uf one) on the left power amp board (the one that had problems) and now it works almost perfectly; the relay clicks, misic plays nicely, no hiss or crack etc etc. almost. just one thing is wrong.

Left board offset!

I even changed VR1, (the old one was ok, anyway) but the offset stays after 10 mins at 62-68 mv quite stable with the pot turned all counterclockwise (clockwise it raises). No way to set it at 0 mv. Q1 and Q2 are matched.
Moreover, strange things I’ve found: I tried every transistor I pulled out and Q12 has hFE=0 (should be >70), the other ones tested ok; I haven’t a big experience with transistor but I’ve never found a transistor with hFE=0 (and I’m pretty sure the tester is good). On the other hand, the power supply caps were approx. 30% more of the nominal capacity.
 
Q11 and Q12 are power limiters. Q12 hFE=0 means that it really isn't working, and perhaps bending the output to your offset reading.

Was that hfe reading done out of circuit? What are the transistor numbers used in each position of that channel's amp?

I would just try another Q12 and see how that affects the situation.
 
My bad, I forgot the bourne pot is a 25 turns :whip:
So now after 20 min, outputs and inputs disconnected, volume to min and other pots in central position, no speakers connected, I was able to set dc offset at 0mv;
Many many thanks to larryderouin, llwhtt, MTF, tsd71 and all the people in this forum that have, with their posts, contributed to resurrect another sx-1010!
NickG
 
Wow... That was some tense reading for me. I have an SX-1010 that has sat for years in a dry corner of our downstairs unused apartment. it has fired up and runs awesome... BUT I know I need to have some of the components refreshed before it has problems. Right now I am getting the panel lighting in order with new LEDs. I do have very little in the way of test equipment (just a mid-level VOM).

I have the "SM" for the 1010 so I guess I could start by looking at the TP levels.

The wire-wrapped board connections seem to restrict the free movement of the boards to access the backsides.

I have not pulled the bottom cover yet so perhaps that is the key to exposing the other areas. (Yes/No?)

Based on what I have read here... I should just start by getting the PS board updated and take it in stages from there.
20200930_185223.jpg
 
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