Another SX-780 with issues

Nolannniah

Active Member
Hi all,

This is my first post, and did some extensive reading and followed a few posts instructions. There are some very knowledgeable people here at AK.

I’ll start from the top:

Had no sound and no relay engagement, did some troubleshooting.
Removed the PPs, after, I could not get good steady readings on them. Tested one at a time-Voltage vs. shorts etc…Found that one of the packs were bad. To double check, I reinstalled the “good” one. Got relay pop and output. SO, ordered new ones. Very hard to find from a good source, I might add-still don’t know if mine are up to par yet. Have not tried to push them above 10w yet…Then I noticed that my 2 channel meters were not producing the same output and was slightly noticeable in the speakers. Read up a little more-did the DC adjustments/readings checked at 10, 30 and 60 intervals, then checked again in 2 hours. Good there, but still have the slight power level drop. I hooked up an external input to listen to some music (via RCA cables). When, I switched over to Tape Monitor, heard a loud POP through the speakers. Ok, no big deal (came across a “how to” post on cleaning procedures on switches and stuff), read up, followed instructions and cleaned all. Then went to try it all again. Have the same issue. Cleaned a second time, had the same results.

Steps I have taken: Re-soldered switch joints, switched inputs (RCA source), switched RCA cables (cleaned in and outside of the RCA inputs on the unit), switched speaker ports (A & B), AND still have same issue. This is no doubt what caused one of the packs to blow. When this happens (if volume is turned down or slightly up-no difference), it causes the meter reading needle to almost “tap-out” on the other side (when this happens relay disengaged then back on). If I slowly guide the switch to the other position, all works well. Just when you flip the switch down/up (like you would do for normal operation-I’m not slamming them down/up), it does this. I have noticed when it flips down/up a little harder the pop sound is noticeably louder. This issue occurs on all flip switches (except the Loudness one)-Mode (Stereo/mono), Tape 1 & 2, Low Filter, and FM Muting switches.

Also would like to note that the 3 in a row transistors w/heat sinks (Q25, Q19 & Q20), they were all touched up with new solder. Although, Q20 (the one closest to the front of the unit) is the one getting pretty warm vs. the rear one (and looks like it has been for a while-not from my repair I’ve been working on). Also, the resistor (R323, 2.4kOhm @1W) in front of Q20 is getting pretty warm as well. I am measuring roughly 140+ degrees Fahrenheit on both. Is this normal? The other ones are cool to the touch.

Sounds like I have a ground problem or a bad connection somewhere. Kind of at a loss now, cause don’t I really don’t know where to start troubleshooting from here…I am guessing that is dealing with a preamp-voltage from somewhere that causes this to happen. Am I wrong? Ideas? Anybody have this issue before?

I replaced both PPs and the .22kOhms for both. They were different. One side had the original style (with two) and the other the one with 3 pins (the 2 in one model).

I have had this receiver for about 20+ years now. Sorry to the owner of the pic I stole.

Thanks in advanced-
 

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I'd go ahead and check the balance between the channels. Here's a pdf of the procedure.
 

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That has been done. My readings are +/-7mV from 0mV, for both. Both channels are not steady they fluctuate.
 
CAREFULLY take voltage readings of the pa3004 protect chip (pins 1 - 8? with a possible gap) near the relay and the transistor IMMEDIATELY (ecb) next to it.

We are looking to see if the protect is justified in operation, or if the relay driver transistor has fallen down on the job.
 
Welcome Aboard!

That's O.K. Even though I posted that photo it belongs to AK now - Or the public domain.

What is the voltage at the emitter of Q25? I'm curious if the DC voltages are close to correct and not floating way out of spec until a switch is closed.

Is Pin 22 of the audio board solidly grounded? It may be good to verify ALL ground connections, since some of the circuits are grounded through some indirect paths.

What are the DC voltages across C307 and C308? What about the AC voltages in the same places?

Incidentally, this cleaner diagram may also be of use: http://www.wwdsltd.com/files/SX-780Schematic-Wide.jpg
 
For Markthefixer:
Pins:
1. +15.00v
2. the "gap"
3. .00v
4. .00v
5. .00v
6. -7.32v
7. -.02v
8. +7.11v

You may be on to something here Sir. When I was checking voltage across some of the pins, I was able to recreate the "pop" sound.

For Watthour:
Pin 22: Grounded well
C308: +42.16vdc/0.00vac
C307: +43.48vdc/0.00vac

For the transistors (as you looking at schematics) 1=Left, 2=Right, 3=Middle
Q25:
1. +14.97v
2. +15.58v
3. +16.44v
Q19:
1. +43.52v
2. +44.16v
3. +49.02v
Q20:
1. -42.22v
2. -42.78v
3. -52.11v
 
well, the 13v is high... check the D4 voltage. which is also q25 base. It sets the voltages for the entire unit.

and pin 3 is 0.00v which means it is not trying to turn the relay on, it is unhappy.

just go left to right on the transistors, with the lettering facing you.

measure the output voltages at r330 and r331 (75k) on the "hot" side of the relay contacts.

and carefully measure and post the voltages on each darlington pack.

I mean by carefully: shorting pins = dead pack instantly. Insulate all but the tiniest portion of the END of your probe. Office tape is fine.
 
PPs
Left:
1. -1.17v
2. -41.20v
3. -14.7mv
4. N/A
5. N/A
6. N/A
7. +41.23v
8. -7.5mv
9. +41.22v
10. +1.15v

Right:
1. -1.14v
2. -41.17v
3. +8.0mv
4. N/A
5. N/A
6. N/A
7. +41.21v
8. +12.1mv
9. +41.23v
10. +1.17v

Pins #7 on both are not connected to Main CCA

I have to give these to you again-I measured across them-not Ground to "Hot" side-My fault...Did this at 0530 this morning, didn't read the post correctly-eyes were still fuzzy. :drool:
R330: +23.5mv
R331: +4.6mv

D4: -15.54v

Also, in your first sentence where you are referencing the 13v, where is that coming from? Is that a typo? Kind of lost on that one...

Will note that when the voltages for PA3004 were taken the relay had already engaged (unit running and sound output). If that makes a difference...
 
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R331: -7.7mv (going to Main CCA pin #25)
R330 +10.2mv (going to Main CCA pin #27)
Readings not taken @ Main CCA pins
Input is (other side of the R's), -13.7mv
 
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The "13V" Mark wrote refers to the regulated 13.5VDC power supply controlled by Q25 (affected by D4). Your measurement of 15V on Pin 1 of the PA-3004 should have been closer to 13.5VDC. Your measurement of 15.54VDC at the zener (D4) indicates the regulation point of Q25 may be too high.

Further, the measurements at Q19 and Q20 seem high.

What is the AC line voltage?

Is this a multi-voltage (European) model? If so, is the voltage selector plug set at 110V or 120V? Rumor is that there are some of those out there.
 
Image originally provided by MTF:

sintoamplificador-pioneer-sx780_MLA-F-4432516769_062013.jpg
 
This is what I have:
120Vac 60Hz
SN: ZC3602051Y
Also mine says Made in the Rep. of Korea vs. Japan

There is not a switch of any kind I can see.
Looking at your Pic...No not my model
 
So, what is the measured AC line voltage?

Also, re-check the collector voltage at Q25. It appears pretty low. That can be compared to the voltage across C316 and the voltage at the end of R1 - the large cement resistor clipped to the main heat sink.
 
I haven't calculated the ratios, but it looks like all supplies are proportionately higher due to the 14.0v specified diode actually at 15.54 volts. It is a possibility as the integrated circuits used in the preamps have a upper voltage range, above which they would not be happy.

The pack voltages are good. The DC offsets are good.

Let's explore this pop issue from the front panel perspective only for right now, which switches pop, in what sequences are pops generated.
Basically everything gets flipped, pushed or rotated to see what the effects are.

Did you use flux when soldering (might be a conductive type - like for copper pipe soldering - which must be cleaned up afterward), or is there a lot of gunk spotted around the bottom of the board that's different from the original factory uncleaned gunk?
 
Watthour:
This is the exact model I have.
Line Voltage is 121.5V~

Q25:
1. +15.41v
2. +16v
3. +16.9v

C316: +16.91v
R1: +16.91v

Markthefixer:
I am using electric solder with flux core.

Ok-the right side/channel is the problem. When trying to recreate the pop noises when flipping my first switch-"POP", to the right channel (checked PPs afterwards), relay tripped and the pack went...relay never came back on line, So back at square 1 for me, but have narrowed it down to the right channel

So, unwillingly I removed it. And everything works as it should. To including flipping switches with no pops. I have 2 more PPs I at my disposable.
 

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You are operating it with no right side pp at all?

When it did not come on, that was the exact time to take pp readings.

right channel. right power pack was zorched? We can't go on popping them....

how about putting it back in, and taking pp measurements?

WHICH switch did you flip, and I had NO idea the pops were so violent.

usually pop complaints are an understated -pop- in the backround

not - POP - like a cherry bomb in the room.
 
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LOL yeah, they're like cherry bombs-Will make ya jump for sure!
Yeah too easy to solder the joints again/PP is still in place in unit. I can put another one in if needed (but won't be flipping the switches-rather I'll guide them into place). Or we can use the old one for now.
Just let me know where to start. Its 2300hrs here and I have to be up at 5 so I'm calling it a night-
 
I flipped the tape monitor 1 switch, next to the mode switch. At that time both PP were hooked up and running. POP...Unit trips relay and stays that way. What I did was unsolder the right pack after I took a couple of measurements from it and saw it was toast. The pop sound would come from all switches except the Loudness one (with both PPs installed and running). Yes I was only operating with the left running. Wanted to make sure I didn't fry anything else and relay engaged. I tried testing all the switches at that point. With only the left channel going, everything operates fine. NO mini cherry bombs going off.
 
OK, I have to think through a search and destroy mission for some stray DC somewhere in the right channel preamp...

The DC causes the mighty thump when it is switched in or out and then propagates as an AC bass pulse.
 
Well...Re-cleaned the board all along the right channel path. De-soldered, re-soldered joints and then put the PP back in the board to start taking voltage reading again. Flip power switch...and relay kicks in-and I have sound-from both channels...But pins 1 and 2 on PP had same reading the other day.
Strange...same PP and all works. I let it run and made it work...roughly about 2 hours @ 10+ watts per channel. Must have had a loose joint some where...Also, no pop when selecting input sources...I can't call this one-never seen anything like it...
 
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