Pioneer SX-780 No click

[ed123456]

Hello,

This is my first post.I have combed this site for this problem and I either have a tough one to fix or I missed something so here goes.
I picked this 780 up from a guy for $25 for parts he said he thought it was struck by lightning,so I thought I'd give it a try to fix nothing to loose but time and ordered parts..
1; upon powering it up it clicked immediatly and the right meter went half way up and I noticed a thread of smoke coming from the area of the sips and the nearby resistors.( now I'm going from memory about 20 hours of time,so I may jump from topic)I tested the 2; a979 sips and the 2; c1775 which were bad also sub'd an A844 in place of Q22 a A904 I removed and tested (on a B&K 520B) the 3 regs with the U shaped heat sinks all are good. I replaced PA3004 thats the pioneer ic near the relay and disconnected the 2 0050 ic's the relay problem was cured,it clicked on after a few seconds.I then checked the 4 .22 resistors out of circuit then added the left channel IC and had a click and then sound (yay)then I took the second 0050 installed it and had click and sound (just to be sure the IC's were good)then I resolderd the right IC turned it on and NO CLICK.I re-checked even swap the IC's back and forth L+R to R+L and no go.Even if I try to run the only the right channel and NO chip in the left no click.I took the 3 transistors and 2 sips from the left an put them on the right and still the left channel works no right channel.I have High 34 volt readings on pins 1,2 and 9,10 I don't have that problem on the leftchannel so here I am.The .22's are good and I have checked and replaced some of the over heated but still good resistors that are mounted above the board.I must be missing something or just burned out.

Does anyone have any idea's?
Thank you.

 

[Tom B]

pin 9 should have +34v. Pin 2 should have -34v. Pin 1 should have -1.4v and and pin 10 should have +1.4V.
I don't recommend any swapping from side to side as you could be creating more damage.
Tell me what the voltage reading is on pin 3 and pin 8 and both emitter resistors for that bad channel. Take a reading on both sides of each resistor. It should be less than 40mv at all 4 points. Take that chip out of the right, and get the left working as you did before with audio.
Now take voltage readings (With the chip out), of pin 1, 2, 3, 8, 9, and 10

 

[ed123456]

pin 9 should have +34v. Pin 2 should have -34v. Pin 1 should have -1.4v and and pin 10 should have +1.4V.
I don't recommend any swapping from side to side as you could be creating more damage.
Tell me what the voltage reading is on pin 3 and pin 8 and both emitter resistors for that bad channel. Take a reading on both sides of each resistor. It should be less than 40mv at all 4 points. Take that chip out of the right, and get the left working as you did before with audio.
Now take voltage readings (With the chip out), of pin 1, 2, 3, 8, 9, and 10

Ok. will do when I get back Monday. I removed the right IC and have the left channel working.I set it aside today and wrote the above post.
Thank you.

 

[ed123456]

Left channel playing fine,relay clicks.

Ok here is what I have,the relay clicks, right channel with IC unsolderd ,I have Pin1= -45v..2=-34v 3is..0v Pin8=0v..9=+34..10=+34 The 3/ .22ohm resistor voltages are 0v 0v 0v

With the Right ch. IC solderd in place now NO relay click Pin1=+34v..2 -34v..3=+34 8=+34.9=+34 10=+34..The 3 resistor points are +34..+34...+34.I don't understand how the IC isn't burning out instantly.

I just found D1 1S1555/ecg177 shorted subbed a 1N4148/ecg519 just to take the short out.Checked the c1775 and a912 test ok

 

[ed123456]

Anyone else have any ideas on this one?

 

[SVI2004A]

With pins 1 and 0 (10) both at +34... I’d expect +34v at output... - it’s not bbqing resistors as the pnp side is not conducting...

I’ll have a look for a schematic.. I’d be interested to hear results if driving transistor to pin 0 is unsoldered...

 

[SVI2004A]

Q15 or Q16... (whichever is on the right channel) taken out should see output drop from +34v and should go -34v

 

[ed123456]

With pins 1 and 0 (10) both at +34... I’d expect +34v at output... - it’s not bbqing resistors as the pnp side is not conducting...

I’ll have a look for a schematic.. I’d be interested to hear results if driving transistor to pin 0 is unsoldered...

Pin is soldered on Q16 that has a line right to pin0 I removed Q16 now its 0v Bare in mind that I have the stk0050 unsoldered at the moment.If I solder it back in the relay won't click so it's protecting the speakers.I don't understand how the IC is still good but it is.

 

[SVI2004A]

With the STK connected, you should have -V at output

Sounds like work is needed on the differential/drive area


** often (in sets I see) effectively pin 0 would be pulled down to -V via 8K ohm give or take - this will drift output to -V (think of it like an elastic band)

and inside the STK between pin 1 and 10 is a bias control transistor (this will allow volt drop between pin 1 and 10 - when it’s hotter it will cause less volt drop across the pins- making sure npn and pnp are just conducting at right current)

Connected to pin 10 is the driver transistor... this is controlled from the differential pair array that compares what the output is like compared to input

What effectively happens now is the drive transistor will driven to pull voltage up till the output voltage is 0.000V (if all is well with differential)

So imagine it like a tug of war with a person holding the end of a 2.8 Metre rope (which has a red marker halfway on it representing output) with the other end of the rope attached to a bungee cord.... the person should be standing 1.4 Metres towards him from the 0 line drawn in the sand....

If he steps back, the centre of the rope heads in his direction.... if he eases up or let’s go, the bungee rope will pull the red marker it’s way.

A referee would be telling the person to pull tighter or relinquish - based on where the marker is supposed to be...

I specifically set rope length to 2.8M to reflect how for 0 V there is 1.4V at pin 0 and -1.4V at pin 1....

What is happening in your set is that the person is pulling the rope all the way into his territory... (it’s a good thing that the pull down transistors haven’t also gone bad)

So all in all the STK isn’t under light stress... if transistors on pin 1 and 10 went short... it would be bad news for the STK.

Hope this helps you see what goes on with the DPP series STKs... (my better Technics amps use Darlington output IC)

——

Surely some of the pioneer gurus will know of culprits to check over — im Not keen on transistor pulldown... resistor pulldown is a lot less problematic

 

[ed123456]

With the STK connected, you should have -V at output

Sounds like work is needed on the differential/drive area


** often (in sets I see) effectively pin 0 would be pulled down to -V via 8K ohm give or take - this will drift output to -V (think of it like an elastic band)

and inside the STK between pin 1 and 10 is a bias control transistor (this will allow volt drop between pin 1 and 10 - when it’s hotter it will cause less volt drop across the pins- making sure npn and pnp are just conducting at right current)

Connected to pin 10 is the driver transistor... this is controlled from the differential pair array that compares what the output is like compared to input

What effectively happens now is the drive transistor will driven to pull voltage up till the output voltage is 0.000V (if all is well with differential)

So imagine it like a tug of war with a person holding the end of a 2.8 Metre rope (which has a red marker halfway on it representing output) with the other end of the rope attached to a bungee cord.... the person should be standing 1.4 Metres towards him from the 0 line drawn in the sand....

If he steps back, the centre of the rope heads in his direction.... if he eases up or let’s go, the bungee rope will pull the red marker it’s way.

A referee would be telling the person to pull tighter or relinquish - based on where the marker is supposed to be...

I specifically set rope length to 2.8M to reflect how for 0 V there is 1.4V at pin 0 and -1.4V at pin 1....

What is happening in your set is that the person is pulling the rope all the way into his territory... (it’s a good thing that the pull down transistors haven’t also gone bad)

So all in all the STK isn’t under light stress... if transistors on pin 1 and 10 went short... it would be bad news for the STK.

Hope this helps you see what goes on with the DPP series STKs... (my better Technics amps use Darlington output IC)

——

Surely some of the pioneer gurus will know of culprits to check over — im Not keen on transistor pulldown... resistor pulldown is a lot less problematic

 

[ed123456]

In the sx680 there is a resistor hidden between the 2 large power supply caps that causes the same type of problem the 680 doesnt have a relay circuit to protect the speakers so the 34v dumps right into the right channel pinning the right meter without popping the 0029.It's also the right channel same as this 780.This receiver was storm damaged that took out the right channel.I had a simillar problem on a technics sa202#? it use's the 0029 but the problem was a shorted right channel phono preamp ic ta7129 this ic would pop the driver transistor messing up the right channel of the technics without popping the 0029.
I have replaced / swapped everything I can see in the 780, there has to be a simple pull down resistor I'm missing.If you think of anything that might help let me know,this will happen to someone else and these receivers are to good to part out,thank you for the explanation.

 

[merlynski]

Try soldering in a 1k resistor between pin 1 and pin 3 and another between pin 0 and pin 9 on the bad channel (STK not installed). This will close the dc feedback loop and stabilize it to allow dc tests.

 

[ed123456]

Try soldering in a 1k resistor between pin 1 and pin 3 and another between pin 0 and pin 9 on the bad channel (STK not installed). This will close the dc feedback loop and stabilize it to allow dc tests.

Ok will do,but one question 1 and 3 and...9 and 0 ?or 8 and 0?

 

[merlynski]

Ok will do,but one question 1 and 3 and...9 and 0 ?or 8 and 0?

You are correct, 1 and 3, 0 and 8. I looked at the schematic at least 3 times, felt something I wrote was wrong, but couldn't see it. Sorry about that . This is a tip I saw on another post, have not tried it myself (yet), but several others seemed to have good results. Can't hook up speakers, of course . . .

 

[ed123456]

With the STK connected, you should have -V at output

Sounds like work is needed on the differential/drive area


** often (in sets I see) effectively pin 0 would be pulled down to -V via 8K ohm give or take - this will drift output to -V (think of it like an elastic band)

and inside the STK between pin 1 and 10 is a bias control transistor (this will allow volt drop between pin 1 and 10 - when it’s hotter it will cause less volt drop across the pins- making sure npn and pnp are just conducting at right current)

Connected to pin 10 is the driver transistor... this is controlled from the differential pair array that compares what the output is like compared to input

What effectively happens now is the drive transistor will driven to pull voltage up till the output voltage is 0.000V (if all is well with differential)

So imagine it like a tug of war with a person holding the end of a 2.8 Metre rope (which has a red marker halfway on it representing output) with the other end of the rope attached to a bungee cord.... the person should be standing 1.4 Metres towards him from the 0 line drawn in the sand....

If he steps back, the centre of the rope heads in his direction.... if he eases up or let’s go, the bungee rope will pull the red marker it’s way.

A referee would be telling the person to pull tighter or relinquish - based on where the marker is supposed to be...

I specifically set rope length to 2.8M to reflect how for 0 V there is 1.4V at pin 0 and -1.4V at pin 1....

What is happening in your set is that the person is pulling the rope all the way into his territory... (it’s a good thing that the pull down transistors haven’t also gone bad)

So all in all the STK isn’t under light stress... if transistors on pin 1 and 10 went short... it would be bad news for the STK.

Hope this helps you see what goes on with the DPP series STKs... (my better Technics amps use Darlington output IC)

——

Surely some of the pioneer gurus will know of culprits to check over — im Not keen on transistor pulldown... resistor pulldown is a lot less problematic

With the STK connected, you should have -V at output

Sounds like work is needed on the differential/drive area


** often (in sets I see) effectively pin 0 would be pulled down to -V via 8K ohm give or take - this will drift output to -V (think of it like an elastic band)

and inside the STK between pin 1 and 10 is a bias control transistor (this will allow volt drop between pin 1 and 10 - when it’s hotter it will cause less volt drop across the pins- making sure npn and pnp are just conducting at right current)

Connected to pin 10 is the driver transistor... this is controlled from the differential pair array that compares what the output is like compared to input

What effectively happens now is the drive transistor will driven to pull voltage up till the output voltage is 0.000V (if all is well with differential)

So imagine it like a tug of war with a person holding the end of a 2.8 Metre rope (which has a red marker halfway on it representing output) with the other end of the rope attached to a bungee cord.... the person should be standing 1.4 Metres towards him from the 0 line drawn in the sand....

If he steps back, the centre of the rope heads in his direction.... if he eases up or let’s go, the bungee rope will pull the red marker it’s way.

A referee would be telling the person to pull tighter or relinquish - based on where the marker is supposed to be...

I specifically set rope length to 2.8M to reflect how for 0 V there is 1.4V at pin 0 and -1.4V at pin 1....

What is happening in your set is that the person is pulling the rope all the way into his territory... (it’s a good thing that the pull down transistors haven’t also gone bad)

So all in all the STK isn’t under light stress... if transistors on pin 1 and 10 went short... it would be bad news for the STK.

Hope this helps you see what goes on with the DPP series STKs... (my better Technics amps use Darlington output IC)

——

Surely some of the pioneer gurus will know of culprits to check over — im Not keen on transistor pulldown... resistor pulldown is a lot less problematic

It took a while but I found it. It was the 2291 sip which was temperature sensitive and a flaky ta7136P sending spikes when the volume was touched preventing the relay from clicking on.I'm guessing the spikes from the IC took out the a979 sip (it was shorted) and the 2291 was damaged.Luckly a sx780 main board appeard on ebay for 32.50 and I was able to scavenge parts from it.It however had the hitachi op amp IC's and these did not work with out allot of noise and volume lag.I've found a source on ebay for ta7136a not P but I'm hoping this will work.Thank you all for the help.

 

[John Crowell]

I have owned an SX780 for over 20 years. Over the years the relay problem got worse and worse. I took it to a tech who kept it 3 months and thought he had it fixed. But he did not. After much research I ran across a post (somewhere) that stated 90% of the time when the SX780 relay keeps cutting off it is due to a bad solder joint on one of the three transistors with the U-shaped heat sinks. So I pulled the cover and turned it on. The first time the relay cut it off I used an insulated probe to push on one of the heat sink/transistor assemblies. It immediately came back on I repeated this several times and determined it was the rear transistor.. I pulled off the bottom cover and could see that one of the leads appeared to have a cold solder joint. So I re-soldered all three of the transistor leads on all three of those transistors. It has run now for several hours over several days and has not clicked off at all. It's amazing that something so tiny and difficult to find can cause so much frustration! I hope this helps someone else.