Pioneer SX-1080, after re-cap problem w. center voltage & idle current - diyAudio
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Old 29th March 2014, 12:53 PM   #1
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Default Pioneer SX-1080, after re-cap problem w. center voltage & idle current

Hi all

I decided to do a total re-cap on my dear SX-1080, which I brought back to denmark i 1980, since the sound quality eventually was at poor transistorradio level.

So I ordered a bunch of caps, a couple of pots, transistors, diodes and resistors.

To cut it short I now have the problem, that there is no output

On the left channel I can easily adjust idle current to 30mV, but center voltage is (pin #10 at power amp) at +3.15 V.

On the right channel idle current can't be adjustet at all - it's "dead" at 0.0000V, as center voltage is at -1.18V.

I also replaced the 4.7 Ohm, 1 watt resistors at the main heatsink assy (all 8 of them), even though they all were reeading 5.05-5.30 Ohms, but that didn't do any difference.

I'd really appreciate any input that could come up with a hint where to look.

I have taken a lot of readings all over I can post, if needed, but I just want to open with this for starters.

/Ole
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Old 29th March 2014, 01:36 PM   #2
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Hi Ole, can you show on schematic what exactly caps you have replaced?
Or just name them (I've got the service manual). This may help to come up with some ideas...

BR,
Valery
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Old 29th March 2014, 06:09 PM   #3
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Quote:
Originally Posted by vzaichenko View Post
Hi Ole, can you show on schematic what exactly caps you have replaced?
Or just name them (I've got the service manual). This may help to come up with some ideas...

BR,
Valery
Hi Valery

Sorry 'bout the late reply - had to take junoir to a Basketball game.

Yes, I got the whole list and I have uploaded it.
I have also uploaded my observations after the initial power on after re-capping - perhaps that can be of use as well?
Attached Files
File Type: txt ctr-volt Idle-curr.txt (4.5 KB, 14 views)
File Type: zip Pioneer SX-1080 Cap list-DYI.zip (54.4 KB, 21 views)
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Old 29th March 2014, 11:04 PM   #4
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Ole, you also changed some transistors with substitutes - are you sure B-C-E are at the right places?

If yes, let's do some troubleshooting.

1) First of all, I would disconnect the output stage collectors from the rails, just to be on a safe side (it's easy to kill them, also if some of them are already dead, they will not affect the rest of the circuit). So disconnect the wires from pins 14 and 21 in both channels. Without the output load the drivers' (Q6, Q7) current will be enough to balance the amp.

2) Power-on, check the rails - pins 8, 1, 12, 22 (+/-60v, +/-62v respectively).

3) Check what you've for at the output and at Q6, Q7 bases.

4) Check the voltages at R11, R9 and R12 (1.2v, 0.9v, 0.9v).

Tell me what you see )

P.S. Be careful when measuring - easy to shorten something... 120v between the rails is rather serious stuff )
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File Type: jpg Pio-1080.JPG (286.6 KB, 155 views)
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Old 30th March 2014, 07:52 PM   #5
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Hi Valery

I trust you got my PM...


Quote:
Originally Posted by vzaichenko View Post
Ole, you also changed some transistors with substitutes - are you sure B-C-E are at the right places?

If yes, let's do some troubleshooting.

1) First of all, I would disconnect the output stage collectors from the rails, just to be on a safe side (it's easy to kill them, also if some of them are already dead, they will not affect the rest of the circuit). So disconnect the wires from pins 14 and 21 in both channels. Without the output load the drivers' (Q6, Q7) current will be enough to balance the amp.

2) Power-on, check the rails - pins 8, 1, 12, 22 (+/-60v, +/-62v respectively).

3) Check what you've for at the output and at Q6, Q7 bases.

4) Check the voltages at R11, R9 and R12 (1.2v, 0.9v, 0.9v).

Tell me what you see )

P.S. Be careful when measuring - easy to shorten something... 120v between the rails is rather serious stuff )
Well here are som numbers:

HTML Code:
Power amp assy, Left CH			
+	-	V(actual)	V(Specs)
1	chass	-0,15		-60,00
8	chass	+64,60		+60,00
12	chass	+62,50		+62,00
22	chass	-62,50		-62,00
------------------------------------------
R9		+0,90		+0,90
R11		+0,76		+1,20
R12		+0,93		+0,90

Power amp assy, Right CH
+	-	V(actual)	V(Specs)
1	chass	-2,00		-60,00
8	chass	+64,00		+60,00
12	chass	+62,50		+62,00
22	chass	-62,00		-62,00
------------------------------------------
R9		+0,00		+0,90
R11		+0,00		+1,20
R12		+0,00		+0,90
So, Left R11 is off, as is all three Right R9, R11 & R12.
Also pin 1 on both Channels are way off.

I must confess that I do not know what exactly you are asking in "3) Check what you've for at the output and at Q6, Q7 bases."
Connecting one lead to base I get, but I don't know where to connect the other lead ? To GND ?
emitter?
collector?

Sorry for being such a noob.

Also I must confess I'm not exactly sure if all transistors are placed correctly b-c-e wise.

I have been leaning on "hopjohn"s project on Audiokarma.org, and comparing this photo

SX-1080 What to know before diving in? - Page 2 - AudioKarma.org Home Audio Stereo Discussion Forums


Click the image to open in full size.

to mine:

https://drive.google.com/file/d/0B-K...it?usp=sharing.

Again I know this close to a guessing game, and I suspect myself this can be where the problem lies. I did try to find an explantion, but without success, sorry ;-)
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Old 30th March 2014, 08:27 PM   #6
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Hi Ole, we've got a very good result right away. We're missing -60V regulated rail.

So now we move to the Power Supply Assembly and see what we can find there.

Start with checking -76V, then chek the other couple of points marked by red boxes. Also see if there are some burnt resistors, etc.

If -76V is in place, then the problem is in one of the transistors, most likely Q4.
Before we get -60V back, it does not make sense to do anything with the power amp - it will not work anyway.

Let's see what we've got

Valery

P.S. In 3) i meant measuring against the ground, you are right.
Attached Images
File Type: jpg PSU Assy.JPG (232.9 KB, 120 views)
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Last edited by vzaichenko; 30th March 2014 at 08:39 PM.
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Old 30th March 2014, 08:30 PM   #7
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Quote:
Originally Posted by vzaichenko View Post
Hi Ole, we've got a very good result right away. We're missing -60V regulated rail.

So now we move to the Power Supply Assembly and see what we can find there.

Start with checking -76V, then chek the other couple of points marked by red boxes. Also see if there are some burnt resistors, etc.

If -76V is in place, then the problem is in one of the transistors, most likely Q4.
Before we get -60V back, it does not make sense to do anything with the power amp - it will not work anyway.

Let's see what we've got

Valery
Coming right up
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Old 30th March 2014, 08:38 PM   #8
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Quote:
Originally Posted by vzaichenko View Post
Hi Ole, we've got a very good result right away. We're missing -60V regulated rail.

So now we move to the Power Supply Assembly and see what we can find there.

Start with checking -76V, then chek the other couple of points marked by red boxes. Also see if there are some burnt resistors, etc.

If -76V is in place, then the problem is in one of the transistors, most likely Q4.
Before we get -60V back, it does not make sense to do anything with the power amp - it will not work anyway.

Let's see what we've got

Valery

P.S. In 3) i meant measuring against the ground, you are right.
Was there supposed to be a picture with your answer where you refer to "the point marked by red boxes"?
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Old 30th March 2014, 08:39 PM   #9
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Sorry - forgot to attach the picture - now it's there
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Old 30th March 2014, 08:49 PM   #10
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Quote:
Originally Posted by vzaichenko View Post
Sorry - forgot to attach the picture - now it's there
Phew - this is hard - I need to find and end-connect to R8...

OK, I read:

-86V between R8 (end away from edge) and GND.
-0.11 on pin 27

Am I doing this correct:
Black lead to GND

Red lead to pinmarked "13V" on Q3?: Reading is 0.00008V

Red lead to pinmarked "13.6V" on Q3?: Reading is 0.00007V

Last edited by ultraspark; 30th March 2014 at 09:06 PM.
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