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Old 5th August 2015, 05:45 PM   #1
Depaj is offline Depaj  France
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Default Scott A 426

Hello,

First time here on Diyaudio.
Here's a quick intro before asking what really is on my mind.

I'm living in Lyon, France so you'll have to excuse me for a few mistakes from time to time.
I have been around audio from the 70's and 80's for a long time, the music as well as the equipment and recently I discovered the pleasure of opening them up and try to understand them a little more, repairing and recapping when needed. So I am looking around on the internet for nice pieces.

I currently have a Technics su 8080 with a pair of ditton 66 legends, a technics sl-B2, a teac UD-H01 dac and a philips CD 624.

As secondary setup I have a Dual Pa 5060 saved from being thrown away with technics speakers also saved from a horrible death, a technics SL-PG580A and an ITT turntable (quite rare apparently).

I'm here to learn mostly but will do what I can with my little knowledge to help.

So now the real deal, I'm looking for help with a Scott A 426 I picked up recently.

When I got it nothing worked so I started by isolating the problem : the power amplifier section. I checked all the transistors and most of them were shorted, never seen that much in one single amp. Also a few resistors and diodes were bad so I replaced everything (some with originals some with equivalents but same on both channels).
(the red circled ones have been replaced)
IMG_0406.jpg

But after all of this the problem doesn't seem solved, I powered it up with a lamp connected in series and it lit up so there has to be something else.

I couldn't find anything so if anyone here has any idea of what could be causing this I would be very grateful.

Thanks
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Old 5th August 2015, 06:04 PM   #2
Mooly is offline Mooly  United Kingdom
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When faced with something like this, the only answer is methodical fault-finding. If you have a circuit that we can see then I'm sure we could advise better on how to go about it.

(and well done for using the bulb tester)
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Old 5th August 2015, 06:24 PM   #3
Depaj is offline Depaj  France
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I actually have a second amp, same model but working and I have been comparing every single components resistance between the two and have found no difference except for a few ohms on one resistor.

What do you mean by circuit, the other side of the board or the schematics ?
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Old 5th August 2015, 06:46 PM   #4
Mooly is offline Mooly  United Kingdom
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Schematics. Its the only way to do it on here... different story when its in front of you.

First thing would be to disable one channel by removing the output devices and work on the other. You could short out the vbe multiplier or whatever arrangement is used for biasing (this is why a circuit diagram is needed, because I can't determine that without having the board in my hands to spin around and study) and begin fault finding from a known starting point.
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Old 5th August 2015, 06:56 PM   #5
Depaj is offline Depaj  France
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Here they are (if the resolution isn't high enough I can send the hole pdf file.
I tried that but not the way you say; I just unplugged the +/- 30V coming from the filter caps for one channel and then the other but both times the problem remained the same.
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Old 5th August 2015, 06:59 PM   #6
Depaj is offline Depaj  France
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Sans titre.jpg
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Old 5th August 2015, 07:03 PM   #7
Mooly is offline Mooly  United Kingdom
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OK, that looks pretty straightforward. Q804 on the top channel is the vbe multiplier, Q814 on the lower. If you link collector to emitter that would force the bias to zero whatever other problems there may be. If you do that on both channels and the bulb still lights brightly then there is a problem with the driver or output transistors. No speakers connected at any point while working on it.

I'll be back later......
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Old 5th August 2015, 07:30 PM   #8
Depaj is offline Depaj  France
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I just tried that, the bulb still lights (not very bright but as bright as before).
I checked for the voltages of power supply and I only have 11.6v against the 30v I should have and do have whenever I disconnect the power amplifier board. So the problem doesn't seem to come from q804 nor q814.
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Old 5th August 2015, 07:54 PM   #9
Mooly is offline Mooly  United Kingdom
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I would have been surprised if they had been faulty, but what shorting them out does is force a zero bias condition.

Leave the short in place on both channels (so that is as if that 0.047uf cap across those transistors was shorted out).

You need to disable one channel to work on this. Whichever channel you choose, you need to lift one end of both of the 0.27 ohm emitter resistors in that channel. You also must lift R817 and R820 or R842 and R844 (both 330 ohms) depending on the channel you decide. You can then work on the other channel to repair it.

You need to bring it to that state before we can go much further. If the bulb still lights then there is a problem with the drivers and/or output transistors on the channel you are repairing. With the vbe multiplier shorted they can not conduct and draw current between the rails.

See where you get to with all that
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Old 5th August 2015, 08:32 PM   #10
Depaj is offline Depaj  France
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Okay, so I disconnected the resistors on one channel, bulb still lit.
Then I disconnected both channels and no problem, but when I reconnected the first channel the bulb lit again so I assume both channels have faulty output and/or driver transistors. The drivers are q816 and q813 for one and q806/q803 for the other channel right ?

So far you have been a lot of help so thanks, I hope we get somewhere with this .

I might see your next answer tomorrow as it is evening here and I work early.
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