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Old 19th May 2015, 10:07 PM   #1
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Default reparing old amplifier tectronic sa 535

Hello everyone,

I'm trying to repair an old amplifier named tectronic SA 535. More for the challenge than for the sound but yeah also for the sound (curiosity)!

From what i understood, the schema is the same as scott A417.

The problem remains is about an output dc offset really high (-37v ) on the right channel. The power transistors (and drivers...) was checked using diode mode of dmm and nothing different from the left channel that works.

So, I would like your opinion in order to identify the cause.

I suppose that one of transistor (power pnp ?) is fully closed, so the based should be saturated. right ?

My first idea is to replace electrolytic capacitor that should have near 40 years old now. Maybe one is in short circuit ?

In the past, i saw a resistor cause problem on an old metrix multimeter. So , checking resistor is the second idea.

And finally, checking all the others transistors, maybe for found one that is "blocked".

maybe there is better way (shortcut ?) to indentify the problem ?
What could you recommend to do ?



thank you for your help.
by a noob!
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Old 19th May 2015, 11:23 PM   #2
rayma is online now rayma  United States
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Quote:
Originally Posted by moonLeo View Post
The problem remains is about an output dc offset really high (-37v ) on the right channel. The power transistors (and drivers...)
was checked using diode mode of dmm and nothing different from the left channel that works.
Check all of the internal fuses, the emitter resistors in the output stage, and also the nfb loop electrolytic capacitor to ground.
Did you test the B-C junctions on the outputs?
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Old 20th May 2015, 04:09 AM   #3
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When there is a fault, nothing can be predicted. You have to check the power transistors. they mostly go short from Collector to Emitter. In such a case, it is also good to test the smaller transistors at the front end of the power amp stage.

Gajanan Phadte
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Old 20th May 2015, 07:45 PM   #4
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nothing can be predicted

@Gajanan
Please keep comments like that to your self . Electronics is not about prediction its about knowledge and procedure nothing else and yes fault can be "predicted " based on a method and proper measuring ....

Based on the statement that the amp is the same with Scott

-37 will first mean that the internal setting of the voltage of the mains for this amplifier is set up for 220V while now you feed it with 240 which is the mains in France ( i expect) Rail voltage is 31+31 for this amplifier . Set it up for 240 V and voltage will drop to about 31-32 volt at no load conditions

-37 will mean that one of the Lower NPN output is blown It could be many other things but statistics will call this first ( your amplifier seems to be Quasi complementary output stage so all transistors are NPN )
Measure outputs see for short between C-E you may as well remove it from the circuit to see if offset will drop

Correct all capacitors with no exceptions are to be replaced , Then before powering up you need to check drivers and peripherals

Outputs are easy to get all NPN D525-526 you will find them easy ...

Check that , replace capacitors , post here for safe start procedures after the repair

You need to verify bias settings before closing the amp ( this might be tricky in your case since there is no adjustment but we will get to that also )

Kind regards
Sakis
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Last edited by east electronics; 20th May 2015 at 07:49 PM.
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Old 20th May 2015, 07:53 PM   #5
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@Rayma ... may be cup of coffee before posting ???

----If emitter resistors are open i don't expect the amplifier to latch in the one or the other rail

----This amplifier features no rail fuses since it will latch to any of the rails if the fuse is blown

----If transistors fail with a 99% rate they short between C and E not B and C .

regards
Sakis
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Old 21st May 2015, 05:58 AM   #6
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Quote:
Originally Posted by east electronics View Post
nothing can be predicted

@Gajanan
Please keep comments like that to your self . Electronics is not about prediction its about knowledge and procedure nothing else and yes fault can be "predicted " based on a method and proper measuring ....

...

Kind regards
Sakis
That means you should be able to tell which component has blown, can you.

Yes, I know. There was a thread of repairing an amp and they were using SIM to find the fault. I know you have ENOUGH knowledge as to know when an electrolytic has shorted.
I don't have any repair shop for boasting my abilities, but I suppose you have.

Edit:...and by the way, your advice is like carpet bombing. OP, please be careful while doing all that is advised. You will not have any track of anything.

Gajanan Phadte

Last edited by gmphadte; 21st May 2015 at 06:16 AM. Reason: added content
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Old 21st May 2015, 08:03 AM   #7
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First, i would say, thank you at all, for your answers, it really help me in understanding and investigates.

from what I saw :

after extract the output transistor and the drivers, with dmm on ohmeter, it didn't indicate a short between C and E on each. Transistors not reinstalled by now.

One resistor on the feedback was about 1,1k instead of 10ohms announced on it. so replaced. Sometimes, on amplifier schema, the resistor technology is recommanded, for example on pass F5, there are recommendation of carbon resistors.
I suppose that recommendation was made because of sound quality. Is it important for feedback loop ?

All capacitors on the main amplifier card are now replaced. I didn't replaced on the others cards (tone/preamp/...), maybe it is needed ?

The nexts steps is to reinstall the transistors.

Is it safe to check the amp by power up with no input/no speaker and control afresh dc offset output ?

As you said, one transistor can be short between C and E, and the outputs is saturated. Is it relevant to try to reverse follow (from the output to the component to identify) the "signal" path in order to identify the component that make this signal high ? Maybe too much risky operation ?

Thank you for your help

kind regards
Adrien

Last edited by moonLeo; 21st May 2015 at 08:10 AM.
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Old 21st May 2015, 08:23 AM   #8
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Gajanan ...forum is about helping each other and exchange of opinion and knowledge From a personal point of view I very much dislike only a couple of things ...Suggestions that including terms like yours and urban legends commonly seen in the internet .

To the OP

---Transistors are checked on diode check on your DMM not ohm meter
---Transistors often measure OK outside the circuit but fail under voltage rare but possible
---10 ohm resistor in the feedback doesnt sound right please explain which resistor you are talking about according to the schematic .Look in comparison with the working ch to see what is going on there Messing up with the values is a destruction recipe.
---Small capacitors are to be replaced also in the pre area
---No its not safe to power up ....Get familiar with what is called a bulb tester which is a 60W bulb in your case in series with mains to work as an active power limiter in case of fault


Kind regards
Sakis
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Old 21st May 2015, 10:52 AM   #9
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@east electronics

Gajanan ...forum is about helping each other and exchange of opinion and knowledge From a personal point of view I very much dislike only a couple of things ...Suggestions that including terms like yours and urban legends commonly seen in the internet .

... and what do you think I was doing.
This place is not for expressing your personal opinions.
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Old 21st May 2015, 12:00 PM   #10
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Okey, so about testing transistors :

I was thinking that an ohmeter between C-E should identify a short. so wrong...

@east electronics if the transistors should be ok outside of circuit but fail under voltage, as they are extracted by now, is it relevant to test its in minimal condition(simplest circuit) with a base to a function generator and compare waveform? ( same waveform on input and output) should identify a good condition ?)



The resistor circled in red in this schema indicates what resistor was changed.


Click the image to open in full size.

Kind regards
Adrien
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