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Technics su 3500 help needed

rrmitko

rrmitko

New Member
Hi all,
I have this nice vintage amplifier but few days ago the sound from the right chanel become distorted. I cleaned the balance pot and the problem disappeared but came back again yesterday and I decided to swap the power transistors between the boards and the problem remains ,so I removed the transistors from the equation. I also connected the preamp to another amplifier and it is working absolutely fine. The distortion occurring for 10-15 seconds and disappears for 5-8 seconds. I removed all other transistors from the board and they all tested good. No signs of burned resistors. Any help will be appreciated !
 

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I had a problem just like that with a Marantz model 140 power amplifier. The distortion did the same thing, came and went, and back again. This was before I restored it.
It drove be crazy at first. Same thing, all the transistors and resistors were fine.
No bulging or leaking caps. It turned out to be one electrolytic capacitor in one channel. Once I found it I recapped the amp. But it looks like your amplifier has been restored already. Do you have any freeze spray? When the noise starts, spray the transistors on the amp board for the offending channel one at a time.
A transistor can test good but still be flaky in use. I've found a few failing transistors with freeze spray.
 
Unfortunately I don't have that spray. The amp has been reccaped and has been working fine. When sounds distorted on collector of TR405 I'm getting between 5-6v instead of -1.1v so I swapped that transistor with the one from the good board but still the same:(
 
All switches has been cleaned but still the ssme.I will keep swapping elements between the boards until I find the faulty one.
 
Thanks for the reply! All capacitors are new and the problem moving with that particular board. At the beginning was in the right chanel, when I swapped the boards moved to the left. Even swapping the output transistors between the boards didn't sort the distortion. I will replace C406 and C401 again just to be sure ,that they are not causing the problem.
 
I measured -37.90v on the collector of tr402,suppose to be -38.40v. When the sound is distorted drops down to -37.60v. Voltages on tr401 are ok. Tomorrow I'll swap them with the pair from the other board and see what will happen.
 
Check for cold solders on board. (looks pretty clean). I'd change out the 47 ohm resistors (413 and 417) as they could also cause the described problems. Intermittents can drive you crazy...my 2 cents.
 
Swapped the transistors between the boards and I'm getting exactly the same voltage on the collector of TR402, a bit lower than what the the SM states. Resistors 407,413 and 417 tested good and within the specs.I went through all solder joins again.So I played music all day yesterday and today and still no distortion ,maybe there was a cold solder joint somewhere. TR401 and 402 have completely different Hfe. Is it worth replacing them with matched KSA992FBU?
 
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Hi all,
Unfortunately the distortion returned yesterday. I replaced R407,413,417 with those from the other board (still waiting new ones to arrive) but no joy again. I think to continue swapping transistors from one board to another and probably go from there?!?! Any ideas guys?
 
Yeah, don't guess.

If both channels work great with no distortion for a time, then it is usually not a bad part so don't keep swapping them or you will get a bad part. In my experience, it still sounds like a dirty switch or cold solder joint somewhere. You might be pleasantly surprised at what this simple fix will do. Might save you a lot of grief. :eek:

I would go through it and touch up All solder connections. Plus, if/when the distortion occurs, wiggle or push every button especially the tape monitor even if you already cleaned them. Work the **** out of them. I can't even begin to tell you how many times my tape monitor switch on my Pioneer SA-8500 still gives me grief.:mad:
 
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Hi all,
I went trough all solder joins. Replaced TR401,402 with matched pair KSA992FBU but still the same. I'm not getting any voltage when trying to adjust the idle current, suppose to be 15mV but there's nothing.
 

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So far I swapped TR405,408 from the other board and replaced TR401,402 with KSA992FBU and still the same.I was checking voltages according the sm but I will measure TR406 after work. All voltages are ok until the problem occurs.
 
inductor said:
no output relays !
you have mentioned you have replaced the transistors between the boards . all of them ?
or send me the board and i will try to repair it ?, just a thought
Click to expand...
Swapped TR405,408 from the other board, replaced TR401,402 with KSA992FBU and now TR406 still the same. I will replace some of the resistors with new ones. I haven't check the capacitors yet.
 
I manage to short somehow TR409 on the good board while I was measuring:( Now I need output transistors as well. Any good substitution?
 
New output transistors are in. Everything works but still distortion from time to time from that particular board.
 
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