Restoring a Technics SU-V6

[PC91]

Hi everybody!
a friend of mine has a service center, and i've found on his backyard some monts ago a Technics SU-V6 very damaged, he gave it to me, and i want to return it to it former glory.
the power transformer was missing and the output transistors of the left side were shorted.
after replace the power transformer (for a sony one of something sony don't know what, huge transformer) and removing the shorted transistors it turned on and smoke comes out of a resistor (after testing i've found that was for not having the transistors).
anyway. it was working well the right side, and decided to replace all the transistors (boths sides) in case the shorted ones have blow something else, after doing this it worked amazing, but (stupid me) i forgot of putting the heatspread and the output stage (right side) blow out, also the fuses blow out, when i replaced the output stage (2SB755 and 2SD845) it blowed instantly after turning on, and the resistor (is between the bases of the output transistors) that used to smoke instantly become in charcoal. there was -35v in one side and +35v in the other side of the resistor. after doing nothing the voltages become stable and the resistor barely smoke, but i'm affraid of putting new transistors in case of blowing them again (aren't expensive but are hard to get). what should i measure before try again? the left side is untouched yet, just removed some component so i don't have smoke on an resistor for the lack of the output stage.
i left you a picture of how it looks and maybe it gives pity to someone and helps me :P http://i44.tinypic.com/11wgc9i.jpg
actually now it's just some boards on a table beacuse i dismantled for cleaning and fix the house of it :P
thanks
btw sorry for my bad english

[nigelwright7557]

If its a BJT amp then its likely taken out the driver as well as the output transistors.

[PC91]

Hi nigelwright7557, i've switched the drivers and tested again, nothing blowed up, i don't know if i got audio ok because i've dismatled it for some cleaning, but hooking up a speaker to gnd and to the relay i obtain a very small hizz (it hasn't preamp yet is in cleaning :P) anyway, i seems that is now working fine, thanks a lot for the fast reply, tomorrow i will ensamble a little bit more to see if theres audio on it

[PC91]

Hi again, i've tested it and blowed again, the output get shorted after i connected the rest of it, after getting the service manual of a SU-V8 wich is almost the same found that the board where the inputs are is called "Equalizer / Voltage regulator" one of those transistor i had to replace because it was fisically broken, had the legs chopped of, on the test points it sholud be -17.5v and +17.5v but theres -17.5v and +27v, after replacing the transistors on that board is still the same, i'm stuck'd right now, don't know what to do next.
thanks!

[Nrik]

Did you replace the output transistors again or only the first time?
(...Because when you replaced the drivers you didn't mention that you also replaced the outputs) You should ofcourse have replaced the drivers and the outputs simultaniously.

The voltageregulator part is definitely still not working properly. You should get that down to 17,5 volts before anything else.
Did you only replace the transistors that you could see was blown, because a transistor can appear fine, but still be dead.

If you can share ( or link) to the schematic, we will be able to help you a bit further.

Edit: found it myself: http://www.hifiengine.com/manuals/technics/su-v8.shtml

[Nrik]

...it looks pretty strange thta you can measure -17,5 volt on the negative regulator but not +17.5 volt from the positive, since the negative regulator is using the positive as a reference - meaning it MUST be at +17,5V.

Anyways: check if the other voltages in the voltage reg circuit has the correct values, and get back.

[sakis]

replacing semis as a procedure to trace down the fault is very bad practise

first problem will be that the replaced semis are not of the same quality of the original ...
second the trouble free start up you use is absoluelly wrong and it is almost sure that will drive again in smoke issues

amplifiers is more about symmetry stabilty and most of all procedure

here is a tutorial about vintage audio .... if you follow it step by step it will take you a few hours and a few dollars to make it back to life for another 30 years

there you go vintage amplifier repair/upgrade manual

[Artie]

Originally Posted by PC91

. . . after replace the power transformer (for a sony one of something sony don't know what, huge transformer)

All transformers are not created equal. How do you know if this is even putting out the proper voltages?

[PC91]

hi!
to nrik, yes i've replaced the output again together with the drivers and now i'm out of them i'll have to wait to trip to buy more of them because where i live theres no electronics shop. i ve replaced all the transistor not only the blown, actually no one has a damage that i can see, i will follow the power lines from the transformer disconnecting stage by stage to see if i can find where the problem is located.

to sakis, thanks for the link, i will have it on mind for following it when at least can turn it on with all the components and without smoke hehe,

and finally, to artie, when i found the amp, i start to look in a scrap pile to find transformers, after measuring, this one seems suitable, the voltages are lower than the original (original should deliver 59.2v and -59.2v after the diode bridge, the replacement gives 40.5v and -40.5v after the diode bridge) it's not as good as the original but it used to do the work fine until i blowed up forgotting the headspread (yes i know).

thanks to everybody! i will keep tracing where the voltage goes crazy

[PC91]

hi! it's me again :P
after some test i finally get some kind of normal readings, where should it be +17.5v theres 19v and where should be -17.5v theres -19v, found it by completely disconnect the left channel, and following the path of the failure i've got inside of the pre-drive board, the part of the left side, after removing the µPA74V the readings on the regulator stabilized, but on the predrive board there are some issues witch i'll be looking for it tomorrow, i'll let you know, cheers!