Technics SU8011 repair

[andymac62]

Hi
I am repairing this amp for my son, and wondered if i should put a 10 ohm to 22 ohm resistor in series with the emitter of Q305.

Many other amps have a small resistor here.
The output module STK0029 (Vbe multiplier & output stage) is blown & Q305 is blown also.
Q305 is a 2SC1885, which i will replace with a BC639.
I am not entirely sure why Q305 is connected like this, unless it gives much higher gain this way. I would think a lot more stress is placed on the base junction of Q305 in this configuration, and the addition of a 10 ohm to 22 ohm series emitter resistor would improve reliability, but may reduce gain.
I will bypass the emitter resistor with a 47u cap.
If all works well, i will make this change on the other channel.

Comments, suggestions ?

btw, the service manual has been recently added at http://fileshare.eshop.bg/equipment_mfg/Technics_5.html
and shoudl be at Jan's site http://www.audio-circuit.dk/ soon

[andymac62]

i guess Q305 is the standard VAS amp.
If i add in an emitter resistance, i may reduce the overall gain, and maybe output linearity, due to the local feedback effect of the emitter resistance.
If no one with greatly more experience than me has advice, i will try a 22 ohm emitter resistance & 47uF in parallel, and see how the output goes. Then i will short out the emitter resistance & see how it affects the output.

Hope some one can clarify things for me.

[h_a]

I'm not sure I understand why you are fiddling with Q305?

If the output is blown, that usually means the amp was shorted, so why bothering with the VAS?

I didn't investigate your particular amp in detail, but generally you will reduce open loop (DC) gain and affect stability. If the amp is well engineered, this mod will not be needed.

Have fun, Hannes

[destroyer X]

from colector to emitter..... this way, not exceding the small transistor current limits,you may use the advantage of high gain of those small devices....and this when high gain is usefull. I do not think you will be happy with the resistance there... the current requirements to the output transistors are huge when face deep drops of impedance you have in speakers..there are frequencies where the speaker impedance drops..the impedance is not linear, impedance depends on frequency....it is a frequency dependant value..it is specified into 1 Kilohertz, but, in different frequencies it is not that nominal 8, 6 or 4 ohms written into the speaker. The lower the speaker impedance, the higher will be the current need into the output stages..and the transistor gives you all the voltage and current drive is this stage... the Voltage Amplification Stage (VAS) is the main stage into an amplifier..responsable by the whole voltage swing, as in the output you have only current gain... the voltage gain is 1..this means... the voltage that will appear into the output is the VAS stage voltage swing. Voltage and current depends one each other...are two different expression into the same reality..one is quantity and other is intensity.... reducing current, using emitter resistance will reduce your amplifier performance into the frequencies you may have impedance dropping. I think it is not a good idea to reduce current.... better is to increase current and to substitute the small transistor by a higher power device that can give you reasonable voltage gain..alike BD139 for instance, that can give you 200 of gain..when the original will give you 250 to 300...so.... not so big loss of gain when you can increase current, as BD139 can hold more current and dissipate it . Some folks use to load the VAS using resistances..increasing current and loading it... this use to produce some interesting effects into the harmonics. You should try by yourself..... and you will feel that sound was better after your introduction of such resistance..this is not real, unless you compare one channel (the tweaked one) with the other one (keept as original) you may feel advantage...but this is much more psychological fooling ya technique then real world result. When we tweak, having belief something will change, and that will change for better..then we are persuading, convincing ourselves, in advance, that this result will be obtained... when we believe that..we will perceive this way...sadly, we are not precise machines to evaluate things....we use to brain wash our minds..there are folks that change speaker cable and say sound changed..ahahahahah...only his bank account has really changed...dropped for less...as such fooling ya devices are expensive.

regards,

Carlos

[tiefbassuebertr]

the emitter resistance of Q 305 determine the open loop gain of this second voltage amplifier stage (and the whole amplifier). To optimize the value of this resistor you need the possibility of cad design (pspice simulation), because the influence of the whole behavior is great.
Because the first gain stage (differntial amp dual transistor Q301) haven't resistors in the emitter line, it is hard to believe for me, Q 305 don't need emitter resistor (risc of oscillation in the MHz aera).
Follow URLs I have found, that could be helpful:
http://www.electronica.ro/audio/STK0029.shtml
http://www.datasheetcatalog.net/de/d.../STK0029.shtml
http://pdf1.alldatasheet.com/datashe...C/STK0029.html
Best regards Andreas Kirschner

[destroyer X]

You have enough knowledge.

Go ahead and tell us how to do that thing.

regards,

Carlos

[andymac62]

Thanks everyone for your comments.
I am just trying to find the best fit of parts for my repair.
The 2SC1885 is 150V 50mA, with Hfe of 150 at 10mA

If i look at similar amplifiers, Sansui AU-117 has two 2SC1845 set up as darlington in this position, with a emitter resistor.
Luxman L30 has 2SC1951, with no emitter resistor.
btw, i own and still use a Sansui AU117, i love the sound of this amp with my old 1979 12" 3 way speakers, and havent changed my setup in 25 years except to add a subwoofer. There is a lot of Sansui info at http://fileshare.eshop.bg/equipment_mfg/Sansui_5.html, as well as Jan's site in my first email in this thread.

As the main output module STK0029 has a blown internal transistor (top npn to positive supply rail), and the VAS transistor has blown as well, i was just wondering if i could improve the amplifier without affecting stability. I am concerned if i introduce too much gain i will make the amplifier unstable, and if i have not enough gain, i wont get the same output.

I have an STK0029 on order, and I need to find a suitable replacement for 2SC1885, ones that are readily available from my local store in AU
BD139 may be ok, but its gain is typical 80 at 10mA
BD681 is a 100V darlington with Hfe of 750
MJE340 is 300V with Hfe of 70 at 10mA
TIP41C is only low gain Hfe <75
BC639 has a gain of 120 at 10mA

I am thinking the BC639 or BD139 is the best replacement.
btw, there is a resistor from the emitter of Q305 (and the same transistor in the other channel) to the negative rail, so i dont think i will use a emitter resistor in my repair.
If all goes well on this channel, i will change the other channel VAS transistor as well.
The 2SC1885 has lasted 25 years of service, and probably failed when the output module failed, so BC639 should be my best choice as a replacement.

Any comments on my logic, or comments generally, and thanks again for your help clearing up my muddy thinking.

[jaycee]

To be honest these old Technics amps that use the Sanyo output modules are nothing great. The modules seem to be very prone to failure.

I have a Technics SU-Z55 which i got with one channel distorted, and during measuring, quit altogether. The output module is probably toast. It's based on an STK2038B with the AN7062 chip as the frontend.

It's a terrible design anyway - the "protection" circuit consists of 2 thermistors near the transformer, that attenuate the volume if the transformer gets too hot!! I plan to gut it and build 2xLM3886 or something like it in there.

[andymac62]

to everyone, many thanks for you replies.

I also dont like the power modules as they are full of too many compromises, but this is the simplest way to repair the amplifier.

If it fails again, i will just replace the models with LM3876
The LM3876 has the negative rail on the heatsink, so i would have to isolate the heatsink, as currently it is connected to pcb 0V. This is more work & i am basically lazy.

At least i have the diagram now, so it will be easy to replace the power amp section if i need to later.

I also dont like the old technics much, but my son likes it, so i will repair it.
I still have my 30 year old Sansui 25W, which i think sounds wonderful. I will repair it if it ever dies on me.
I had a listen to many surround amps, and i would need to spend more than $4k on an amp to get as good a 2 channel sound as my 30 year old Sansui.

[linuxguru]

Originally Posted by andymac62

i guess Q305 is the standard VAS amp.
If i add in an emitter resistance, i may reduce the overall gain, and maybe output linearity, due to the local feedback effect of the emitter resistance.

I have an SU-8011, in which these transistors have been upgraded at the
factory to the more robust 2sc2383. IMHO, that's not a good VAS transistor, and something like a Toshiba 2sc3423 or Sanyo 2sc3953 is more appropriate as a VAS transistor.