Quad Sansui problem; help???

[Betomon]

NO NO NO........do not swap or desolder anything......you are going down a road to disaster here......DO NOT listen to your friend.....

We can help you but you need to stick with Jim and myself and forget what your friend is saying.......

The transistors themselves have polarity a 2SA is a PNP and a 2SC is an NPN......the reason the holes dont line up is because you are only supposed to be able plug the transistors in one way......otherwise the Emitter and Base will be reversed, this is not a polarity issue, its just simply a wrong pinout issue.......thats why they dont fit......

Forget about the stamp on the back of the transistors, the originals were made nearly 30years ago on the other side of the world......the one thing that remains consistent is the pinout of the case style of transistor.....

The MJ21193 is the PNP which goes where the 2SA went
The MJ21194 is the NPN which goes where the 2SC went...

This is what we mean by polarity..........

Do you understand???

If you try and turn those transistors around to and then try and match things up you are going to have a complete disaster on your hands.....

By the way you have used WAY too much thermal paste on those transistors too.....

We want to help, but stay with us, and stop going out on these wild ideas.....there is a lot going on with this amplifier and you need to do exactly what we tell you otherwise we wont get anywhere....

Okay, so summarize where you are at and we will start again.....??

Ok phew sorry about that. He told me the same thing about the print and pinout.... but, i guess I got him confused. Thanks for clearing it up for me.. (I think he might post... but I'm not sure..)

And I understand now about what you mean by polarity, thank you

So for some reason I still got the polarity mixed up, because the 94s are on the outside and the outside is where 2sa went.

So, I'm fixing it right now but that will be the last thing I'll do tonight.. my dad comes home tomorrow so he can give me a hand in checking other things.

 

[Betomon]

Oh and about the thermal paste... I did not use any that didnt already come on them.. its just that they came with a lot and I got a little sloppy i guess..

 

[kevzep]

Oh and about the thermal paste... I did not use any that didnt already come on them.. its just that they came with a lot and I got a little sloppy i guess..

You could tidy that paste up, and re-apply a small amount, now you are putting the MICA INSULATOR on? This goes between the transistor and the heatsink, it is the same shape as the transistor...there is one for each transistor.
If this is not there the transistor will short out......
Then triple check you have the correct transistors in the correct sockets....

Now having the polarity of the transistors wrong would indeed prevent the bias current from flowing......

Still there will be other problems as well so we need to proceed with caution.....

Okay then, good work!!!

 

[Betomon]

Alright guys Im finishing putting the transistors on right and checking and rechecking

Also, I actually never took the micas off, some of them came off with the old transistors but I put them back just as they came.

I'll report back soon.

 

[kevzep]

Alright guys Im finishing putting the transistors on right and checking and rechecking

Also, I actually never took the micas off, some of them came off with the old transistors but I put them back just as they came.

I'll report back soon.

Good man, small steps, lots of patience........I might not be around until a bit later today....it 6am in the morning here on Sunday.....give me about 6-8 hours....

 

[Betomon]

Oh ok I see. But I put everything back how it was supposed to be...... turned it on after making sure the bias was right... got light for about a split second and then nothing. So I turned it off.... what did I do wrong now? :'(

Fuses are all fine.. nothing smoked, no fireworks, no smells.

 

[Betomon]

Update.. bad news:

I took off the bracket that holds the 4 10000 µf caps. The purple cable that connects them to the power supply board was loose... pretty much all the way off. My dad redid that. Also, we redid the 2 resistors under the caps to make the leads shorter to prevent any kind of grounding to the metal bracket.

I checked the 3.9s on the power supply board........ they're toast. No readings on any range. Not even continuity.. they are just gone. No signs of burning or anything though.. they look clean.

Since the leads were so bad for the 3.9s we are going to have to redo the leads.. it looks pretty rough, but my dad is on it. Looks like I'm going to have to buy spares now..

What could be the cause of this? I'm pretty dumbfounded right now.. :sigh:

 

[Trnsfmr]

When you turn the unit on, it draws enough current to burn up the wirewound part inside the resistor. Normally, the caps charging up is a big current draw. I would say that something else is adding to that, like a short in the output section or a bad transistor that is essentially a short.

When you buy new resistors, get at least 4.

it's discouraging, but we'll get there.

 

[Betomon]

Will do.. I'm thinking 10 for a "price break"

I'll check transistors, probably starting with outputs

 

[Betomon]

Remember how I said one A899 was blown (visibly burnt)? I tested the second one on the front board and it tests both ways .. that means its bad right?

I'm in the process of determining this resistor is blown for sure.. I have replacements already

UPDATE: I took it out of the circuit and it was fine.... I'll keep checking..

 

[Betomon]

I think I found the suspect......

C1708 6XG

On the front left channel, I took it out and tested it.. I got very low numbers using the diode testing function... 40s, while the one on the opposite side (right channel) was getting 600s. I replaced it (I bought like 8 of them).

Hopefully this is it? I've been testing other transistors but they are fine. Also, the problematic board seemed to be front (more specifically the left channel).

I'm ordering 10 3.9s in case this wasent it..

 

[Betomon]

The + and - connections are down, the ground is up, and I have a 12 uf polypropylene cap across the both banks of caps, from - to +.

Ive been curious as to why you have that cap there... can you tell me what benefit is gained by doing this? Since I'm ordering stuff I can throw a couple of those in there

 

[Trnsfmr]

Ive been curious as to why you have that cap there... can you tell me what benefit is gained by doing this? Since I'm ordering stuff I can throw a couple of those in there

Well, this is in the netherland of Audio Mod Paradise. The idea is that just like for use anywhere else in the amp, or in a speaker crossover, the power supply caps might not respond real well at high frequencies. Paralleling a poly cap helps the high end response of the power supply.

Now, I'm an engineer, and I can tell you that from an engineer's point of view, that is complete BS. If you watch the rail voltage on a scope, you won't see it dip or anything because of high frequencies, you just see it collapse when the amp starts to clip.

BUT, I can also tell you that if you do this, along with a few other audio paradise "fixes" like bigger and bigger supply caps, that it does seem to sound better. I like the sound and since I am 1/2 engineer and the other half voodoo pseudo electronics designer and experimenter, well, I go with what sounds better.

You see, a lot of other people hear it too, and we only have harmonic distortion, and IM distortion to blame everything on, and it's not enough.

 

[Betomon]

Ahh, I see Jim.. and it does make sense I guess.. even if its just psychological

And guys I found the suspect... a C945 tested very low, so then I took it out of circuit.. and it still tested low and both ways. So I bought some replacements and also for the remaining transistors so it will have all brand new transistors

(just missing the MJ complimentary pair for the rear board.. oh well those all seem to be fine)

Now I'll be checking some diodes while I wait for the package to come in..

 

[Betomon]

Well I checked diodes and resistors... they all seem fine.. (although I did not test all resistors)

 

[Betomon]

So I replaced the TRs, and caps I didnt do last time.... then I put in the resistors, unplugged all the quad boards, hooked it up to the DLT.

I took out the driver boards as well.. just to make sure it would start without them...

And it didnt... I checked and rechecked connections, checked the new resistors (they are fine) and fuses (fine). The light came on and dimmed just a bit, but then went back to full brightness. The little bulb for the fm balance lit up very dimly and stayed lit while the DLT was lit. I noticed a bit of cycling as the light would get bright and go dim several times in quick succession.. at the same rate as a tiny click inside the qrx.

Any ideas?

 

[Trnsfmr]

Hi Betomon,

Sorry, but I don't have any experience with the DLT. Don't know what to say.

 

[Betomon]

:

What would happen if I try it without the DLT?

I checked diodes on the PS board and they are fine... as well as the resistors, and.. I'd imagine the caps are all good since they are all new.

So do I just run the risk of blowing transistors?

 

[kevzep]

So I replaced the TRs, and caps I didnt do last time.... then I put in the resistors, unplugged all the quad boards, hooked it up to the DLT.

I took out the driver boards as well.. just to make sure it would start without them...

And it didnt... I checked and rechecked connections, checked the new resistors (they are fine) and fuses (fine). The light came on and dimmed just a bit, but then went back to full brightness. The little bulb for the fm balance lit up very dimly and stayed lit while the DLT was lit. I noticed a bit of cycling as the light would get bright and go dim several times in quick succession.. at the same rate as a tiny click inside the qrx.

Any ideas?

Remember you basics here. Why does the bulb light up?.............
If its dim, the unit is pulling current......
If its bright, something is shorting out.........if its cycling, something is pulsing, or oscillating and about to fail, or shorting out........

So turn it off and start looking at possibilities.
If the driver boards are not in there, then I would first look at your DC rails and output transistor arrangement........

Measure from the base of the ouptuts to ground, and the collectors to ground, and see what you get......it should be a high reading.......

 

[Betomon]

Okay, thanks kev, I'll look at the outputs. And thanks for the reminder about the bulb