Input stage caps. (Dc offset...)

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ReneBMW
Posts: 88
Joined: Mon Mar 01, 2010 1:18 pm

Input stage caps. (Dc offset...)

Post by ReneBMW »

Hello!
Soooow, today i fired-up my titanium600.2 on wich I replaced al power transistors and powersuply mosfets with upgraded ones and replaced the power supply capacitors for the recommended Panasonic FCC series 5600uf.
It fires up nice, and it plays realy nice!
So the work I,ve don on it for now def makes a difference.
Keep in mind that during the whole life of this amp it only had 7 working power supply mosfets because of a pcb that showed a incomplete trace... Out of factory...
But I jumped this pcb fault by scrubbing the lacquer of and soldering a small piece of de-solder lint on it.
It doesn't look beautifull, but then again, someone else has worked on this amp and fucked up some parts, so I made the best of it.
But 1 thing...
It had a dc offset of 30 something MVP, and it still does...
I understand that this prob is caused by some old electrlytic caps in the input-stage, not blocking the dc.
But... What is the input stage?
Where to loop and wich caps to replace?
I presume it's the board wich is upside-down, but just want to be sure...
I would not have a problem with replacing ALL electrolytic caps.
All output transistors turned out to be ok after checking.
Any ideas?

Ciao!
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Eric D
Short Bus Driver
Posts: 4255
Joined: Sat Dec 30, 2006 8:50 am

Re: Input stage caps. (Dc offset...)

Post by Eric D »

Can you post a photo of the "incomplete trace"? I am curious if all of these amps have this problem, or if just yours did.

Large DC offset can be caused by a lot of things. Are you measuring the DC offset with your DMM on the speaker outputs, or with one probe on the input ground, and the other on a speaker output?
Got "schooled" by member shawn k on May 10th, 2011...
No longer really "in tune" with the audio industry, and probably have not been for some time.
Hands down the forum's most ignorant member...
Don't even know what Ohm's law is...
ReneBMW
Posts: 88
Joined: Mon Mar 01, 2010 1:18 pm

Re: Input stage caps. (Dc offset...)

Post by ReneBMW »

Hey Eric!
Yeah, I measure the offset on the speaker outputs, but I will try measuring on the input ground.
But where is it?

As for the picture, I only have one where I allready scraped the lacquer of.
And it's not that sharp.
But I'm pretty sure that there are more titanium 600.2 with the same fault, I might have found something new after more than a decade! :P
If you find something, will you let me know?
I found this fault after I checked al the gets after I seen 1 that was blown, I checked for continuities, and when I cleaned the board I couldn't believe my eyes haha
Attachments
Incomplete trace with the lacquer allready of.
Incomplete trace with the lacquer allready of.
trickyricky
Posts: 1653
Joined: Sun May 06, 2012 8:01 am

Re: Input stage caps. (Dc offset...)

Post by trickyricky »

I have both the ZX600ti and the TI600.2 and I repaired the power supply for the TI600.2 and I didn't see any open traces near by (or where yours is). So you checked continuity between the middle pads of the fets and the one you scratched wasn't making any continuity with the rest? That's not good, lol, glad you fixed it.
ReneBMW
Posts: 88
Joined: Mon Mar 01, 2010 1:18 pm

Re: Input stage caps. (Dc offset...)

Post by ReneBMW »

Yeah, I'm glad I fixed this haha, my first amp to repair myself, with some help on info offcourse.
Like zeropoint0.5, he gave me part numbers for upgraded fets.
I just started a few weeks ago with doing more then only listening to these amps until it broke, and replacing it with another.
The info on this forum also helped a lot with repairing vias and replacing ps caps.
Now the only thing that bugs me is the dc offset, and I want to know where to start looking so I can replace parts.

@ Eric, I thought the dc offset shield be measured on the speaker outputs, wich I did...
By "input ground", do you mean the ground next to the +?

I'm gonna get me a book about audio amplifiers...
trickyricky
Posts: 1653
Joined: Sun May 06, 2012 8:01 am

Re: Input stage caps. (Dc offset...)

Post by trickyricky »

http://www.diyaudio.com/forums/car-audi ... set-2.html the last three posts will help. This is what i had to do to control the dc offset on my ti600.2.
ReneBMW
Posts: 88
Joined: Mon Mar 01, 2010 1:18 pm

Re: Input stage caps. (Dc offset...)

Post by ReneBMW »

Hey tricky, thanks man, I will try to find some heat sinks tomorow and see if it makes a difference, good one!
ReneBMW
Posts: 88
Joined: Mon Mar 01, 2010 1:18 pm

Re: Input stage caps. (Dc offset...)

Post by ReneBMW »

Hey tricky!
Today I checked dc offset again.
When I put my fingers on an output, the dc offset goes down, the channel with 30 mv offset went down to 1, 2 mv, just like the "good" channel.
I took a small piece of metal and some thermal pass and glued it on the backside to see what happen, et voila! 1mv dc offset!
I know it's just patching symptoms, but I ordered some heat sinks for the outputs, TO-220 case, and put those on the faulty outputs.
Replacing the outputs would be better, but those differential pairs are discontinued, and I don't know if ther are any replacements/ upgrades.
So the heat sinks wil do just fine for now, and when the new outputs are in, I will put heat sinks on al the outputs close to the front.
These things get pretty hot, why aren't they cooled?
trickyricky
Posts: 1653
Joined: Sun May 06, 2012 8:01 am

Re: Input stage caps. (Dc offset...)

Post by trickyricky »

Do as my friend stated, instal them under the pcb (similar to the old soundstream) and use the bottom of the amp as a heat sink. This issue will return regardless on how new the predrivers are. Some of the m series experience the same issues.
ReneBMW
Posts: 88
Joined: Mon Mar 01, 2010 1:18 pm

Re: Input stage caps. (Dc offset...)

Post by ReneBMW »

Sooooooo, this morning i die some small checking.
Checked for shortage once again, to be sure. Checked the replaced powersuply fets (irf3205, these are a bit higher so you have to vile some of the screws (what's the word... Take some material of the screws that hold the plate to the fets, otherwise they touch the housing)
Checked rail to rail voltage, wich is 96 volts (?!?! Isn't this high?!)
And set bias.
Strange thing is that it pulls 2 amps at idle, with bias fully closed!!
So I set bias just like I set it on my zx475, wich sounded waaaay better after that, at 1.2 mv, put the housing on and let it rest for an hour to check if idle current has risen, and it did, a bit.
It puls 2.65 amps at idle, and on the manual it says "750 mv"
Weird, as my amp doesn't get warm, and plays wonderfull!
I have no scoop to check, so I haven't tryed it at full power offcourse, but the way it sounds I don't need it to go louder.
Don't know what I did, but it seems to be working.
I set my 500.4 titanium the way I was told, and I set it at 0.2 mv.
I was told that this was the way so set it, but now it sounds like crap, now I know what some people mean with "warmth" and " detail", just like the 600.2 sounds now.
500.4 drains just under 3 amps at idle, and doesn't even warm up.
What does the "typical current draw at idle" mean??
If I stay under the typical current draw it soundalike ass, if I go higher, it sound great, but it gets slightly hot.
Btw, what is normal rail to rail voltage on the titanium 500.4'and 600.2?
Can it be that the new power supply fets cause this, and is this good or bad?
As for dc offset, I ordered some heatsinks to solve the dc offset temporarily because it works (thanks tricky) but I will try to replace the inputs with some newer versions wich are still available (2SC5171 and 2SA1930 from toshiba)
I will try this on my old zx475 first, as it's still only getting better after testing and improving my skills, but it's still my "amp for parts" (not for long as it keeps going this good)
Let me know what you guys think!

Cya!
ReneBMW
Posts: 88
Joined: Mon Mar 01, 2010 1:18 pm

Re: Input stage caps. (Dc offset...)

Post by ReneBMW »

Hey guys!

Well, just wanted to share something.
Today I recieved some parts to get my last zx475 working.
This zx475 I wanted to use as a reference to set bias on all other amps.
Turns out that 1.7 mv is a not to high setting, so I set all zx ti like this.
While I was at it a tryed some new input transistors without matching them.
So I expected high dc offset.
While I was replacing the transistors I saw that someone has "fixed" some issue
By replacing a 52ohm resistor with 2 100ohm resistors in parallel.
As I have a amp for parts I took out the correct resistor and replaced it.
Powerd it up to find out dc offset is much lower than before!
Now I know where to put a pot to adjust dc offset in the future!
I put a small heatsink on a input transistor, and now dc offset has gone from 35 mv to -2!
On the other channel also!
So I guess this does the job temporarily, but in the future I wil order some pots to replace the 52 ohm resistors so I can adjust dc offset perfectly without having to change other parts.
Btw, I found that toshiba 2sa1930 and the 2SC5171 are a good replacement for the eom input transistors, as these are hard to find, especially in matched pairs.
So now I will take my zx475 for a test and adjust my titanium500.4 just like the zxti, and I have no doubt it will sound waaaay better then it does now.
I will post pictures from the resistors and where they are located later.
This is my first succes in repairing and tuning amplifiers.
Thanks to Eric D, trickyrick and zero point 0.5, without the help of these guys I would know where to start.

Cya!
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