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PSU/AMP Trouble shooting question
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Uncle Krunkus
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PostPosted: Sun Sep 02, 2012 1:41 am    Post subject: PSU/AMP Trouble shooting question
Subject description: Is the real problem a leaky cap?
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This is a PSU (and partial amp) PCB for a bass amp I'm trying to fix.
It's pretty obvious TR4 (at the top) has shat itself from the smoke and burns on the PCB. It's a 2SD600K which I'm in the process of tracking down or working out an equivalent. (anyone know of any good free transistor equivalence tables on the net? Something you can fiddle figures on, and it just updates the list of replacement parts automatically? Am I being unreasonable there? Have I just invented it? Laughing )
The other power transistor is a 2SB631K which looks okay.
The two on the side are 2SA1268, and look good.
The other smoke mark is where the mystery transistor was. Same case as the 2SAs, but I went to wipe the smoke off the case, and the part number simply wiped off with it! Laughing

So,....
Has anyone any idea how to work out what it was? Or does anyone want to help me work it out from first principles? Laughing

BTW The amp is a "Magnum CG 30B". Looks about 15-20 years old, made in Korea.


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Last edited by Uncle Krunkus on Fri Sep 21, 2012 5:45 pm; edited 1 time in total
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bubzy



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PostPosted: Sun Sep 02, 2012 2:42 am    Post subject: Reply with quote  Mark this post and the followings unread

this is kind of a pointless post but,
i have a transistor selector book here, its got over 27,000 transistors in it but your one doesnt come up, the 2sd600 is in there but has no equivalent, unless someone has manufactured one since this book was written. but if you need any others, give me a shout and i can check the book.

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JingleJoe



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PostPosted: Sun Sep 02, 2012 4:37 am    Post subject: Re: PSU Trouble shooting question
Subject description: What is this transistor?
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Uncle Krunkus wrote:
(anyone know of any good free transistor equivalence tables on the net? Something you can fiddle figures on, and it just updates the list of replacement parts automatically? Am I being unreasonable there? Have I just invented it? Laughing )
The other power transistor is a 2SB631K which looks okay.
The two on the side are 2SA1268, and look good.

http://www.rapidonline.com/ has a parametric search function for some items but not transistors unfortunately, however you could check other big suppliers like farnell and see if they do.

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PostPosted: Sun Sep 02, 2012 9:30 am    Post subject: Re: PSU Trouble shooting question
Subject description: What is this transistor?
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Uncle Krunkus wrote:
This is a PSU (and partial amp) PCB for a bass amp I'm trying to fix.
It's pretty obvious TR4 (at the top) has shat itself from the smoke and burns on the PCB. It's a 2SD600K which I'm in the process of tracking down or working out an equivalent. (anyone know of any good free transistor equivalence tables on the net?


http://alltransistors.com/transistor.php?transistor=19779 scroll down to the yellow link "2SD600K Equivalent Transistors"

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Uncle Krunkus
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PostPosted: Sun Sep 02, 2012 1:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks guys
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stewpye



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PostPosted: Mon Sep 03, 2012 12:23 am    Post subject: Reply with quote  Mark this post and the followings unread

To work out what the transistor with the missing markings is you'll have to trace out part of the surrounding circuit. It will be either a 2SA or 2SC I suspect, but without seeing more of the circuit I can't say if it's NPN or PNP.

Cheers,
Stew.
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Uncle Krunkus
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PostPosted: Mon Sep 03, 2012 2:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah I agree,
I'll try tracing some of it out today and see what I can find.

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stewpye



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PostPosted: Tue Sep 04, 2012 12:47 am    Post subject: Reply with quote  Mark this post and the followings unread

If you could take a photo of both sides straight on, mirror the bottom side and somehow make it transparent (apart from the tracks) so you can lay it over the top sides it might make it easier.

Stew.
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DES



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PostPosted: Tue Sep 04, 2012 11:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

I did a quick search but couldn't find anything on the Magnum, but did find a picture of a Rogue CG 30B bass amp.
http://cnj.craigslist.org/msg/3154672336.html

It's on Craigslst. If it apears to be similar to the Magnum you search for schematics for the Rogue version...

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Uncle Krunkus
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PostPosted: Wed Sep 05, 2012 1:22 am    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, exactly the same. Thanks for that.
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Uncle Krunkus
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PostPosted: Mon Sep 10, 2012 7:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Okay,
Looks like this is more the actual amp than the PSU.
Looking at the schem so far, it's fairly obvious that I'm looking at a small signal amp PNP. Probably a 2N3906 would work fine, as this is not hi-fi, it's a basic bass amp.
Any other ideas?
If you need me to expand the schem in any direction, let me know.


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stewpye



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PostPosted: Mon Sep 10, 2012 11:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think it's more likely to be an NPN transistor. I'd like to see more towards the transistor that drives it, and the resistor etc at the top of the diodes.
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Uncle Krunkus
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PostPosted: Thu Sep 13, 2012 2:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Okay,
here's a bit more.
What do you think?


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Uncle Krunkus
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PostPosted: Thu Sep 13, 2012 9:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Now that I look at it like that, I think I agree. An NPN would fit in there just right. Smile
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PostPosted: Tue Sep 18, 2012 4:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Call off the search!! Surprised

The suspect, a 2SC3198 was last seen hanging out on an exact same PCB which came inside and exact same bass amp,..... in perfect working order! Cool
I s'pose it's one of the perks of working in an op-shop.
Now I can fix this one, AND get a bass amp of my own. Very Happy

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PostPosted: Fri Sep 21, 2012 3:44 am    Post subject: Reply with quote  Mark this post and the followings unread

I put the new transistors in, and, although it sounded quite good, I had a feeling there was something else lirking behind the obvious fault.
I played along with some tracks, but after about 15mins, the amp faded and went quiet. BTW This was the original fault report.( so it wasn't all just intuition.
Turning it off and then back on gave back the full sound again straight away. But then it took just 10mins, then 5, then 1,... I think you get the picture.
My first idea was a bad power resistor warming up, but the fact that you can cycle it so quickly with the power switch, makes me think now it may be a leaky cap.

What do you dudes think?

PS I'm gonna re-cap it all anyway in the meantime. Wink

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PostPosted: Fri Sep 21, 2012 8:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

The other reason I think it's a cap is that there is quite a thump when you power on, when you turn on the brightness, when you switch the EQs, etc.
Now a bad cap might put a DC component on the ground line yeah? So the bypass cap for each section would make a thump as they were switched in or out. Does that make sense?

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PostPosted: Mon Sep 24, 2012 3:43 am    Post subject: Reply with quote  Mark this post and the followings unread

I don't know if anyone is still out there, but I'm gonna document the end of this anyway, just for reference sake.

Okay I removed and ordered the main filter caps, (2*3300u42V) and while I was on the E14 site, I also found the main transistors, (which I also haven't checked yet) and they could be handy, cos if one of them has gone short base->emitter, I'm pretty sure that would cook the preceeding transistors. Any substance to that?

So I got 4 of the caps and 3 each of the BDX53/4C pair. Remember, I now have an exact copy of the amp for myself, so some spares are a good idea, as I plan on getting to know this amp fairly well. (I'm thinking about tricking mine out with a chorus or something grungy, or both! Cool )

In the meantime, I've replaced the rectifier, power resistors, the other 5 caps, and all the higher wattage resistors. Mostly just cos I found I had them all in my spares, and I'm coming around to the idea of using them, cos why else would you keep all this stuff if you don't use it? Confused
If the problem's still there, it would have to be the power transistors. Or an evil spirit! Laughing

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PostPosted: Mon Sep 24, 2012 4:52 am    Post subject: Reply with quote  Mark this post and the followings unread

It's been an interesting project... I'm looking forward to hearing how it finally turns out, that you eliminated your thump and all is good. Hang in there.

<m>
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PostPosted: Mon Sep 24, 2012 10:02 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes I've been watching your progress too, from afar, like some kind of compulsive spy. It's been interesting Smile
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PostPosted: Tue Sep 25, 2012 4:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

The 2SD600 and the transistor next to it just form a darlington pair, just to give it a high output current. (as hFE-darlington = hFE1 * hFE2, for us nerdies)
Any small signal transistor, for the 2SD600 a NPN, will work. I would use replace the small signal PNP on the bottom side of the PNP darlington pair with a same series transistor though, to keep the circuit a bit symmetric.

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