p3a blowing 2sc5200 / 2sa1943 problem - diyAudio
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Old 28th October 2008, 04:29 AM   #1
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Default p3a blowing 2sc5200 / 2sa1943 problem

Hi,

I've just finished building a pair of p3a using 2n5551, mje350/340 and tip3055 and tip2955. I've designed my own pcb's using the schematics posted on Elliot's web site.

It works nice. I correctly set the Iq, and checked all connections and voltages several times.

I tried it for several hours. No problems at all. No heat problems, no sound problems.

Then, I removed the tip3055 and the tip2955 and replaced them with toshiba transistors 2sc5200 and 2sa1943. They shorted within minutes of testing. It happened twice: they worked for a few minutes, then after powering off and then back on, they shorted badly.

Should I make any mod to the amps to make them work or could them just be fakes?

Thanks in advance for your comments.

Best Regards,
Guido.
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Old 28th October 2008, 05:33 AM   #2
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markoramius,
Oscillation!
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Old 28th October 2008, 06:26 AM   #3
kalmara is offline kalmara  Bulgaria
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The 2SC/2SA devices have much higher switching frequency ones than the TIP ones...are your miller caps in place? (C4/C6) And what value are they?
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Old 28th October 2008, 08:49 AM   #4
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Default Guys Come On !!!!!!!

VERY SIMPLE

when you replaced the transistors you need to do again idle adjustment .....from scratch

may be 1943+5200 are better transistors but they are not "drop in " replcements ....

i will be very happy to see your pcb .....
in my testings BD139-140 sounded better than 340-350

if your transitors are fake they will probably work fine ...but only up to a low level of power and load .....push them a bit harder and then boooooom !!!!!! be carefull about that also
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Old 28th October 2008, 08:55 AM   #5
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Default yet again same story .....

Quote:
Originally posted by Lumba Ogir
markoramius,
Oscillation!

people miss the obvious and look on satelites that might go to mars ......i am breathless ....
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Old 28th October 2008, 09:22 AM   #6
jaycee is offline jaycee  United Kingdom
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Two other possibilities:

1) Your PCB layout is not good enough. The P3A uses a CFP output, and this is notorious for oscillation - the PCB design is crucial here. Long traces between the drivers and the outputs is a no-no.

2) Your transistors are fakes, without meaning to sound bad, this is quite likely considering where you are from

If you have not put it on your design, put some small electrolytic decoupling caps near the output transistors, say 330uF. They should be near to the output transistors, as should the 100nF decoupling capacitors Elliot shows.

You cannot set the bias with one pair of output transistors, and then swap in another. You must start with zero bias and bias them with the new pairs.

I would recommend MJE350 for VAS, and BD139/140 for drivers.
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Old 28th October 2008, 10:41 AM   #7
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sakis,
would you tell us what driver/output device combination was used in your CFP amp, please. (may help markoramius)
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Old 28th October 2008, 11:26 AM   #8
rabbitz is offline rabbitz  Australia
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I've used 2SA1943 / 2SC5200 on Rod's earlier P3A boards and experienced no problems. The miller caps were standard value and used silver mica.

It's been mentioned but VR1 needs to be set for max resistance and safety resistors installed and then go through the complete checks and biasing procedures.
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Old 28th October 2008, 11:33 AM   #9
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Hi,

Thank you all for your replies.

I read a lot thru the forum about oscillation. I tried to do things in order to avoid it.

Sakis: thanks, I'm posting pcb layout here. I would really appreciate to hear your comments about it (please be gentle, this is my first diy audio project!!!). Small transistors are 2n5551. Input cap is 4u7 polyester. C2 / C4 / C6 are ceramic disc capacitors: I KNOW THEY ARE NOT RECOMENDED, but I couldn't find a supplier for better materials.

When I've installed the toshiba pair I performed the Iq adjustment with patience, according to Mr. Elliot's project web page.

Jaycee: I know the layout is crucial. It is my first. Maybe is no good at all. I read a lot thru the forum and looked at several examples and things to take into consideration. Regarding the transistors, I suspect they were not original, because of price and appearance. The 2sc5200 didn't say toshiba at all.

Thank you again for your posts. I really enjoy doing this project and learning a lot while I'm doing it. I hope to make it work.

Best Regards,
Guido.
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Old 28th October 2008, 12:17 PM   #10
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Default i am pretty sure

that when you poewered up your amp idle was on maximum thatswhy you fried the transistors ....

and/or your calculations about idle were wrong .....

thats ok though we all been in this mistakes more than once ....

i also made my own pcb since i had my own criteria about it and the best i ve heard about this amp after many many tests was the version that included 68pf multileyer vas cap .... notorola bd 139 and 140 military specs and original 2SA 1302 -3281 toshiba transistors .....

the think is that i have accses to many weird semiconductors and still have left about 150 pairs of these output transistors originall made .... so a get the chance to play arround with difrent kinds of semis ....

once i try to see if there is audible diference between bd139-140 of philips and st ....philips sounded better but may be i am worng .... then again compaired to all these i have from a police stock ...original motorola's bd 139-140 gold plated tips sounded the best !!!!!


i plan to use them in the future in my own amps ....NOT for sail OR for costumer repairs ....

finally i agree with jay cee that pcb is critical for this amp ( others also ) and ground rooting signal returns and so on can make a big diference .... for shure rods pcb will be one of the best options ( also costs nothing )

at least by this way you are on the safe side about a lot of things
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