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srinath 20th December 2012 08:54 PM

Transistor 7805p question.
Q: What would happen if a 7805p transistor received too much ripple input voltage.

Back story: On my HK avr 7000 the smoothing cap on a 3.3 and 5v line, a 10000 mf 16v cap was swollen.
That feeds to a few transistors and the 3.3v and 5v - 4 different voltage outputs infact have other smoothing caps - both electrolytic and smd caps. I found all 4 electrolytic caps swollen.

I am replacing all 5 of the electrolytics. However I am wondering what will have happened to the transistor and the rest of the mess of components in the way and past them.


davidsrsb 21st December 2012 01:11 AM

No damage at all. The 7805 regulator is almost indestructable. It would have been running COOLER as the capacitor failed and the input ripple increased. Eventually the ripple could get so bad the regulator drops out and the 5V circuitry could misbehave with brown outs.

srinath 21st December 2012 01:28 AM

OK that is reassuring. Great, now would it have fried the caps down stream of it ?
Or is that set of caps frying what caused the 10000 mf 16v cap to cook ?
Some kind of what happened first type question. Thanks for your answer.

gmphadte 21st December 2012 03:38 AM

A damaged filter cap is equivalent to swallowed poison. You may not die. If u r lucky, everything will be fine.

Gajanan Phadte

srinath 21st December 2012 03:34 PM

See this amp worked with the swollen caps but only in 2 channel.
It turned on without the dts board in.
After I replaced the smaller caps the amp never got out of stand by.

I either screwed up something or the other swollen cap gave up in the few times I powered it on after the removal of the dts board and it refused to get past standby state.

I guess I put the amp back with the new cap and see what it does.


indianajo 22nd December 2012 03:10 AM

When one electrolytic cap goes, and the amp is over 15 years old, I change them all. Low grade consumer products, e-caps are suspicious after 5 years. My DTV converter came from ****-Mart with bad e-caps- diagonal lines across the screen that disappear after a couple of minutes. ****-Mart always has better "bargains" than every other store.
If the manufacture is saving one cap, they saved money on them all. How many times do you want to take it apart? And diagnose what is wrong this time?

srinath 22nd December 2012 10:20 PM

The amp is commingup on 15 yrs, maybe in a few years. 2002/03 if I recall.
I cant replace all caps ... not just yet, its just too many. Lets see if it powers up tommorow. If it does, I will be good. I may work on it a bit more after that replacing stuff. The bugger worked just days before I pulled the board to replace the dts board caps.

indianajo 23rd December 2012 04:11 PM

If you have been into it and it died soon after, then bad connections are always a possibility. Powering down, pulling and reseating boards is useful.
As far as all the little E-caps, I bought a couple of dozen fresh 47 uf @ 35 v ones rated 5000 hours last year for $.09 each, so they are not all as expensive as the $5 main power supply cap pair. But the little ones can stop the sound just as surely when dried up. Have recapped an organ, a FM radio, a disco mixer, a preamp and 3 power amps since retirement. The difference is between day and night, except the preamp (had a bad paper cap). Although I replaced other parts to two power amps and the mixer, too, after I was sure the >20 year old e-caps were all good.

gmphadte 24th December 2012 04:02 AM

If u have pulled out any flat cables, just pull them out and check again. These also create such problems.

Gajanan Phadte

srinath 24th December 2012 08:02 AM

Actually that 10,000 mf 16v cap wasn't even bad, it had a plastic top that swelled, but when I pulled it off, under that the metal top of the cap with the score pattern so it will pop when it swells, it was intact.

I pulled a lot of flat cables, and have re connected them a plenty. It still wont power up.
I have it nearly all back together, but a pulling it apart again, I left out 1 7 pin connector from the front board to the main board.
I am nearly certain the problem is in that small supply and relay board in the front - and I refitted the amp so I can take it off without pulling the whole amp apart.

Thanks for all the help guys, I am going to fit it back proper and try it and see, maybe do a system reset after that and see what gives.


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