computer amplifier zener diodes overheating?

[DoomPixie]

Hi all,
I just opened up my computer's amplifier to check see fi i could see any other mod's i can do to it. it's a typhoon amplifier, came with two little sattelite speakers and a shitty little sub, i have noticed that two little zener diodes apear to be overheating and have scorched the pcb a lot, they havent blown i dont think ( amp works perfectly) the amp uses Tda3020 chip's. the zener diodes apear to be something to do with the subwoofers amp. only mods i have done so far is to add an extra 40V 100uF capacitor to each of the rails, i have been running it with a 10" 8ohm sub instead of the little 6" sub that came with it.. but i have been doing this for a good year and a half now.. i did notice it looked like the zeners were getting hot when i opened it to add the capacitors but the board is black underneath them now and i dont like it..lol..
next mod i think wil be a new transformer, i experimented with capacitors and no matter how much more i added after 100uF there didnt seem to be any improvement, it helped a bit, before the lights on the front of the amp were goign almost completely off under heavy bass at even 1/4 volume after adding the 2x 100uF extra cap's it holds up pretty well at 1/4 volume and if the bass is turned down there are no problems atall, had this with the shitty little sub that was supplied with it aswell, if you can call it a sub, it didnt go down past 100Hz and the crossover point in this amp is meant to be 120Hz ( according to the manual)
anyway, any idea's what could eb causing it? mabey i can replace the two zener diodes with higher wattage ones?
many thanks,
Owen

[poobah]

A higher wattage zener would, of course, be a good measure... but the amount of heat generated would be the same. The trick here to mount the zeners as high off the board as possible. This allows for better airflow, but also the increased lead length increases the surface area available to convect away heat.

Be VERY careful replacing these parts. You will discover the copper near "burned" areas has lost strong adhesion to the board.

[DoomPixie]

good thinking, i will have a look and try and work out what value the zener's are so i can order some new ones and i will mount them as high off the board as their lil leg's allow. seems to me that another year tops and this lil amp would have failed which would have been a real shame had i not noticed this as it is a good sounding little amp ( for the £20 they cost) as i said it has a few little bugs but nothign majour and it sounds good. with a decent sub instead of the one that came with it and still useing the original sattelite speakers it sounds quite decent and is LOUD for what it is..
Owen

[poobah]

Yep,

If you want see good examples of this; pop the the back off your TV... for God's sake, don't touch anything... just look.

[DoomPixie]

i know about tv's..lol... not as bad as the defribulators we used to work on in university though... serves me right for specialising in medical electronics though..lol.. they have a nasty bite..
Owen

[poobah]

Yeah,

Loooong time ago, I had a friend who was real BS'er... thought he knew everything (like me... except he really didn't). He told me that picture tubes could charge themselves up... I'm thinking to myself... here we go again.

Some years later I'm actually woirking on my TV. So I get a long screwdriver and a clip-lead, hold it in a rubber glove, and discharge the CRT. Phones rings, I'm back a half hour later with pair of needlenose to get tha little clicky wire thing out of the side of the tube.

Wham... now I hate BS'ers even worse. This was before I really knew (more than textbook) about dielectric absorption.

Lesson learned: 1) hate BS'ers and let them know about it (sometimes they are right, but how can you tell), 2) If your TV breaks, hurl it... it was secretly programmed in a LSI chip to self destruct anyway.

[DoomPixie]

yeh, the tv in my room i bought new with a 3 year guarantee and after 3 years it stopped working.. all it does now when you plug it in is put's a blue key symbol on the screen..it says "keycoded" on the front of the set aswell but ever since i bought it i havent had any code or anything and there is nothing in the manual about it other than it is to help prevent people from stealing the set but how does it help prevent someone stealing the set cause as far as i can tell there is nothing in the manual about it other than that statement! i tried phoneign them up but they tried to get me to send it to them and pay for them to look at it cause it was out of waranty..
Owen

[Corvus corax]

Are you sure you are looking at ZENER diodes, here? I would find it unusual to see a zener being used for a power supply application in a small power amplifier. More likely you are looking at the power supply rectifiers.

[DoomPixie]

nope.. they arent in the power supply section, they are in the amp section behind the subwoofrer amp chip, another thing they could be doing is supplying the op-amps as there are two op-amps on the board aswell. they are zener diodes, they are marked ZD1 and ZD2 on the pcb.
Owen