If it works dont fix it..

[kriss99]

Hi everyone,

I have an (elektor) amp built many years ago and it worked up to yesterday.I decided to revamp it and changed some elec caps in the power supply,new case etc.All looked normal so I wanted to check to quiescent current to adjust it and close the case.Channel one ok,was 75 ma ,set to 80ma as said in the magazine.Switch off and on again and OMG(!!) in the other channel the power resistors R18,R19 started to roast. I switched off this "thing".

The darlingtons were dead,the T4 dead,the T3 was ok with the multimeter but in the component tester gave bad curves.I changed it as well as the power resistors. Switch on again and now the resistor R12 is very hot but the other components ok.
After hiting my head against the wall some times ) I try to understand what caused this.Tomorrow morning I can go to a store to buy all of the semiconductors or throw it from the window.Any ideas would be appreciated
Many thanks.Kris

[djoffe]

P1, if old may have decided to go open when disturbed. That causes horrendous quiescent current. T5 would have tried to limit the current, and perhaps it died in the process. If R12 is hot, perhaps T5 has a collector-base short. T7 might have died again also.

Before you power up again, short from T3 Collector to Emitter. That will make for zero quiescent current, but at least things shouldn't blow up.

Probably a good idea too to limit current...Would it be painful to drop 120 Ohm power resistors in series between the rail and the amp under question. With 80 mils of Quiescent current, it would drop 10 Volts, but that still leaves 30 volts per each rail. That way, if there's still something pretty broken, the 120 Ohm resistors get hot before the transistors die...

Alternately, you could bring it up on a variac, or remove the fuse and put a 20-40 watt bulb across the fuse.

Good luck...

[Nelson Pass]

That's why your brighter designers place the pot as a shunt
in the bias circuit between base and emitter. That way if
it opens up it simply shuts down the bias.

[ostripper]

Like this , ( A - B - C below) consider what EVERY trimmer will do in a open wiper condition.

A - (CCS) will decrease current source to under 1ma , causing underbias at output.
B - (offset) will open , cause slightly worse offset.
C - (Vbias) .. open , will cause output stage to underbias.

OS

[jaycee]

theres so much wrong with that elektor circuit its not even funny :S

[Lee Knatta]

I've seen sooo many amp designs with the stupid trimpot connection. It's hard to believe...

And we have a broken Elektor amp here every week and the cause is almost always bad design practice from Elektor.

By the way: IMO the design looks crappy altogether: No tail CCS for LTP, no Emitter degeneration for LTP, no current mirror for LTP, no output inductor network... Could it be more shabby?

[kriss99]

Thank you very much.To be honest a get bored of the so many idiosyncracies of the elektor amps,and i have built many of them.I changed the darlingtons amongst other transistors and after several hours I found out that the bdx66 was fake,not functional or i dont know what,and I bought it knew.Maybe this amp doesnt worth futher attention.Thank you again.

[Mooly]

Whatever it's faults it's a basic circuit and as such is easily fixable.

Do as suggested and disable the bias generator and also power up with a bulb tester untill you are sure it's OK. Then still with the bulb see if the bias adjusts. If it is OK return it to minimum, then apply full voltage and reset correctly.

[kriss99]

Thank you Mooly.But first I must find BDX66-67 replacements because I cannot find in the market(!) What about MJ2501-3001?

[kriss99]

One think I forgot is that the T4 was hot from the first time I fired up this amp.It was something about 40 (celcius) and I had had it with a small heatsink but it was this way many years.Is that normal?