Problems, again, with building the amp...

[Mlaen]

I'm trying to build amp from attachment but normally I have problems...
When I first connected the amp voltage drop across 0.22resistors were 0V... I was adviced to remove output transistors, remove 220R resistor that is connecting the emiters of drivers and install 120R res. to the emiters of drivers, so now the drivers are output tranzistors with 120R emiter resistors... man I'm complicating...
So, now I have ~4.5V drop on those resistors no matter in what position trimmer is!
Any idea where to start searching for error?? I checked all the components more than once... I also made another board but with same results (

[jan.didden]

What about those people that gave you that "advice", where are they now? Ran away?

I would advice to built it up in the original configuration, and then apply a revolutionary concept: measure!

There appears nothing wrong with the schematic. I promise you that once you start measuring the DC conditions in the amp, the error will quickly become clear. Especially with 0V across the Re, that's much easier than a burning output stage!

Jan Didden

[edl]

let's insert back the outpot transistors.
now in the Vbe multiplier remove the 4,7K resistor, and use and change the 1,8K to 1K. use a trimpot rated 1K.
then heat up the amp, and let's try to get a normal bias.

[AndrewT]

Hi add a link from pot wiper to upper end of 4k7, just in case the pot fails open cicuit.
Add some local decouling to the output cap ends of the supply rails and some smaller caps to the driver ends of same rails. Connect all decoupling grounds to the power ground not the signal ground. Use insulated wire point to point (P2P)

[Mlaen]

First of all, thanx for answering so fast!!

Originally posted by janneman
What about those people that gave you that "advice", where are they now? Ran away?

it's harder over the internet...

eld:
about that 4k7 res, after I remove it, do I have to short two holes he's been in?

First thing I'll do is get some sleep...I'm working on this amp all day (today I printed the PCB on paper), drilled the holes, assambled all, and realised that it's not functioning
SO I'm a bit frustrated...and I'm also frustrated for my bad english

[Mlaen]

instead of going to sleep I made a few of those changes you adviced me to...
I short-circuted the 4k7 resistor, replaced 1k8 with 1k and replaced trimmer to 2k5 (i don't have 1k right now...)
So, ,voltage drop across Re is stil 3.7V min and ~4.3V when trimmer is at full left...

PS.
Andrew, I didn't quite understood your post, my english terminology is not so good...

[moamps]

Bok,

Najcesca greska koju sam vidjao u ovakvim dizajnima su pogresno okrenuti tranzistori MJE340 i 350 (odnosno BD139/140). Kad ga gledas sa strane natpisa redoslijed je slijeva nadesno E-C-B, dok je uobicajeno B-C-E. Mozda se ta greska i tebi potkrala.

Za daljnje pronalazenje greske bilo bi dobro da izmjeris napon C-E na tranzistoru BC556 pri krajnjim tockama trimera.

First, check the polarity of the driver transistors.
Second, measure voltage across C-E pins of the transistor BC556 at the end points of the trimmer.

Regards,
Milan

[Mlaen]

Thanx I will!
Instead of using MJE340/50 I used MJe15030/31 wich have pinout B C E when viewed from front...

[Mlaen]

Ok, ,here it is...
when trimmer is full right Ur=3.7V and Uce=0.69V, and when full left then Ur=4.3V, and Uce=2.3V

PS.
I still have instaled only driver transistors and 120R emiter resistors... Dr. Jagodic, the guy that designed this amp adviced me to do that temporary until I fix the problem...

[moamps]

Originally posted by Mlaen
..... and Uce=2.3V ...

Correct readings for Uce=2.3V