Eti 480 trouble

[Hayden]

I’ve built an Eti 480 amp (not the best amp I know) the problem is that the output volume is low, when I turn up the input signal the amp gets to about 30% of its potential then its distorts. I have checked all the components if their in working order and if their in the right spots. I keep setting vr1 to get the right voltage but it keeps changing.
Can any one help? Are there any well known problems with this amp?

Thanks

 

[Netlist]

Hayden
Do you have a link to a schematic?

/Hugo

 

[Netlist]

Found it...
What happens to the rail voltages? Are they stable?

/Hugo

 

[mikee12345]

The general consensus ive seen is -it was stable with Transistors available at its time of design but now,not so!

http://www.alphalink.com.au/~cambie/ETI480/ETI480.zip

http://www.alphalink.com.au/~cambie/

 

[Hayden]

rails are OK
drops about 5v when inputs are high

 

[Centauri]

Check voltage across zener ZD1 (should be about 5.6V) and voltages across resistors R5 and R10 (should be about 5V each).

Also, with input shorted, is there any measurable DC or AC voltage on the output?

Cheers

 

[Hayden]

OK just blown the 2 protection resistors
Where can I get the schematic that shows the voltages of each component etc?

 

[Centauri]

Protection resistors?? this amp has no protection. Which resistors do you mean?

BTW which version did you build - the 50W (2 transistors) or 100W (4 transistors) ?

Cheers

 

[Greg Erskine]

hi Hayden,

Did you look at the Cambie site as mentioned by mikee12345?

http://www.alphalink.com.au/~cambie/

This site has more info on the ETI480 than you probably want to know.

 

[Hayden]

100w it is
100 ohms across the fuse went up in smoke.
don't know why I haven't changed anything since I last powered it up.

 

[Centauri]

The 100 ohm resistor across the fuse should only be there for current setting. If they have burnt out, then this means the fuse is blown for whatever reason (maybe blown output transistor).

Cheers

 

[Hayden]

I have replaced all the output transistor, still didn’t fix it “something is drawing massive amounts of currant that gets the 1/5 watt resistors smoking) so I pulled out Q7 bd139 and Q8 bd140 and the resistors didn’t get even warm so the two bd’s have something to do with the problem am I correct?


“While I remember” I did have Q9 and Q10 insulated from the output and I had it powered up then would that have blown anything?

THANKS

 

[Centauri]

I would guess that Q7 & Q8 could be shorted - very common if the output transistors have blown. Check between C and E (centre & LHS leg looking at front of transistor) of these with a multimeter set to ohms range - if you get a low reading they are toast. Do the same check with Q4 & Q5 (no need to remove them for this).

If Q7 & Q8 are shorted, also check resistors R14, 18 (47 ohm) and R15, 17 (33 ohm).

When any parts identified above are replaced, solder a shorting wire between RHS leg (B) of Q7 & Q8, leave the fuses out (but with 100 ohm resistors across them intact) and fire it up. The resistors should not get hot and the voltage on the shorting wire should be close to 0V. Remove the shorting wire and try it again - if there is high current draw again then Q6 may be open circuit.

Good Luck

 

[Hayden]

Just replaced all transistors and checked over all the components to see if their in working order (yes) and no shorts anywhere no broken tracks or dry joints, all polarities are correct. That’s it that’s all I can do there is nothing more I can do

I can’t even set the voltage because the 100ohms resistors across the fuses get too hot

I need to solve the resistors getting hot business first.
2 seconds and their smoking (major current drawing)

 

[Centauri]

Ok, next step....

Remove the output transistors only (the 2N3055 & MJ2955s) and retry.

The voltages on the base of Q7 & Q8 should be around 0.6V & -0.6V . The voltage on the output (speaker) line should be close to zero. The voltages on the collectors of Q7 & Q8 should be around 0.6V less than each supply rail (ie. 36.4V & -36.4V assuming 40V rails).

Let us know how this goes.

Cheers

 

[Hayden]

I can’t power it up for more than 2 sec otherwise the 100ohm resistors smoke.
Maybe the bios voltage is set too hight to start of with…..just wait I check it.

 

[AKSA]

Hayden,

Set the bias to the lowest setting possible on the trimpot. You can verify this by measuring the voltage across the two driver bases, but since it blows fuses so quickly just try moving the wiper to the other side.

If the amp still blows fuses on switchon, then the chances are the output stage is oscillating - a very common problem on CFP amps which are incredibly sensitive to bias adjustment.

Oscillation is fequently cured by increasing the capacitance across the base/collector of the driver transistor. This hobbles it at high frequencies, but sucks the lifeforce from the music.

You can't have it all - this is not a particularly good audio circuit.

Cheers,

Hugh

 

[Eva]

This may sound a bit obvious, but :

If the 100 ohms 1/5W resistors blow 2 seconds after power is applied thus not allowing to do any measurements...

Why not using a pair of 100 ohm 10W inexpensive ceramic resistors temporarily connected with wires to the fuse holders in order to have enough time to do some measurements and find the problem?

These resistors will limit the current consumption to non-destructive levels. Furthermore, 10W power rating is enough to sustain a short circuit to ground permanently with +-30V rails so you will have enough time to do as much measurements as you want

 

[Centauri]

I can’t power it up for more than 2 sec otherwise the 100ohm resistors smoke.

Does it still do this with the output transistors diconnected ? - if so then driver transistors shorted or Q6 open circuit.

 

[Hayden]

I Fixed the problem with the resistors,
There was leakage between one of the outputs transistors and the heat sink (q10 I think), my meter didn’t pick it up but when I took it to work it did (I need a new multi meter)
Now I just got to set it up with the oscilloscope.

Thanks so much