Citation Twenty - Four ...

 

Anyone got knowledge of this power amp ? Particularly looking for bias setup
procedure.

TIA

Arfa

 

On Wed, 14 Apr 2010 17:42:57 +0100, "Arfa Daily"


Never did one but if you hadn't a print for it here's some.

:http://akdatabase.org/AKview/displayimage.php?album=18&pos=7

It's #8 of 24

VR404 is idle current trim on right ch, 403 on left. VR402 is DC
balance on right ch, 401 on left. I suppose the balance is pretty
simple, just zero out. Don't know where the test points are for idle
current. But with a quad of silicone planars on each side wouldn't it
be around 30ma?

 



I was guessing at around 20 to 25mA, but I now have a copy of the service
manual, courtesy of Trevor over on uk.r.a. It's actually a bit of an 'odd'
setup procedure in that they give you four pairs of test points, but then
you have to glue 4 x 100 ohm R's around each two pairs, and then adjust for
32mV between the junction points of these R's ...

The DC balance adjustment is also a little 'odd'. First you set the O/P
impedance selector to 4 ohms, then check the offset at the speaker
terminals, and adjust, if necessary, for 0V. Fair enough, and exactly what
you might expect. But then you reset the O/P impedance for 8 ohms, and check
that the DC offset is " +/- 60mV " ...

I'm sure that something has got lost in the translation there, and what they
actually mean is that first you set for 0mV on 4 ohms, and then check that
on 8 ohms, you do not get more than a maximum 60mV offset. Would you agree ?

Arfa

 

On Thu, 15 Apr 2010 01:08:04 +0100, "Arfa Daily"


Glue? Where is the electrical connection, on the emitters?



Yep sounds perfectly reasonable to have a max of 60mv plus or minus
into 8.

 



No "glue" as in sticky stuff ! :-)   Just a term that gets used here in the
UK electronics repair business, to indicate any sort of quick and dirty
electrical connection, as in 'just glue a bulb across the battery, and see
if it lights up' or 'just glue a cap across the resistor, and see if that
cures it'. Solder is often referred to as 'electric glue'.

Dunno where the test points are. Didn't bother to actually check on the
schematics for the output stage. There are a pair of test pins sticking out
of the board at one side of each channel's power amp, and then another pair
at the opposite side. They are not particularly close to the emitter
resistors. What you have to do is hook two 100 ohm Rs in series, and then
connect them from the outer pin of one test pair, to the outer pin of the
test pair at the opposite side. Then the same again between the inner pins.
Repeat for the other channel. That leaves you with a pair of resistor
junctions between the test point pins on each channel. It is then between
these junctions that you measure for 32mV.

I did it this morning, and it actually went exactly as the manual stated,
with 32mV being easily and accurately achievable. All ran nice and cool, and
no signs of x-over distortion on the 'scope, so good result. Very difficult
though to set the DC offset especially close to zero, as the pots have a
rather enthusiastic response. Still, was able to get it close enough, and
certainly within the quoted limits

Arfa