On Thu, 17 May 2012 09:04:02 -0700 (PDT), klem kedidelhopper
wrote:

On May 16, 11:41*pm, wrote:
On Tue, 15 May 2012 12:03:47 -0700 (PDT), klem kedidelhopper









wrote:
I'm working on an old Bogen CHS60 60 watt *PA amplifier. This is a
direct coupled amplifier and almost every transistor is blown. A real
mess. Except for one small signal transistor,, a 2SA539Y *all the
other TO92 types are 2SC945L. In addition there are two TO220 drivers
and two TO3 outputs. I have been replacing the 945 TO92's with 2SC1815
types which is what I have in stock. Both types seem to cross to an
NTE 85 so I figured this would be a safe bet. However I just noticed a
problem. The problem is the two remaining good 2SC945L's on the board
seem to test OK out of circuit however the base is definitely in the
center on both off them. The board is marked EBC for all of these
types as well but the spec sheets for both the 945 and the 1815 types
shows that they are ECB.. The interesting thing here is that the spec
sheet for the 539Y shows EBC as it *is marked on the board. I have
never seen where two transistors bearing the same number were
configured differently but is this possible? It certainly seems that I
have installed all these 1815's incorrectly. Does anyone have any
further insight on this? Really confused. Thanks, Lenny

Lenny,

Check on the back to see if the amp was made in South Korea. *If so,
Bogen used Korean versions of Japanese transistors which can have
different pin outs. *The beta and the bandwidth of the transistors,
especially the outputs, are critical in these amps. Check for spurious
oscillation and or notch distortion after the repair. *If you see
these anomalies on the scope, use an older transistor type for the
outputs or order the parts direct from Bogen. *I think *2N3055s works
well as the output transistors. Chuck

I have two of these amps. The other one is about as bad as this one
is. Transistors went down like dominoes. I replaced all defective
small transistors in this amp and the outputs using 2N3055's as well.
Except for a momentary slight "thump" when first turning it on I still
can't seem to get a peep out of this thing. The output center point is
unbalanced too. With 55V on the top collector and 35V measured in the
center. Perhaps as you said the transistors may be that critical and I
would really have to use originals as there are no adjustment pots in
the thing. If that is actually what is happening here I probably will
just **** can both of these units. I was going to fix one because I
have a job coming up where I could have used it. And I would have
liked to sell the customer something I had, and don't need at a
discount rather than go out a buy a new one for them. I'm starting to
think though that saving this customer some money is hardly worth it.
This unit was made in Korea and by the way Bogen no longer supports it
nor do they have any parts for it. If as you said the transistors are
that critical and I've had to replace almost all of them how would I
ever get this thing to work without originals? Lenny


Lenny,

These amps have capacitor coupled output so there will be dc at half
the power supply voltage at the output of the amplifier. Bogen will
email you the schematic for free. I usually used parts from Bogen
since I worked at a warranty station, but when parts weren't available
in time, we had to use subs. If notch distortion was a problem, we
changed the bias diode chain or changed a resistor value. If you stick
with the original output number, oscillation shouldn't be a problem. I
have worked on over a thousand of these amps and never had one I
couldn't repair so you shouldn't have to toss them. I'm on vacation
until after Memorial Day, but I might have a schematic at home. If I
do, I can help you get these amps repaired. Chuck