Re: stereo amp cleaning

Dave wrote:
"Mark D. Zacharias" <spammenot@xxxxxxxxxxxx> wrote in message
news:0sX2i.6296$RX.639@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Dave wrote:
You can swap Q405 and Q406 and see if the problem moves to the other
channel. Resistor R417 (560 ohm) can be checked in circuit. R405, a
68K, would probably need to be checked out-of-circuit. Experience
tells me that high-value resistors can change value or go
open-circuit for no good reason. Worth keeping in the back of your
mind sometimes as you're troubleshooting problems such as this.

R417 and R418 both test around 560 in circuit

R405 and R406 both test 66k in circuit

I'll try swapping Q405 and Q406 and see if the problem follows the
transistor. I found them relatively close (4 hour drive) in stock,
or I can order them in to my local supplier and wait a couple of
days/weeks to receive. How many do you think I'd have to order to
find a pair with DC gain matched to within, say, 15%? The 2SC2603
crosses to NTE289, which is available as NTE289AMP, a matched pair
for use in amplifiers...
For sure Q405 is not turning on. I watched the output of Q406 and,
about 5 seconds after turning power on, it goes from -0.4 to -7.1V. Q405
just stays at -0.4V.

Dave

If they are using a 2SC2603 then it is not critical. The 2603 is a pretty
standard signal transistor - nothing special. A 2SC945 would be fine, heck,
even a ECG / NTE 123AP. Just make sure the basing is correct. An American
type transistor will usually go EBC rather than ECB as viewed from the
front.

Mark Z.


.

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