Trying to bring a Acoustic 260 head back to life.
Had a blown fuse, replaced fuse and plugged it using my light bulb series power tester, bulb
stays fully lit.
Pull all four of the output transistors, easy to do as they are plug in socket mounted, short
is gone, bulb briefly lights bright and fades down to barely glowing.
Using a multimeter in diode test mode all transistors show they should be good.
Checked the two driver transistors the same way and they check good as well.
Output transistors are 40411 NPN RCA's
Anyone else dove into one these, schematics seem to be non existent,
Would it be worth just replacing the outputs with something like a set of MJ15024 or 2N3773 transistors and see
where that takes me?
Mike C.
Had a blown fuse, replaced fuse and plugged it using my light bulb series power tester, bulb
stays fully lit.
Pull all four of the output transistors, easy to do as they are plug in socket mounted, short
is gone, bulb briefly lights bright and fades down to barely glowing.
Using a multimeter in diode test mode all transistors show they should be good.
Checked the two driver transistors the same way and they check good as well.
Output transistors are 40411 NPN RCA's
Anyone else dove into one these, schematics seem to be non existent,
Would it be worth just replacing the outputs with something like a set of MJ15024 or 2N3773 transistors and see
where that takes me?
Mike C.
Off the top of my head, maybe a output bias transistor short, or shorted diode in the power supply?
Probably not power supply, but if you lose one side of the voltage rail going to the outputs, ya got problems.
Check the power supply voltages. Could be a shorted electrolytic cap or P.S. rectifier. +-~35 v usually, but if there is an output cap series speaker, single supply ~70 v. Voltage will be lower with the mains light bulb on. Front end could have own ~15 v supply or resistor drop from main voltage to around that value. Warning V>24 v from one hand to the other can stop your heart.
Transistors can measure good at 2 v (meter test voltage) and fail at rail voltage. Iceo test predicts failure better. Put 12 v or higher power supply series a 47k resistor series 200 millamps scale of a DVM. + to C - to E npn transistor. Current >couple a microamps, the transistor is shorting.I use a car battery charger parallel an e-cap, and some alligator clip leads.
40411 is a 2n3055 selected for ability to go above 60 v. Homotaxial process so Ft is about 200 khz. Modern epitaxial replacements with ft> 1 mhz can oscillate ultrasonically in some layouts. Even modern parts labeled 2n3055 will have the high Ft. The factory that made homotaxial parts is now a NJ county park. 10 ohm 1 watt resistor between driver & O.T. base can eliminate this in some cases.
Transistors can measure good at 2 v (meter test voltage) and fail at rail voltage. Iceo test predicts failure better. Put 12 v or higher power supply series a 47k resistor series 200 millamps scale of a DVM. + to C - to E npn transistor. Current >couple a microamps, the transistor is shorting.I use a car battery charger parallel an e-cap, and some alligator clip leads.
40411 is a 2n3055 selected for ability to go above 60 v. Homotaxial process so Ft is about 200 khz. Modern epitaxial replacements with ft> 1 mhz can oscillate ultrasonically in some layouts. Even modern parts labeled 2n3055 will have the high Ft. The factory that made homotaxial parts is now a NJ county park. 10 ohm 1 watt resistor between driver & O.T. base can eliminate this in some cases.
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The supply is a singled end 70volt.
The supply it self is good, I've disconnected it from the amp section and it is fine.
Pulling the outputs and feeding test tone to the amp I can hear audio output at the driver transistors.
I did find a hair line crack on a trace by shining a flashlight through the circuit board, jumpered over that
.....no change!
The supply it self is good, I've disconnected it from the amp section and it is fine.
Pulling the outputs and feeding test tone to the amp I can hear audio output at the driver transistors.
I did find a hair line crack on a trace by shining a flashlight through the circuit board, jumpered over that
.....no change!