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Appropriate transistor to use in Heathkit AA-15 diver circuit.

saabracer23

saabracer23

AK Subscriber
Subscriber
My father asked me to rebuild this small integrated and for him. He remembered when he was a younger kid that his friend‘s dad had one and he was always so impressed with it. He got this one from a friend a long time ago because it had issues. I took a look and one channel is completely gone. I’m replacing the outputs with MJ802G, I replaced the drivers which are RCA 40409 and 40410 with Toshiba TTC004b and TTA004b.

IMG_5505.jpeg

For the pair of 2N3393 at the beginning I’m not sure yet, maybe KSC1845. My question is mainly for Q203. It’s a 40408 and I’ve seen a few posts about this transistor being referred to as the VAS stage, where it’s marked as a predriver on the page. Is it the VAS stage? If so I would imaging it wouldn’t need the 700 mA current rating and could use something more appropriate like 2SC2911, if not then maybe another TTC004b?

Thanks!

Dan
 
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looks like its being used as a phase inverter. Lower transistor (Q205) DC to the collector will get signal 180 out from the upper one (Q204) cap-coupled to the base. I think the voltage amplification is coming from Q201 and Q202.
 
Yes, Q203 is the Voltage Amplifier Stage, it's the last stage of Voltage gain everything after that is Current gain. Phase inversion is handled by Q205.

Craig
 
llwhtt said:
Yes, Q203 is the Voltage Amplifier Stage, it's the last stage of Voltage gain everything after that is Current gain. Phase inversion is handled by Q205.

Craig
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I placed Q201 and Q202 with KSC1815, looks like an overall better match to the originals. What are your thoughts about using something like the KSC3503 or 2SC2911 at Q203? I know you weren’t a low capacitance device, not sure what the capacity of the original is but being ready for 700 mA I’m guessing on the higher side.

Dan
 
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Ran into another bad transistors RCA 417-116 which crosses to a “S2091”. I really can’t find any information on either of those. The best description being “PNP 80V 1A”. Maybe a KSA1013?

Dan
 
Doug G. said:
2N3393s are readily available.

Doug
Click to expand...
I did see that, but would prefer to use something. I have on hand already. If I had to place an order, I’d have no problem, throwing a bunch in my cart, but I recently placed an order and don’t want to wait on these. It did make me take a second look though, alltransistors has these as 100 mA devices, but it actually looks like they are 500 mA. So I’ll probably pull out the KSC1815 and put in something like 2N5551 or 2N4401.

Dan
 
Does anyone else have an opinion on the transistor being used in the VAS stage? Can a traditional lower current device be used here? Like the KSC2383 or 2SC2911?

Thank you
Dan
 
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