Vintage RCA audio preamp I'm trying to fix/learn from

[rp55]

BTW I simulated the entire preamp using falstad's circuit designer. Does it seem like it is a representative simulation? (full screen the window to see all of it)

http://www.falstad.com/circuit/#$+1...0.0+0.00625+1+-1 o+35+64+0+35+10.0+0.05+2+-1

 

[rp55]

In taking a guess at some things that are happening:

It appears C6 removes the dc bias of Q1. A new bias is put on Q2 via R11-R12. The manual says something about C8 bypassing Q2's emitter which causes more current out of its base and hence more voltage on R13. Then the last two stages look like a push-pull design for the output. That's about all I can see given my current knowledge. I guess those inner caps are some sort of feedback balancing between transistors as their reactances related to frequencies don't appear relevant (or perhaps I'm off base there... likely).

 

[rp55]

So I seem to get an antenna receiver (when in boost mode via connecting E4 and E5 together per spec) when E3 is not connected to the transformer per previous replies. The boost has worked in the past. I get radio white noise unless E3 was connected to the transformer body. Also get some very low hz oscillations happening when switching out of boost (E4 and E5 not connected). No Earth ground or 0v connected to the body otherwise. It is fixed by connecting (grounding?) E3 to the transformer body. I don't understand why and I'm unsure about ground reference differences between various nominal considerations of ground (if that makes sense).

Perhaps it's merely (after just thinking about it) just having a long wire hanging off of -30 (E3) which creates an antenna?

 

[tfr000]

Did you notice C3 and C5? MMFD is a “Milli Micro FaraD” which is a nano farad = nF.

MMFD is micromicrofarad = pF. Very old schematics always list caps as uF or uuF. nF are more recent.

 

[Baluncore]

You are correct, apologies. A senior moment I believe.

 

[rp55]

You are correct, apologies. A senior moment I believe.

I can relate to that. Funny when I read your explanation I recall just skipping it because my mind was already full with what I was thinking about (but I knew you knew) I couldn't deal with thinking about it. Certainly your initial response to this thread was in line with the solution (as well as enlightening me about other things such as differential input being handled by transformers plus other stuff I still don't understand).

So anyways I feel like this thread can be closed at this point. Thanks very much to all who helped out with input. I'm planning on opening another thread soon as I have questions about some basic transistor stuff... with the hopes that I can come back to "this" thread via a new, more specific one.

This thread helped me fix the problem I was having initially which was (as proven by the responses) fixed with a different methodology than needing to "understand" the circuit per say but did get me on a new track to get deeper into this.

But I can certainly appreciate how things can get mixed up and forgotten as there's so much! I mean I just spent all day (another one) verifying transistor workings confounded by conventional vs real current flow (with the stupid diagrams constantly being mirror imaged)... It's mind boggling stuff but very cool!