Puzzled by these inductance values

[AlfBaz]

I have this centre tapped inductor. It's out of a really old car radio's transformer coupled Power amp section which I'm trying to come to grips with

Resistance measurements seem to indicate that it is centre tapped with the outer wires being red black and green the centre tap

red - black = 378.83 ohms, 0.53mH
green - red = 187.66 ohms, 7.94H
green - black = 191.25 ohms, 7.96H

I would of thought red-black would be the series addition of ~~ 8H coming to a value of 16H

Is my crappy wavetek Meterman LCR55 meter playing silly buggers or is them some coupling effect because of the way its wound?

[c4757p]

The winding is probably black->green and red->green, rather than black->green->red, so that measuring black->red goes once clockwise and once anticlockwise.

Also, inductance doesn't add in series when it's coupled, it's actually roughly (A series B) = (sqrt(A) + sqrt(B))², so 8H series 8H ~= 32H. (Consider that inductance is proportional to the square of the number of turns. If you double the turns, you'll quadruple the inductance.)

Granted, I don't really know what the use of such a transformer would be, but then I've no experience with audio stuff old enough to be transformer-coupled...

[AlfBaz]

Thanks for that. The winding topology you mentioned seems to make sense and the relationship between coupled series inductors is interesting, I'll put that on my "list of things to look up"

As far as transformer coupled audio amps go, I know precious little about them. This whole radio fault finding exercise is a PITA. I thought the output transistors where faulty, turns out they're germanium 
I try to google some of the components and all I get is pictures of them in someone's antique component collection gallery

Here's what I've traced out so far for the power amp section. Any tips would be greatly appreciated, especially any info on Q1/Q2. I have to replace them as one of them had a lead broken off right at the body.

[c4757p]

I thought the output transistors where faulty, turns out they're germanium 

They are faulty - damn silicon's picked up a few extra protons from somewhere....


Looking at the input to that circuit, I think one of us has something wrong about that transformer. There were generally only PNP germanium power transistors, so just like with vacuum tubes, an inverse signal was usually generated using a transformer to drive a push-pull amplifier, and I'm guessing that's what that one is supposed to be doing. But if the windings are the way it looks like to me, then it's just going to give a non-inverted copy of the signal (which is why I was confused about its purpose). Are you sure your measurements are right? And someone else - please double-check me, if I'm missing something I'm too tired to see what it is...

[AlfBaz]

I just tried the following.

Injected a sinewave into the red lead, made green the ground attached probes to incoming on red lead and another on black and they are inverse. If you look at L1/L2 combination on the above schematic (not attached the rest of the cct) signal in to ?, centre tap common, inverted signal at the bottom

Something like this...

[David_AVD]

Looks like a phase splitter.  What's the overall fault ?

[AlfBaz]

They are faulty - damn silicon's picked up a few extra protons from somewhere....

Damn, missed the transition through silver, will have to wait until it gets to gold

[AlfBaz]

Looks like a phase splitter.  What's the overall fault ?

No output...
I started with the PA in an attempt to trace back to the radio's output. Haven't quite got there yet.
I'm tempted to put it all back together with a sub for the TT365... the broken lead on it was probably what failed

[David_AVD]

I haven't seen germanium transistors for a looong time.  Not much to go wrong with that circuit.  Replacing the broken transistor will probably see it spring to life.

[AlfBaz]

I was thinking along those lines also but I had to desolder a heap of wiring to get it to a stage where I could freely inspect and trace stuff out.

Some of the external wiring to the radio was missing as well so I wanted to try and identify key sections in it so that I knew where to probe if it didn't work.

Might just bite the bullet and do just that.

The radio is out of a HQ Monaro. I was tempted to get in touch with the owner and ask him if he wanted the guts ripped out of it and throw an ipod in there 

[David_AVD]

I repaired a few radios like that back in the 80's.  Some (most) of them are a real bitch to get apart.  Lots of wires and no connectors!

[AlfBaz]

You've just described this thing perfectly!

I started off my interest in electronics with AM radios, but that was nearly 30yrs ago, I have trouble remembering how to spell superheterodyne let alone know what it is

Have to admire the lengths they went to get things done. Here are some snaps of the tuner. The intricacies in the mechanical push button station selector is just fantastic

[David_AVD]

Yeah, those press button station "memories" are a mechanical marvel.  Not much ever went wrong with them.  A bit of a clean and and lube would get them back into action most times.

Actually, thinking about it, it was the late 70's when I worked after school at a TV repair shop.   

[vk6zgo]

I was thinking along those lines also but I had to desolder a heap of wiring to get it to a stage where I could freely inspect and trace stuff out.

Some of the external wiring to the radio was missing as well so I wanted to try and identify key sections in it so that I knew where to probe if it didn't work.

Might just bite the bullet and do just that.

The radio is out of a HQ Monaro. I was tempted to get in touch with the owner and ask him if he wanted the guts ripped out of it and throw an ipod in there 

Gee,I would have thought that by the early '70s,they would have been well into Complementary Symmetry & Silicon transistors.

A lot of the locally made portables & the like around that time had dumped germaniums & transformer coupling,looks like car radios were a bit of a holdout of the older designs.

Maybe the bean counters were calling the shots even back then!

"We can't afford new designs!----Use all the old stuff we've got in stock!"

What brand of radio?
Airchief were the OEM radio,made by Astor,but AWA,& I think Philips,turned out after market models.

"The radio is out of a HQ Monaro. I was tempted to get in touch with the owner and ask him if he wanted the guts ripped out of it and throw an ipod in there "

Oh the horror!!

Both the after market AWA in my EH Holden,& the Airchief in my 1974 Premier were great performing AM radios,both in sensitivity & selectivity,as they were designed with AM as the priority instead of an afterthought-----ipod,indeed!!

[G7PSK]

Looks like a push pull impedance matching transformer to me. There was usually a transformer to the input of the power transformer and on the output  to the speaker, with a lot of old transistor radios you have to think valve as they were designed by people who previously only had valve experience, must have been strange  to have a bunch of transistors dumped on your bench and told make a radio out of these new fangled things. What are the transistors by the way?

[SeanB]

I will bet AF119's in the Rf and IF stages, and some Ac126's in the audio side.

[rsjsouza]

GP7SK is correct; maybe you would like to redraw the circuit with some known circuits in mind:

http://www.angelfire.com/planet/funwithtransistors/AJ6-1.html
http://qmsslab.com/af/amp%20push-pull%20tr.png

And a brief explanation is:
http://www.circuitstoday.com/push-pull-amplifier