Sir Richard9025 . . . . .
Let's just see if the following info, can create in your " minds eye ", just exactly what is occuring in your audio output stage.
ALL without the use of your non available oscillosope or even your then having the full experince in using one.
Consider the use of these vector displays being shown below in depicting a pure sine waveforms gradual transitioning . . . . at progressively increasing audio input levels.
A PURE SINE WAVES ANALYSIS . . . . .
Shown, at the very top row, is the nice, pure sine waveform in the
BLUE outlining.
Its further color enhanced markups, depict the pitfalls that may befall it at higher amplitude audio input levels.
As the waveforms upper + node approaces your power supplys maximum + voltage threshold capabiliy, the very top tip of the waveform starts progressively compressing and crunching down into a more rounded waveshape, as is being shown in the
YELLOW marked in outlines.
Meanwhile, its lower - node waveform tip is also approaching ground, and it just can't go BELOW ground . Expect the - node to be taking on the same compression and gradual transition into the compressed and rounded
YELLOW - node depiction.
Now with there being an even further increase in audio input level . . . . the
YELLOW rounding of the sines + and - nodes start a transitioning of the node tiplets into a flattening.
We now have ourselves a waveform transition into a quasi squarewave, and that result will not sound good at all. It even sounds progressively worse at higher input levels.
The next cluster of sine depictions are split up into three audio input levels as is being marked as Performance 1-2 and 3 levels at the right side of the plots..
The left and right halves of each level present the effects upon the + node of the sine on the left and the negative node on the left.
Finally . . . . . for making an Instant Visual Evaluation . . . . . all of the sine portions are being color coded as per the decline of quality of the waveform.
GREEN's are basically ALL of the units being unaffected, and staying pure sine waveforms.
YELLOW is being a noticable degradation of the pure sine waveform.
ORANGE is being a severe degradation of the pure sine waveform.
Now lets see how the three performance levels relate to your unit, by initially observing the relevant portion of the schematic now placed just below.
AUDIO OUTPUT SECTION SCHEMATIC . . . . .
Initial point of interest will be the three
YELLOW marked up resistors that make up a voltage divider bridge that halves the supply voltage with about a 10% swing above and below 1/2 of the 9VDC supply voltage.
This is used for setting the base bias on the VT2 transistor which is the first in a sequence of 5 direct DC coupled transistors, to the very output of the amp, where it finally transistions to using AC coupling into the single speaker.
SINE PLOTTINGS . . . . REFERENCING AGAIN . . . .
Now in referencing back to Sine Waveforms
Performance 1 Level it is split into the left half in the effect upon the + sine portion of the waveform and the right half relates to the - node portion of the sine waveform.
it is showing the handling of a low level input waveform that is of such low level that, no distortion will be noticable.
Take particular attention of the fact that the left half illustration is showing a sine wave form that is imposed upon a trio of +4.5 Vdc . . . .a +4.8 Vdc and a +5.1Vdc levels as are obtained by adjusting the R18 trim pot.
Then the trim pot is adjusted in the other direction, such that the same effect is noticed on the -node of the sine waveform as is being shown on the right half of the picture.
Now in either situatiuon of the extreme adjustments of the control, both the + and - nodes show no distortion . . .yet.
That situation is being reserved for the
Performance 2 Level plots which are just below.
In that situation the audio input has been gradually increased . . .such that . . . . .
In the first plot, the bias is set at one half of the supply voltage and the + and - nodes will then just fill in their amplitude capabilities.
Run the bias on up to ~+4.8 and the + sine waveform starts taking on a rounding compression of its + node.
With the bias up to +5.1 and the sine waveform takes on a harsh transition into a harsh quasi square wave waveform . . . . .that's being one unpleasant sound.
Look to the right half of that
Performance 2 Level plot and you can see the same effect upon the - portion of the sine wave, with the -4.2 and -3.9V bias settings of R18 control..
The last
Performance 3 Level plot is where the bias is set at +4.5 Vdc and the audio input level is applied to such a high level that the sine waveform is even showing the start of tip clipping at that level.
Then as the different possibilities of bias settings are made, it only gets worse in all of the settings.
Look at all of the
GREEN positive and negative color coded waveforms, and it reveals that R18 should be adjusted such that when the pot is set at ~ one half the 9Vdc supply voltage that the max level of input AC waveform should not show any effect on its + and - node tips.
Figure that as your max input level that you would want coming into this amp input. That does not seem to be the case on the level of audio which is coming into the audio output amp from your post amplifiers after the AM and FM detector stages. That is being even after the somewhat high losses of passing thru the loudness compensation and tone control networks located just before it.
Let me later, show how to incrementally reduce that excess audio input coming into the audio input stage, on down to that level that the amp stage will handle without overloading.
CROSSOVER DISTORTION . . . . .
A last point to consider on this unit is the fact that the low level transition of a sine wave from O volts amplitude on up to 700 millivolts will be cut out due to the turn on need of a silicon transistor in requiring that level of input voltage for its base to initially start reacting to it in creating any progressive collector conductance.
700 MV on both the + and - node is quite a click-click " dead space" to account for.
Soooo . . . . seems like this circuit design has set up a series biasing voltage loop thru the output transistors, so that they will always be slightly conducting, which THEREBY leaves a pre conducting path for that very low level audio to ride upon and share that very same DC current loop path .
In now referring back to the SCHEMATIC snippet, one will be seeing the marked in series of
GREEN resistors, which make a current loop conductive path, it starting from the +9 DC supply line and then going down thru R30-R34-R37-R38 pot-R39-R41 and the conducting transistors to ground at the emitter of VT10.
AGAIN . . . . just as was done with the R18 symmetry pot . . . . . and in this case, R38 pot, they have minumized the adjustment range of those pots by assigning their resistance change variance, to have but a minimum of range adjustment.
Basically, its having just enough swing to bring upon correction of any associated manufacturers parts variances and aging with time.
If you figure out the current loop resistance values, they permit right at 1 milliampere of current passage. That then gets multiplied on up by the gain of the 2 output transistors and their 2 driver transistors. Those are being configured, not as darlingtons, but instead, they are using a very high current gain
Sziklai ***** pair circuit configuration.
Sziklai ***** . . . . .
https://image.ibb.co/dHWSxS/Sziklai_explanation.png
THE POWER AMPS SITUATION . . . .
It seems to be operating normally, with the limiting range effect of both the R18 symmetry pot and the R38 two trim posts making it impossible to GROSSLY misadjust them, by virtue of their only having a very limited range of adjustment / effect.
AN INSERTION . . . . .
Just after having seen your posting of scope waveforms.
That display of waveform peak nodes just confirms the symmetrical clipping of both of the waveform, with too much audio input.
That scope just reminds me of Telequipment of the '70s which was a cheaper knock off of the Tek scopes. Of course, your unit might even happen to be a Russkie wannabe of those units.
You seem to be in AC coupling mode into the scope which will settle the waveform to the center of the Y axis trace pattern.
I think that the option for DC coupling mode, which would ADDITIONALLY let you see any combined DC element , with the AC waveform . . . . . is being a 3 position slide switch at the very bottom of the front control panel.
I think that it is marked as
DC---GND---AC as its two modes. With the GND position letting you place the sweep X trace position either at mid screen graticule line or at the very bottom graticule line.
YOUR GAIN SITUATION . . . . .
You now neeed to refer to the very bottoms schematic snippet, which depicts the FM and AM detector outputs and their following, single post ampolifiers:
Using the Reference 1 left half . . . . .
You will see that they have selected beta options of both a VT11 NPN and VT12 NPN transistors that match the required needed gain of the higher level FM detectors audio output, versus the lower output AM detectors audio output.
PLUS . . . they are using a
R52 COMMON SHARED collector resistor, with the actual audio input being selected by the switching off of power to the respective AM or FM RF stages.
The
GREEN ARROW path shows the AM signal flow, while the
RED ARROW path shows the FM signal path flow. They combine at the C61 coupling capacitor to then flow to the very lossy tone and volume circuitry just prior to to C14 at the input of the power amp section.
Now move over to Reference 2 right half . . . . .
Lift R52 and sub in a 5K trim pot and take its center tap to C61 where you have since lifted it from the collector connnection of VT11.
Extreme CCW terminal of trim pot connects to B+ supply junction of R49 & R48, with extreme CW terminal of trim pot to shared collectors of VT11 and VT12.
You should then be able to power up and use the trim pot to bring your excessive audio levels down to the input level that does not have clipping until the max adjustment of the Radios MAIN volume control.
DETECTOR SCHEMATIC REFERENCING . . . . .
Thasssssssit . . . . .
73's de Edd
.....