Well, some of this riddle may be solved. I've been assuming I'm supposed to see 120VAC at the AC side of the rectifier on the Sunn because my brain was thinking this was the line, but it's not. It's the secondary of the transformer. Looking at the schematic, I'm only *supposed* to be seeing 66VAC. The GOOD news is that I *have* been running the amp at pretty much full voltage! Which means it only draws 220mA at the outlet.
That's all good news.
That out of the way, here are my findings with a 1KHz 240mVp-p sine wave injected directly into the power amp (bypassing the preamp board for now, which has its own problems), and the master volume turned up to about 3 or 4 (hard to tell as it's not mounted) we get a 5Vp-p sine wave at the output at full power. Actually, we get that even at around half power, where it starts to drop off as I turn the variac down.
Line levels are about 1.4 V pp for consumer products. See:
https://en.wikipedia.org/wiki/Line_level
1 kHz is good.
Increase the gain until the amplifier clips. Then reduce it slightly and look at the +-symmetry.
All we really care about at this point is amplification and symmetry and no excess power draw.
At full power, there is detectable crossover distortion, but pretty tiny--might not see it if you're not looking for it. Around half power the output wave begins to lose amplitude and the crossover distortion grows more severe as we get down to 1/4 power, 20% power, etc.
Good.
At 10% power the amp starts singing Elvis Presley tunes. Not sure what that's about.
Not sure what this means.
Other finding: Sometimes I had trouble getting the signal to appear at the output. I had to reset the function generator a couple times and/or disconnect the signal wire and reconnect. I remembered you had said to the inject the signal as I was turning up the variac, so maybe this is normal? Next time I run tests I'll have a 2nd probe at the input to see where the signal might be getting clamped down, if it is.
250 mV is tiny anyway.
No other distortion of the sine wave as I was turning it up.
Q15/Q6 etc is actually the VI limiter.
Initial thoughts:
* Bias regulator is messed up based on earlier tests.
* Get it just below clipping. It should almost reach the power rails. i.e. A few volts less.
* Take a peak at the -15 V supply with a signal applied.
* Take a peak at the DC output voltage (speaker terminals) with a signal applied.
Symmetry. One you have a nice sine wave at close to the rails, use GND reference and check symmetry.
Comments:
Not sure what oscilloscope probes your using, if any.
Crash course in SCOPE PROBES
Standard oscilloscope input is around 1 M || 22 pf. There are 50 ohm input scopes and some that are switchable.
There are other capacitance values.
Why capacitance?
The scope probe (e.g. (1x/10x/gnd) has a variable capacitor in it. in 1x, the likely is covered or non-adjustable. The probe has 1 M input Z at this point.
In 10x the variable capacitor comes into play. It tries to make the probe cable purely resistive and compensates for the cable capacitance.
Compensation:
Is using the square wave calibrator output on the scope and adjusting the capacitor, until you get a "square wave". Fourier theory says that the square wave can be constructed of the sum of an infinite number of odd harmonic + fundamental sine wave. The probe must be >1X to work.
Mentally, you generally have to take the 0.1 V/div and multiply it by 10 and realize it's 1 V/div in 10X
The input impedance of the scope with probe using the 10x setting (variable or fixed) is 10 M. I think it's 10 m for all other attenuations like 100x etc.
So, when selecting a scope probe bandwidth is most of the cost. A 400 MHz probe on a 10 MHz scope is pointless and MAY NOT work. The amount of capacitance that the scope can compensate for is also required.
The compensation adjustments can be at the input connector or probe. You can get 10x, 100x and 500x and other fixed attenuations. Tektronix and I believe HP had a tab on their scope probes that would change the scale.
On a 3-prong plug scope, the ground clip is not your friend. It's connected to earth ground.