Measuring output power

[bsco]

Can you inject your signal straight in to the R1/C2 junction in the power amp?
If you can get good power out, then you still have an issue on the Daughter board.

How come the discrepancy between your scope and meter? 25Vp-p is not 12VAC rms.

I don't know why I am getting different readings......I double checked everything......and everything seems to be working.....I did notice something....

I placed my hand on the individual heat sinks and the heat sink for T11 is not as hot as T9 and T10....The heat sink for T12 is not as hot as T11.....I did some in circuit resistance measurements....I was not looking for exact measurements because there are lots of components connected together in various loops...

so I started to look for a difference in readings between the different resistors in the section of the output transistors....R16 and R17 measure 37.9 ohms in circuit...I realize that they are 120 ohm resistors...but if something in the two loops were out of wack, then there would be a difference of a few ohms when doing a resistance check....both read exactly the same....

R22 and R23 are 150 ohm resistors....when measured in circuit both read exactly 43.2 ohms in circuit...so everything in those loops seem to be balanced to each other.....

R18 and R20 are 100 ohm resistors... however, in circuit, R18 measures 37.8 ohms and R20 measures 43.1 ohms.....so there seems to be an imbalance here in these two loops....

R18 is in the base/emitter circuit of T7 which is associated with T11 and T12.....
R20 is in the base/emitter circuit of T8 which is associated with T9 AND T10....

This might be a clue.....maybe not....I am going to pull one end of each resistor to see if the values have changed in one of them and if not, then the only thing I can think of is to replaceT7 and T8 with transistors with the same Beta......(I think)....correct me if I am wrong....maybe this slight difference is not a big deal.......as far as the difference in readings between the meter and scope,I have no idea.....yet..
Cheers.....
I checked those resistors....R18 is 99.8 ohms and R20 is 100 ohms....so they are ok....

[TomCarlos]

Hey guys.... I am trying to catch up on this one. BSCO, you mentioned a DMM. What type of DMM are you using? Is that a True RMS meter? It might give you a different reading compared to other meters. Maybe the others can weigh in on this.

When in doubt, make sure you measure a known wave and compare the RMS to PP as shown above.

[bsco]

Hey guys.... I am trying to catch up on this one. BSCO, you mentioned a DMM. What type of DMM are you using? Is that a True RMS meter? It might give you a different reading compared to other meters. Maybe the others can weigh in on this.

When in doubt, make sure you measure a known wave and compare the RMS to PP as shown above.

No Tom...It is not...just a regular average DMM....nothing special.....
@g-one....tried to inject the signal like you suggested.....the signal is completely out of wack....for some reason there is all kinds of weird crap there....completely un-useable....
shen I go thru the efx return jack, the signal is nice and clean....

[g1]

So put your signal in the FX return again. When you get full power (whatever that is right now), is the waveform still good at C2?
And how about your frequency dependent output level, is that also present at C2?

[g1]

One other thing, the speaker output routes through the switch in the phones jack, make sure you have good contact there.

[bsco]

So put your signal in the FX return again. When you get full power (whatever that is right now), is the waveform still good at C2?
And how about your frequency dependent output level, is that also present at C2?

I re-connected the signal back to the efx in jack......the signal is clean and it is also good at C2....I will check that freq dependent output level at C2 as well and let you know.....I will double check the headphone jack......
Cheers

Ok. I checked that out and it is good.....The headphone jack is fine as well.....Will take a look at it again later tomorrow evening.....Thanks for the advice and help so far.....

[J M Fahey]

however, if I lower the freq, the AC voltage out goes up

Mmmmmhhhhh, it stinks of poor Multimeter.

Since you are using a Scope, set it up this way, always DC coupled:
1) look at output waveform, rise it until it just clips.
2) touch the + rail , and raise scope sensitivity until visible horizontal line (which shows V+) just touches the top of the screen scale.
3) check that now the -V rail just touches the bottom of the screen scale.

we are maximizing/optimizing/matching the actual voltages to what can be displayed on screen, for better "visual/optical" precision.

4) reattach scope probe to speaker out.
Waveform peaks will not reach screen top or bottom, the difference is loss/drop across transistors and ballast resistors.

I expect some 4V loss.

If so, search no further, you have what you have.
Visually measure sinewave peak/clipping voltage and calculate RMS , compare that to what Multimeter shows.

If much higher, say 10/12V loss, you have a big problem, that amp can't drive that load (low Hfe?)

If scope measurement is reasonable but multimeter "reads" much less ..... your Multimeter can only measure 50/60 Hz ... period ( a few do).

Just curious, what is your wall voltage?

+/-39V idle looks low to me.

[bsco]

Ok...Thanks Juan.....I'll give it a try later today.....
Cheers

[bsco]

Ok. Been away from this for awhile.......I did as you suggested Juan....There is a "MAJOR" problem here...it is not delivering the correct power out.....it is more like 20W at full volume...probably even less.....You mentioned about the hfe of the output transistors.....some of the heat sinks are not as hot as the others.....I am going to pull the board again and remove those O/P transistors and check them on the transistor tester....I'll let you know what I find......