Measuring output power

[g1]

So did Gerald cut off that video when he realized he was going to have to say it was a 3 watt amp?

Bernie: if I'm only wanting to check power, going into FX return (or power amp in) means I don't have to worry about clipping any preamp stages, or having to fiddle with the tone controls to keep the sinewave sinusoidal.
JM explained about the 2:1 power ratio not working out in real terms due to supply loading. I think it may be more like 2/3 power when doubling impedance (going from minimum rated load to double).
This VS100 is designed to be able to handle a second 8 ohm speaker in the second spkr jack for a total of 4 ohms.

[nashvillebill]

Is there any reason you're dead set on using the speaker instead of buying a non-inductive 8 ohm dummy load? $14 from Parts Express. 8 Ohm 100W Non-Inductive Dummy Load Resistor

[bsco]

Ok guys....I am using a dummy load As the amp came with an 8 ohm factory speaker, I am using an 8 ohm dummy load.... Just tried to go into the effects return......there is an effects mix pot which has to be set fully cw.....

scope set for 5V/div....35Vp-p......when the signal gen is turned up to the point of clipping, the top peaks flatten out and the bottom peaks are ok......the DMM reads 13.9VAC.....am I having a power supply problem here I wonder.....I will have to remove the chassis again to do some power supply measurements.....


I did originally find two 100uf 35V caps down 50% and replaced them and added a couple of heat sinks to the two low voltage regulators....and replaced the tube....when the tube was replaced the volume came up....but apparently not enough so something is not right.....


I am going to go downstairs again and pull the chassis.......be back shortly....ok back.....
plus and minus 39VDC on the power rails.........maybe I am doing something wrong....I will go through this whole procedure tomorrow afternoon as I have a few other things to get out the door...... it is midnight here so I am going to give this up for tonight........anyway, thanks for all the info and help so far....I saw that video before....very funny.....
Cheers

[J M Fahey]

when the signal gen is turned up to the point of clipping, the top peaks flatten out and the bottom peaks are ok.

Recheck that the signal reaching power amp input (meaning the capacitor which feeds the first differential transistor base) is still clean.

Since you have that lack of symmetry also check both rails under load, one of them might sag appreciably more than the other.

[bsco]

Thanks JM...Will do that tomorrow...
Cheers,
Bernie

[Jazz P Bass]

Marshall VS100R Schematic

Here is the schematic for those who want to join in.

[bsco]

Ok. Here is what I found......when no signal is applied, the voltage rails are plus and minus 39VDC.....when I connect a signal and run it to just before bottom peak of sine wave starts to oscillate, the voltage on both rails drops to 31VDC approx...The waveform at C2 still has a flat top to the sinewave when the signal is pushed....backing off the signal level will produce a clean peak at the top but the level is reduced....double checked the sinewave going into the amp at the return jack and it is fine...but at both sides of C2 the top of the peak is flattened......there is no voltage sag in one rail compared to the other.....they both decrease by the same amount......I did check the filter caps for capacitance and esr...checked good.....so I am going to add a couple more caps to the 2200uf that are there just as a test to see if the filtering improves....and if somebody has any other ideas they are most welcome.....
Cheers....

[g1]

Pull that mute fet and re-check

[bsco]

Pull that mute fet and re-check

You are referring to T16??? I will do that and let you know shortly.....
Thanks

[g1]

Yes. If that doesn't do it, then it's off to the Daughter PCB.

[bsco]

Yes. If that doesn't do it, then it's off to the Daughter PCB.

OK.....it actually made the waveform worse...the bottom peak is now clipped as well but not as bad as the top....are you referring to the board that has the mix and reverb controls??

[g1]

Don't know, it would make too much sense to label the pot functions on the schematic.
But yes, it has 2 pots on it and a level switch for FX loop.
Look at the signal at pins 2 & 4 of the 13pin connector.

Daughter PCB is the name of the schematic page.

[bsco]

Don't know, it would make too much sense to label the pot functions on the schematic.
But yes, it has 2 pots on it and a level switch for FX loop.
Look at the signal at pins 2 & 4 of the 13pin connector.

Daughter PCB is the name of the schematic page.

OK. Will do and let you know.....I'll re-install the mute fet as well.....
Cheers
P.S. Yeah.....it would cost Marshall a ton of money in ink and labour to actually produce a clearly labeled schematic.......(this one has three pots-two for reverb and one for effects mix).....I'll check that out shortly......

[bsco]

Ok...Found IC1 leaky..TL071CP...installed socket and installed a TL071CN....that is what I happen to have on hand.....Now the waveform is much cleaner at 1Khz...power supply voltages still down to 31V on both rails at max volume....at minimum volume they are 39V on both rails........however....but only about 12vAC on the DMM....25Vp-p on the scope....however, if I lower the freq, the AC voltage out goes up....I am using an 8 ohm dummy load....I did hook up an 8 ohm speaker as well...when you get to say 1 or 2 o'clock on the controls, I find the amp loud......but at half volume It does seem kind of low...at least to me it does.........any thoughts anyone???.....

[g1]

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.