What does this scope signal look like?

[beasleybodyshop]

Hey everyone, been wracking my brains on a 1976 Ampeg VT40. Gets a really loud hum when i flip off the standby. All voltages check ok, all tubes test ok. Amp has been properly biased, and recapped. Diode checker on the FWB rectifier shows .5v drop across each, which indicates they are ok.

Attached is a picture of the output waveform at the speaker. Does this look like filter caps humming? I try pulling tubes and the hum does disappear after I pull the PI, but I can follow this waveform from all the plates from V1 to the output.

Any clues? Could this be bad grounding?

[beasleybodyshop]

schematic:

[beasleybodyshop]

Hum sounds really close to the B note on the 5th string of my guitar. 120hz?

Ok, so i did some digging on the forum. This looks like a bad filter cap. however, this amp has several, and I have recapped everything in the power supply. I have used my capacitance checker, but they all indicate to be ok, none of them are shorted through.

Can just one bad filter cap cause the noise?

And is there another way to try to identify the bad cap?

[gui_tarzan]

Did you check the effects loop jacks?

[vibratoking]

And is there another way to try to identify the bad cap?

One cap could cause noise. Measuring a cap for shorts with a multimeter won't tell you much. One trick is get a known good cap and clip it in parallel with an existing cap. It's easy to clip on and you will hear a difference if the cap of interest is bad.

[dorrisant]

Also, some bad caps will look just fine on the bench but will fail when a high enough voltage is introduced. I use an old Eico 950 RC Bridge Comparitor. It will test for cap failure up to around 450v.

Tony

[beasleybodyshop]

Ok, so I made a little cap checking tool, thanks to some help on the forum.

I soldered alligator leads onto a .1uf 630v mallory 150 signal cap. I connected one end to my scope, and the other end to each filtering node...the reasoning being that the ripple is still AC right? if there is no ripple, it wouldnt show up on my scope.

so i do this, clip it into all the caps, and when i get to the 100uf/100uf connected in series, i see this almost sawtooth like waveform.

I didnt see any other waveforms other than DC on the other caps. looks like this was the culprit.

[tele_player]

Why did you use a cap for this? All scopes I've used, which is many, have a switch for AC or DC input coupling.

Robert

[Structo]

Generally speaking, I believe when the hum is 60Hz it is the heater circuit.
Could be lead dress and or faulty grounding.

If it is 120Hz hum, it is the power supply and filters.

Can you post your voltages?

[LOUDthud]

The waveform looks backwards. It's not the typical hum you see on a supply rail. It doesn't even look like what you would see on the bias supply (a negative rail).

Does the hum stop immediately when the Standby switch is flipped from Play mode to Standby mode or does the hum fade out for a second to two?

[martin manning]

The waveform looks backwards...

Or inverted...

[DonMoose]

If it's the lower cap in a series pair that's bad, would that wave make more sense?

[John_P_WI]

Looking at this on my phone and really can't see the scope display clearly. Time of one wave period looks like 8ms or 120 hz, while the hash looks like roughly 200 us or so = approx 5khz.

Is there a signal injected to the amp? Sure doesn't look balanced and something sure is ringing. Additionally, what is triggering the scope to read 226 khz? That's in switching power supply range??

[beasleybodyshop]

The waveform looks backwards. It's not the typical hum you see on a supply rail. It doesn't even look like what you would see on the bias supply (a negative rail).

Does the hum stop immediately when the Standby switch is flipped from Play mode to Standby mode or does the hum fade out for a second to two?

hum immediately stops when the standby switch is flipped off.

All voltages read within 10 percent of the posted ones in the schematic. The only discrepancy is pin 6 of the PI - should be 104v, its actually around 82v

I went ahead and moved all the ground connections for the filter caps to a single point - waiting for new 100ufs from mouser. The lead dress in this amp is ridiculously awful. straightened that up as well. i don't even see how it worked before.

[beasleybodyshop]

Looking at this on my phone and really can't see the scope display clearly. Time of one wave period looks like 8ms or 120 hz, while the hash looks like roughly 200 us or so = approx 5khz.

Is there a signal injected to the amp? Sure doesn't look balanced and something sure is ringing. Additionally, what is triggering the scope to read 226 khz? That's in switching power supply range??

no signal injected into the amp. this is just what the output reads with it plugged into a dummy load and to my scope.