Help with fixing hum in Rocktron Velocity 150 amp

[GuitarPlayer]

So, I've started dabbling in basic electronics repair, and one of my recent projects was my Rocktron Velocity 150 guitar amplifier.    Someone on this forum previously linked to these schematics of the Velocity 300 which is nearly identical to the 150, only with a beefier power section & cooling fan.

http://www.audunmelbye.no/wp-content/uploads/2011/04/rocktron-velocity-300-main-schematic.gif
http://www.audunmelbye.no/wp-content/uploads/2011/04/rocktron-velocity-300-power-section.gif

One of the op-amps had gone bad causing the volume & tone controls to react very noisily with even minor adjustments.  I fixed that, and while I was in there, I went ahead and replaced all the TL072 opamps in the main signal path with lower-noise NE5532As since the amp had always had a low level hiss I was hoping to eliminate.

To cut to the chase:  the amp is now working mostly fine, but while the hiss is gone, I've now got a persistent hum in both channels.  The hum is there even when the volume is all the way down, though it does get a bit louder as I turn the volume controls up.  The hum is easily audible even with my poor hearing, so it's definitely well beyond expectation!

I heard this kind of problem could be caused by bad filter caps, so I bought an ESR meter and tested them, but they all tested perfectly OK.

So, now I have no idea what to look for next and am hoping for advice. This is a learning project for me, so I don't mind investing more in this project than I will get out of it!

thanks in advance.

[J M Fahey]

Besides measuring cap ESR (which is fine) , measure the actual ripple voltage at each rail, both high voltage and both preamp ones.
Beware that cheap multimeters (typically only 2 or 3 AC scales, such as 700V and 200V, *maybe* a 20V one) confuse DC present with AC being measured, and might show, say, impossible 60V or higher ripple.
A good multimeter (which start low, such as 200mVAC/2V/etc.) has no such problems.

[GuitarPlayer]

Thanks for the response J.M.!  My multimeter has multiple bands so I think I'm good there.  I'm glad to hear that's all I need as I was worried I'd need a scope for measuring ripple.   Now for some dumb questions if you don't mind: Can you clarify what voltage I would expect to measure if things are working as expected?  I take it the values would differ for power vs. preamp stages?

[gearhead63]

Actually, if you want to accurately measure the ripple in the power supply you will need an o'scope. Your multimeter will not accurately measure it!
I checked the filter caps in my VTX, in circuit. There are 2 parallel sets of 2 in series across the neg & pos rails, and without unsoldering one end, they showed to be ok with a Huntron Tracker. But one of the caps on the +15v rail was bad! And you have the same kind of setup that your power supply schematic is showing. C42,through C67 are all interconnected between the pos & neg voltage rails and ground.You may not be getting a proper reading unless you unsolder one side of each cap before checking it.In fact after looking at the main schematic, that amp has quite a few "bypass"/filter caps in it that go to ground. Each opamp power pin has a .1uF cap. One of those could possibly be a problem. Did your amp have the hum before you replaced op amps?

[GuitarPlayer]

Hi gearhead, thanks for the reply.     I did check all the caps with the ESR meter, though only in circuit.  The board is very difficult to desolder unfortunately.   Indeed there are a ton of those bypass caps. 

As far as I recall the amp did NOT have the hum before the op amp swap, but the preamp was so unstable before that, it may have been hard to tell.

[Roly]

Each opamp power pin has a .1uF cap. One of those could possibly be a problem.

Not 50/60Hz hum.  These are high frequency bypasses.  The preamp supplies are +/-18V regulated, and if these are correct there is little chance any hum is getting in via this path ('course if they aren't correct, there's a clue...)

Similarly with the main +/-50V supplies which should be close to this voltage and very similar.

[GuitarPlayer]

Rail voltage was +/- 48VDC for power section, +/- 17.6VDC for preamp which I assume is within tolerance.  I did not try to see if I can get any measurable AC ripple yet.

[g1]

  If the ripple is so low that a good meter will not accurately measure it, then it is not a concern. 
Yes, it is nice to be able to see it on a scope, but if there really is a problem, it should show up on the meter as AC volts.

[DrGonz78]

Most times I am trying to measure AC ripple it confuses the meter. I remember Enzo saying to put cap in series to the meter to get a more accurate reading. That would be like a coupling cap that blocks the DC on the test point but allows you to read the AC. I can't remember what thread that was part of on this site but I remember reading it.

[J M Fahey]

You should have no ripple in the prea,mp rails (+/-15V or thereabouts), meaning, say, <100mV ripple.
On +/-50V rails you might have less than 1V ripple when idle and probably much lower, say 100 o 200mV, and up to 5 or 7V ripple with the amp driving 150W into a load.

[Roly]

Most times I am trying to measure AC ripple it confuses the meter. I remember Enzo saying to put cap in series to the meter to get a more accurate reading. That would be like a coupling cap that blocks the DC on the test point but allows you to read the AC. I can't remember what thread that was part of on this site but I remember reading it.

It's a reasonable assumption that a meter would only respond to AC on its AC ranges, reasonable but wrong.

Most inexpensive meters, moving coil or digital, will also respond to DC on their AC ranges and produce wildly inaccurate readings.  Until I started using an "AC millivoltmeter" (which goes up to 200VAC!) and specifically responds to AC only I kept a 1uF greencap handy to block DC when I needed to read AC with DC present.  Many older moving coil meters had an "Output" socket which included such a cap internal to the meter.  Modern up-market DMM's have DC-isolated AC measurements, but you really have to know exactly what your particular DMM does internally.

[J M Fahey]

Yes.
Most cheap digital multimeters read only DC , no matter what they say.
And to "read AC"  they still use the DC meter, just with a 1N4007 diode in series, and multiply reading by 2.2X , the ratio between RMS voltage and the average value (the DC component) of a half wave rectified sinewave .
It works, sort of (and costs 5 cents, the bulk price of a 1N4007) , to measure "AC only"  voltage, such as a wall outlet or a transformer tap, but any DC present will be multiplied by 2.2 and displayed as "real AC present", so , say, a 50V DC voltage (your +V rail) with , say, 5V ripple present, will show a whopping 50x2.2=110VAC ripple !!!!!!
Which of course is impossible, worst case would be 35VAC ripple, the winding tap AC voltage if the filter cap is open.

[GuitarPlayer]

OK, well I'll give it a try this weekend, with and without a coupling cap!