120 Hz Hum in Solid State Amp

[Fred2202]

My nephew brought me his ailing ASI solid state guitar amp to take a look at. It is a solid state Q-tube 70 guitar amp probably from the 1980's. It has a single 12AX7 in the preamp (along with a bunch of op amps) and a Mosfet power section. It has a low level hum that sounds like 120 hz AC leakage. The volume control does not seem to effect the hum level, so I suspected the large electrolytic caps in the power section were shot due to age. The amp is laid out on about four separate circuit boards so I disconnected the preamp board from the power amp and the hum remains.
I replaced every large and small electrolytic cap on the power amp, and for good measure replaced the bridge rectifier and some small IN4007 type silicone diodes. Tried the amp out and the low level hum is still there. I went back in and reflowed all of the solder joints and checked the grounds and they looked good. Checked all the power resistors around the power transistors and the readings look good. After all this the amp is still humming.
At this point I have run out of ideas as to what to check next. I do not have a schematic for this amp and I have not fooled around with transistor amps very much. The amp seems to have plenty of power (I think it is about 70 watts and is loud as hell) and sounds good (for a solid state amp) except for the low level hum.
There are some small non-polar caps on the power amp board that may be coupling caps... I was going to try replacing them next.
Could a bad output or driver transistor cause this type of hum?? Any suggestions for troubleshooting would be much appreciated.

[Fred2202]

120 Hz Hum in Solid State Amp

Another thing I forgot to mention in the post above is, this amp makes a strange sound when once power is shut off. If makes a putt putt sound that fades off like a capacitor or something is discharging. It does not do it when the amp is turned on.

[Enzo]

Bad transistors don't make hum as a rule, and the power off oscillation is nothing.

It could just be weak design, you know. I'd be looking for a ground return somewhere not making good earth. Power supply ripple currents sharing copper with signal grounds.

[Mars Amp Repair]

The ususal questions apply,
does the hum change at all with the volume or any of the controls up or down? That might help to isolate the stage the hum is being introduced...if it is a ground loop as Enzo mentioned, then things begin to get a bit difficult to trace.

It may also be possible the the power stage is oscillating at an inaudible frequency & loading the power supply to create the hum. However if this were the case, the power stage would most likely be getting verrrrry hot.

If you have one, hang a scope on the speaker output leads (ground your scope to the chassis, not the negative speaker lead!) & see if you have an ultrasonic frequency there.

Those problem can sometimes be real buggers, too. glen

[Fred2202]

Thanks very much for the replies. The hum stays at pretty much the same volume level regardless of where the volume knob is set. That is why I suspected bad power supply caps. There is only one ground off the power section and power amp circuit boards to the chassis. I probably could separate the ground off of the power supply section to see if that will help.
I did change out a bad output jack to the speaker. Put in a non-insulating type that grounded to the chassis and the amp squealed like a pig. I changed it back to an isolated type like the original jack and that solved that problem.
I do not have a scope...but I did turn the amp on, hooked up a cd player and let it run on 1/3 volume for about three hours. All of the components including the transformer were only moderately warm to the touch.
The amp did work flawlessly up until a few months ago. That is why I thought there is a bad component somewhere in the power amp section. All the caps and rectifiers/diodes in the power supply are new at this point. I will take another look at the ground connections and see if I can separate the ground from the power supply section and power amp section and put connect them to different points on the chassis.

[lundee]

Is the amplifier noise you are experiencing actually 60 cycle hum?

"Humming noises can be caused by several things. First, it helps to know if the noise is 60 or 120 cycle hum. A very low frequency hum indicates 60 cycle hum. 120 cycle hum sounds more like a buzz than a hum. 60 cycle hum can be very difficult to find and correct. Grounds are usually the culprit. Check all the grounds on the amp. Check the grounds for the power transformer. They are soldered to the chassis next to the transformer on most amps. Having all the grounds for the whole amp soldered to the grounded lugs next to the power transformer can help the hum problem, but this is usually not required. Find all the grid and cathode wires and lift them up away from the metal chassis..."

Quoted source: www.hoffmanamps.com

[Gtr_tech]

I'd check the 15v rails to the op amps. You may have a leaky zener regulator dragging one rail down, or possibly a bad op amp.

[Fred2202]

Wow I did not expect replies to this old thread. Thank you for the replies. I did a little more work on this amp since the last post but right now it is still sitting on the shelf waiting for me to get back to it. The hum is 120 HZ as stated in the first post. I can tell the raspy 120Hz type hum from the deeper 60Hz hum. Grounds are good as I have quadruple checked them There are only a couple of grounds coming off the circuit boards anyways. There is one preamp board, one power amp board, and one board containing the rectifier bridges and large electrolytic caps. I completely rebuilt the board containing the large E-caps and replaced the diodes on the rectifier bridges but this did not help. With the 120 HZ hum I thought a simple cap job would fix this one.
Since this is a solid state amp everything is on circuit boards so I really cannot move much in the way of wires around. It is all modular plug ins with short leads. I replaced all the IN4007 type diodes in a bridge type arrangement thinking I had a bad one that was not rectifying correctly...no change. I isolated the problem to the power amplifier board as I can disconnect the preamp board completely and the noise is still there. The power amp board has some driver transistors, zener diodes, small film caps, and two power transistors. I was thinking about replacing the zener diodes as there are about 6 or 8 of them on the power amp board and they are fairly cheap. They are tiny and I will have to break out the magnifying glass to read the numbers on them. IF one of these zeners is leaky could it cause an imbalance that could allow a 120HZ hum to not cancel out in the power amp circuit?
I am more of a tube circuit guy but I guess many of the same principles apply to solid state.

[booj]

Ever get anywhere with that Q-Tube 70? I have one here with the same symptoms. Puts out about 1.5 - 2 volts 120hz AC to the output with a DC offset of about .5 volts.
Everything looks good on the output board.

[Fred2202]

I did finally fix this about 4 months ago. I resoldered and replaced the wires on the grounds and replaced some of the little diodes (I think they were 1/4 or 1/2 watt zeners) on the power amp board. I checked them all with the diode setting on my VOM and two of them did not read correctly. Had to get out a magnifying glass to identify them. This did the trick and the hum stopped. I previously replaced the big electrolytic caps, the diode bridge, and some of the 1n4007 diodes in the power supply. This initial parts replacement did not stop the hum. Anyways the amp is hum free now and sounds good. It still makes a crazy putt putt sound like a outboard motor when you turn the power switch off.
As far as your amp goes....try disconnecting the preamp from the power amp. Turn it on and see if it still hums. If the hum goes away after you disconnect the preamp then the problem is in the preamp board. If the hum is still present then the problem is in the power supply or the power amp board. Check the grounds off the boards and make sure they are good. If they are not good the amp will hum like crazy or squeal like a pig depending upon which ground is bad.

[booj]

Thanks for the reply! I looked at it again and resoldered the connections to the output board and the leads of the output components including what seemed like a loose .22 ohm 5 watt resistor as well as the power supply bridge rectifier. Turned it on and it works!
Just needed a little kick in the right direction!

[McgillicuttyMAN]

I just found your thread while I was researching a way to fix the hum on an old combo Q-tube 70 I have laying around. Thank you for your posts I think they will be very helpful. Im messaging you bc the amp also has a significantly higher volume on the clean channel. When switched to gain the output drops. Does yours/nephews do this? is it normal? It is cool to hear you got it to clean up!

[Elderlyemo]

It's a really old thread but I\ll leave my 2 cents. Managed to restore an old Peavey age humming at 120 Hz (hum only, no input sound, hum is consistent in volume regardless of the volume knob) by checking and replacing a couple of zener diodes 1N4148 and rectifier diodes 1N4003. Ground was fine, I checked it with a separate wire connected to the mains ground, to eliminate possible original ground wire failure. Before that I had a similar hum issue with a Peavey Backstage but that one at least reacted to the volume knob and it turned out to be bad filter caps (2200 ones) which I simply replaced. Anyway, thanks again for the tilt in the right direction!

[Mars Amp Repair]

Yes, that is going back a ways! It's what I so appreciate about this forum. There's a plethora of valuable information going back for all to be able to access. I appreciate your addition to the thread, as these Peaveys last forever and still are viable choices for players to own.
It's particularly nice to see the entries that Enzo contributed throughout. He was a very talented and gracious contributor. Glen