60 cycle low hum that doesn't change in volume Crate BX50

[dadroadie]

Hi To All,
I was invited to this forum by Enzo from another website. He suggested that you may be able to assist me in trying to troubleshoot one of my student’s amps. It’s a Crate BX50
The issue with the amp is a low volume 60cycle hum (I was able to verify this frequency by running my audio generator through the amp and matching the tone). This hum does not change in volume no matter where the volume is set. Something I did find interesting is that the pitch will change going from low to high and back to low as the master volume is tuned this doesn’t change in volume just a little in the pitch.
So far I have checked the bass and the cables by substitution with no change. Also the hum does come through the headphones and an extension speaker when hooked up so the speaker doesn’t appear to be the issue. I have moved the amp to different AC lines in the house with no change. I also opened the amp up and could not find any bad solder joints. I removed the + and – voltage filter caps (4700uf at 25v and100uf at 35v and tested them with my cap tester and they tested within range. (I unfortunately do not have a scope). The power supply voltages appeared correct (TP4-7).
The amp otherwise works fine and will run for hours without overheating or shutting down. This isn’t a loud hum. It’s just in the background and if playing with others it isn’t heard. He got the amp on Craigslist very inexpensively. I’m not sure but the amp may have been sitting for some time. The jacks were very tarnished so I cleaned then with CAIG along with reseating the internal connectors.
My main concern is reliability. He wants to take it back to college to use. Right now he is using my Peavey. I would hate to have this go south on him during a jazz recital. Also the hum is annoying when during silent passages. I heard that some solid state amps because of the design will always have some noise. I don’t know if this is true and I haven’t heard other Crate amps to compare. My Peavey is very quiet. Any help or suggestions would be appreciated and again thanks to Enzo for the suggestion. I,m not allowed to post the schematic but I do have it if someone can help in getting it uploaded
All the Best
Dad roadie

[Randall]

"This hum does not change in volume no matter where the volume is set."

Are you referring to the Gain control here? Does the Contour switch have any effect? Do the tone controls have any effect?

23701H9_.pdf

[dadroadie]

Hi Randall,
Thanks for the reply and posting the schematic! To answer your question I was referring to the master volume control not the gain.I did check all the controls per your question. I tried the Gain,Contour,Contour Switch,Low,Low Mid,High Mid,High and Master Volume controls in all different combinations on the controls and rotated each control completely during each test. None of them changed the frequency or the volume in any combination. The master volume does lower the volume slightly when its set at its full output. He has been running the controls as follows Gain at 3, Contour switch on and Contour set at -1 (center is flat or 0) Low at +1 ,(center is flat or 0) Mid Low,Mid High and High at 0 and the master set at 6. If I can Answer any other questions just let me know and again thanks!
dad roadie

[g1]

It's still not clear just how much hum we are talking about. Louder than your quiet Peavey does not necessarily mean abnormal. Could well be, but maybe not.
Can you put your meter across the speaker set to measure AC volts. The measurement will tell how much hum is there. If your meter has a mV range for the AC volts setting, that will give the best result.

[dadroadie]

Hi g1,
Just to make sure I follow. I have a Fluke Digital VOM. Set the range to AC MV. Hook the positive (red Lead ) to the +terminal of the speaker and the negative (black lead) to the -terminal of the speaker. turn it on with no guitar plugged in. Should I set the gain and master volume at any specific level? Since its AC I Imagine the polarity of the leads wouldn't matter but that's how I'm seeing that you want it tested. The amp is a sealed cabinet so I'll have to take it apart to do the test. I just want to be sure I'm doing it correctly.
Again thanks
dad roadie

[g1]

Correct. Just use the settings that would give an annoying level of hum during quiet passages. We want to put a number on that.

[dadroadie]

Hi g1,
I went down in advance and took the amp apart to do the test after I heard back. Believe it or not I got only 1.8mv AC. Took it twice to be sure. Doesn't seem like much. To give you an ideal of the volume level.The room is about 10' by 18' and if nothing else is on you can hear the hum softly throughout the room. Maybe this is normal for a Crate unit . I don't have any Crate amps to compare it to. Let me know if there is any other tests you or other members would like me to do. Again thanks for the Help!
Regards
dad roadie

[g1]

That doesn't sound like much. If you still have it apart, I would disconnect one of the speaker wires and power up the amp to see if maybe it is mechanical hum coming from the transformer or something.

[dadroadie]

Hi g1,
Thanks again for the help
The hum is definitely coming out of the speaker.
dad roadie

[nevetslab]

Hi g1,
Thanks again for the help
The hum is definitely coming out of the speaker.
dad roadie

Can you provide some pictures of the insides of the amp? As it is 60Hz Hum, I'd suspect AC Mains field radiation from the power xfmr being picked up by the wiring, or even in the PCB layout, or the AC mains wiring being poorly routed near the input circuitry. Hard to tell without photo images, but we're here to guide you. AND, Welcome to the Forum!

[Enzo]

2mv of hum on a speaker lead does not sound like much to me.

If the master volume has no efect, that pretty much puts the problem in the power amp, which is two TDA2050 ICs.

Look at your schematic, left center near power plug. There is a lit of test points. The main power supplys are the +22v and -22v. Test points TP4, TP5. The schematic says 22v while the chart says 18.5v. DOn't worry about that, anything in the 18-25v range will work fine, but you do want both to be about the same as one another. 18.5v is the DC, but it also shows AC volts of 0.166v p-p. p-p means peak to peak. Set your meter to AC volts and measure there. 0.166v p-p is about 59mv. That is ripple on the supply. SO 58mv is OK by the schematic. Ideally we would have zero ripple, but dream on.

The other side has a different number for some reason, but is 0.195v p-p. WHich is 69mv ripple. SO we are looking for less than 100mv ripple. Are they both at similar levels?

By the way, 60Hz and 120Hz are the same note, just one octave apart, so if you set your generator to 60Hz, a 120Hz signal sill sync with it. It can fool you. Test points 6 and 7 are the 15v rails, make sure they are both at about the same voltage - other than polarity - and they should be clean of any ripple.

[nickb]

2mv of hum on a speaker lead does not sound like much to me.

Quite. 2mv is 0.5uW i.e. 0.5 millionths of a watt! Looking at it another way that's a signal to noise ratio 184dB. That's better than most amps and you'll be hard pressed to improve on it.

If your problem is you need it for low volume bedroom use then, since this has a very low output impedance and no current feedback, a simple resistive attenuator of say 20dB (1/10th) between the speaker and amp will give move the hum to negligible levels.

[dadroadie]

Thanks to all that replied.
Ill go down and check the things suggested. Ill also try to take a photo. One thing I did notice is the sound from the speaker sounds very similar to the sound that a fluorescent light emits . I have one over the workbench. This is always unplugged during the testing as I know they can add all kinds of noise. This sound can also be heard through the headphone jack and the extension speaker jack if that helps .
Again thanks
dad roadie

[dadroadie]

Hi to All,
I went down and took the following readings. Enzo I found TP-4 (21.4VDC 72.6mvAC) TP-5 (21.4VDC 80.6mvAC)
TP-6 (15.9VDC .1mvAC if that) TP-7 (-14.8VDC .1mvAC if that) I have tried to load the photos requested I'm not sure if I did it correctly. .

[nevetslab]

Hi to All,
I went down and took the following readings. Enzo I found TP-4 (21.4VDC 72.6mvAC) TP-5 (21.4VDC 80.6mvAC)
TP-6 (15.9VDC .1mvAC if that) TP-7 (-14.8VDC .1mvAC if that) I have tried to load the photos requested I'm not sure if I did it correctly. .[ATTACH=CONFIG]52946[/ATTACH][ATTACH=CONFIG]52947[/ATTACH]

I cleaned up the two photos so we could see them clearer:



As the power xfmr is at the opposite end of the chassis from the input circuit, and all of the primary/secondary lead dress looks appropriate, and the core is down below the chassis, that kind of leaves it to the field sensitivity of the radiated stray fields from the power xfmr to the circuits layout, I think. I"m guessing the ribbon cable between the PCB and the short vertical board in the middle of the chassis is to the power xstrs/drivers. That seems fine.

I would extract the main PCB from the chassis, after disconnecting the power xfmr primary/secondary leads, and look for bad or cracked solder joints, particularly on all of the control solder terminals, input jack terminals. The packaging looks well built/designed, and the hum you're getting seems higher than what would have left the factory.

And, as Enzo pointed out, it sounds like the hum issue is coming from your power amp circuit, since turning down the Master Volume has no effect. You could also try replacing the two LF corner feedback caps on the two power amp circuits, C23 & C31. Not sure why two different values were selected, as the 2.2uF cap sets a LF corner @ 106Hz, while the other a LF corner at 23.4Hz. I had hum problems on an older Studer power amp in these same locations, replacing those with fresh caps cured the hum. I'd use the same value for both (10uF). Just a thought.