Fender Passport 250 hum issue - when nothing is plugged in

[Performer]

Hi folks, I have been searching for a post or thread to do with a problem I'm having with this old Fender Passport (250) I have acquired recently.

Basically, the thing works well, but when it's turned on with nothing plugged in, (just the speakers), there'a a hum, which sounds to me like a ground issue, but it's not, at least it's not external to the unit.

I have confirmed a good grounded outlet for starters, no other audio gear connected to the outlet has the issue, so I am guessing it is internal to the Passport.

After reading about how nighmarish these units are to work, on, before I open it up, I thought I'd ask if there is a known issue or a typical failure, such as maybe a PCB mounted jack or pot lifts from the board etc, that might cause this thing to pick up 120 hz hum (I'm in Canada btw 120V-60hz). I'm expecting to go in and have a general look for anything not connected, but it sounds like when you plug in a 1/4" phone plug & touch the end with your hand, only not quite as bad as that.

All in all, the unit makes for a great little practice space PA, but this hum is making me crazy!! If I can nuke the hum, I'd love this little thing.

Any advice from the experienced Passport folks (who all seem to hate these things) for the uninitiated before diving in, would be greatly appreciated.

Btw, I am familiar with doing some minor repairs to various electronics, soldering and resoldering cold solder joints etc, but my diag skills are limited to basics.

[Jazz P Bass]

First thing that I would check for is unwanted DC volts on the speaker.
ie: blown output stage.
Next check would be the power supply.
Are the supplies loaded down.
Is there any extreme AC volts ripple on any of them.

[Enzo]

Isolate the problem.

Does turning the master volume controls to zero kill the hum?

Do ANY of the controls affect the sound of the hum in ANY way?

We don't like them because they are totally inconvenient to work on. And some guys don;t like working on switchmode power supplies, I don't mind that myself.

[52 Bill]

Enzo is right, isolate the noise to track it down. Also try and separate the power amp from the preamp by plugging into the break/FX loop jacks. If the hum stops when the signal is path is broken from the preamp to the power amp, then the hum is coming from the preamp. If it continues, then the problem is in the power amp.

[Enzo]

And does the hum come out both sides or just one speaker?

[Performer]

Thanks for the replies, greatly appreciated.

So far, I have only had time to confirm the following:

The hum is there with nothing plugged in except the speakers and it comes through both sides, even with the mode switched between stereo and monitor/main.

It does change in volume when the master volume controls are changed and it goes away completely when they are turned all the way down.

With the main volmes up a bit, the hum is not affected by the input volume levels of any channels, just the typical channel noise increases as you turn up those input levels all the way up, but that additional noise is insignificant.

The hum does not change when plugging in a stereo cable to the aux send/return, but that cable was not plugged in to anything , ie; it was a broken loop.. I should also mention that I have tried powering the unit from various outlets, all which were tested with one of those little 3 prong outlet testers, but each showed a good ground and proper polarity (we are fairly new to the house, so you never know).

At this point, it seems to me that the problem is with the power amp or the connection to it, or something along those lines, but I could be way off base.

Aside from this hum, the unit works perfectly and has all the volume I expect it should have from both channels, it just sounds like something is ungrounded inside the thing.

Also, I'm not sure I know how to check for DC voltage at the speaker or how to differentiate it from the ac voltage from the amp. I am guessing it's not as obvious as using a DMM on DC volts...

As for the power supply, well, that will have to be when I open the unit up, which time has not yet permitted.

Thanks again for the help!!

[52 Bill]

It does change in volume when the master volume controls are changed and it goes away completely when they are turned all the way down.

Then the problem is in the preamp section, somewhere before the Master Volume controls.

Try turning down all of the controls except the masters, is the hum still there? Try and find out if the hum can be localized to one specific set of controls.

[Performer]

When all the controls are turned down, except the master volume controls, the noise is still there.

Basically, if the master volumes are up, the noise is there, no matter what else I do to the other controls.

Cheers.

[52 Bill]

Now you need to find out if the problem is mechanical, like a loose connection or electronic, like a bad cap or chip.

Does plugging into any of the front panel jacks cause the hum to change? If you give the amp a thump on the front panel or along the sides of the case, does it change?

[Performer]

I can confirm that plugging something in to any of the jacks does not affect the hum/buzz, nor does giving it a few smacks from the front and sides.

Looks like I'll have to open it up to see if I can find another mechanical issue, then go from there, but that will probably be a few days before I get to it.

Thanks for the suggestions!

Cheers.

Now you need to find out if the problem is mechanical, like a loose connection or electronic, like a bad cap or chip.

Does plugging into any of the front panel jacks cause the hum to change? If you give the amp a thump on the front panel or along the sides of the case, does it change?

[Jazz P Bass]

Once open, check the power supply.
The +15 & -15 should have no volts ac riding on them.

[DheudE]

I have one of these passports as well.
The issue is actually the digital reverb.

[tboy]

The issue is actually the digital reverb.

Please elaborate.