Blues Junior Reverb Hum

[woodrow f call]

So, I got a good deal on a USA made Blues Junior that wasn't working. I got most of this amp sorted out. New reverb tank, killed some noise due to cracked solder joints, new switchcraft input jack, hooked up the speaker wire.

Now the only thing I have left is some hum when I turn up the reverb. I've looked at the solder joints, reflowed the RCA cable joints on the board. It sounds like 60 cycle hum. I move the reverb tank closer to the speaker baffle and get less noise. I tried vertical, but it gets worse. I moved closer to the back of the amp and it gets worse.

Anything else I should look at? The noise isn't terrible until reverb is past 4 (which is just too much reverb anyways), but I'd like to get this dialed in as best as I can.

[martin manning]

Did you try flipping the tank end-for end? The return transducer can pick up 60Hz hum if it's close to the power transformer.

[woodrow f call]

Did you try flipping the tank end-for end? The return transducer can pick up 60Hz hum if it's close to the power transformer.

Yes, I did. It surprisingly didn't change.

[maxkracht]

Could try new rca cables, the ones that came with it are not great. Might also double check that you got the right tank?

[Stevem]

Check to make sure that the case of your new tank is at ground potential like it should be as should the shielding of your reverb return cable.

[woodrow f call]

Well, there was a problem with the cables. Reverb stopped working..... Removed, stripped and reinstalled, reverb works. Hum seems worse, re-did what I just did. No change.

There is a pronounced sound in the background of this hum.... Almost like a cap charging and discharging maybe? Again, only with the reverb turned up. Everything goes away when reverb is on zero.

I jumpered the capacitor going to ground on the input shield wire wondering if it was bad (using the same value cap). No change.

The return shield goes straight to ground. Continuity checks good.

I reflowed the reverb pot solder. No change.

It's starting to look like I'm going to need an oscilloscope.

[nuke]

Check that the replacement tank is correct, there are many variants. You should have an isolated input socket, and a grounded output socket. 8EB2C1B is the number that should be on the tank, at least for most of production. There are some number decoders for reverb tanks online to see what you have in hand. Make sure you have the cables arranged properly as the "to/from" naming sometimes gets confusing. (we've all done it)

Then try this: pull the two RCA plugs from the pan, clip a jumper on each so that the pin and the ring an each plug is shorted together. Not to anything else, nor to the other plug. That should produce no hum when you turn up the reverb.

Check the +15 and -15 power supplies. If one of them is off voltage, the reverb will do wonky stuff. Might be the chip, but of course, whittle your way down to it.

The usual source of hum in the FBJ are the ribbon cables. The two cables that run to the EL84 tubes should get pushed down toward the chassis and the you'll have to experiment with the ribbon for V3 (12AX7 nearest the output tubes) to position it for lowest hum.

[Stevem]

The part mentioned in the above post about the 16volt + and - supply is a very important thing to check and it jarred my memory.
I once had this hum issue with one of these amps with the reverb humming and one of the 4 filters in that reverb chips supply section was well on the way to completely failing .

If the DC voltage on that chip is not balanced enough then one of the results is 120 hz hum.

Compare what you have for AC ripple voltage on each leg of the supply to that reverb chip.

Also each leg of that supply has a zener diode to regulate that + or - voltage and either being bad will make for a imbalance in the voltage.

[woodrow f call]

I do have a the correct tank. Got a different project to work today, but will check these items.

I ordered probes and a signal generator so I can use the Tektronics 485 I have.

[Stevem]

If you have clip leads and another amp then pump the signal from the send rca into that other amp, does that amp hum?

Another simple test with a clip lead is to short across the return rca cable .
With that shorted turn up the reverb pot, if you still get a hum then your issue is on the return side of the reverb circuit.

[nuke]

yeah, check the +15 and -15v supplies. They are known to pop a zener diode from time to time and lose regulation. That will very likely cause hum, and probably pop the op-amp chip as well.

[lonote]

The usual source of hum in the FBJ are the ribbon cables. The two cables that run to the EL84 tubes should get pushed down toward the chassis and the you'll have to experiment with the ribbon for V3 (12AX7 nearest the output tubes) to position it for lowest hum.

I had to replace the tube board in my BLJr, & while I was at it, I replaced all the ribbon cables with separate conductors & used shielded on the grid wires. Not as bad a job as some make it out to be.

Very quiet now.

BLJR Replacement Tube Board.JPEG

[woodrow f call]

Ok, I shorted the RCA cables as directed. As predicted there was no noise.
Checked the voltages on the IC pins (4 and 8 ).... One read 15.01VDC and the other was 15.00VDC.
I did not see any AC using AC or mV settings on my Fluke 117.

[woodrow f call]

Here is a video of the hum.

Can someone verify if it is 60hz or 120hz?

https://youtube.com/shorts/EPHGWbICn-s? ... ROZcLA0c9_

[martin manning]

I think I hear both the low hum of 60 Hz and the buzz of 120 Hz. My tuner seems to agree, it says F#3, 180 Hz, which is 60 + 120.