Troubleshooting Blues Junior Reverb (Update question on Hum, see OP)

[Woodrow F Call]

Ok, I got most of this amp sorted out.  New Tank, killed some noise due to cracked solder joints, new 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. 




Original Post Below:
Hey everyone. I got a good deal on a USA made Blues Junior that wasn't working.  Input jack was smashed and a speaker wire was not hooked up.  Easy peasy.  I put in a switchcraft jack and started testing it.  No reverb.

I tap the tank.... I get the springy sound you'd expect. Looking at the schematic (attached), I think this tells me the problem is upstream of the reverb tank.  I check for voltages at the test points TP21 and TP22.  The note says values may vary with the tank. I get 20mV at TP21 and 0.0mV at TP22.

This leads me to think.... solder joint on one of the components or the chip is bad. 

Am I missing something? 

BTW, I don't have an oscilloscope to check other stuff.  I need to get one just haven't gotten there yet.

I appreciate the help! 


EDIT/UPDATE: I get infinite resistance on the input coil of the reverb tank.....  I guess that's probably it?

[tubeswell]

Try a new reverb pan

[SEL49]

I check for voltages at the test points TP21 and TP22.  The note says values may vary with the tank. I get 20mV at TP21 and 0.0mV at TP22.

Voltages do not vary with the tank. Read note 2 again. You will not be looking for dcv at TP21 and TP22.

EDIT/UPDATE: I get infinite resistance on the input coil of the reverb tank.....  I guess that's probably it?

Good suspect.

[Woodrow F Call]

Try a new reverb pan

Well, I just tried the one in my other tank... yup. Bad tank.

[Woodrow F Call]

I check for voltages at the test points TP21 and TP22.  The note says values may vary with the tank. I get 20mV at TP21 and 0.0mV at TP22.

Voltages do not vary with the tank. Read note 2 again. You will not be looking for dcv at TP21 and TP22.

EDIT/UPDATE: I get infinite resistance on the input coil of the reverb tank.....  I guess that's probably it?

Good suspect.

When I wrote that, I was meaning from tank to tank... is says from pan to pan.  I figured that's why my input was 20mV and not 27mV per the drawing.  I think my meter was set correctly. 

[SEL49]

When I wrote that, I was meaning from tank to tank... is says from pan to pan.  I figured that's why my input was 20mV and not 27mV per the drawing.  I think my meter was set correctly.

Did you have a signal generator set to the proper level/frequency and connected to J1?

[Woodrow F Call]

I don't have a signal generator or a O-scope (well, one with the leads anyway).

I need to fix that. 

[SEL49]

I don't have a signal generator or a O-scope (well, one with the leads anyway).

All those test points with AC voltages (except the 4 test points in the power supply) require you to connect a signal generator set for 10mVAC @ 1KHz to J1. Those test point voltages are meaningless without the sig gen.

[Woodrow F Call]

Thanks!

Update Question in OP.

[stratomaster]

Rotate the tank 180 degrees in place.  I often find jacks facing baffle to be quieter.

[Woodrow F Call]

Click the link to hear the hum. Can anyone verify if this is 60hz or 120hz?

I have tried a few other things:
Tank is correct.
I've moved the tank around, vertical even. No real change.
Ok, I shorted the RCA cables tip to sleeve. There was no noise.
Checked the power supply 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 for the IC chip power supply.