Mojotone 18 watt TMB ground loop issues?

Martinoils

TDPRI Member
Joined
Jul 3, 2014
Posts
57
Location
Tucson, Arizona
Ok, here’s where I am so far. I corrected the soldering and added the jumper from pin 3-8 on V3. Voltage readings are good now within 10-15% of chart.
I get sound and volume but if you tap the chassis or the turret board or wiring you get horrible popping and crackling.
I’m sure the wiring is correct at this point.
Could there be a ground loop or could I have a damaged transformer? I’m just shooting in the dark.
Anyway to test the PT and OT?
Anyone have suggestions?
Much appreciated!
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rdjones

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Nov 6, 2010
Posts
604
Location
Music City, TN
Ground loops don't cause crackling and popping, only hum.
You've tested the power transformer by confirming that "voltage readings are good".
If the noise only occurs during tapping the chassis or board, that seems to indicate that there's an intermittent connection of some sort.

I'd tripple check all connections for poor solder connections.
I'd pull the preamp and PI tubes and run the same tap test to isolate what section is cracking or popping.
It may also be a loose fitting socket/pin.
<carefully !> Observe the voltage readings during the tap test to determine if some voltage is being affected during the popping/cracking.
 

Paul G.

Friend of Leo's
Joined
Mar 17, 2003
Posts
4,229
Location
Rhode Island
Popping and crackling when tapping means a poor connection somewhere. I would use my chopstick and find where the popping originates. It seems daunting, but this is a very simple amplifier with relatively few connections. Personally, when I finish an amp, I use a magnifying mirror and bright light to inspect each and every solder joint. Even with that, I sent an amp out just this month with an unsoldered connection on the reverb driver socket. The amp worked great for about a week when I got a call telling me the amp is "crackling and popping". Sound familiar? Took me about 2 minutes to find it.

Also: if you find a bad joint, reflow it, then continue testing because there's no guarantee that the first bad one you find is the only bad one.

Make sure you practice safety protocols -- unplug amp from the wall and discharge the filter caps before you put your hands inside the amp. Every time, without fail.
 

Martinoils

TDPRI Member
Joined
Jul 3, 2014
Posts
57
Location
Tucson, Arizona
Ground loops don't cause crackling and popping, only hum.
You've tested the power transformer by confirming that "voltage readings are good".
If the noise only occurs during tapping the chassis or board, that seems to indicate that there's an intermittent connection of some sort.

I'd tripple check all connections for poor solder connections.
I'd pull the preamp and PI tubes and run the same tap test to isolate what section is cracking or popping.
It may also be a loose fitting socket/pin.
<carefully !> Observe the voltage readings during the tap test to determine if some voltage is being affected during the popping/cracking.
Thanks I’ll try it
 

Martinoils

TDPRI Member
Joined
Jul 3, 2014
Posts
57
Location
Tucson, Arizona
Popping and crackling when tapping means a poor connection somewhere. I would use my chopstick and find where the popping originates. It seems daunting, but this is a very simple amplifier with relatively few connections. Personally, when I finish an amp, I use a magnifying mirror and bright light to inspect each and every solder joint. Even with that, I sent an amp out just this month with an unsoldered connection on the reverb driver socket. The amp worked great for about a week when I got a call telling me the amp is "crackling and popping". Sound familiar? Took me about 2 minutes to find it.

Also: if you find a bad joint, reflow it, then continue testing because there's no guarantee that the first bad one you find is the only bad one.

Make sure you practice safety protocols -- unplug amp from the wall and discharge the filter caps before you put your hands inside the amp. Every time, without fail.
Thanks Paul
It does seem daunting but I’ll stick with it!
I always unplug and drain the caps before getting in there
Thanks again
 




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