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Marshall Amps/Gospel Radio

3/24/1998 8:57 AM
Ted Breaux
Marshall Amps/Gospel Radio
Ok gearheads, the problem of the day:  
I recently saved a heavily (and stupidly) modded '69 Marshall Super Trem 100W. I chose to rewire the entire amp according to the nice JTM 100 plexi schematic at the top of p204 in the doyle book. The amp sounds great. There is one problem soon as I plug in the footswitch for the intention of operating the tremolo (Channel 2), I get this Gospel radio station coming in loud and clear. It matters not whether the trem is in operation or not, but as soon as the switch is plugged in and I turn up the volume of Channel 2, I hear demons being cast out, poisonous snakes being handled.....everything. I hope this is a good omen, but in the meantime, I would gladly entertain any suggestions or hints before I dive back in to it!  
3/24/1998 9:07 AM
Steve Watson

Holy Cow!  
It's a sign!  
It's a Miracle!  
Are there any outlines that look like the Virgin Mary on this thing?  
What Are you doing standing around?!  
Get Jerry Falwell on the Phone! Call Pat Robertson! You've got something there that'll get you top billing on the 700 Club!  
Now that I've had my fun, this is just a guess from an unknowledgeable person, but is the footswitch cable shielded?  
3/24/1998 9:17 AM
I recently had a Phillips NOS 12AX7 that picked up radio pretty well. Couldn't get anything religious out of it though. Could be worth swapping some tubes around.
3/24/1998 10:44 AM

Look for "cold" solder joints. They can act like diodes and, along with parasitc capacitances, behave like an radio detector. Since the "receiver" is not tuned, you get the strongest station nearby.  
Also shield everything you can (beware of ground loops). Make sure all your preamp tubes are biased. Try rearranging the wiring. Is this channel 2 a high gain channel? If so extra care is needed.  
You may need to put some very small capacitors in strategic places.  
Good luck.  
3/24/1998 10:46 AM

I think Steve's got the right idea.  
Hopefully the cable is a shielded one.  
Regardless of how the rectifiction is happening, I bet it's the cable.  
The cable is acting like an anttenna and since the circuits are high impedance there is a lot of RF current floating around on it.  
Try bypassing the cable to ground right at the chassis with some very small caps like .001uF and run the cable through a few small Ferrite beads. Anything that will offer a high series impedance or low impedance to ground at RF should knock it out.  
Anything to choke off the RF currentd on the cable as it enters the chassis.  
3/24/1998 11:08 AM
Ted Breaux

I appreciate the posts! I am concurrent with the cable being an antenna theory, I just am not sure where the rectification is happening. The footswitch cable I used is shielded, so that rules that out. I may try the small caps from the cable leads to ground however. I will double check all connections, and end my catechism lesson via my Marshall!
3/25/1998 4:32 AM

To check if it is the cable or not, substitute it for a short-circuited plug (tremolo always on) or plug nothing (tremolo always off). If the radio interference disappears, then for sure it is the cable. Try better quality cable, the shorter the best, and the decoupling tips suggested in this thread. If the interference does not stop, better double check your soldering, grounding, shielding and the layout of the wires.  
What kind of tremolo circuit have you got? Does it use a photocell or does it modulate the power tube's bias? If it is of the photocell kind I doubt it may be the cable, cause the footswitch is just enabling/disabling a LFO that couples to the audio path via a lamp and a photocell. Don't think RF can pass through all this (slow responding) stuff.  
If you turn the volume completely down does the interference remains? If you don't plug anything but turn the volume up is it still there? Do you have a MV? This may help to locate the rectification.  
Have you tried your amp in other places? I worked in a lab once where we would supply 5V to TTL inverters and measure over 8V at the output! The totem-pole output stage of the TTL was rectifying a nearby TV station signal and along with the capacitance of the scope, acted like a detector. To make things worse, a professor that worked there made some calculations and found that the room was a resonant cavity for that particular frequency. So circuits used to work everywhere except inside that lab.  
If nothing else works, try some praying... (sorry, I couldn't resist)  

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