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Re: Marshall Amps/Gospel Radio

3/24/1998 6:59 PM
stephen delft
Re: Marshall Amps/Gospel Radio
Re: Your gospelling amp...  
Never mind all this tech-head stuff about screened cables and ferrite beads....go to the market, buy a small goat and two live chickens... ;-)  
Buy it a 'SacredHarp" hymnal - if nothing else, you'll learn something about acapella.  
and... Check that any chassis connections (especially the input jack) are making secure connection to clean metal.  
3/25/1998 10:48 AM
First, is the tremolo on a separate board, or on the end of the same board as all the other parts for the amp. The earlyest ones were on a separate board rite next to the main board. I have both types and have had this prob.  
3/25/1998 11:13 AM
Ted Breaux

The trem section is at the end of the mother tag board. The trem uses a PNP transistor (I suppose as an oscillator), a 12AX7, and appears to alter the bias of a preamp tube (from memory). The phenomenon occurs in different rooms, and is audible only when the volume on the trem channel (Ch 2) is turned up with cable in. I will short the cable jack pins. If that fixes it, I will decouple the cable. If not, I am going to check all solder joints and grounding. I may have to shield some runs of wire in the preamp as well. I will let you all know how turns out.  
Hmmm...this gives me an idea.....if I service the amp of someone I am not too fond of, I'll just build a crystal radio into the backlside of the tag board, and wire the output directly into the preamp. That should be fun to diagnose!  
Ted B.
3/25/1998 4:40 PM
stephen delft

Your last reply gives another possible clue...Check the schematic around that PNP transistor. I think some Marshalls used a trem circuit where the 12AX7 is the trem oscillator, and the transistor acts as a modulated shunt from signal path to which case, it might be one of the semiconductor junctions in that transistor which is acting as a diode for the rf signal. Try hanging small 100pf ceramic caps in the obvious places...c-b...c-e...b to ground. ..maybe also high-loss ferrite bead(s) right on the base and/or emitter leads.  
And if the trem footswitch wire connects anywhere near that transistor, you could try putting rf choke(s) or a damped c-l-c filter in series, where it joins the board. If you have a really bad rfi problem, you might also need to filter the outer (shield) of the same wire.  
Might also be worth swapping the transistor.  
BTW, I wonder if anyobne has checked the harmonic spectrum of that gospel station recently?  
Good luck, Stephen
3/25/1998 7:51 PM

Hi Stephen,  
Did you mean to imply an RF PI or T network?  
The T would shoot the RF right in and the PI would attenuate of course.  
What frequency are we talking about? I missed that.  
If it is an AM station you'll need bigger lump C and L then if it is a 100mHz FM station.  
I'd try the 100pF cap shunt on the input first. Or a simple 1mH RF choke in series to the first section might stop it without effecting the audio bandpass.  
I had a Prescription stomp box that did this and the cure was an Amidon ferrite bead ( i think it was a mix-73 or 43) on the input, the + battery lead and a small 220pF cap to ground on the jack.  
This stopped the RF and had no discernable effect on the tone.  
3/25/1998 10:45 PM
stephen delft

Did you mean to imply an RF PI or  
T network?
Pi...cap to ground, series L, cap to ground  
but only as a last resort. I'd try a simple series RF choke and/or small caps across the transistor junctions first.  
Thanks for the stomp box RFI fix.  
- Stephen
3/26/1998 5:40 AM

Now, this transistor has high probability of being where the detection takes place. I agree with Stephen that decoupling this transistor or even swapping it can have a good effect.  
How is it wired? Emitter to ground, modulation signal applied at the base and the collector coupled to audio by a capacitor (no DC supply for collector)? How is the "Depth" pot wired? In series with the transistor? If so interference remains with the "Depth" at minimum?  
I'll take a look at the Marshall schematics I have to see if I can find one that matches your description.  
Good luck,  

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