RF Mystery. My Supro is picking up radio stations. Need help..

[cluster]

I'm sure this subject has been done to death but I still haven't been able to crack this one.

Ok, I have a 1959 Supro 1615T that catches radio signals (RF Interference) when turned ON with NOTHING plugged in. Here are the symptoms:

1. I can eliminate the RFs if I plug a regular guitar cable into "treble" input of channel 1 or channel 2. This is weird because it's usually the other way around, that is: usually RFs happen when you plug a cable into an input (not the other way around).
2. Here is a twist: pluging the guitar cable into the "Regular" input of either channel does nothing to get rid of the RFs. BUT, plugging into the treble input will eliminate the RFs.
3. The RFs only effect channel 2 (the trem channel) and not channel one. That is: If I turn channel 2 volume down, I can eliminate RFs. However, with channel 2 volume off, I can turn channel 1 volume up and no RFs. Also, pluging the guitar cable into channel one's "treble" input will eliminate the RFs.
4. As you may know, these Supros use grounded switchcraft jacks that ground right to the chassis. As well, the Treble inputs of each channel are soldered directly to a shielded plate that covers the input section. In turn, the plate has a wire soldered to it that runs to a ground lug on the tag board.
5. Also, I don't have to insert the cable all the way into either treble jack to eliminate the RFs. I just have to insert it enough so that the tip touch the hot end of the jack.

I tried cleaning the jacks, swapping the preamp tube and re-soldering the solder joints in the input section but no luck. I'm looking for suggestions on what I may have missed. FYI, I have a 1959 Supro 1690T (exactly the same amp except for the speaker) right next to it and it is RF free). I don't want to deviate from the design by adding a 68K resistor to the grid or anything like that. The fact that RFs can be eliminated on both treble inputs tells me that it's probably not a bad jack. I'm sure there is a simple fix that I'm just not seeing. Any suggestions??

here is a schematic of the amp: http://www.dirtygirlamps.com/Amp_sit...ro%201690T.pdf

I'll see if i can post a picture of the input section later.

[Enzo]

Notice on the schematic the trem footswitch says "isolated"? Make sure that jack is NOT grounded to the panel.

Check that the grounding contact on the tip has continuity when the jack is empty.

Looks to me like whoever designed it was worried about RF as well, look at all those little caps hanging off channel 2 input stage. Have any of those been removed?

Has the power cord been updated to a grounded three-wire type?


So with nothing plugged in measure resistance from pin 7 of V1 to chassis. You should see 82k, more or less. Do you? Of not then 470k? And if not is it open?

And the 6.8M to ground on pin 1 grid?

[pdf64]

How about if you swap tubes between the 1690 and 1615?

[dai h.]

won't the ".005" (5nF) at the input act like antennae? If the 500pF shunting the grid to ground, (assuming they are cheap ceramic--assumedly not more expensive temp. compensating type) have aged (since somewhere around half a century has passed--usually downwards) maybe they aren't effective (or effective enough) anymore? Oh, and if the same "aging downwards" phenomenon applies to the .005 (assuming ceramic), maybe they are smaller in value now (and making worse/better) antennas?

[Enzo]

No more of an antenna than all the wires and resistors and anything else normally inside an amp.

[dai h.]

hmm... well it looked like it might be problematic since the hot end is floating (my thinking was that the gtr. pickup when plugged in provides a much lower impedance to ground hence the description of the RF problem not being apparent with that input).

[Valvehead]

maybe try some shielded cable for the input jacks wire ?

[TimmyP1955]

Look at the schematic - they buggered the inputs. The Normal jacks are shorting, but the Treble jacks are not - so there's a nice antenna on each channel. Plugging into the Treble puts a source impedance across the input and swamps the RF. When plugged into the Normal, there's 100K in series with the source, so the source impedance is too high to swamp the RF.

What I don't understand is how plugging into Treble 1 could stop the RF in channel 2.