I finally pulled a 38-7 table model off the shelf I purchased a while back and recapped it and found a bad 6K5 that was making all sorts of hisses and squeals in the audio. Replaced the offending tube and the radio works well except for a buzz in the audio which sounds much like an unshielded audio cable in other equipment. While it isn't extremely offensive, I know it's there and don't recall my 38-7 console having the same issue. Unfortunately I have the console in the garage awaiting warmer weather for refinishing behind other consoles. The 'buzz' varies with the volume control and does diminish although not completely when I connect the chassis to earth ground. At first I thought it was induced noise because of incorrect lead dressing but moving some of the wiring around doesn't seem to change it. I did find by accident that if I put something metallic (IE screwdriver, needle nose pliers) in the area behind the volume control most notably by coupling cap 28 the buzzing increases. It almost seems that there is some sort of field being generated in that area and being picked up by the coupling caps. All of the caps used were new 'orange drop' type and even tried changing cap 28 with a yellow film type to no avail. Removed the wire from the volume pot going up to the tuning shorting switch but no change there. Has anyone run into this before? TIA
Did you change the .015 caps in the bakelite as well? I just dealt with the same issue on a model 60 I'm working on. Changed the cap where the power cord connects and the buzz went away. I don't claim to know what I'm doing, but that worked for me in that situation.
Yep - restuffed the bakelite with 2 type Y2 safety caps. I bought an assorted set of safety caps recently from JustRadios. Prior to buying these, I used to just use orange drops.
Could be lead dress and/or component placement in the det/first audio stage when you inadvertantly "moved" wiring or maybe the replacement cap(s) are not in the same place?
Lead dress on grid wires, coupling caps and resistors can cause this too.
Placing grid connected components closer to the chassis pan surface may help shield them.
This may not be a solution,more as a reminder, that florescent type or neon gas lighting nearby can cause similar, or otherwise weird hums into tube type equip sometimes, especially if a nearby lighting transformer is getting ready to fail. Modern day household appliances can introduce all sorts of RF interference, especially washing machines, dishwashers, going thru their cycles and feeding RF hums back into the AC lines. Are all the orig tube shields in place?
Also, come to think of it, try using a shielded piece of wire to the dial muting switch. Be sure to ground the shield at both ends, as the "grounding" of the dial mechanism may be less that perfect.
Tex has a point too with the tube shields. Be sure the socket shield rims and the shields have shiny metal contact to be effective.
Thanks for the tips - next time I get to work on it will check into what you all suggested. One thing I already did try was to remove the wire from the muting switch from the center terminal of the volume pot altogether and the buzz was still there. I didn't really relocate any replacement caps and when I mentioned moving wiring thinking it was a lead dress issue, I basically moved some of the wiring that was close to the twisted filament wiring away slightly (as much as the slack would allow) to see if that would make any difference. Tube shields are on, but I'll try and clean the rims and shield contact points with some fine emory paper. The part I still find odd is placing something metallic near the caps on the volume control increases this buzz.
Hopefully I get some time to get back into it and check it out further. Thanks!
Your hand is touching the metal object and capacitively coupling house AC wiring fields (hum) into the circuit. It's called hand capacitance effect. Your body is essentially a big "capacitor." Same effect when you touch the grid cap on an audio tube to check if the audio section is alive. You hear the hum.
Exactly the way I check for audio also Chuck!! Center tab of the vol! A wet toothpick comes in handy indeed sometimes!!
Hi Tony,
Just something else to look at, is any tie point to chassis. Occasionally, a rivet will develop resistance, even a couple of Ohms, and cause a buzz.
I have found two sets, one a Philco 84 IIRC, the other a Truetone that used a tube socket's rivet and a terminal lug for grounding. That rivet had a little over 2 Ohms. Jumping the connection, the buzz disappeared. Worth a try.
Gary.
Reminds me of a buzzy Z-nith chassis I once worked on. Apparently it had been an ongoing problem because a previous repairguy had replaced the 6F5 G with a 6SF5 and rewired everything accordingly. Still had symptoms like you describe. Get anywhere near the wiring and the increase in hum was noticeable.
Ole Luddite me put it back the way it was supposed to be, did the routine resoldering of things, ran a new shielded grid lead and somewhere in the process I fixed it. Can't rightly say what the initial problem was but once it was cured all of the finickiness went away.
-Bill