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Post by premiumplus (Dave) on May 19, 2017 16:22:42 GMT -7
I pulled my old 1960 Tweed Princeton out of the closet the other day and noticed a low level hum when nothing was plugged into it. Figuring it was a tube, I went down the line and substituted known good tubes, to no effect. After a few minutes of checking resistances with the amp turned off, I found the root cause. Instrument input jack #2 has a shorting arm on it and the 57 year old contacts were really badly oxidized. I was measuring over 5K ohms across the switch contacts which were closed! I took my burnishing tool to both jacks and now good contact has restored the proper input impedance (and proper signal reduction on jack#2) and that was it. No more hum.
I thought I'd post this here because it is something that I've not seen in many years. In addition to the hum going away, the amp sounds better. Way better, and it makes me think that maybe the jack was making intermittent contact while playing, and creating artifacts in the tone. I've owned the amp for a long time and it's never sounded this good. It just struck me as strange, and the amp sounds so good now that I had to post about it.
So remember, every 5 or 10 years you need to burnish your contact switches in your amp. This usually causes problems in the Effects Loop jack when it's not used for a long time. I had sprayed it with DeOxit several times and that wasn't enough to clean it up, hence the burnishing tool.
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Post by BritInvasion on May 19, 2017 16:47:01 GMT -7
Great info Dave , thanks for sharing this !
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Post by purpletele on May 19, 2017 18:58:55 GMT -7
I just revitalized a stock DRRI for a friend that was distraught after her purchase.
They send them out pretty cold bias settings. So everything was getting muddy.
Now the amp is happy, my friend Juli is happy, and I am pleased.
It was your inspiration to work on Amps today Dave! I'm having fun with it as well.
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Post by zpilot on May 20, 2017 0:25:56 GMT -7
It's just for this reason that I run a very short cable between loop jacks when they are not being used. That disables the switch contacts on those jacks. I will unplug/plug that cable once in a while just to keep their contact points clean.
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Post by benttop (Steve) on May 20, 2017 8:38:43 GMT -7
Closeup of the contacts:
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Post by Maddog on May 20, 2017 9:06:29 GMT -7
I had an identical experience with one of my tweed bassman amps.... I never expected a "hum" from corroded contacts; I expected "static." It (the hum) drove me nuts. I was banging the hell outta those poor capacitors, resistors, and trannies trying to see which one was crapping out... Finally, Michael Clark (Clark Amps), the tweed amp guru, told me to go ck the input jacks..... Brilliant! ....
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Post by dcarver on May 20, 2017 12:53:44 GMT -7
Very cool info Dave. Thanks for posting. One question : what do you use for a burnishing tool ? Some 600 sandpaper, a small wire brush, whatever works best to get the crud off ? Or is there a specific tool that works best for this task ?
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Post by zpilot on May 20, 2017 13:00:41 GMT -7
I have a practice amp with a headphone jack that mutes the speaker when a headphone is being used. One day the speaker quit working. I finally traced the problem down to the switch contacts in the headphone jack. It was partly because they were corroded and also because, since I used headphones a lot and kept them plugged in even when not using the amp, the cheap jack's contact spring had become weak.
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Post by zpilot on May 20, 2017 13:11:27 GMT -7
Very cool info Dave. Thanks for posting. One question : what do you use for a burnishing tool ? Some 600 sandpaper, a small wire brush, whatever works best to get the crud off ? Or is there a specific tool that works best for this task ? I don't want to remove the plating on the contact point so I don't use sandpaper. I cut a strip of paper and soak it with D5 and work it back and forth several times between the contacts. If they are especially dirty I hold them slightly open and blow the crud out with canned air. Then I clean them again.
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Post by premiumplus (Dave) on May 20, 2017 14:26:00 GMT -7
Very cool info Dave. Thanks for posting. One question : what do you use for a burnishing tool ? Some 600 sandpaper, a small wire brush, whatever works best to get the crud off ? Or is there a specific tool that works best for this task ? I have one of these Burnishing tool You used to be able to find them at Radio Shack but at least MCM still carries it. It's basically a small file shaped more like a feeler gauge that's impregnated with diamond dust, at least that's the way they were described back in the day. They used to be called "Relay Contact Burnishing Tool". zpilot, like you I usually try using DeOxit sprayed on a strip of paper that I pull through the contacts (as I hold them shut). That didn't work in this case, that jack was oxidized good and proper!
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