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Posts: 565
Location: Denmark | ... as Greta Garbo once said.
I have a FET preamp fitted O which hums at 50Hz (normal AC frequency here). The hum reduces when I touch the cable shielding or the amplifier so I guess it is a shielding issue. Any tip from the group here is welcome and appreciated. |
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Posts: 3611
Location: Pacific Northwest Inland Empire | Try a different cable, maybe? |
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December 2004
Posts: 565
Location: Denmark | Thanks. I tried a few good cables so I am fairly sure that I need do something inside. That could be connections and cabling an if relevant shielding of the part picking up the noise. Unplugging the pickup from the preamp does not change it a lot, so something between preamp in an guitar out.
Edited by tpa 2022-11-06 6:22 AM
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Posts: 3611
Location: Pacific Northwest Inland Empire | So, you have a shielded cable from the preamp to the output jack? If so, you have a gremlin in the guts of the preamp. You might have to open up Pandora's Box and troubleshoot the circuit. Could be a bad capacitor or grounding fault somewhere. I think the FET schematic is on file at ovationtribute.com |
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December 2004
Posts: 565
Location: Denmark | Thanks again. Yeah its probably a humming gremlin in the big black box. I'll have open it. |
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December 2004
Posts: 565
Location: Denmark | Any tips on getting the FET out in a nondestructive manner? |
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Joined:
November 2002
Posts: 3611
Location: Pacific Northwest Inland Empire | Can't remember what the preamp enclosure looks like, but it should open up pretty easily. Once exposed, then follow the circuit until you find where the grounding/shielding problem is. Frankly, this level of investigation & remediation is probably above my paygrade & should be deferred to more experienced & competent technicians.
http://www.ovationfanclub.com/megabbs/forums/thread-view.asp?tid=37...
This is from an old entry by Steve (numbfingers) in 2013. He is pretty proficient. You might want to drop him a PM on this matter. |
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Joined:
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Posts: 565
Location: Denmark | Thanks again 
Edited by tpa 2022-11-07 11:09 AM
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Posts: 1118
Location: NW Washington State | You Rang?
I was reading this yesterday, and feeling guilty that I've never fixed my 1624 with 2-knob preamp that's been hanging next to my desk for 10 years. It hums unless I touch the volume/tone pot shaft- easy to do since I never put on the new knobs I bought. 
If I ever get around to it, I think I'd look for bad/weak solder joints, particularly anything that goes to ground. Maybe start by plugging it in to amp set at low volume, reaching inside and wiggling any cables I could reach. Then I might remove the output jack and reflow any of its solder joints I could reach. Is the jack enclosed? Does it have some extra contacts inside that could be a problem?. After that it would be removing the preamp box and reflowing solder joints- particularly those that go to ground.
Any cables, plugs, jacks inside could be a weak point. Looks like at least one cable on mine might be connected with a 3.5mm jack.
Could be that shielding the box with copper foil like we use on solidbody guitars would help- but then all of these guitars would be humming without it. Do they? The 1624 is my only Ovation acoustic- I've got a bunch of solidbodies.
This seems like a lot of work, and I don't really need the pickup. Let me know if you fix yours and then maybe I'll fix mine. 
-Steve W.
Edited by numbfingers 2022-11-07 8:29 PM
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Posts: 565
Location: Denmark | Ha ha, I have lived with it for some time as I rarely (etc. etc.)
The urge to fix it pops up occasionally as part of string change preparation. Now I have removed the old strings. Let's see what wins, the laziness and urge to play it compared to doing the job (or at least attempt to do it). |
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December 2004
Posts: 565
Location: Denmark | Step 1: strings off Step 2: Remove FET3 
Step 3: Strings back on so I can play. Step 3 means that I have no stress with the further work as I havn't used the guitar much plugged in. If I can get it hiss and hum free that may change though. I now know is that it also produces hiss and hum while out and that the connetions to/from the jack are good. When the time is up I will open to what gremlin may be loose inside. I will take care to keep her away from any water :-)
Edited by tpa 2022-11-19 3:09 PM
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December 2004
Posts: 565
Location: Denmark | So I opened Pandora's box, looking for reasons for the mentioned hum and also a hissy kind of unwanted noise coming and going. What I found: - If I cover the socket jack with metallic foil connected to shield the hum reduces significantly
- The electrolyte capacitors look OK
- Apparently Ovation did not clean for flux residues back around 1990
- The diagram on Ovationtribute has a minor error in the wiring of the bass tone control. Luckily they apparently did not follow that diagram.
I gave the PCB a cleaning to remove flux residues. It may seem pedantic but with such high impedance parasitic capacitance and current leak may have an influence. I gave the potentiometers - incl. the trimmer - a good cleaning and reflowed a few of the solderings. This removed the hissy noises and (I think) some of the low frequency hum. Believe it or not - it sounds better, and is quite useable as it is and I am quite happy.
I haven't yet decided how to shield the output jack, but I will do as it seems to reduce the low frequency hum further. The diagram below was drawn based on the diagram at Ovationtribute but corrected based on the the actual amplifier. The inductor has no printed information on it so I could not verify size (2H) Controls are: R2: Treble, R8: Mid, R3b: Bass, R5: Volume R7 is the trimmer which is used to set the working point of the FET. In the FET preamp described in the thread referred to above by seesquare there is no trimmer but a single 22k resistor instead of the 15k resistor + trimmer. 
While at it I ran (just for the fun) a few simulations to see the effect of the tone controls.
Edited by tpa 2023-05-25 4:53 PM
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I'll hum it for you ...
