Gibson Chet Atkins CE 1986 that hums!

[Scott Wackerman]

Hi,

 

I've checked all the solder connections and they are fine. It seems the pre-amp is compromised. The owner wants to keep the original volume and tone switch in the rotational orientation as from the factory. So far, most that I've spoken with resolve to change out the pre-amp, volume and tone parts. The problem is; the switches will not be in the same rotational orientation with the after market parts.

Any help would be most appreciated. If you are unclear about my problem then please review the photo's or get back to me.

 

Gibson_ChetAtkins_CE_1986_%20008.jpg

 

Gibson_ChetAtkins_CE_1986_%20013.jpg

 

Lastly, when I contacted Gibson, their solution would change out the original configuration! Arrrgh!

 

Regards, Scott 

 

[Russell Vance]

Given that the hum is probably not from the volume and tone pots...what is the problem with connecting up  a new aftermarket preamp to the existing pots.   I take it you wish to have the volume and tone pots go the same way up and down relative to the how the existing volume and tone controls work.   This, I think, is doable depending on how you wire the preamp to the pot connections (providing the pots are linear, and even if they are not).  More information perhaps.  

Also, go read up about the difference between pots and rheostats and how to configure a pot as a rheostat and how to change the direction of rotation for the increase and decrease of signal - then we can talk a bit more. Rusty.

[Pierre-Antoine Roiron]

I'm with Rusty on this one. Plus if you really need custom parts, contact MEC (they make parts for warwick framus, etc... and are part of warwick-distribution.de), they can make custom pots (value, curve, rotational orientation,...) for a decent price.

[Rob Mercure]

Also being in total agreement with the others is it possible that you could define further what you mean about a changed configuration?  The pots can move around that bracket and are held at a specific point either by a small tab - which might mean drilling a new hole to retain the orientation - or by two sides of the threaded pot shaft being flat - this being kept the same by drilling the hole out round and then locking the shaft at a specific orientation with "lock-tite" or other chemicals (or disassembling the pot and swapping the threaded shaft to the new one). 

Or are we talking about pots with a differing "taper?"  Which doesn't appear to be what you're asking about.

Essentially, as the others are pointing out, the pots are separate from the preamp and don't have to go with it.  But a hum problem is generally a grounding problem and since your preamp is battery powered (direct current, no source of hum) I can't imagine how 1/2 dozen resistors, a few caps, a trim pot and and IC could hum without failing.  So the next question is "what are you calling hum?"  To most of us "hum" is the sound of the power line's alternating current getting somewhere undesired and being amplified - 60 Hz in the USA, 50 Hz in much of the rest of the world.  It's the sound, with some distortion, of touching the tip of a guitar cord with the amp powered.  If the frequency you're hearing is different then we're not dealing with "hum" as commonly described.

Lastly most any competent technician should be able to repair the preamp - if it is the source of the noise whatever it is - without involving Gibson (a company which proves over and over how incompetent is - I wouldn't trust their diagnosis).  I noticed one electrolytic cap that might need replacement due to age, a couple diodes that might be noisy ("whooshing" sound) and the IC ("fuzz, or whooshing, or crackling" distortion likely - complete failure much more likely) - the IC is probably a dual op amp.  Op amps are pretty much identical the difference being voltage ratings and slew rate but I doubt that these would be that noticable in a guitar (this is where the hifi crowd come to blows - most musicians would rather play than argue!) so the cost is probably less than $10.  In fact, if you had to replace every part of the preamp the total parts price would probably be less than $25.

I'd recommend taking the guitar to an electronics tech you trust who has an oscilloscope, determining what's actually wrong, and replacing the part or two (or resoldering the bad ground - maybe cleaning the output jack) that's most likely the source of your symptoms.

 

Rob

[Paul Verticchio]

Rob & Len give great advice.

Here's what I'd do after performing Len's recommended transducer test (only if the transducer tests 'good'):

 

Remove the PCB from the guitar;

 

Remove & replace all factory wiring with new [and a much better grade of]) wire and MUCH higher quality solder connections;

 

Put a non-conductive barrier between the PCB & the control cavity.  Scrap thin sheet plastic works great; 

 

Make sure that the metal body of the 9v battery is not coming in contact with the output jack.  It looks pretty 'iffy' in that pic. Installing a battery retainer clip would be advisable;

 

Personally, I'd replace the output jack...it's just something I do routinely to give me a "known" good part & starting point;

 

Shield the control cavity (pic's indicate really poor workmanship for such an expensive instrument). 

 

If all that doesn't work, install an aftermarket pre-PCB.  At $90@hr, rebuilding the PCB probably isn't a cost effective repair.

 

Also...Rob:  I just rant & rant about Gibson and no one seems to listen.  I really like your terse description of their current state of delusion.  Thanks for the grin of concurrence (:

 

.

[Scott Wackerman]

Hi, Every post has been most welcome. 

Some answers to questions asked and things I've realized by all of the valuable suggestions:

- The humming is pronounced 60 cycle that diminishes when I touch the jack plate or the volume control mount (photo shows the thumb controls).

- pickup is a hex piezo

- Shielding is indeed non-existent

- better grounding is prudent

- Battery absolutely needs a mount

- Jack absolutely should be replaced

- Run a pre-amp test

- Remount pre-pcb with better insulation between pcb and body

- It's cheaper and more cost effective to buy a new pre-pcb "but I need better source - CAE is local but they have a bad rep"

- Need to find op amp!

-Trouble shooting and rebuilding isn't a viable option. I've got to believe that here in Silicon valley I can find a replacement amp cheaper and more cost effective than building one myself.

Regarding Gibson? That company is in a downward spiral. Of course, not everyone at Gibson is a buster but the quality and consistency is a huge disappointment. I would be nice to see the company change direction. Off my soap box.

 

Again, I just want to get this guitar up and running especially since it has Chet's name on it!

Thanks! Scott

 

 

 

 

[Rob Mercure]

Well, I wasn't suggestion rebuilding the PCB - just trying to illustrate how cheap each individual component was.  Like I said an experienced tech should be able to use a "scope" to determine where the hum it coming from and change only the specific components that are bad - and who gets $90 an hour?  When I had my own shop mine was $35 when I started and now, when I work, $45 - but even at Buddy Roger's Music in Cinty we only charged around $60 an hour.  With due respect if an electronics tech can get $90/hr then s/he should be working for NASA or Raytheon or some giant defense contractor cuz their own assembly/repair techs don't make that much money (I've got a friend working on air to air rockets for a major firm in a major city and he doesn't make that much money <can't tell you where or what> due to his security clearance - sorry).   But, hey, if I ever get my health back and you know such a place please lemme know - usually you have to be part of a mortgage derivatives ponzi scheme insurance banker to get that kinda dough.

Sticking an insulator in there's a good idea - "fish" paper is the old standard but I've also used gasket material I bought at an auto parts store for almost nothing for misc. insulators.  It takes high temp and didn't break down at 1,500 VDC (as high as my variable bench supply goes - if I really wanted to test it I've to a 20 KVAC neon sign tranny which I could feed with a variac - but I rarely work with anything over 600 DC).

 

Rob

[Scott Wackerman]

HI Rob, Your point about the cost of individual components is well taken and understood. The pictures I took are of the instrument "as-is," for my own photo documentation. I realize the battery, shielding and insulation are compromised at best.

 

As for needing to be a derivatives broker, many of those types live in this area and drive the cost of living up. Here, a bag of chips are 6.00 for a regular bag. A House, ~1,500 sq. ft. depending on the city, ~ $1 mil. Therefore wages are inflated but nobody has any more pocket money and probably less than a non coastal region. But, alas, the cost of those homes are dropping rapidly but the mortagages are constant......... Whoops! I digress!

Thank you for your help! Scott

 

[Paul Verticchio]

Hiya Rob,

 

$90 is the high end of the SHOP RATE range, not what the individual tech gets.  At this particular shop, the standard diagnostic fee is $80! This is an average representative rate at most central-midwest (STL & CHI) PROFESSIONAL tech shops that aren't part of a retail "music store" operation. 

The shop I refer folks to in STL (when an electronic repair is out of my 'comfort zone') employs factory trained EE's to oversee all tech work as they're a "platinum level" factory service center for almost all of the premiere pro gear manufacturers. This particular tech center only serves the MI & SR community. Y'know, guys that serve national touring acts on a 24/7 basis and get the parts sourced and the gear fixed within totally unreasonable time frames.  BTW: their bench rate was $75 @ hr this time last year. Oh that wacky economy!

 

I'd rather mildly high-ball the estimate a bit than come in too low.  Customers seem to appreciate paying less than the estimate (:

[Scott Wackerman]

Hey!

First, I need to thank everyone that chimed in about the problem with this Gibson Chet Atkins CE guitar.

• Eliminate Pre-amp as a source - I suspected basic component failure (wires) after the components passed the test Len Biglin suggested.
• One thing that helped me extensively was Paul Verticchio's suggestion that got me thinking.
• I constructed a shielded test box where I could place the components, wire harness, piezoelectric pickup in a environment free of electromagnetic noise.
• Utilizing the test box; I was able to virtually remove one component part at a time and expose it to a electromagnetic source (electric razor). While simultaneously watching my signal on a O-scope. I saw peaks in my wiring harness. 

• I replaced all the wiring with quality wire and shielded all the guitar cavities with conductive shielding paint. 
• Properly grounded and double checked all connections for continuity.
• Installed a proper grounded 9 Volt battery clip.

Put it all back together. She's now knows the words and has stopped humming! Excellent! New life for this guitar that was put out to pasture and damn near shot!

 

Thanks for all the help and I'll pay it forward! Regards, Scott

 

 

[Len Biglin]

Hi Scott , I'm not sure if I missed an earlier post , but to locate the hum I would remove the pickup (black shielded wire) from preamp and that will most likely stop the hum .If so its the pickup humming.If it still hums , try lifting the preamp away from the wood, a sensitive area may be touching the body. If the hum is sensitive to your body being close then touch a grounded part ( jack sleeve ) if that works you need to ground the strings , and so ground your body .

[Timothy Tucker]

Hey Scott,

 

I think I might have found a possible soultion to the hum.  Check out my attachment for reference (I used your original and editied it with Paint...I'm no artist).  It appears as though the preamp board has been modified from its original state.  The areas on my picture that are white, they originally had 5 more trim pots.  These Hex pickups had 7 wires coming from them and each wire represented one of the six strings in which you could adjust the volume for that particular string via the trim pot.  The colored dots represent the color of the wire from the pickup that went to that lug on the preamp strip.  If you notice, the first position would have been red (where you now have your ground).  This would have been the lead for that particular string/trim pot.  The top lug on the strip is black, which is a common ground for the entire pickup assembly.  It appears as though the pickup has been changed to a regular two conductor version, then somone omitted the extra trim pots and lugs, creating a master trim pot.  You might try putting your lead from the pickup where the red dot is and seeing if you can get your ground to go in the area where the black dot is (it looks like the post might have been cut in the picture) to ground it.  Hopefully that will correct the issue.  Of course, if the pickup has been like that for a while and the hum recently developed, it could be a component issue as the others suggestd.  Let me know what you find out.

Timothy

Gibson Repair

[Scott Wackerman]

Hi Tim,Thank you for the very astute observation. I didn't notice that on the pcb until you brought it to my attention. Now that you point out the blank spaces I've realized that I have previously scoured the web and noticed the multi trim pots but I failed to correlate those to the system I am working with now. I divined those to be different models or iterations of my current units. I have yet to find a 1986 with the back plate off - image, but I did see - maybe a 1987 with those trim pots. You might be onto something! Thanks for the edited photo which paints (pun intended) a profound and clear image of what you are talking about in this post.

I'll run the test along with other suggested tests soon and post the findings. I cannot be the only guy who needs this information on this planet! 

Regards, Scott