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 Post subject: Resurrecting Another Fender Amp.
PostPosted: Dec Sat 31, 2016 5:35 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 410
Location: Fayette County, Pa
My Turn!

I just got in a Fender Showman Amp Head (1960s era) that has seen better days. Guy wants it fixed / restored, cost no factor. It had water damage on the preamp side of the component board. (Was laying on its side when the water got in) Also had signs of moisture throughout. Tubes missing, and the power board butchered. Somebody had removed the diode strings and replaced them with a bridge rectifier of unknown value. Power cord rotten and some other signs of being worked over.

First step was to determine if it was even worth going any farther. I pulled the output transformer to inspect it for damage since water damage was obvious very near it. Fortunately no damage was apparent, I did a high voltage test of it as well as an AC voltage test to check for internal shorts / leakage. It passed.

Next test was the power transformer. I disconnected the HV leads and bias supply lead and applied power to it from a variac. Raised it to 110% of rated voltage under no load. It held this for 30 minutes without showing any sign of over heating. Thus the first two major components were tested and found to be good. Now the work begins.....

I pulled the fiber insulator from under the component board to confirm no damage, found extensive moisture damage and mold buildup. I assumed the entire under surface of the component board was also in the same condition. I raised it up enough to get a toothbrush under it and proceeded to clan the underside with denatured alcohol. After that I dried the entire component board for 4 hours under a warm air blower. I also fabricated a new insulator board and replaced the mold-covered original. At this point the component board appears good, no leakage between connections.

I considered doing a test here but upon inspection I found two electrolytics shorted, signs of leakage on another in the doghouse. All need replaced so parts on order..... No way to determine what else may be wrong until I can power the amp.....


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 Post subject: Re: Resurrecting Another Fender Amp.
PostPosted: Dec Sat 31, 2016 7:42 pm 
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Joined: Mar Fri 25, 2016 6:00 pm
Posts: 129
Location: Battle Creek MI
its not the worst you could see though, one time i found an old amplifier (could not identify what model or anything) outside behind a family members house, it was rusted to hell, all the caps were basically looking like piles of wax and tinfoil and it had an ant colony living in it, i decided to salvage the tubes that did not shatter when i tried to pull them and abandon the thing surprisingly the tubes still worked 6BUB, 6EA7 and a couple 6FM8s the rest unfortunately are bits of glass. status of the colony is great though :lol: apparently according to my grandmother that thing had been sitting out there for 50 years.

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 Post subject: Re: Resurrecting Another Fender Amp.
PostPosted: Jan Sun 01, 2017 4:55 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 9189
Location: Powell River BC Canada
Your fiber insulated board should be Non-Hygroscopic . Have you a way of
knowing that it is ?

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VE7ASO VE7ZSO
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 Post subject: Re: Resurrecting Another Fender Amp.
PostPosted: Jan Sun 01, 2017 3:55 pm 
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Joined: Oct Tue 30, 2012 9:33 pm
Posts: 72
I would be wary of the fiber board that was wet. I've had to replace a few that got funky just sitting extended time in a basement. They are available and yes it will take time, but you could could be chasing some gremlins if you don't.


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 Post subject: Re: Resurrecting Another Fender Amp.
PostPosted: Jan Sun 01, 2017 5:15 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 410
Location: Fayette County, Pa
I used the simplified term "fiber insulated board" for the piece I replaced just to keep from typing so much! Sorry for any confusion! It actually is a puncture resistant laminated polypropylene sheet that is intended for high voltage insulation applications. It is non-hygroscopic and has a breakdown voltage of >10 kV for the thickness I used.

Regarding the other piece which has the components attached, after thoughly drying it, I will be spraying with a conformal coat which is designed to prevent mold / mildew. I use this for outdoor applications and is the same material used by the military for use in jungle environments. While this won't reverse any breakdown that may have already occurred it will stop future deterioration. I won't know if any gremlins may be present till I power it up, but all indications as of now is there is no leakage between adjacent eyelets. It will be something that will need a close watch out for though, especially on initial power up.

I did replace a few internal wires which had insulation issues where heat had worked on them , hard / brittle, just to be safe.

Also cleaned the tube sockets of dirt and smoke residue. (This thing may have spent time in a bar or someplace similar due to yellowish coating over everything) Wasn't the worst case of this I ever saw, but something that definitely helped its appearance.

Caps came in late yesterday, so far no time to change them out yet. I will post results once I do additional work


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 Post subject: Re: Resurrecting Another Fender Amp.
PostPosted: Jan Thu 05, 2017 3:34 am 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 410
Location: Fayette County, Pa
Caps installed and I powered it up with some used 6L6 tubes to replace the missing ones. It came up and played, although I found a couple issues.

First, no tremelo. I looked again at the component board and the neon bulb / photocell assembly is missing...... Guess that explains the no tremelo! Fortunately i do have one here to replace it, will do that later.

Reverb works, delay line is good so I pulled it out and cleaned up the pouch. Had a ton of dirt on it.

I still need to go through and check voltages on various tubes and compare to specs. That should tell me the condition of resistors, etc. Tubes were all checked, good (except for the missing 6L6s.)

I will do a burn-in once I get new 6L6s installed and will do a bias set up after that. Looks promising as of now.....


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 Post subject: Re: Resurrecting Another Fender Amp.
PostPosted: Jan Thu 05, 2017 11:40 pm 
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Joined: Oct Tue 30, 2012 9:33 pm
Posts: 72
Ah, The surf bug as it was called by the late Jon Bessent. I bought his custom made ones that had matched components to provide the least amount of signal loss. Got a great tremelo in my Twin Reverb now....


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 Post subject: Re: Resurrecting Another Fender Amp.
PostPosted: Jan Sun 08, 2017 4:25 am 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 410
Location: Fayette County, Pa
The amp is now working great. Tremeloe unit was replaced and that restored that function. (Who knows why it was removed originally.)

Cleaned controls, Installed 4 6L6 tubes and matched the set. Adjusted bias, did a 10 hour burn-in, no issues. Did final bias set up, all good.

Found one of the "Bright" slide switches defective and replaced it.

Also replaced the AC cord with a three wire grounded one. The original had degradation to the outer covering as well as the ground pin pulled from the molded plug.

Ready to go back to the guy that brought it in for electrical restoration. Case still needs some cosmetic work, I don't do that here, I limit my work to getting the electronics running. He is going to remove the vinyl and replace it with new, as well as the front cloth. My work is done, all amp functions are working now with no issues.


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 Post subject: Re: Resurrecting Another Fender Amp.
PostPosted: Jan Sun 08, 2017 3:21 pm 
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Joined: Dec Sun 25, 2016 5:53 am
Posts: 69
Good Job. I worked on a number of Fenders. They were very "robust" and quite reliable but need a bit of TLC every 20 years or so..

My "favorite" messy amp restoration was a Dynaco St-70 that a co-worker sold me. It had been drowned in one of those infamous Pennsylvania super floods. The entire interior was potted with hardened river silt that had to be chipped out just carefully enough so as to not to icepick the printed wiring board. Had to strip it down to just parts, polish the chassis, repaint the transformers, replaced all tube sockets, and all electronic components with modern caps and resistors. Replaced the ratty wiring and put in four new Mullard EL-34's and an NOS Tung-Sol 5AR4. A long and tedious project but worth it. ST-70's are currently "enjoying" a period of high prices of late. Shoulda woulda coulda kept it (as with 99% of all the gear I've had).

Nice to see a quality repair like you did.

BTW what sort of gear typically crosses your bench?

Regards, C-N-E

_________________
1970's Avionics Tech, USN Attack Squadron 147, "ARGONAUTS" A-7E A/C


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 Post subject: Re: Resurrecting Another Fender Amp.
PostPosted: Jan Sun 08, 2017 6:06 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 410
Location: Fayette County, Pa
I do two types of jobs here, both part time since i am officially retired. . First I do custom design work. That involves taking an idea someone has, discussing the project, then designing and building the device to meet their specs. (It actually is a bit more complex when you get to the details, meeting various codes, certifying the safety aspects, etc.)

The other which is more inline with what we do here is amplifier work. I have been doing repair work on audio and radios since the 1970s, mostly under contract with other dealers. (Don't have to deal directly with the customers that way). Word gets around and I also do restoration for local antique radio buffs. Fills in between the custom electronics jobs!


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 Post subject: Re: Resurrecting Another Fender Amp.
PostPosted: Jan Wed 11, 2017 8:17 am 
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Joined: Dec Sun 25, 2016 5:53 am
Posts: 69
CaveRat wrote:
I do two types of jobs here, both part time since i am officially retired. . First I do custom design work. That involves taking an idea someone has, discussing the project, then designing and building the device to meet their specs. (It actually is a bit more complex when you get to the details, meeting various codes, certifying the safety aspects, etc.)

The other which is more inline with what we do here is amplifier work. I have been doing repair work on audio and radios since the 1970s, mostly under contract with other dealers. (Don't have to deal directly with the customers that way). Word gets around and I also do restoration for local antique radio buffs. Fills in between the custom electronics jobs!


Aha, similar here, except I now build custom gear for the shop. Re-purposing of cool chassis and equipment that has scant probability of continuing its original function but has good basic hardware to work with. Rack-mount types I prefer.

I have the shop and "M-Cave" in an RV. Visitors don't know if I am a spy or mad scientist when they see the gear. I have quite a few pieces of test gear of types that I used in the service and cost at the time more than a Porsche 911. A few more priced as much as a modest home then.

Boatanchor radio gear is also great if the cost is reasonable. I got an SP-600 for free a few years ago, but its condition
was so poor that it would need $2500 and many hours. Always looking.

I guess it's just Techno-stalgia.

-C-N-E-

_________________
1970's Avionics Tech, USN Attack Squadron 147, "ARGONAUTS" A-7E A/C


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