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Fairchild 260 rebuild questions

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1. I'm adding 3 prong grounded power cords to my Fairchild 260s. I recall reading somewhere that simply grounding to the chassis can cause a 60Hz hum in old amps. So I'm wondering how to do it in a way that will avoid this problem.

2. I'm replacing the selenium rectifier with a 1N4007 or a UF4007 diode and wonder, is it a simple swap 1 for 1? The diode is tiny compared to the rectifier!

3. I've read a lot about star grounds and busbar grounding wire. The stock amps have grounding points attached to the transformers. Would I benefit from upgrading to a single bus bar?

Any suggestions and or schematics to accomplish the above will be greatly appreciated!


:xmastree:
 
WBS said:


It is attached to a couple of 20uF caps which are connected to the bias control pot.

Yes, that is what i thought. You can just drop any rectifier diode in there, no problem. As far as the 3-prong AC modification... the additional green lead is just a chassis safety ground and should not effect the performance in any way and may just save your life, so I say just do it, and just solder a ring terminal to the lead and screw it into the chassis near where the AC enters.

I would personally leave the layout and grounding alone, and just replace the inevitably dried-up capacitors and old resistors, which have probably drifted with age and heat.
 
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SY will steer you in the right direction regarding safety grounding, but ground loops are a concern. I have added 3 wire cords to most of my vintage gear and have had to deal with ground loop issues in all cases.

Incidentally the Fairchild 260 is considered an extremely rare and desirable piece, so tread very gently if you want to retain its collector value. (Do nothing that cannot be easily reversed. Keep the old parts too.)
 
Yeah, but the idea of rebuilding them to factory specs with today's precision parts makes me drool! Plus, even though they're a little banged up, I plan to repaint the trannies so they'll still look really cool!

Funny though. While I'm looking forward to their sound and appearance, the wife recently asked, "Where do you plan to put those?"
"On a shelf, right below the TV", was my reply. She was pretty quiet while she noodled that one.

I may need to upgrade the "entertainment center" to garner a higher WAF. :D
 
kevinkr said:
SY will steer you in the right direction regarding safety grounding, but ground loops are a concern. I have added 3 wire cords to most of my vintage gear and have had to deal with ground loop issues in all cases.


Interesting... I have not had that problem and I have dealt with a lot of vintage repairs, all of which I have added a safety ground to in the case of a hot chassis condition. Do you mean just by adding a modern IEC with a safety ground or in some other way altering the grounding scheme... or maybe in the case of AC/DC gear?
 
SY said:
Too late, he got those amps from me and they'd already been fairly thoroughly disassembled. One of them even had another circuit put in it at one time. This is an iron-and-chassis deal.

I have a pair of 255's which were in the basement of a friend's father's house -- then the flood hit. Some of the Sprague caps looked like your 3 year old after a bad dinner of mussels and clams.
 
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aletheian said:



Interesting... I have not had that problem and I have dealt with a lot of vintage repairs, all of which I have added a safety ground to in the case of a hot chassis condition. Do you mean just by adding a modern IEC with a safety ground or in some other way altering the grounding scheme... or maybe in the case of AC/DC gear?

Yeah, basically adding an IEC, but I haven't worked on anything with a hot chassis, just hifi gear where just grounding the chassis has usually (but not always) resulted in low level buzz, particularly in the instance where other devices in the system are powered by more than one branch circuit and are also grounded. Cable TV connected systems are the worst.

Every component in my system has a safety ground with the exception of my TT, but except for my tuner (added 3 wire cord) and sacd player they are all homebrew - I no longer have any ground loop issues but getting rid of them was not trivial. I did a lot of work revising the internal ground buss structure in my pre-amp as well as using the 10 ohm resistor in || with back to back diodes trick in some of the gear.
 
Interesting note, I also have a pair of Fairchild 260s and I have been unable to identify a selenium rectifier of any sort, unlike my other power amps. But the pair seem to work fine. Do I need to add in a diode for the rectification? May I learn a bit more about the design philosophy of this set up? Thanks in advance!
 
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