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Old 28th December 2004, 12:25 PM   #1
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Default traps in to3 construction

As part of my rebuild of a ooold used & abused fet amp I was doing some testing & probing of various stuff & uncovered a trap for young players.

The source resistors in this amp are .33 ohm so I draged out the insulation tester which has a good solid low ohms function. All the resistors measure fine & closer tolerance than I expected.
BUT
I found that I had significant resistance in the connection between the solder tabs, mounting bolts & metal case of the TO3 output fets. some were over .5 ohm.
After dismantling It was obvious why, corosion (well gunge realy) on the solder tabs & bolts.

So chaps It pays to look carefully at the mounting & connection hardware. I can see why some insist on brass screws & nuts.

A good scrub and a little metal fortified grease on the threads on reasembly & they measure sooo much lower better than .1 ohm whic is the res of the meter.

cheers
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Old 28th December 2004, 01:29 PM   #2
anatech is offline anatech  Canada
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Hi lowlevel,
Attention to detail on a repair make all the difference. When the amp gets older, the repair becomes more of a restoration. Use a split washer to make contact while you are at it.
-Chris
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Old 29th December 2004, 12:05 PM   #3
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I've never been a big fan of spring washers myself, But in this case probably not a bad idea.
This particular amp is beyond a restoration project more a resurection project. Was very uggly.
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Old 29th December 2004, 08:46 PM   #4
anatech is offline anatech  Canada
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Sounds like the standard bar amp we used to get. Rusty, smokey and nasty all over. Normally not the first repair attempt either.
I like rebuilding lost causes sometime as well.
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Old 29th December 2004, 10:42 PM   #5
markp is offline markp  United States
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Quote:
Originally posted by anatech
Sounds like the standard bar amp we used to get. Rusty, smokey and nasty all over. Normally not the first repair attempt either.
I like rebuilding lost causes sometime as well.
LOL....I know exactly what you are talking about. The oily grunge that builds up after a few years in a smokey bar. It's part nicotine, sweat, smoke, and dust bunny.
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Old 30th December 2004, 05:18 AM   #6
Stocker is offline Stocker  United States
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I got a computer free from a workmate... it lived by his kitchen and the power supplies (2 times) died from overheating due to grunge buildup...lots of fried food!
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Old 31st December 2004, 11:52 AM   #7
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yep this is an oooooold rock & roll amp & spent the last few years in a pub, yep many atempts at repair in the past, some of it was real scary.
how about the 3 x 3.9k resistors zippytied together & wraped in tape to replace the 12k 5w in the bias chain.
I could have sworn the heatsinks were yellow anodised till I scrubed them with thinner.
then there was the mains wiring
There is a certain gratification gained from taking a complete piece of #### & bring it up to spec.
"Redeming the prostitute"
I think I dub this amp "Mary Magdaline"
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Old 31st December 2004, 02:43 PM   #8
anatech is offline anatech  Canada
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Resurrecting the dead. That's what we do.

I am hoping to pick up some old 70V PA amps (dead) to use as a design bed for new amps. I can lose the 70V output transformer and I have a case, power transformer and heatsinks. These were rated at 250W mono. Over-built American units should be okay. My target power range is anything from 50W to 100W per channel in stereo. The supply voltage will determine that.
-Chris
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Old 5th January 2005, 12:16 PM   #9
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Old public address amps arent a real good target because the rail voltages are usualy low and often single rail.
24 volt is common with a view to battery back up.
They use the output TX to get the voltage swing.

Bogen (a term of dirision amongs the young in aus chuckels to self) built some big 100 volt amps years ago.
Big ugggly 450 watt monsters they only had dual 24 volt rails.
A mate has some old ALTEC 350 watt units 24 volt rails.

I'd be looking for some old hideous ex rock & roll amps & toss everything but the case & psu.
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Old 5th January 2005, 08:14 PM   #10
anatech is offline anatech  Canada
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Hi lowlevel,
Yup, generally. These have a bipolar supply. I just can't remember the voltages. Besides, they are free. I can throw out what I don't want.

You know, rewire those other types as full wave instead of bridge and you'll have a 12V supply. The car amp guys are getting excited now I think.
-Chris
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