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Old 17th September 2007, 12:29 AM   #1
Plundering the Planet From the Comfort of Home
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Default Tube Amp Startup

I got me a minty Scott 233 amp and it looks like it hasn't been switched on in 30 years. Seller said so and for $10 with the matching tuner and both with their rosewood-like cases (Knight speakers, too!), couldn't pass. I have the extension cord rig with the 40 watt bulb ready to go and I'm raring to start her up.

I've done a visual and all looks well, except the 5AR4 tube which has some white frost on the topside. Most tubes appear original, one or two of the smaller ones up front look real old but might be replacements. I popped and re-set each tube.

What should I be looking for at power up and are there any measurements that I should be taking? I do have an amp clamp and several different meters. I've read that one thing to check is the transformer temperatures by touch, but I really don't have a feel for what it should be, so that's not very helpful.

Should I leave it on with the 40 watt bulb arrangement and if so for how long? What next, full power up or move up to a 100 watt bulb?

I do understand that there will be some cap and rectifier replacements in my future, but I'd like to confirm some minimal level of functionality before I dive in and start spending cash.

There is also a loose wire with one end screwed to the chassis. What do I do with the other end?

I'll post some pics soon.
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Old 17th September 2007, 12:54 AM   #2
SY is offline SY  United States
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Quote:
except the 5AR4 tube which has some white frost on the topside.
Boom! It's dead.

As for the dangling wire, you should have, or should immediately install, a three wire power cord. The third wire (green) attaches to the chassis. This is necessary for basic safety, so don't neglect this step.
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Old 17th September 2007, 07:59 AM   #3
Akita is offline Akita  Malaysia
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What are the tube in the scott 233? Can you identify it?

1) Unplug all the tube. At the power transformer, find the center tap of the secandary power supply, there should be 3 cable, 1 connect to pin4 (5AR4), 1 to pin 6(5AR4) and the center tap connect to the ground(or at the negatif of the 1st filter cap) measure the dc resistance from the center tap to pin4 & 5 with DMM. It should be the same or very close. If not, there maybe short circuit.

2)Find the heater pair, if you have 2 type of heater then you may found 2 pair, each pair may be shorted to it self but not to others, so check the short circuit between each terminal. Do the short circuit test for others terminal as well.

If they use 12.6v as heater you might found 3 cable shorted(one is center tap)and not per pair, For heater this is normal , but should not more then 3 terminal.

3) If you're sure that no one have been modified the circuit, u can turn it on and measure the HT, it should be with in the range of the allow power tube plate supply(anode).

Note: Again ground it to the chasis 1st. b4 turn it on
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Old 17th September 2007, 12:28 PM   #4
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Thanks for replies so far. Is the rectifier tube fried? I'm not clear on the "boom" statement. I can slip this into my next partsexpress order if necessary. I'll get the grounding cord, no problem and I'm looking for the schematic. Pix coming soon.

The seller said it had not been turned on, but he was too young to be the original owner (may have been his parents, though), said he "did not know if it worked." I did notice that one of the heat shields was on an improper tube and that some screws were missing from the bottom access, so someone saw fit to open her up at some point.

Thanks.
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Old 17th September 2007, 01:47 PM   #5
SY is offline SY  United States
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It is not only fried, it's fried in such a way as to guarantee smoke and sparks. Sorry not to be clearer.
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And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
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Old 17th September 2007, 02:21 PM   #6
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>It is not only fried, it's fried in such a way as to guarantee smoke and sparks. Sorry not to be clearer.<

If that is true (that it was fried at some point) are the measurements described above sufficient to establish that the unit is worth salvaging? I'm guessing some of these parts are impossible to get. Don't want to do a bunch of work and find out it was all for nothing.

THANKS!
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Old 17th September 2007, 02:33 PM   #7
SY is offline SY  United States
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If the output and power transformers are OK, everything else is trivial. You will need to replace the power supply caps anyway, along with any other electrolytics, and those are the kind of parts that get taken out when a rectifier fails.
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And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
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Old 17th September 2007, 03:45 PM   #8
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by DreadPirate
>It is not only fried, it's fried in such a way as to guarantee smoke and sparks. Sorry not to be clearer.<

If that is true (that it was fried at some point) are the measurements described above sufficient to establish that the unit is worth salvaging? I'm guessing some of these parts are impossible to get. Don't want to do a bunch of work and find out it was all for nothing.

THANKS!
I don't know anything about that unit, and I'm fairly new to this hobby. But, I can say that there is likely nothing in there that can't be replaced with the original, or something close enough - with the help of folks here or on specialty sites for the specific brand/unit. Unless you have to replace transformers, the rest is available at pretty low cost, including good used tubes. The hardest part of making tube stuff is chassis work anyway, and you've got that covered.

Sheldon
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Old 17th September 2007, 04:07 PM   #9
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Quote:
Originally posted by DreadPirate
...I'm looking for the schematic.
Google didn't take long to find this:
http://hhscott.com/pdf/fs/233_299C.JPG

The 7591A is a pretty good sized tube, and should be worth a few watts. For $10, I'd wager this amp is easily worth the trouble to fix it up...
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Old 17th September 2007, 04:19 PM   #10
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Some info on this unit:

http://hhscott.com/integrated_amps_stereo.htm
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