LK-72 mis-wired or am I confused? - diyAudio
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Old 21st March 2010, 03:15 PM   #1
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Default LK-72 mis-wired or am I confused?

I have purchased an LK-72 off of CL, and it is my first tube amp. It appears from what I have read to be an "A"; brown on gold faceplate the rear having one (not 2) accessory receptacle.

After being on 120VAC for a couple of minutes it was smoking - wire insulation near one of the 2 high wattage resistors. On mine, the one causing all the heat is unlabeled (as to its value) and wired to terminal 2 of the 5AR4. The next resistor is clearly labeled as a 1250-ohm 10W, not even close to anything shown on the LK-72a schematic, but close to the 1.2k/10W shown near this location in the LK72b schematic.

Do I have some other variant of this product or just a horribly mis-wired example of a kit amp? Thanks in advance. I plan to recap anyway but I am worried that my best bet is to fully disassemble it. I plan to restore this to original function.
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Old 25th March 2010, 01:21 AM   #2
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Maybe I should have used less words. Is it more likely that this is wired per some other 3rd schematic that I don't have, or more likely that it is simply miswired? I guess in short, how common was it for people to miswire these kits?
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Old 25th March 2010, 02:36 AM   #3
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I don't have the diagram so I can't help. But to answer your last question, in a kit anything can go wrong. Many people build kits without consulting the instructions. Others can't follow instructions. It's a crapshoot.

If something in the diagram disagrees with what you have, try to understand what's going on and see if a circuit change will fix anything. A smoking unit is not good.

Shoot me a diagram.
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Old 25th March 2010, 02:55 AM   #4
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Okay I found diagrams for the LK-72A and LK-72B. The resistor and capacitor values in the power supply are quite different but I suspect if you change to the B version you will be okay.

Let me know what you have and I'll direct you if I can. The two versions have either a 20 Ohm or a 33 Ohm resistor coming from the 5AR4, but they don't come from pin 2; they come from pin 8.
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Old 25th March 2010, 04:59 PM   #5
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Welcome to the world of H.H. Scott where what you think you have is not necessarily what you have. Countless times I have run into this in both kits and factory assembled amps. Note also that the factory often assembled a significant number of kits during the development process, and these have invariably ended up on the market at some point. They don't always follow the existing documentation - poorly or undocumented running changes seem to have been common at Scott.

I bought an LT-110A tuner years ago that looked like a standard LT-110A until I turned it on and discovered that it had a stereo indicator that actually illuminated when a 19kHz pilot tone was detected. (Extra tube and coil on the chassis should have been a clue.) Taking it apart it almost had to be factory assembled or built by someone at Scott as it had all factory parts and the chassis was punched, not hacked for the extra coil and tube. (Build quality looked just like any factory product on the inside.)The LT-110A did not have this feature.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine

Last edited by kevinkr; 25th March 2010 at 05:01 PM.
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Old 27th March 2010, 01:05 PM   #6
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Based on the (low) quality of the soldering on this unit, I believe it is a consumer-assembled kit. If anyone has a manual for either of these, it would be greatly appreciated. I have never seen a manual for this or any similar product and have no idea what H.H. Scott was instructing people to do.

I'm thinking of just desoldering all of the components, clean it up, and resolder it all.
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Old 27th March 2010, 03:32 PM   #7
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Originally Posted by griff199 View Post
Based on the (low) quality of the soldering on this unit, I believe it is a consumer-assembled kit.
An LK number does indicate a kit.

community sites,, ........ commercial site planet10-HiFi
p10-hifi forum here at diyA
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Old 27th March 2010, 04:33 PM   #8
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The electrolytic capacitors are probably bad (shorted). Replace them all and replace all of the coupling capacitors too.
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