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HH Scott LK-72B

richiem46

richiem46

Retired and enjoying life and music!
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I recently received a Scott LK-72B from the usual place. It is very clean and all original except for the tubes. Both 6U8's were bad, one shorted and two of the 7591's were extremely weak. I put in two NOS RCA 6U8's and s decent pair of 7591's and brought it up on the variac. It powered up ok and plays very strongly. I am quite pleased to say the least. The seller said the balance control is frozen and it is, I have not investigated why as of yet and ordered a replacement.

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The balance control has an extra lug which is tied to ground buss and does not appear in the schematic. I was wondering what the purpose of the extra log to ground was? Thinking maybe to reduce the probability of the balance control introducing hum?

Here is a picture from the assembly manual. The tone pots have the extra lug also but is not tied to anything. Does anyone have any insight into the purpose of the lug?

Thanks in advance for any help!

LK-72 Balance pot connections-from snip tool.JPG
 
Wish the timing was a little different right now.
Recovering from back surgery at the moment with no heavy lifting for some time forward.

Both of my units are enough different IIRC to be of little help.

Not mine but pics off the net....

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"Sometimes" you can free a frozen control by applying heat to the shaft from something like a large soldering iron and then using penetrating fluid, like Kroil or WD40. You would want to take the faceplate off of course to avoid any damage. As you know, they are easy to remove. It takes time but often a control will free up after this procedure. I have been known to use a very, very small torch flame carefully. I guess you have really nothing to lose if you are careful because you may have to replace the control.
 
Thanks for the suggestion kvflyer, I will try it. I did some checking with an ohmmeter and it looks like the extra lug on the balance control is a "center tap" of the resistor element which is tied to ground. The rotor is also attached to ground so at mid-point the rotor lug and the "extra" lug are effectively at the same potential which is ground. The replacement pot I ordered has no such "extra lug" which does seem redundant and as I stated earlier, is not in the schematic.

Guess I will try the "Heat Treatment" to free the control, then ponder the implications of replacing it with a control that does not have the extra lug.
 
Tonight I tried using a small torch to CAREFULLY heat the shaft of the Balance control. After several attempts the control shaft moved a bit. I then uses some contact cleaner followed by WD-40 which helped a bit more. I switched to multiple applications of fader lube over about a 10 minute period. The control loosened up gradually and now rotated freely. I am VERY pleased that I do not have to change out the pot as I was not sure how it would behave without the "Extra" Tap being grounded. I am still curious as to why the lug is in the assembly manual and not represented on the schematic.
 
An update to my delayed LK-72B restoration. I installed 10-ohm cathode resistors so I could monitor the bias current each 7591 output tube draws when setting and balancing the bias. I also replace a lot of the coupling capacitors in the driver and output stages and the negative feedback path. I replaced the two small 4MF electrolytic caps and the old selenium rectifier in the Bias/heater power supply. I received a set of replacement electrolytic caps from Hayseed Hamfest and installed the two 30-30 MF caps in the power supply. The amplifier played quite well before I installed the two caps but afterwards you could really hear the difference. It played stronger even without the loudness setting on. You can see some sort of black "stuff" oozing out of the old caps in the picture below. The 270 ohm resistor across the sections of the first cap read 630 ohms when I tested it so I replaced that resistor also. I will replace the remaining two caps today or tomorrow and install grid resistors on the screens of the output tubes.

I will test the remaining ceracap caps but will leave them in place as I do not want to change the tone of the amp from where it is now. I have read various things regarding the ceracaps but also noticed a change in tone on an LK-72 that I restored where I replaced all of the ceracaps. Not that the tone was bad, just "different" so on this one I will leave the rest in-place unless they test bad.

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Caps with black "stuff" oozing......

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Chassis underside as of today with replacement caps installed.
 
I installed the last two Filter caps in he LK-72B yesterday and checked the Ceracaps that remain in the LK-72B. They all tested good so I left them in place. I played the amp for several hours today and it sounds absolutely great, I am very pleased.

As I write this I am waiting on Fedex as I expect delivery of the Rotel RA-1312 with the RT-1024 tuner which will be my next project. I have been looking for a RA-1312 for a while and the RT-1024 is a very nice bonus.

Here is a picture of the Scott LK-72B with all of the Hayseed Hamfest caps installed.


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