My Scott LK-72 Restoration -LONG Journey But Not There Yet
- Thread starter fredi898
- Start date
Calling it DONE (Phase 1 anyway) finally! 
Got the bias current dialed in exactly at 35ma after reverting the grid reference resistors to 330K. I did have to move the bias voltage from the last to second last resistor from the heater voltage dropping resistor string. Also managed to get the 100-ohm screen resistors added.
Will take a break and enjoy the fruit of the labour for a while
. Upgrading the 0.1 uf caps to something better may be in the future but I think i won't worry too much just yet.
Thanks Jef for all your help! I would have struggled for another month for sure.
A couple of questions you asked earlier: Yes the stock balance pots has 3/8" bushing. I was contemplating to replace the Allen Bradley pots I used for Bias pots, which have 6 mm bushing. It turned out "audio taper" pots worked just fine as you said. Yes the wire from the 10-ohm cathode resistors are connected to test points on top of chassis. Will post pictures in a separate thread.
Hope your fine-tuning project is going well too.
Cheers,
Fred
Got the bias current dialed in exactly at 35ma after reverting the grid reference resistors to 330K. I did have to move the bias voltage from the last to second last resistor from the heater voltage dropping resistor string. Also managed to get the 100-ohm screen resistors added.
Will take a break and enjoy the fruit of the labour for a while
. Upgrading the 0.1 uf caps to something better may be in the future but I think i won't worry too much just yet.Thanks Jef for all your help! I would have struggled for another month for sure.
A couple of questions you asked earlier: Yes the stock balance pots has 3/8" bushing. I was contemplating to replace the Allen Bradley pots I used for Bias pots, which have 6 mm bushing. It turned out "audio taper" pots worked just fine as you said. Yes the wire from the 10-ohm cathode resistors are connected to test points on top of chassis. Will post pictures in a separate thread.
Hope your fine-tuning project is going well too.
Cheers,
Fred
Fantastic Fred!
You have a very nice unit!
How are your B+ voltage compared to the schematic? I see the CL-80 installed.
And very important, how close are the heater voltages? Do you have -45vdc or so going to V4 per the schematic, close to 6.3vac going to V5 thru V10 string?
Yes, my fine tuning on my LK-72 is about done. Ready to put it in the wood cabinet.
Jef
You have a very nice unit!
How are your B+ voltage compared to the schematic? I see the CL-80 installed.
And very important, how close are the heater voltages? Do you have -45vdc or so going to V4 per the schematic, close to 6.3vac going to V5 thru V10 string?
Yes, my fine tuning on my LK-72 is about done. Ready to put it in the wood cabinet.
Jef
With CL-80 and input voltage of 120 Vac, B+ voltages are very close to spec. The preamp filament voltage is -48.8, which I know is higher than spec, but I figure it is still safe (less than 12.6 volts per 12AX7). Heater voltage for other tubes is close to 6.3 vac.
Before the bias adjustment circuit was added, I ran the amp with new Tung-Sol tubes for a couple hours (not knowing better). I didn't have a way to measure the cathode current then and I am sure the tubes were driven pretty "hot". Knock on wood, the tubes having passed this "stress test", will now live a long life with properly biased operating condition
.
Fred
Before the bias adjustment circuit was added, I ran the amp with new Tung-Sol tubes for a couple hours (not knowing better). I didn't have a way to measure the cathode current then and I am sure the tubes were driven pretty "hot". Knock on wood, the tubes having passed this "stress test", will now live a long life with properly biased operating condition
.Fred
I wonder why Scott run the series string heaters at 12.6? I think every schematic I've looked at show 45ish volts..........
I made this graphic yesterday morning Fred but you were too quick and didn't need it. But so it's not totally wasted, here it is:
So how it sound to you Fred??
Jef
I made this graphic yesterday morning Fred but you were too quick and didn't need it. But so it's not totally wasted, here it is:
So how it sound to you Fred??
Jef
HBTim
Critical listener of beer fizz
Not sure but for me when I posted the popcorn it means I am watching this thread like a movie...
Thanks for taking time with the graphics Jef! Your earlier description was pretty clear already . I am sure someone will benefit from it.
I only played a couple LPs last night. Pretty happy how it turned out. I don't recall ever hearing all the details before with my SS phono preamps. To my untrained ears the vocal was natural and transparent and has impressive presence. The highs were a little on the harsh side. I will let it "burn" in for a few weeks before making any conclusions/further improvements. After all a lot of new parts have been installed (caps. film resistors, 7591 and 6U8A).
One improvement that I can definitely declare is the temperature of the transformers was a lot cooler after playing a couple of hours than before.
Cheers!
Fred
. I am sure someone will benefit from it.I only played a couple LPs last night. Pretty happy how it turned out. I don't recall ever hearing all the details before with my SS phono preamps. To my untrained ears the vocal was natural and transparent and has impressive presence. The highs were a little on the harsh side. I will let it "burn" in for a few weeks before making any conclusions/further improvements. After all a lot of new parts have been installed (caps. film resistors, 7591 and 6U8A).
One improvement that I can definitely declare is the temperature of the transformers was a lot cooler after playing a couple of hours than before.
Cheers!
Fred
HBTim
Critical listener of beer fizz
Oh, no worries man... did not even take offense. I was wondering if I was using it wrong
I've been playing my LK-72 into some Dynaco A25 speakers. Really sounds damn good! Great clean detailed sound. Very quiet between tracks.
It matches the LK-72B schematic from 8/26/63. Date codes on the transformers and cap cans all date to summer of '63.
I've done about everything to it that can be done restoration wise I believe. But kept completely stock. All cap's replaced cept for the silver mica's. Can caps restuffed. Almost every resistor changed to metal film, the 1 watt plate resistors are now carbon film. In the power supply, only a couple of resistors changed as needed. All four PEC's replaced with metal film resistors and NP0 cap's.
Right now it has new production Tung-sol 7591's, NOS matched JAN Phillips 6U8A PI tubes (from Blue Glow Electronics) and the four 12ax7's are matched Genalex Gold Lions. The 5AR4 is the original Scott tube. It had all the original tubes when I bought it 10 years ago and all were getting too weak except for the 5AR4.
I plan to listen for a period of time then install four NOS Westinghouse 7591's. I just bought two Matsushita NOS 5AR4's, may sub one in to have a listen.
Jef
It matches the LK-72B schematic from 8/26/63. Date codes on the transformers and cap cans all date to summer of '63.
I've done about everything to it that can be done restoration wise I believe. But kept completely stock. All cap's replaced cept for the silver mica's. Can caps restuffed. Almost every resistor changed to metal film, the 1 watt plate resistors are now carbon film. In the power supply, only a couple of resistors changed as needed. All four PEC's replaced with metal film resistors and NP0 cap's.
Right now it has new production Tung-sol 7591's, NOS matched JAN Phillips 6U8A PI tubes (from Blue Glow Electronics) and the four 12ax7's are matched Genalex Gold Lions. The 5AR4 is the original Scott tube. It had all the original tubes when I bought it 10 years ago and all were getting too weak except for the 5AR4.
I plan to listen for a period of time then install four NOS Westinghouse 7591's. I just bought two Matsushita NOS 5AR4's, may sub one in to have a listen.
Jef
Nice restoration Fred, I just restored my first Scott that I received back in 2015 (299-C). It was quite a challenge after learning what I know on Fisher and Dynaco equipment. I had to start and stop a bunch of times to study and get an understanding of the chassis. Now on to the second 299-C, which is a later unit that has the black knobs and adjustable bias, the early version only has balance adjustment. I dug up a pair of old Sylvania coin base and Sherwood 7591's I had stashed away and installed them, they seem to pair up well and sound great. Can't wait to do some comparisons between the Scott's and Fisher's I own.
HBTim
Critical listener of beer fizz
Man that is great!! I have two LK72’s.. one champagne face and one Chocolate face. I really really really love the look of the Chocolate face. It needs to be gone through. But, the champagne face plays like a dream. I was a huge SS guy and had all kinds of SS... after listening to the Scott, only one SS will stay. The Pioneer SX-1010 fully restored. To me, the 1010 is as good as SS can get. Now, I also have a S-5500 IV Sherwood in really really nice shape that also needs to be gone through. Can’t wait to hear that, but can’t imagine it being better but maybe different than the Scott. Anyway, just thought I would share.
Depends on if you're married, and what the wife will put up with.
Yes, relatively easy......
For the tone control PEC's which were connected to the Scratch switch, I drilled out the two rivets and removed the OEM terminal strip near the scratch switch. I mounted three new terminal strips using the rivet holes. Then based on the schematic, I sketched out individual component layouts showing the new resistors and cap's going between the Scratch switch and new terminal strips. I also replaced the neon power lamp with an LED powered off the 6.3v heater ckt. I used a grommet to hold the LED in the original yellow lamp plastic jewel. The LED used the 3rd terminal strip.
For the phono PEC I added a terminal strip and strung resistors and cap's between the Equilazation switch (where the PEC was mounted to) and new terminal switch:
Jef
fredi898,
Great work! Just picking up on this thread. My LK-72 is out of rotation right now but needs to be put back in--jazzed after reading this thread. Sorry I didn't weight in earlier, but you got excellent support from the gang. (and thanks for the props, guys!).
Dave
Great work! Just picking up on this thread. My LK-72 is out of rotation right now but needs to be put back in--jazzed after reading this thread. Sorry I didn't weight in earlier, but you got excellent support from the gang. (and thanks for the props, guys!).
Dave