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 Post subject: Scott LK-72
PostPosted: Jun Fri 26, 2009 11:11 pm 
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Besides replacing the rectifer and all paper caps is there anything else I should be looking for. Do I need to replace the electrolytics or can I leave them in. I powered it up with a variac, It pulls 1.1 amp @ 117v and the rectifier is reading -48v. Thanks for your insite.

Wayne

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 Post subject:
PostPosted: Jun Sat 27, 2009 2:43 am 
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Replace the electrolytics. When I get home I will give you the links to the caps I used. I will also list what value resistor I used to replace the first resistor in the bias supply.


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 Post subject:
PostPosted: Jun Sat 27, 2009 11:56 am 
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If you're only getting -48v on the bias rectifier after replacing the selenium rectifier with a silicon rectifier (MANDATORY!), the quad cap (4x75uf, positive case ground) and/or the 50uf 75v cap (B model only) feeding the bias pots are probably bad (leak/short). You should be getting between -56 and -62v or so, IIRC, at the output of the bias rectifier. You should get about -47v (actually, -43v to -45v is the stock value depending on model (A or B), but anything between -43v and -49v is OK) after the dropping resistors on the quad cap (for the heaters of the input tubes).

I've had to replace this bias supply cap on a number of Scott units... some that had otherwise been recapped before. Many think those low-voltage electrolytics don't fail much- however, this one is starting to be a problem on many of these, I'm finding.

Regards,
Gordon.

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 Post subject:
PostPosted: Jun Sat 27, 2009 8:42 pm 
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I haven't replaced anything yet, I powered it up slowly on the variac and took some voltage readings as a reference point. The
-48v is with the stock selenium rectifier.

Wayne

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 Post subject:
PostPosted: Jun Sun 28, 2009 1:09 am 
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Wayne,

Both Gordon and I have done a lot of HH Scott amplifiers. (Right now, listening to Peter Cetera/Amy Grant via Sirius Satellite and what do you think the amp is? Well of course, an HH Scott LK-72 brown faceplate amp of course :wink: ) Seriously,

The coupling capacitors, those white ceramic American Radionic Ceracap capacitors should go. Now in you LK-72, you may have two .05 µFD disc capacitors connected in parallel to your 7591 grids. The early ones did. You might want to replace them with good modern .1 µFD poly caps. Price is very cheap for new modern caps. No need to purchase expensive designer capacitors for $10.00 each, seriously.

And of course, the Siemens selenium bridge rectifier that supplies filament voltage for the preamp 12AX7 tubes and most importantly, the bias for those expensive 7591 tubes. When you replace the selenium bridge with a silicon bridge, the voltage will be higher since the silicon is more efficient than the selenium. You will most likely need to change the first resistor after the bridge. Many LK-72/299 amplifiers have a one watt 18 Ω resistor. You will most likely need to increase the value of it. 22 Ω or 33 Ω will usually bring that voltage right back to where it should be. You might also want to use a five watt wire wound resistor for that job.

I will always replace all capacitors in the audio path but that is a matter of opinion. Electrolytic capacitors are also a time bomb waiting to explode. I have been lucky and most of my electolytics are original. But very soon, I will be removing them and stuffing the original caps with new guts (to preserve the original appearance).

Also, the bias capacitor, the 75-75-75-75 µFD seems to now be failing in many amplifiers. If you see fit to replace it, you must remember that it is a common positive, not common negative. I suspect no one is making them right now. You can use individual caps below the chassis or do a re-stuff thing if you want.

What you have is a very nice performing amplifier when it is brought up to standards. Listen to mine every day!

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 Post subject:
PostPosted: Jun Sun 28, 2009 1:10 am 
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BTW, I have all of the schematics for the various versions of the LK-72 and lots of experience with it. Don't hesitate to contact me...

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 Post subject:
PostPosted: Jun Mon 29, 2009 5:10 pm 
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My left channel doesn't work in the stereo mode. Put the switch to mono and the channel works. Swapped all tubes from the right to left channel but the same thing. Voltages on all tube plates are in spec. Voltages on the 7591 plates are 412v for the left and 407 on the right. I worked the selector switch back and forth but the problem doesn't seem to be in there. What am I missing?

Wayne

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 Post subject:
PostPosted: Jun Tue 30, 2009 2:40 pm 
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Wayne,

Couple of questions;

Did it ever work correctly?

What input are you using, Phono, Extra, Tuner, or Recorder In?

Do you have the correct schematic? There are several.

Try working backwards from the 7199 or 6GH8 depending on the vintage of your amp. You can inject a signal or use a capacitor to inject hum into the grid of the tubes, working back toward the preamp sections.

Remember that this was a kit and if you have only recently acquired it, it may have never worked correctly. I had one LK-72 that I thought was working just fine... until I plugged in a turntable. Left channel was missing, right was OK. So, as you mentioned, all voltages were present and correct. I replaced all of the components around one of the 12AX7 preamp tubes and the problem was still there. Getting frustrated, I marked the chassis with a note, put it away and worked on some other items. A few months later, I took it back out and it worked properly. Don't know what was wrong, could have been as simple as a bad connection on a tube socket, even though all contacts, etc. had been cleaned.


BTW, I was using a signal generator and watching the output of each section with a scope. That first section of 12AX7 had signal in, no signal out and proper voltages. I even thought that the following stage may have been dragging it down. Still don't know what was wrong :cry:

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 Post subject:
PostPosted: Jun Tue 30, 2009 3:23 pm 
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If there is speaker output (that sounds OK) on both channels in mono mode, the problem has to be in the circuitry of one of the 12AX7s. It can't be in the amp stage (6GH8/7199 and 7591s), because they're after the mono switch.

I'd be looking at the circuitry surrounding V3 if the sound is out on channel A on all inputs in stereo, or V1 if it's out ONLY on phono. If it's channel B that's out, then it'd be V4 or V2 respectively...

Regards,
Gordon.

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 Post subject:
PostPosted: Jun Tue 30, 2009 10:32 pm 
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GordonW wrote:
If there is speaker output (that sounds OK) on both channels in mono mode, the problem has to be in the circuitry of one of the 12AX7s. It can't be in the amp stage (6GH8/7199 and 7591s), because they're after the mono switch...


Absolutely. A lot of trouble shooting can be done with the normal signal connected to the amplifier and analysis of the results of switch position when referenced to the schematic.

GordonW wrote:
I'd be looking at the circuitry surrounding V3 if the sound is out on channel A on all inputs in stereo, or V1 if it's out ONLY on phono. If it's channel B that's out, then it'd be V4 or V2 respectively...

Regards,
Gordon.


And I agree also. The circuitry of HH Scott amplifiers is rather straight forward... and it works and sounds well...

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 Post subject:
PostPosted: Jul Wed 01, 2009 7:31 pm 
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I was able to get a signal through each stage through the grids of the 12AX7's. I'm using the tuner mode BTW. Well after all the time spent now neither channel works, all I get is a tinny sound on both channels with the volume turned up. I was hoping for a quick fix to get it working while I'm waiting for the new Solen caps to arrive. The problem is probably bad caps. After I recap I'll report back, Thanks again for your help.

Wayne

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 Post subject:
PostPosted: Jul Thu 02, 2009 12:00 pm 
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Most likely bad caps. Don't power up the amp again until the new caps are installed. Solen caps are a good choice. That's what I run in mine. If you get in any trouble when restoring your amp (provided it is the A version) let me know. I can easily pull the bottom off mine and do voltage measurements if need be. it may seem unnecessary, but I recommend replacing every resistor except the two ceramic ones that stick up through the top of the chassis. It tends to make the amp more reliable. Plus if you use metal film resistors from Mouser Electronics it will be relatively cheap to get every resistor. The only resistors except the two ceramic ones not replaced are a couple power resistors in the power supply section. Wayne, I think I may have mentioned this in either Email or PM, but make absolutely sure your tube socket pions are clean. If after you clean the tube socket pins there is still noise when the tube is moved in the socket replace that tube socket. I had to replace the one for the 6GH8 in the left channel recently. Also check the filament wiring. I was using mine last summer for some DJ work. The amp was running then all of the sudden I saw smoke. Turns out the heater wiring shorted somehow farther on down the chain so I had to re-wire the whole heater circuit.


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 Post subject:
PostPosted: Jul Thu 16, 2009 9:06 pm 
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It's been recapped and all out of tolerance resistors replaced. I found a lot of bad solder connections and one wire that was not soldered at all. The left channel still did not work but the problem seems to have been in the switches, especially the stereo selector and rumble switches. A lot of Deoxit cleaner seems to have fixed the left channel problem. I have the service bulletin and I'm confused about the balance adjustment. One section says to adjust the bias pots to 70ma measuring from pin 5 on the 7591's. Another section has you connect an audio cable from the bias test point to the respected channel inputs and moving the bias switch back and forth while listening for clicking sounds. Wouldn't adjusting for 70ma be easier? Thanks for you help.

Wayne

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 Post subject:
PostPosted: Jul Fri 17, 2009 12:36 am 
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I am not at home to check the manual, but IIRC the service manual I have says to pull the driver tube on one channel, connect a 16 ohm load to the 16 ohm speaker terminals, connect a voltmeter across the load and then adjust the bias pot for that channel until you get the lowest AC voltage across the 16 ohm load you can get. That works provided you only have two bias pots.


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 Post subject: Re:
PostPosted: Nov Sun 20, 2011 1:48 pm 
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Don Cavey wrote:
BTW, I have all of the schematics for the various versions of the LK-72 and lots of experience with it. Don't hesitate to contact me...

Don, would you have the schematic for a LK72, with the 7199 tube, has no bias or balance controls.
THanks
Dan


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 Post subject: Re: Re:
PostPosted: Nov Sun 20, 2011 3:12 pm 
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danno999 wrote:
Don Cavey wrote:
BTW, I have all of the schematics for the various versions of the LK-72 and lots of experience with it. Don't hesitate to contact me...

Don, would you have the schematic for a LK72, with the 7199 tube, has no bias or balance controls.
THanks
Dan


Yes, I do. I think I have a hard copy that I could mail to you. Send me an address via PM and give me a couple of day to look for it.

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