Restored X-202 worries

RS Steve

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This X-202 is destined to end up with my brother to pair with his K-horns, it was full of problems being bone stock when I received it. After a full recap that included HH can caps, screen stability resistors, modded bias setting test points, and new 1% large resistors throughout, it had a dead right channel. I went back and triple checked my solder joints and components, feeling confident all was good, I turned my attention to the controls. After finding a few bad connections on the volume pots, I re soldered them and started to get good sound out of the right channel. Thinking I solved the problem, I put the bottom back on and moved the amp from its side to sitting like it should. I turned it back on and got horrible crackling in the right channel while turning the volume up, but once I turned it past 1/4 volume it cleared up and sounded perfect. I again deoxed all controls and tried again, with this test showing the right channel crackling without even moving it. :confused:

By this time I was really frustrated, so it was time to take a step back and breath deeply. I slept on it overnight, and no that doesn't mean I used it as a pillow, :p but I did think about it enough to consider there could be a piece of junk in the right volume pot that moves around and causes these problems. I decided to blow them out with compressed air, and add deox again. This process cleared up the problem, :) wheeeew, talk about a relief!

Now after getting things working properly, I notice a couple things that slightly worry me. One is that there is a slight hum in the power supply, I will consider loosening and re tightening all the screws and see if that helps.
The other concern is the two 250 ohm 7 watt resistors I replaced, the temps that I am getting from them is down right scary. My heat gun is showing well over 300 degrees :yikes: and putting off a pretty good smell of hot electrical odor. I have only seen tubes reading this kind of heat and never something underneath the chassis. Now the original resistors did leave quite a heat stain in this area, but with temps that high, I'm not comfortable until I get some assurance this is fairly normal.

I will test my other X-202 to see how hot these get in that unit, but I don't remember them getting near that hot, it could be just this brand of resistor. :dunno:

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Steve; Those 2-250ohm 7w are in the power supply,(R80-R81) change them to a single 125ohm 15w-20w as they are in parallel. IIRC from the TA-600 they are now currently running flat out or nearly so for dissipation and are really undersized for the job. A 140ohm 15-20W might be a better bet if the B+ is running high in the 1st place plus being able to shed the heat better. Check the farside voltage of the resistor on wall voltage. It should be 395VDC (Farside meaning away from rectifier) nearside is closest to rectifier (should be 415VDC) Adjust as needed. I would get a 125ohm, 130ohm,140ohm, 150ohm, in 15w or 20w as available in stock. 15w might be right on the edge, but 20w would be a good bet. It probably won't be any cooler but it WILL be able to withstand the heat better.

Dave; Shoot me down if I'm blowing smoke out of my a__ here. to me it sounds as if the 250ohm 7w are undersized and doing the same thing as in the TA-600. Bumping the wattage up makes sense to me and adjusting the voltage to keep B+ in line here also makes sense to me. (This is one of those things I'm still learning and Steve is a good Guinea Pig (LOL! just kidding Steve.)
 
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Steve; Those 2-250ohm 7w are in the power supply,(R80-R81) change them to a single 125ohm 15w-20w as they are in parallel. IIRC from the TA-600 they are now currently running flat out or nearly so for dissipation and are really undersized for the job. A 140ohm 15-20W might be a better bet if the B+ is running high in the 1st place plus being able to shed the heat better. Check the farside voltage of the resistor on wall voltage. It should be 395VDC (Farside meaning away from rectifier) nearside is closest to rectifier (should be 415VDC) Adjust as needed. I would get a 125ohm, 130ohm,140ohm, 150ohm, in 15w or 20w as available in stock. 15w might be right on the edge, but 20w would be a good bet. It probably won't be any cooler but it WILL be able to withstand the heat better.

Dave; Shoot me down if I'm blowing smoke out of my a__ here. to me it sounds as if the 250ohm 7w are undersized and doing the same thing as in the TA-600. Bumping the wattage up makes sense to me and adjusting the voltage to keep B+ in line here also makes sense to me. (This is one of those things I'm still learning and Steve is a good Guinea Pig (LOL! just kidding Steve.)

Dave mentioned bumping them up to two 10 watt units, I believe this is what he has used in his X-202 restorations. It's crazy having all that heat directly under the power transformer if you ask me.
 
Take the power transformer temps. Dave once gave a max of 190*F but anything under 150*F is usually ok. I like them as cool as you can make them. Any way you can move them away from under the tranny??? If my recommendations are different from Dave's go with Dave's as he knows the math, and I don't (I get migraines just looking at letters in an equation) so I'm spit-balling it. But I think I'm spit-balling very conservatively.
 
Take the power transformer temps. Dave once gave a max of 190*F but anything under 150*F is usually ok. I like them as cool as you can make them. Any way you can move them away from under the tranny??? If my recommendations are different from Dave's go with Dave's as he knows the math, and I don't (I get migraines just looking at letters in an equation) so I'm spit-balling it. But I think I'm spit-balling very conservatively.
I wonder if using two in series helps spread the heat out better than using just one 125 ohm, I will step them up to 10 watt units just to be safe, but I wish there was some way to shield the heat better. There isn't much room to place them elsewhere, they really made sure not to have anything else near them for good reason. I think they had the originals up against the chassis, that's why there is the heat stain there, I have these with plenty of air space around them. But 300+ degrees is just plain scary to me, anything could catch fire touching them.
 
If you have them out in the air, and the bottom on, you shouldn't have any problems. I've got a couple of resistors in my R-3 that hit 350+ all the time. They are on a terminal strip out in the air near the bottom of the chassis. Lots of depth in this one, but everywhere is covered in parts. So they get the remnants of the heat. With the bottom on and 3/4" feet the cabinet doesn't even get warm. But I understand your worries. Not a lot you can do to alleviate the heat inside other than mounting them on top, but then you get into electrical hazards, etc. I'd just set them up in the air as close to center as possible and just keep an eye on them.
 
If you have them out in the air, and the bottom on, you shouldn't have any problems. I've got a couple of resistors in my R-3 that hit 350+ all the time. They are on a terminal strip out in the air near the bottom of the chassis. Lots of depth in this one, but everywhere is covered in parts. So they get the remnants of the heat. With the bottom on and 3/4" feet the cabinet doesn't even get warm. But I understand your worries. Not a lot you can do to alleviate the heat inside other than mounting them on top, but then you get into electrical hazards, etc. I'd just set them up in the air as close to center as possible and just keep an eye on them.

Yeah, Dave responded the other day that the originals were in there for 60 years with no issues, so most likely not a problem. I just didn't realize they got that hot.

On a positive note, I took a chance polishing the face plate not knowing if it would damage the lettering too much, to my surprise it came out freeking amazing.

How about a little before and after! :thumbsup:

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OH YEAH! Kinda looks like the faceplate on the 80-R I have. You could shave in the dark with a butter knife.
 
So I received my new 250 ohm 10 watt resistors and installed them, I feel better that they should take the the abuse better without failing. I guess one concern I have is that the heat they generate could cause the solder joints to get hot enough to melt or lose some contact. Maybe using a higher heat solder at these points might help? :dunno:

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Steve; like Dave said, if they've lasted 60 years I wouldn't worry about it. The solder usually melts at over 450*F for standard 60/40 IIRC. Never had a solder joint fail on any of these type of resistors with hi current flow and voltage drop but I don't cut any length off either, which in this case would be minimal at worst. They'll be ok.

Larry
 
Steve; like Dave said, if they've lasted 60 years I wouldn't worry about it. The solder usually melts at over 450*F for standard 60/40 IIRC. Never had a solder joint fail on any of these type of resistors with hi current flow and voltage drop but I don't cut any length off either, which in this case would be minimal at worst. They'll be ok.

Larry

Yeah, limited space and position made the leads have to be shortened some, should be good to go now.
 
After snugging up the screws that hold the bells on the power transformer, all the slight hum disappeared. This X202 has the least amount of cross talk out of the 3 I have had on my bench, very, very faint. So with that being said, this lovely Fisher is finished. Super good outcome, the sound these produce is nothing short of stellar!

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Super looking faceplate but your indicator jewels are in the wrong order. "A" is green, "B" is amber and red is power/center channel. Switch the green and amber on the left...
 
Super looking faceplate but your indicator jewels are in the wrong order. "A" is green, "B" is amber and red is power/center channel. Switch the green and amber on the left...
I was hoping someone else would notice, I like it when people pay attention to the details. :beerchug: I popped them out to polish around that area and got those first two jewels mixed up. I noticed after taking those last pictures and fixed the error.
 
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