KR-5150 restoration

I've been just listening to the receiver on it so far and that sounds pretty good but decided to hook a laptop up to it and stream some old favorites through it. I have to say it sounds pretty amazing. The detail is really great. Still a few little issues to work out and I was getting a little down on this thing but listening to it for a while gives me some motivation to stick with it. The things I've read about these Kenwoods seem to be true. Really good tuners and great sound.

Here's a pic of the inside to show where it's at overall. I replaced all the brown mylar caps because their coatings were chalky and coming off in places. They tested ok but I've read before that when the coatings are compromised they can fail so I figured it wasn't a bad idea to get them out. The replacements are WIMA polypropylenes which should sound better anyway. The 1uF PP's don't fit in perfectly. They're a little large for the space and sit on top of some other components a bit but they worked. Everything bigger than 1uF are still electrolytics since I couldn't find film caps that were small enough. I tried to replace all the signal path caps with bi-polars as much as I could. Some that were higher voltage are audio-grade polarized though. I left the two green mylar caps in because they looked fine and I didn't have an exact value replacement on hand. I went with 6800uF for the output caps.

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I decided to work on making the lighting a little better so I took the faceplate back off and took a look at the lamps with the dial off. The bulb on the left did look just a bit cooler than the others. I'm pretty sure I looked at them before putting the dial back on. I guess I didn't notice it but oh well, that's what bulbs from ebay gets you. I had a few warm white LED bulbs that I got from Mouser and tried them. Much better. I like the blue dial. The meters are still a blue green but I think I can live with it.
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Now that I have everything running pretty well, I took another look at the big 8W voltage dropping resistor on the bottom of the chassis. It ran pretty hot before but I wanted to see if recapping helped that. It didn't. It still gets up to 130C and it's crammed in right next to the rectifier and some caps. I think I'm going to figure out how to relocate it up to the top of the chassis away from stuff where the heat can escape at least.
 
If anyone has suggestions here I'd appreciate them. I think the part of the power supply that feeds the tuner section isn't optimal and needs some improvement. It's already caused me some problems. I've upgraded most of the components already but there's one more change I think needs to be made.
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Here are R302 and R303 on the bottom side of the chassis.
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That big 8W 330 ohm resistor drops 69V down to about 38V so it's dissipating over 3 watts. When I've checked the temp on it, it's about 130C sitting right under and almost touching the rectifier, and right next to a couple bypass caps. That smaller 4W 820 ohm resistor drops the 38V to ground and is dissipating about 1.5W so it's almost as bad. I thought about just relocating those resistors to the top of the chassis in a place where the heat can be a little farther away from things and get out of the case easier. That might still be what I do. But I'm thinking about some other options.

I've already upgraded most of the rest of the circuit and I think it needed it. The 38V coming from R302 goes to what was a 35V cap that I've upgraded to a 50V already. Then the 38V goes to a 1.5K resistor that was 1/2W and I upgraded to a 2W already. That feeds a 14V zener which was 1/4W and I'm pretty sure is what blew and caused some damage in the tuner section. I replaced it with a 1W part already.

The resistors that are in there seem to be holding up fine but they probably need to move from where they're at now. They're too hot to stay there and there's not enough room to put bigger ones where they're at. So I think I'm going to move these two to the top of the chassis and maybe replace them with the same values in higher wattage. I might be able to fit a 330 ohm 15W heat sinked resistor right above where they're at now on top of the chassis. The 820 ohm would probably be ok with a 7 or 10 watt replacement.

If anybody has better ideas, please let me know.
 

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Well, relocating the existing resistors won't work. The leads were fatigued and broke off when I was removing them. So I guess I'm going with new, higher wattage ones which isn't all bad. That's probably the way I was leaning anyway. Just have to wait for the parts.
 
Here's my fix for the hot resistors that were on the underside. It doesn't look exactly factory but I think it's more failure proof. I used some standoffs to lift the 330 ohm 15 watt heat sinked resistor. The 820 ohm 10 watt resistor is lifted up about an inch over the filter cap even though you can't really tell how far it is in the pic below. They both run about 75-80C now so this is a decent improvement from having 130C trapped on the underside close to a few heat sensitive components. The heat should be able to get out of the case pretty easily now and even though they're mounted about an inch from the big caps, the caps are staying at room temp so I think this works.
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I mentioned earlier that the volume pot isn't quite 100% on this thing. It's got a dead spot on the right channel for the first 5 or 10 degrees off zero. Then the right channel lags in resistance the rest of the way. I can adjust the balance to make it sound good but the balance is at 4/10 to the right and my OCD is having a hard time with it. I did some looking for new pots and finding ones with loudness taps is a challenge. I found this though - https://www.aliexpress.us/item/3256804596759743.html . I figured for under $10 shipped to my door, I might try it. Well, I got it today and hooked up a couple meters to it. Surprisingly it tracks really well between the two gangs. It even says Alps - Japan on the back. Chances of that are probably zero but it seems like it might work. I think the only thing I'll have to do to it is take about 10mm off the shaft.
 
I did some more testing on the original volume pot. Wanted to be sure it's really the problem before I go replacing it. With ohmmeters hooked up to the pot, I played around with the pot, balance, loudness, and tone controls. What I found was I can correct the imbalance with a lot less balance control correction than I need to make the channels sound equal. Also the dead spot seems gone now so maybe just cleaning it a couple more times and using it has fixed that a little. So I think I might still have something a little off in the right channel somewhere. I'm going to do more testing before I replace that pot. I did find that the shaft on the new one is actually just the right length. To fit it, I'd have to drill out the shaft hole in the chassis a little but it looks like the threaded part is too short to fit a nut to hold the cover plate on. It might work ok but I'll be happy to keep the original pot if I can.
 
This morning I decided to hook up the signal generator and scope to see how the amp section is doing after the recap. It's quite a bit better. I was getting a bit under 30 watts output before the recap. Now it's a bit over 35W into 8.4 ohm dummy loads with no clipping (and it sounds way better too). Balancing out the channels only takes moving the balance slider 1/10 to the right now after more pot cleaning. I might be able to live with that. Frequency response is very flat from 20Hz-20kHz. Did some square wave testing and checked all the buttons. Everything works just like it should. There's a slight amount of bass cut with the bass knob set to 0. Turning it to +2 makes the square wave flat but it's a slight difference and probably not a big deal. I'm going to try some distortion checks on it later just to try that out but the output on the scope looks good.

I also decided to give the tuner a final alignment and probably should have just left it alone. It was working pretty well except for the stereo separation, which I still haven't found the adjustment for. One of the ferrite slugs in the RF section disintegrated on me as I turned it. Had to fish out the pieces with a little screwdriver and a magnet. So I need to find a replacement for that slug.
 
I've been listening to the 5150 with a streamer quite a bit today and I'm really happy with how it sounds. Once I find a slug for that inductor, I'll be almost ready to button it up. Still need to refinish the wood cabinet too.
 
Well I found a replacement slug for that inductor in the RF section and got the tuner all aligned. This is the first alignment I've done so it took me several hours to get the procedure all figured out, test equipment set up, and make all the adjustments. It's working great now though. I found one of the hardest parts was finding a quiet place on the dial. Where I live there are a lot of FM stations and really not enough space between them to do the things where you want to only hear the signal generator.

After that was done, I got the metal covers back on and started refinishing the wood cabinet. It was pretty worn and had some stains on it so I gave it a light sanding and put some dark walnut stain on it. After that dried, I put on a coat of Danish Oil. I like oil finishes more than urethane. It'll probably take a couple days to get a few coats on it but once it's done and back together I'll put up a pic of it. I'm pretty happy with how it's turned out. I think I've improved some of the minor shortcomings like the tuner section power supply and it sounds great now.
 
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