KR-5150 restoration

Brian1970

Brian1970

AK Subscriber
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I'm finishing up my Sansui AU-555 and started looking for a new project. Found a KR-5150 and I guess I'm developing a soft spot for capacitor coupled quasi-complementary amplifiers. Also it was cheap, there was documentation in the AK database, and I wanted something I could learn to align a tuner on. I won't have it for at least a few more days but I'm already looking forward to digging into it. I did some googling and found a lot of people are pretty fond of this one even though it's not the best looking receiver. I didn't see any particular problems with them but if anyone knows anything to look for, I appreciate any help. I'm still a bit new at this so I thankfully accept any assistance, or just happy to share with anyone who wants to follow along.

Edit: I did notice the 2SC458's on the schematic so those will get replaced. Probably with KSC1815's unless there's some reason to go with something else in this application.
 
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Brian1970 said:
Found a KR-5150 and I guess I'm developing a soft spot for capacitor coupled quasi-complementary amplifiers.
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Excellent. ;)

I liked the look of the 5150 and then found a couple of 6160 units, so I restored those instead. Very, very similar... just more output.

Brian1970 said:
I did notice the 2SC458's on the schematic so those will get replaced. Probably with KSC1815's unless there's some reason to go with something else in this application.
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The KSC1845 is much lower noise than the KSC1815, so they'd be "better" in audio signal path. You just have to be careful about how much current the actual circuit needs in that spot as the 1815 handles more than the 1845 does.

You can find more in the guide-to-Kenwood-restores here: https://www.audiokarma.org/forums/i...epair-recapping-and-servicing-threads.789484/
 
I got the KR-5150 today and didn't have much time to do anything with it. Put it on a DBT and that went fine so hooked some speakers up and tried FM. The left channel's dead and right is distorted. There's some ac hum, a couple bulbs burned out, probably some other minor stuff. Then I had to leave and just got back. Didn't even have time to check the fuses yet. Hopefully I'll get to do a little more checking it out tomorrow. Definitely this weekend.
PXL_20230601_193834432.jpg
 
give all the controls a thorough cleaning and there's a good chance that your dead channel will come back and the other will lose the distortion then you can re-evaluate it. i did i tx140 and a kr4140 that both had the same symptoms(minus the hum). an mcs receiver also, btw.

not to say that there is nothing else wrong but controls are where i'd start. you don't know if the hum is in one channel or both so no point in trying to track that down just yet even tho at that age, i'd plan on a recap anyway.

if you do have to dig into a dead channel, you could most likely do the whole repair with a dmm and a diy signal tracer.
 
I figured I'd open it up and give it a good visual inspection first. At first glance it didn't look too dirty. Just a little dust in there. Not bad really. Then on closer inspection I saw R68. It's cooked and the ohm-meter says it's open. Weird thing is it looks like it's on the right channel side of the board and that's the one that sort of works.

I did clean the controls just while I thought about what to check next. Decided to check some transistors near that burned resistor. Found one on the board that looks a little suspect. It seems to be conducting backward and forward between the base and emitter. Also found one output transistor on the right channel is bad which seems like the more likely cause of the burned up resistor, although I was expecting Q3 to be the bad one, it looks like it was Q4.

Definitely going to have some work to do.
PXL_20230602_212303953.jpg
 
Did some more poking around and found all but one of the 2SC458's look like they might be shorted collector to emitter. You might be able to see D2 in the pic above. I'm not sure if this was a factory change but there's only one diode there, where the board and schematic have two in series. There's a D1 and D3 for the other channel with the same single diode across. Those diodes are open on both sides. Also found a couple other diodes that look a little iffy. They conduct backwards for just a second. Lots of parts to pull and check. I'm just waiting for the fam to be awake so I can fire up the desoldering gun.
 
I have both channels again! I replaced the burned up resistor and the 2SC458's. I pulled those two diodes and my multimeter diode checker still said they were open. Just for kicks, I checked them with a Peak tester. They're some kind of diode with a 1.3V forward drop so I guess it's 2 diodes in one and my meter must only go up to a volt or something. So I put them back in. Once I was done, I fired it up on the dim bulb just to check for shorts and was surprised there was sound from the tuner coming out the speakers. I've got the output transistors out since one was bad and didn't know you'd still get sound that way. I didn't leave it on for long since I don't know if it's good to run it without the outputs. It sounds a little muddy so I'm sure it can use a recap, which it will get, along with new output transistors. If anyone has recommendations, please let me know. I think I might be able to use 2N3773G but I need to stare at the datasheets a little longer.

Unfortunately I'm not going to be able to get this one done real soon. I'll be away for a few weeks and I don't want to make a parts order that'll sit on my doorstep for that long. But it'll give me plenty of time to work up a list of stuff I need.
 
For your output transistors, you could:
  • Look for a TO-3 or TO-204 packaged transistor, which are less and less available.
  • Look for a TO-220 packaged transistor, many more options and lower cost, and just install it in place of the standard TO-3 packaged one. The TO-220 are designed to replace TO-3 packaged ones. How to replace TO-3 with TO-220.

If you choose which way you want to go, post that here and we can suggest the best replacements for you.
 
dlucy said:
For your output transistors, you could:
  • Look for a TO-3 or TO-204 packaged transistor, which are less and less available.
  • Look for a TO-220 packaged transistor, many more options and lower cost, and just install it in place of the standard TO-3 packaged one. The TO-220 are designed to replace TO-3 packaged ones. How to replace TO-3 with TO-220.

If you choose which way you want to go, post that here and we can suggest the best replacements for you.
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That's really interesting. i had no idea you could do that. It looks like 2N3773G will work for me and they are available right now but they are about $8 a piece. If a TO-220 can give better performance or cost less, i'm interested. Suggestions for replacements would be welcome. Thank you @dlucy !
 
@dlucy I had some unexpected time last night to get my Mouser order together and get it sent in with enough time to get it before I take off out of town, so I just ordered some 2N3773Gs. So no need to worry about the TO-220 recommendations. I did try to find some that might work but had some trouble finding anything that matched the 2SC1111 specs and said anything about being usable in audio amps in the datasheet. I'm still not that good at searching for parts though.
 
I got the new output transistors put in this morning and tried it on AM and FM. AM isn't picking up much. On FM the right channel is noticeably weaker than the left but I think the problem is the tuner output. On AM the static is pretty equal I think. Later today I'll try to get the signal generator and dummy loads hooked up and see how it looks with the old caps in it.

One strange thing I somehow just noticed is there are no instructions for setting the bias in the service manual or owner's manual. I guess it should be pretty standard so I can probably figure it out.
 
Here's how I set the bias. If anyone has a better way, please let me know. I checked the board layout and one of the output emitter resistors on each side is connected to posts on the edge of the board so I used those. One weird thing though, at least on mine. One of the pins is marked with the wrong number. So on the left channel I connected the + lead to pin 41, but it's mismarked 44. It's between 39 and 43 so it's obvious after you compare to the service manual board layout, which seems more correct, but still weird. The - lead goes to pin 45. On the right channel + goes to 42 and - goes to 46. If you get them backwards, you just get - voltages on the meter so not a huge problem. Since I'm going across one 0.47 ohm resistor, I set the bias to 13mV. Seems like an ok place to start and I'll adjust if I get crossover distortion when I look at it on the scope. On mine at least, it seems important to let it warm up a while. When I initially set it to about 12mV, it climbed up to about 16 in the first 10 minutes, then went back down slowly until after about 20 minutes, it was back at 13 and I just called it good for now. The output transistors are room temp.
 
Checked the power output and it may be a little low but it's pretty even on both channels. 29.7W on the left, 28.9 on the right. I'm not sure how they tested the output but the owners manual says 40WPC into 8 ohms. The waveforms look good on both.

I also checked the tone controls, loudness, hi & low filters with a square wave and they look good.

Maybe a recap will help with the power output? I'm going to bump up the power supply capacitor ratings and that might help a bit more, not that I'm really going to put even 25 watts out on this thing much.

I think a recap will help with the sound at least. It sounds a bit thin but if you turn the loudness on, it gets pretty muddy. Turning the bass and treble knobs up a few clicks helps and it doesn't sound bad but I think it could sound a little better. We'll see in a few weeks when I get a chance to recap it. So far it looks like this one won't be that hard to get back in good shape.

Edit: It occurred to me that maybe they were advertising peak watts. If so, the math works out pretty close I think. My wattage numbers are RMS volts squared / dummy load resistance.
 
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i've only worked on a handful of pre 1971 units and none of them made rated power. the 5150 was rated at 33wpc from what i've read (made in 1970) so your measurements are right in line with what i've seen. i've seen it mentioned that it was made in 1970. another source says 71-75. another says 1976. i've seen claims of 33wpc, 40 wpc and 50wpc. :dunno:

the way power was measured was different than modern day. it was sometime in the early 70s that they changed the way things were measured.

measure it at 100hz and 200hz and it MIGHT hit rated power in that vicinity. thd is probably getting high too.
 
I just double checked the owner's manual specs and you're right @painttoad 40 WPC is "each channel driven" into 8 ohms so I guess that's mono? Both channels driven into 8 ohms, it says 33 WPC so I'm not as far off as I thought. Plenty of power for me. Here's a screenshot of the specs in the manual.

upload_2023-6-10_15-6-26.png
 
each channel driven is worded strangely enough to confuse people.

modern ratings will say "one channel driven"

i went with a larger main filter cap on the tk140 and the kr4140 and it will help on what i call "short term continuous power" but watch it on the scope and after a bit, you will notice the peaks will start to sag and bounce a bit before it clips.

fresh output coupling caps definately won't hurt either but do not change the value
 
Thanks Doug. That's exactly what I needed. I guess I could have set the bias a bit lower to 9.4mV and I'll do that after I recap and replace the trim pots. The right channel on this one has gone through some things and that trim pot wiper seems to make poor contact when I get it down around 10mV. It seems fine just a little above that so I went with it for now but I'll bring them down a bit more. The datasheet for the new output transistors doesn't specify an idle current but 20mA should be plenty I think.
 
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