KR-5150 restoration

painttoad said:
replace the trim pots when you replace caps.
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Planning to. I got new ones. The original ones are a real pain to get at since they adjust from the side. You need a really short screwdriver and it's still tough to get to them. It'll be worth it just to have the adjustment on top, even if the original ones were in better shape.
 
Finally got a chance to do a little work on the 5150 this afternoon. Got the 3 oil filled caps replaced. They were in surprisingly good shape. Still tested fine but I replaced them with new film caps anyway. Also replaced the main filter cap with a 6800uF Nichicon. The original 2200uF read a little high at 2600uF but still had low ESR. It might have been ok for a while longer. Still have a couple axial caps in the power supply section to do and I'll try to get them done later this evening. The power supply section is probably going to take me as long as the rest of the re-cap. Things are a little tight and all the point-to-point wiring all over the place makes it a little tough to get to things.
 
Got the power supply done and was listening to FM a for a bit. Everything was working great for about 10 minutes, then there was a pop and no more FM. The TO-220 transistor (2SD234) that sends power to the MPX board celebrated the 4th a little early. I checked the voltages right after powering it back up and they looked ok so I'm reasonably sure I didn't make a mistake in re-capping the power supply. I unhooked the wires from the 2SD234 to be sure and it's open from base to emitter, shorted from base to collector. The DBT is still dim so hopefully no major damage. I don't have any subs on hand so I'll have to order some but before I do that, I'll check the MPX board to make sure it looks ok. There is a 2SC548 on the MPX board that I didn't see when I was replacing them on the amp board.
 
I noticed something interesting and it might be why that 2SD234 blew up. The tuner board on this receiver looks like it grounds to the chassis by being screwed down onto it and all the screws holding my tuner board were pretty loose so I may have had a weak ground connection. Worth checking I guess on these models. Looking a little closer I can tell there were a couple places where the board was soldered to the chassis on the underside. Looks like someone removed my board at one point and didn't redo those. Time to bust out the big soldering gun.
 
Waiting on parts is probably good for me. It forces me to think a while which I need. Figured I should look a little deeper for reasons that transistor may have popped. Found the 1S338T diode was open. It didn't look open in circuit but there are lots of other components around it so I'm glad I took it out and tested it. Best I can tell, it's a 14V 1/4 watt zener and I don't have any of those of course. It's in series with a 1/2 watt resistor so I figured bigger would be better. Just ordered a couple 1W ones. If anyone has better info on that zener diode, please let me know.

The 1.5K 1/2 watt resistor reads a little high - 1.67K so I might replace that too. I think I have some 2W 1.5K's. I'm not sure I'm a fan of the power supply. It uses big resistors to drop voltage and uses components that are barely adequate for the voltages and wattage so I'm tempted to upgrade everything in it but I'll try to just get it working first.
 
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Got the burned out zener diode and pass transistor for the tuner & MPX board replaced. Voltages look good now but there was some damage. On FM it'll tune stations but I'm only getting sound from the right channel and both meters are dead. I checked an Aux input and I'm only getting sound from the right channel there too so looks like some damage to the pre-amp or amp too. I put a signal in the power amp inputs on the back and flipped the switch. This is weird but I'm only getting sound from the left channel when I do that. So definitely some problems to work thorough. And I thought this was going to be a simple recap. Oh well, I won't have much time to work on it this week but hopefully next weekend I can start chasing down some of these issues. Since the toasted components provide power to the tuner and MPX boards, I guess I could start there but it seems like it might be good to get the pre-amp and amp working right first. If anyone has thoughts, please let me know.
 
If you think you've toasted components in the stereo, signal-carrying portions of a circuit, you could build and use an Octopus device or Huntron Tracker to compare each component in one channel with the same component in the opposite channel... to narrow down which are damaged.

This won't help you much if you're troubleshooting the prior-to-stereo portions of circuits.
 
I finally had a little time to work on the 5150 this morning. I glad the channel outage switched from one to the other when I flipped the "Normal / Separate" switch in the back since that led me to try cleaning it (which I hadn't done yet). That fixed that at least and now I have sound coming out both speakers through just the amp and also the pre-amp when using the Aux inputs and I'm at least getting mono through both speakers on AM and FM.

When the 14V power supply to the tuner and MPX board went, I don't know what happened first. I guess either the zener went open and pushed about 40V into the pass transistor base and made that blow, or maybe the pass transistor popped and caused the zener to open? Both the AM and FM tuners seem less sensitive than they did before. On AM the signal meter seems to work. I'm getting about 100mV at the meter when tuned to a station. On FM I thought the meter wasn't working at all but it's actually barely working. I'm getting about 5mV at the meter when tuned to a station and the meter is barely deflecting. I'm not getting a stereo light but that may have got burned out if there was a 40V surge. I don't hear any stereo separation though so I don't think that's working. I checked just about every component in circuit on the tuner and MPX boards and don't see any big problems. I think I might just go ahead with recapping and pop a few other components out as I'm doing that to test them out of circuit, then try aligning the tuner to see if that helps. My tuner IF board is a different revision than the one in the schematics I've found and there are some differences but I think it'll be close enough to figure it out.
 
Had a little time today to work on the 5150 and checked voltages in the FM part of the tuner board. Found they were weird around Q1 so I checked that one again. It looked maybe not quite shorted collector-emitter so took it out and it was almost shorted. Very leaky at least. I probably didn't check for C-E shorts last time I went through and checked. Q1 was an SE3001 which looked pretty unremarkable except for a 600 MHz fT. The highest fT transistor I had on hand was a 300 MHz 2N2219A so I tried it and that fixed most of my problems. Sensitivity seems about back to the way it was, the signal strength meter is working, and the stereo light works again. The only thing still not quite right is the fine tuning meter. It's off center and only deflects to the left so I'll have to figure out if there's still something wrong or how to adjust that. The alignment instructions say to use the discriminator adjustment but that doesn't seem to move the needle and seems like it might be a typo. Ran through the alignment and it seems pretty good but I'm not quite happy with the stereo separation. Looks like it could be better so I'll work on that later. It might still have some issues but making progress at least.
 
Went to check out the AM section of the tuner today and no sound from the right speaker. It was the Normal/Separate switch again. I guess that thing's going to have to come apart to be cleaned or be replaced.
 
I think I found at least some of the issue with the AM tuner. Grounding. As I was checking voltages, I checked a couple ground tabs near the input of the AM antenna and there was a little bit of voltage there that went slowly down to zero when I touched the voltmeter probe and the sensitivity got better so the board was finding a better ground through the meter than from the grounding pads touching the chassis. So I put a little solder blob from the ground pads to the chassis and it helped a lot. It's still not picking up as many stations as I think it should but it could just be all the RF noise in the house. That might get better when I put all the covers back on but that's going to be a while yet.
 
I got up early and don't have to go to work until later today so I decided I was tired of burned out light bulbs. I replaced all of them with warm white LEDs. I'm not completely happy with it. Looks like the paint or whatever the colored stuff is on the back side of the dial face got cooked a bit unevenly, I guess from one of the bulbs being burned out for a long time. The bulbs are the same color but the dial looks blue on the left side and green on the right. Looks decent though and at least all the lights work now. I also played around with the knobs. They were all off-center before and I tried moving them around to different positions until I found a combination that made the white marks point as close to where they should point as I could.
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I figured out the fine tuning meter. It is actually the top adjuster on the discriminator can that adjusts it so it works right now.

Started on recapping. Did the tuner and MPX boards so far and pretty much all the electrolytics were 20-40% high in capacitance but ok on ESR so they weren't horrible but probably isn't hurting to replace them. I also got a little bit of the pre-amp/tone control/amp board done and pretty much the same story but did find a couple caps with ESR double or triple what it should be. So even though they're holding up well for almost 50 year old caps, it's probably time for them to go and I expect it to sound better with new ones.

I'm still debating whether to do something about the lighting. I have some green fuse type LEDs so maybe I'll just go green with it. I'd have to change the fine tuning meter LED to a green one too so it matches but I think it might look better.
 
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I got the recap done except for the output caps. I found almost all of them were about 20-40% high in capacitance and ESR was a bit high on quite a few of them. When I got done I plugged it in on the dim bulb and there was a noticeable difference. Before recapping, the filament was dim but lit. Now it doesn't even look lit up. So cumulatively it was drawing more current before. Exactly how much I don't know but looks like it was dissipating a few extra watts I guess. I didn't really give it much of a listen yet. I'm resetting the bias now and giving it 30 minutes to stabilize. Then I'll give it a listen.
 
I listened to the 5150 for a bit today and the right channel still doesn't sound great. It's a bit weaker than the left, which I noticed a little before but it's more noticeable now. Left channel sounds really good but the right is missing a lot of the background of the music, mostly the highs and lows. Since the right channel had the bad output transistor, could be the output cap got a little stressed when it went? I'll replace those sometime in the next few days and see if that's it. I still need to take the Normal/Separate switch apart and clean it too I think.
 
I got the output caps replaced and the new ones do sound better but the output still seems a bit lower on the right channel. It doesn't seem to be the normal/separate switch or speaker switch. I checked the volume pot and I think that's the issue. The right channel resistance is always lower than the left and there's a bit of a dead spot on the right channel in the first couple mm. I can even it out with the balance control but it's pretty far to the right. It's probably time to hook up the signal generator and scope to make some real measurements instead of just using my uncalibrated ears.
 
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