Need tools? Here's a video showing what I use (with links!): • Must-Have Tools For Wo... I eventually "fixed" this Kenwood KR-44 but I'm not too happy with the fix.
FYI: When you got all the noise off the volume pot it told you two things: 1) the pot is dirty, and 2) because of how loud the noise was, you have DC across the volume pot. This is almost always the result of a leaky coupling cap. The result will be varying DC on the arm of the pot, and that can affect the bias of the following stage. Since you replaced all the caps you most likely ended up fixing this DC problem. But as soon as you suspect DC on a volume or tone control, you should stick a voltmeter on the pot to see. If it is there, it will lead to distortion, damage, or both. NTC thermistors were common to prevent thermal runaway. On most amps I ever saw they were mounted to the output heatsink. I was rather surprised to just see these on the board itself. As it gets hotter they will reduce the bias voltage between Q5 and Q7. I can't find the specs on that SDT-35, but since the pot in parallel is 500 ohms, it is reasonable to assume the thermistor should be about 500 ohms (plus or minus a factor of 2) at normal room temperature: 23C or 70F. The 350 or whatever you measured that thermistor at seems reasonable. Since you needed to basically short that pot out to get a reasonable bias, this should tell you that there seems to be too much current flowing in the collector of Q3, resulting in a larger than expected voltage drop across the thermistor/pot pair. Alternately, that one or both of D1/D2 are bad. More likely the collector current in Q3 is too high. There is a pot VR1 that seems like it will adjust base voltage of Q3 and therefore collector current. This is probably the adjustment to set the idle bias current in the amp. The pot across the thermistor is to set the slope and range of the temperature compensation. Since VR1 is connected as a rheostat, it is reasonable to assume that it could be noisy and considerably higher resistance than it should be, resulting in too much idle current. Let's do some circuit analysis. The schematic tells us that the 26V power lead to both amps has 40ma in it. So 20ma per amp. This 20 ma is divided up between Q11, Q7, and the Q7 base bias string. We need to figure out the current in each of these strings to eventually estimate the correct voltage across the thermistor. Let's start by assuming that about 3/4 of the 20 ma is in Q11. We can correct this assumption later. This puts 15ma thru Q11 and R25. So there is about 7mv across R25. Since there is a nominal .75V Vbe drop, the base of Q11 is +0.75V from the 23V center rail. This means there is .75V across R19, so there is 2.3ma flowing thru R19. We can assume as a first approximation that the base of Q11 draws no power, so this 2.3ma is the current thru Q7. Q7 has another .75V Vbe drop, so it's base is about 1.5V positive from the 23V rail. This makes the voltage across R11+R17 = 46V - (23V + 1.5V) = 21.5V. The combined resistance is 6.9K, so the current must be 3.1ma. Now we can check our initial assumption of the current thru Q11. We have 20ma - 2.3ma - 3.1ma = 14.6ma thru Q11. So our initial assumption of 15ma was very close, and 7mv is the correct voltage across R25. But what is the voltage across VR3/TH1? We know the top of D1/D2 is +1.5V from the 23V rail, so at .75V per each diode drop, the bottom of D1/D2 must be very nearly 23V. But what about the bottom of the thermistor? We know that Q11 has 15ma, so Q9 should have 15ma, and R27 should have 7mv across it. So the base of Q9 is .75V above the 0V rail. This makes the collector of Q5 also .75V, so R23 has .75V across it. I=E/R, so the current thru R23 is 0.75/330 = 2.3ma. What a coincidence, both Q5 and Q7 are running at 2.3ma. Since there is 2.3ma in R23, there is 2.3ma in Q5, and 2.3ma in R21. This is 1/10th of the resistance of R23, so since R23 had .75V across it, R21 has .075V across it. We know that Q5 has a Vbe drop of -.75V (since it is PNP). We can add to that the .075V drop across R21. So the base of Q5 is -.825V from the 23V rail. Now we have what we need. The top of VR3/TH1 is at the 23V rail, and the bottom is 0.825V lower. Now if we knew the resistance we could calculate the current. We can assume that TH1 is about the same resistance as VR3, or 500 ohms. But I think you said you measured it at 350 or so. That is reasonable. So we have 350 || with nominally 250 (pot in middle of range), so the parallel value is about 150 ohms. With 150 ohms and .825V we can calculate the current thru the string as 5.5ma. This would be the current in the collector of Q3. But the source of that current is R11, and we already know it has 3.1ma across it. Our numbers don't add up, but they are close, close enough for the wild voltage assumptions we have been doing. Let's continue onward and compute the base voltage of Q3. Let's take 4ma for the collector current (average of our computations). So there is 4ma thru R15, and .4V across it. Using the usual .75V Vbe, we get .4+.75 = 1.15V on the base of Q3. R9 is the source of the current to develop the base voltage. It has 23V on the right, and the schematic tells us 16V on the left. So 23-16 = 7V across it. At 150K, this is 46ua. Since we want 1.15V across VR1, it has to be R=E/I, so 1.15/46ua = about 24K. Since VR1 is 30K, this seems reasonable, or perhaps a little on the high side. So now you have a way to figure out the correct bias, and how to achieve it (i.e.: clean or replace VR1). You should undo the changes around TH1 and set VR3 back to it's original position if you remember it, or mid-scale otherwise. Set VR1 to 0 ohms, power up, measure the voltage across R25 or R27, and adjust VR1 until they are both 7mv.
Hey... lenny here.. I used to be a bench tech for Lafayette Radio in the 70's and we repaired all makes incl Kenwoods.. yes they were cheap but you couldn't kill them. I love watching you diagnose, and repair, reminds me of MY younger days (I'm in my mid 70's) . Thanks for the vids.
@@RodBeauvex , I heard that some early Shortwave stuff, like the pristine HA-800 I bought not working from a thrift shop Ebay seller - the STANDBY connector wires were just cut off, so no voltage to the RF front stages, was built in Japan by GRE, in July 1971, per the Technician signed label on the bottom cover.
Those thermistors are there to compensate for thermal run away, as the current varies through the transistors, a d the temp of the thermistor changes accordingly, it allows for some fine bias regulation, auto compensation. it's likely the potentiometers are dirty or faulty, remember the receiver is over 50 years old, so some components will have drifted, I'm sure the original design, whilst pretty low end, would have been perfectly stable. Nice little bit of history
exactly. not sure which crazy guy left those thermistor on the circuit board itself !! they should be screwed on the o/p heatsink between those transistors. no wonder transistors got sacrificed for this mistake. 😵
Wanted to say Thanks for these videos. I’m probably weird but they are kind of like ASMR to a geek like me! The older guys won’t know that term 😂 btw I’m 56!
I have the exact same receiver that still works - sounds ok - this exact receiver can also be seen a few times on a shelf in the 1968 Elvis movie Speedway.
If you haven't already, clean those trimmers with deoxit. VR1 and VR2 are the balance control, they balance the two output transistors per side. They should be set to make half rail voltage at the positive terminal of C9 C10. VR3 and VR4 are your idle current controls, should be set to around 30ma through R27 and R28, or 15mv across them each. Once set these amps sound quite nice and are really tolerant of component drift. I have fixed one that was left in a shed for decades and most of the resistors are 30 to 50 percent high, the worst I've ever seen in a receiver. Even with those trashed resistors, It sounds decent and it could be set up with just the trimmer controls. So yours could have an issue with the driver transistors. Sometimes they sneak a germanium PNP into Q5 Q6, they did that randomly. Those often fail with age. The thermistor really should be in the circuit even though it should've been designed to mount on the heatsink, it would work much better there. VR5 VR6 are for the shutdown circuit and you shouldn't have to adjust those. Keith
Way ahead of its time with pre out and pre in jacks. That's really a great feature all receivers should have in my opinion. It also makes it easier ( even if it is 50 years later ) to diagnose. Nice to see it being blessed with a new life.
I own a Kenwood KR-6600. Tons of muscle and not cheap. You didn’t think you were going to be able to use your new blanket did you Aiden??!! Makes a great cat bed! Thanks for another interesting video. Keep them coming.
Don’t give up vintage audio gear can be challenging but so satisfying to get it working again my first repair was a HK 630 wish I still have and love ❤️ how great it sounds so much it made me and vintage audio addict nothing wrong with that 👍
I continue to admire your patience and curiosity. I would have been ruminating over the ROTI the whole time. Blanket bingo party was a great distraction.
The video has so much. The chillest fright from the volume knob crackle, the most content cats, showing how use a receiver as a power amp (obvious now that I think about it), and lots of work on that previously unknown receiver! 15:03 would that be considered amplifier "oscillation"?
11:23 😂 The view of the underside of the circuit board looks like someone shoved a cube of compressed metal shavings from a lathe inside the casing!! Man that must've took some work for whoever soldered that. 😂😂
I watched your video... I wanted to point out something that i see you overlooked.... You were perplexed about the bias problem and focused on that thermistor and what to do about it.. what you overlooked was the fact that there is an adjustable two lead 500 ohm potentiometers Vr03,04 that is in line (parallel) with that thermistor which is in essence your bias adjustment pot which ranges from zero to 500 ohms.. The thermistor is there to only create small offsets of resistance as the unit heats and cools to compensate the changes in semiconductor characteristics as there is thermal changes. The thermistor is not used as the primary resistance factor for bias, just supplementary and will not properly compensate if the potentiometer is set too high in resistance or is open. The bias circuit comprises of the two diodes De01, 02 and De 03, and 04, the 500 ohm potentiometer and the thermistor in parallel with it. Your issue is that the bias pot was likely very dirty or bad. When you jumped out the thermistor you were in essence turning the bias to one end...zero ohms. The "fix" you did was in essence you bypassed both the bias pot and the thermistor and created a very low fixed bias adjustment that wont be able to offset when the unit heats and cools. With the bias fixed so low ,you will likely have a higher spec notch distortion by doing so but on a low end unit like this, i am sure it will likely not be very noticeable LOL. If you still have this unit.. replace the bias pots and then you can do the adjustment to where the bias should be and also remember to keep/put back in that thermistor in line as it was..... There is another adjustment pot which is your quasi complimentary balance adjustment pot VR06. Visually it is adjusted so there is even cut off at the tops and bottom waveform point of clipping or lowest point of distortion or it can be adjusted where it there is half positive VCC between the two output halves.. ( for example. if say QE12 collector is 46V, QE10 should be 23V )
Nice Kenwood receiver you have there. However, you must be careful when you are turning-on those earlier Kenwood and many other Japanese made stereo receivers is that you should bring the volume control all the way down before turning-on your receiver which has the power switch combined with the speaker switch as well. Failure to lower the volume can accidentally blow-out your speakers as well. Later Kenwood and other Japanese made receivers moved the power switch to a separate button from the speaker switch which eliminated this problem as well. The old Fisher stereo receivers and amplifiers has the power switch combined with the volume switch which is also a good thing.
Strangely enough I found one of these in San Diego and I actually bought it at a estate sale for a dollar I took it home cleaned it up a little bit plugged it in and it worked I have old Sansui speakers and to be honest with you it sounded like crap but I put it on the Internet and got 50 bucks for it!
I was wondering if you would consider someone shipping you a old receiver, To work on for content? If you are interested I can send you a list of old receivers I own. I would take care of the cost! Love the channel!!!
hi, i would just like to say, germanium outputs need temperature compensator for the bias. pl use a resistor in series with a suitable thermistor for a bias voltage of ~0.1V. and stick that thermistor on the o/p transistor heatsink(important) with clip/screws between each output pair. thermistor will pull down the bias voltage when the output gets hotter and it will stabilize them . also, you can use slightly higher emitter resistance than the originals at the output transistors, to minimize the chances of thermal runaway & have a little more chance to save the transistors from destroying themselves. hope these will help.
You need a proper listening setup now and a good demo of each piece you repair. Maybe sitting on the sofa, camera behind said sofa and sipping an adult beverage.
I love these old silver and wood grain style stereos from the 70's, as far as vintage receivers go I have a 1977 Craig 5503 which IMO is a nicely built unit. I've read up on the history of Craig and anyone who's been into audio in that era has pretty much turned their noses up at them because Craig was known for very cheaply made stuff. But to get into a higher quality line of stereo receivers Craig commissioned a company in Japan to make the 5500 Series units. The model I have, the 5503 supposedly was the most expensive model of that line going for $650 in 1977. Sadly it didn't help them, their name was so stuck to cheap products that people didn't want to buy them. Supposedly the company that made them for Craig made them too well and ended up going bankrupt because of it. Of course this is just info I found so weather or not it is true is another thing. But from my experience with it the stereo is pretty darn heavy, I'd say it has to be a good 35lbs or so, maybe more. I got it back in 2015 from the original owner who is/was an old high school friend of my fathers growing up. My older brother and I were helping him close his furniture store as he was going out of business. So we were helping him finish his last few deliveries to long time customers who bought new recliners and couches. He had the stereo setup in his store on an old tv entertainment stand that had to be from the 90's. It was his main source of music for the store. According to him he tried selling them in his store at one point so he had the one in the store and another in his house he used for personal listening. But they weren't very popular. He had more luck selling SONY products as far as stereos went. I got it from him as partial payment for helping him with deliveries as I was looking for a decent vintage stereo. At the time I wasn't working so I wasn't able to go out and buy something like a Marantz or a Technics. I haven't used the stereo in a bit as it needs some servicing being 45 years old and all. So finding a place able to service it for me will be the fun part.
Nice logo on the file cabinet! You never said how it actually sounded after repair. In my experience low and mid level receivers can actually sound better than their high end models in the same line of receivers.
Very interesting video. I noticed you replaced the big filter cap (next to the transformer), with one of a similar physical size. I'm recapping an old Lafayette receiver which has a similar size cap but replacements are a fraction of the size. Functionally fine, but it leaves a big hole where the old cap was. Where do you source caoacitors that have the same physical dimensions? Thanks.
I guess you got yourself a class B amplifier. It seems to work without too much cross over distortion. The bias current is so low that the end stage is unlikely to run away. I would keep it like this. BTW, Kenwood is indeed quite cheap, nasty almost. But somehow I like it. So it is nice that you saved this very rare piece of audio.
I have the same sort of problem with my late father's quadraphonic stereo, it's got these four VD1210 varistors, no bias adjustment that I can see , just these four trim pots that seem to be balance I guess. No schematic or manual anywhere to be found, one channel seems to get pretty hot, I'm not sure if it's the varistors or the transistors, I'm not sure what to replace VD1210 varistors with in that case. Btw it's a Xonex ET-750 from 1972, doubt you've heard of it, nobody seems to have..
Tbh I’ve been trying to fix a technics sa-ax6 the display wasn’t working a bought a new one and still doesn’t work but other controls work it doesn’t go on protect mode
You're a genius.....I've half a mind to get on a plane with my temperamental Marantz 2226B receiver under my arm and drop it off at your house......there's just nobody capable of fixing it in the UK!!!
Hi jeremy.dont dis everyone in the uk.whats the problem with your marantz?tech repair is a hobby to me now that ive retired,but youre comments intrigue me.
@@mlchaellill4765 Hi Michael, Sorry I flew off the handle a bit* just had a bad experience with some outfit trying to fix it and 'couldn't find anything wrong with it! Don't think they tried hard enough. Both channels are intermittent especially when first powering up btw. Vintage audio just doesn't seem to be much of a thing here, so there isn't the eco system to support it functioning. Unless you know different!!? * plus I might have had a drink or two!
The amp is still oscillating at clipping. You ignored it when looking at the scope. You need to change the psu decoupling caps. The high temperature was due to supersonic oscillation.
That thing needs a full recap! All the caps are worn out. P’s use contact cleaner for the controlles and IPA! So you sure that the contact cleaner dont damage over time the pots! Use IPA afterwards!
P’s If you do a full recap! Make sure you check thé transistors! If they are Good (don’t replace them) because the orginal transistors are way more better, then replica’s!
The schematic appears to show a 500 Ohm bias set control across the thermistor. Did you check this? Preset resistors can go funny, anyone who has worked on old Hewlett Packard test gear will have come across the dreaded Black variable presets in their spectrum analysers.
Way to push through!!!!! interesting thought exercise and nothing here is probably close to correct. The thermister appears to be a NTC version. Your 3 ohm replacement (which normalized bias) and the cold resistance of 180 ohms of the thermister seems to prove this. But, I'm confused as to when the themister would do anything except at start up. I suppose there would be small variations in resistance in the thermister after warm up in sync with the volume increase/decrease. So the thermister doesn't care about anything but the current through it which causes a change in its temperature therefore its resistance. which then could a fine/variable adjustment of bias during operation while VR3 is the static adjustment when doing a bias setup. Or is it simply protection from over current. As current increases and the thermister resistance decreases the bias is pushed lower protecting the output transistors. Your shorted themister experiment seems to have proven this as the bias went to 1mv. So maybe the impact of removing the themister is loss of a protection circuit but not a real problem for bias control. Did you try reinserting the thermister after replacing the output transistors?
Yes, having the thermistor in place with everything else in place caused high bias. The thermistor original part number is SDT-35. There's not much information that I can find online about this part, but what I can find suggests people think it's a 300 or 350 ohm part. I measure about 500 ohms at room temp, and when I breathe on it to heat it up, it goes DOWN to about 350. So the component seems to be working like the thermistor it is, but I'm wondering if MAYBE it's actually a 35 ohm "nominal" part and it has "crept up" out of spec over the years like thermistors are known to do. It makes sense that it would decrease in resistance as it heats up since that's what I've proven we want to see in that circuit. All the transistors on this board tested good, and obviously they must be at least mostly good based on the fact that it's working. But like I've kind of hinted at in the video, it's not worth it for me to spend anymore time on this thing. Onto the next project!
@@AHFixIt as always, a very informative video. It would be really great if at some point in the future, you return this receiver to the bench and get it working like new, you're 95% or more there.
I noticed that sometimes when you play the test song from your phone it sounds like it is going out of tune or slowing down a little bit. As far as i can see you are using vlc player ? Well exactly the same thing is happening to me when i use vlc player on my phone lol.
Interesting you say that. I've been noticing it but haven't thought too much about it. That's behavior I'd expect to hear out of an analog source like a cassette deck so it's quite odd it would happen with a VLC phone app. You must have perfect pitch also :)
I enjoy you learning. I don't enjoy you not learning enough. I feel in this case you have created a monster that will eventually self destruct. That's where "good enough" usually leads... just sayin'
Technically it's not specification perfect, but if it works... The fun is watching you trying to figure out how to fix with a schematic only, diagnostic nightmare. Maybe the old car wash trick would work😉and replacing the thermistors, obviously they weren't controlling the current as when new. Thanks for another great video and persistence to finish.
Had a similar problem with a Pioneer SX-636 capacitive coupled output circuit. The bias potentiometer was becoming intermittently open which was causing the bias to conduct full on and go into thermal runaway. I would suspect that before messing with the thermistor. That gives you some regulation, but not being attached to the heat sink of the outputs, is lame. I would see if you can find the spec sheet on the thermistor and see if it is working per design first before discarding that device for a resistor, just my opinion.
Would it help to calculate the dc load line and Q point values for those output transistors/bias circuits? This guy helps with calculating the optimum bias voltage from your voltage, resistance and HFE transistor values… optimum Q value in the transistors active range and determining saturation etc ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-qKIEyN_y3d4.html
If you are a bench technician, you just committed two sins! Jewelry on your hands when exposed to live voltages in the equipment and liquid drinks on your workbench with live circuits exposed.
I happen to have a KR 77 that I picked up at a garage sale with box, operating manual and sales receipt from 1970. So yeah this Kenwood line was pretty early, had it looked at and it was given a clean bill of health, use it in the garage. I'm a vintage audio enthusiast and love your videos would like to start working on my own stuff doing some minor repairs, so keep up the good work.
I am very sorry, I got irritated with your thermistor hate. "Bad amp design..." Really ? Designed and build by hundreds of people, sold with a garantie for longer than the shop's floor, and you want to throw out the temp control 😂
Thermistors weren't bad design however, when they fail, are pretty much unobtainable now. Good iuck trying to find one with the right compensation curve.
I hate thermistors too! My Maytag Fridge has one that keeps failing. Keeps freezing the food. Oh, um........that's an appliance, not a stereo......My Bad!😉
