I just did a repair video on a KR-720 with some blown transistors. It had similarly undersized heat sinks that were always warm even at idle. It had trouble with biasing because of bad solder joints on the bias resistors. Hardly any capacitors in the amp stages. I love the digital display on these amps although some of the controls are a little cheap feeling. Sounded great after I was finished. I really enjoy your videos!
Found this video cause I recently gambled on one of these at a goodwill and was impressed with the sound compared to my other receivers. I had to clean a bunch of dust out before the tuner dial was able to move again but so far so good.
I always replace with sealed multi-turn trimpots for the bias and dc offset if I plan to keep it! They are expensive and so it depends upon the value of the device as you know. The mid '70s Kenwoods were excellent across the board but they later became less well built and more unreliable. Their best engineers left to form Kensonic Labs, (Accuphase).
There was a similar implementation that was done by Technics, 12V Videos did a repair on one, although the Technics version is better to operate... I have always been a fan of Kenwood car audio equipment, my son recently purchased a new 5 channel car amplifier and I'm impressed that even after all these years they are still putting in a lot of quality into them! My old Kenwood car Amplifiers used to run hot but never failed me even though they were given plenty of abuse on really hot days! The new 5 channel amplifier still generate a lot of heat but I'm not concerned. The Kenwood engineers sure did things differently, at least it didn't show up on your bench pre cooked! Considering how hot it gets with those small heatsinks! But Kenwood was always like that. I have seen amplifiers with larger heatsinks on your bench that didn't survive, reliability and long life is probably what most Kenwood owners are familiar with.. it's what keeps us coming back. Incidentally, just before my son got his Kenwood car amplifier, I had decided to switch to a pioneer car amplifier that appeared to be much better on paper than what Kenwood had to offer but I might a mistake, I should have stuck with Kenwood, the pioneer of today isn't the pioneer that used to be... The pioneer doesn't have the typical pioneer way of doing things that got them their name in car audio, where as the Kenwood has all their traits, the power supply is a beautiful work of art on their current models and the same attention to detail is found on their older stuff, their higher end stuff, like most manufacturers, the low end stuff left lots to be desired...
I believe that "it sounds good" is understatement: the sound of this receiver line is simply amazing. It is far superior to many other receivers I went through - Marantz, Yamaha, Pioneer, Onkyo, etc. KR710-750 are not for playing hard rock at club levels through big speakers, but if you are into acoustic music like classical or jazz, look no further. They will do justice to any reasonably sensitive high end speakers. I have 710, 720, 725, and 750. They all have the same sound character - clear, neutral, detailed, very musical. I like the factory-set highish bias current of the 750 - it is done on purpose to have the first few watts in Class A. All others run output transistors at only a few milliamps. The bias current can be adjusted up to about 300 mA, bur it will require additional heat sinking. I did it with my 720. Heat sinking is indeed weak point of these receivers. I got my 710 and 725 for free because of blown output transistors.
Wow, what a horrible heat sink design. Great stereo to use as a coffee warmer. The Pioneer SX-1250 I restored the other week barely gets above warm and that was with it running full bore at 165 watts for extended periods of time during final testing. The heat sinks in this thing might be ok for a 30 Watt stereo but are grossly undersized for a 65 watt stereo. You can definitely tell the bean counters got involved in the design of this one cutting corners wherever they could. Personally I'll stick with my all tube Fisher Model 800-B AM/FM stereo.
That really needs a fan to be added to it. Looks like they got the tuning part out of their Car stereo division. You should add an analog meter to your setup, that would look cool ☺ Thanks for yet another excellent video. Have a great New Year.
So nice to see you do a non Pioneer receiver, as much as I love them. This tuner is crazy! Can you do a video someday dedicated to biasing? Tubes and transistors! Thanks, Arthur
Another great video. Looks like the intention there was the convenience of tuning in your favorite stations. If you get a chance please go through with the Sencore sg165 on some upcoming videos of any vintage stereo equipment
Fancy looking, but as with many Kenwoods, some of the details that matter are off. Like the knobs for balance, bass and trebble, why not nice alloy or even a plastic alloy imitation knob? All wood and silver, but then they put some cheap car stereo knobs on it? Heatsink? Well compare the heatsink of this 2 x 60 watt receiver to the heatsink of my 2 x 45W Pioneer SA 608 amp, I think the smaller Pioneer has more then double the heatsink. Kenwood was ofcourse prized well bellow Pioneer, but for the money I would rather had wanted a 2 x 40W receiver with all the details right, then spending the budget on the extra watts. Still this one is still running after all those years, so it has to be well made, so not all complaints, don't misunderstand me. ;-)
I think you misspoke and got the age wrong by a decade - that's a ca. 1980 design if I ever saw one. '70s Kenwood receivers also were quite notorious for running hot, the power supply section often needs some TLC. (The companion amp to my KT-80 tuner, the KA-80, also is known to run notoriously hot and fail often if not run severely underbiased.) Also, it seems like the autotuning mechanism is not at all purely mechanical but does in fact use the ratio detector output (plus noise detection) to determine when a station is tuned in properly. It's all rather extensively described in the service manual. The motor drive is even being used for AFC! RTFM FTW, I say. Not a bad little tuner really, it even switches in a 3rd ceramic filter in narrow for whatever that's worth (never got the point of doing it like that, an extra 2 would be better). It's only a 3-gang frontend with no RF or AF AGC in sight, but still. The regulator design has some eccentricities (even beyond everything being referenced to +14V but +24V being used for bias current supply in the +14V reg again), but they obviously knew what they were doing. (Still not sure what they needed R101-103 for, startup I guess.) I would still have an eye on some marginally rated electrolytics in the area, and C58 could stand being a bit bigger than it is in general.
JFET input 12AX7 Hybrid Tube Stereo Preamplifier Bare Board Finesse Sound Preamp from eBay. Please build and test. This is a Jfet input 12AX7 output preamp. PC Board only. Interesting circuit.
