1971 Kenwood Jumbo Receiver • Noisy Transistor Fix • How to Bias • Bench Test • In 4K HDR!

The Kenwood KR-6170 “Jumbo” receiver gets a noisy transistor transplant. I explain and demonstrate biasing. I bench test for wattage power and frequency response. The spring reverb gets tamed. And a quick electric guitar demonstration with the receiver’s guitar input. Shot in 4K HDR.
Here are the topics covered and time stamps:
0:00 Introduction
0:11 Recap of Previous Episodes
0:59 Let’s Fix The Noisy Guitar Input
1:11 Noisy 2SC458 Transistors
1:35 KSC1845 Replacement Transistors
1:40 20 dB Amp Boards
2:25 Did That Fix The Problem?
2:50 Bias/Idling Current
3:10 Class AB Output
3:23 Crossover Distortion
3:41 Calculating Bias Current
4:20 Adjusting Bias Trimmers
4:39 Bench Tests With SpectraPLUS Software
4:52 Power/THD Test. How Many Watts?
7:09 Frequency Response Test
7:51 Spring Reverb Lock Fix
8:25 Putting It All Back Together
8:44 Electric Guitar Demonstration
To see all the videos in this series, please click:
• Kenwood KR-6170
To visit my Amazon Shop, please click:
www.amazon.com/shop/fluxconde...
#kenwood #stereo #stereorepair #4K #HDR

[@paulbennell3313]

What a wonderfully bonkers device! I want one so bad. It'd be the ideal workhorse for my home studio setup.

[@FluxCondenser]

I see these on ebay from time to time. I think there may be one on there now if it hasn’t sold already.

[@FluxCondenser]

Ha!

[@rogerchurch3804]

wow I don't even know what to say about that fabulous machine !! I gotta have one it does so many things Kenwood made some crazy complicated car head units too !! alota people don't know they made one of most coolest super complex boom boxes ever dc-20 made around 1980 very rare I enjoy ur videos I learn a lot !!!

[@FluxCondenser]

Thanks, Roger. I think there may be one for sale now on ebay.

[@marka1986]

In the past have used thick felt weatherstripping so new caps would fit in the clamps, or perf board. The adapt a cap looks like a handier solution.

[@FluxCondenser]

Mark, I was so close to doing just that with the weatherstripping!

[@hifihappiness6387]

I am so glad I've discovered your channel! Your format is easy to follow and so far, quite informative. I started watching due to the Heathkit amp giveaway. Send that I have already won. Will be utilizing your knowledge to help me in my hifi endeavors.

[@FluxCondenser]

Great to hear, Hondo! Welcome to the channel. Glad to have you aboard.

[@Nickutoz]

That's a nice receiver!

[@FluxCondenser]

Thanks! And it's a lot of fun, too.

[@taltongraves2169]

Nice vid!

[@FluxCondenser]

Thanks, Talton!

[@BobWilson84]

Timeout!!! What is that audio analyzer interface ? I like how it’ll show clipping and THD simultaneously

[@FluxCondenser]

Yeah, it's a great interface. Check out the SpectraPlus site. I also use their superb SpectraDac. spectraplus.com

[@theoloutlaw]

Very cool! Maybe one day, would you go a bit more in depth with measuring and replacing transistors? I find this the most challenging part when trying to find replacements, especially in 1960's gear, when vintage components have different numbers and identifications. How far in spec tolerance you need to be when replacing, and what exactly am I looking for? I have one of those simple universal testers that give you hfe, Vf.... but I get confused with so many other options and other specs with new replacement parts, and I don't know how 'far off' I can be if the spec's don't 'line up'. Also, those 2SC458's, did they look 'bad' under test? What is your process that lead you to the KSC1845's? Apart from 'xtraytonyb', I have not found many youtuber's that go into the strategy of understanding transistors, and what 'exactly' to look for in replacing. Thanks for the video, Rick

[@FluxCondenser]

All great questions, Rick and I share your concerns and confusions over replacing transistors. I started in this hobby by restoring antique radios, so I have a better understanding of tube gear. And while I’ve become expert in many facets of radio repair, even with tubes I’m far from an expert on which tubes can replace others in similar circuits. For that, I rely on research, and similarly, that’s where I discovered KSC1845s make good replacements for 2SC458s. Here’s what I do know. Checking for bad transistors usually just entails checking to see if they’re shorted or open. This can be done with the diode function of a multimeter or a basic transistor tester. Checking hFE can be useful when trying to match transistors, but as far as which transistors can replace others, that’s usually done by checking data sheets. Often a data sheet for a transistor will state what it’s a good replacement for. For real-world experience with substituting transistors, nothing will beat going to some user forums on audiokarma and others for some advice. As far as the noisy transistors in this receiver, those were replaced on a hunch. I was aware that many transistors from the period are notoriously noisy and suspected that the 2SC458s in the mic amp fell into that category. A quick search confirmed this and I found many suggestions that KSC1845 are suitable, low-noise replacements. It’s important to note though, that I didn’t think the 2SC458s were “bad.” I didn’t even bother testing them as I knew they were technically working fine. Fine, but noisy.

[@theoloutlaw]

@@FluxCondenser Thanks for the reply. I'm only a 'tinkerer' myself, and I love tube gear, and your early video's are A1 content explaining components and functionality. I come from being a musician to wanting to know how tube amps work, and find I am more interested in the gear these days than the actual playing. Uncle Doug has been a blessing too in first teaching me just how valves work etc etc.... Over time, I have collected some solid state amplifiers, and I must say, I find those more 'tricky' to work out than a tube amp schematic. It's those 'pesky' transistors. I have a bunch of 60's Reel to Reel tape recorders, and they have some oddball transistors in them, and that's where I get stuck trying to work out a similar replacements. Anyway, thanks for the reply back. Recently, I've started to keep a diary of transistors like yours here, and what to replace with if I ever come across some dodgy 2SC458's. :) Thanks! Rick

[@FluxCondenser]

My pleasure, Rick. Keep on doin’!

[@briangoldberg4439]

enjoyed your videos. thanks! one comment: i probably wouldn't put the foam pads directly on the reverb box like that as they will have an effect on the sound. there's no reason you can't put them upside-down on the under side of the brackets (directly above where you put them on the top of the reverb box). You would probably have to cut the edges off the foam circles, but that's not really a big deal

[@FluxCondenser]

The foam pads have absolutely no affect on the sound. The entire reverb is made of metal, so I find it almost astonishing that you’d think adding an insignificant amount of weight to that with foam pads would somehow have an affect. It just doesn’t compute.

[@briangoldberg4439]

@@FluxCondenser my thought was more that the foam would dampen the sound of the reverb chamber, nothing to do with weight. but i'm assuming that the chamber itself plays a role in the sound of the reverb and not just the internal springs

[@FluxCondenser]

I believe the outer housing really just protects the spring and transducers. Even so, again, the foam has relatively no mass so it’s just not going to have any effect. I certainly didn’t hear any differences in performance before and after.

[@briangoldberg4439]

​@@FluxCondenser gotcha. well, thanks for the videos. i enjoy audio equipment and i am also a repair hobbyist. :)

[@thiagotbp]

Thanks for those videos! i actually own a KR-6170 an i'm having some trouble with the right channel, witch is dead. i'm studying your explanations to figure out what is happening. If you have some tip about that it would be very much appreciated

[@FluxCondenser]

My pleasure, Thiago. Glad you’re enjoying the videos. I’m not sure how much experience you have working on electronics, but a dead channel on the KR-6170 would be diagnosed similarly to any other type of audio device. First, isolate when and where the channel is gone. Is it with all inputs? Is it with all speaker outputs? Do any of the controls affect the situation? This will help you isolate the circuits you need to examine. Then, you’d want to do standard signal tracing from input to output so see where you lose the signal. You can also use the service manual to check voltages at different parts of the circuit (available online from Hifi engine, I believe). Having one channel operating is helpful as you can compare the circuit voltages on the good channel with the bad channel to see where things start to deviate. Also note that I fixed the dead FM in my receiver using none of those techniques. Sometimes an issue such as yours can simply be a dirty contact in one of the potentiometers or switches. This could occur in the volume control, balance control or speaker selector switch. Carefully apply a suitable contact cleaner to these areas and see if that doesn’t help. Good luck!

[@cdavidhord]

WAIT A MINUTE-PART 4? KZbin must be shadow-banning your controversial renovations. I have some catching up to do.

[@FluxCondenser]

KZbin just can’t handle all the sex and violence in my videos, I guess ;-)

[@adeeponionbreath]

I hope it smelled like a new receiver. Mine were a treat! Ummm vinyl chlorides!

[@darthbubba866]

Don't park the Kenwood too close to the Dynaco SCA-80Q on the shelf. The Kenwood may get jealous of how much better care you took of the Dynaco. Speaking of Dynaco audio equipment, it's time to rotate out my solid state amps and bring in the Dynaco Mark III amps to help heat the music room now that winter's approaching.

[@FluxCondenser]

Hm. I thought I took pretty good care of this one ;-) Do you think I did wrong by this Kenwood? The 80Q was an unbuilt kit, so yes, I wasn’t going to build it with old parts. This Kenwood is working just fine now despite me not replacing every marginal component. Different beasts, different approaches.

[@darthbubba866]

@@FluxCondenser Truth be told I really had to stretch to find something to razz you about so I indulged in a bit of whimsy :-) Your diagnosis and repair work, as well as your video productions thereof, truly are excellent.

[@FluxCondenser]

Ha! Thanks, Darth. Feel free to razz away. The problem with writing, as we all know, is that tone doesn’t always translate. So, I figured I’d delve into the “better care” comment. And if you had felt that I had done wrong somehow, I’d be genuinely interested to hear how. I’m just a hobbyist and not a pro tech, so I can only gain from constructive feedback. As always, I appreciate your support and comments.

[@reneneron2971]

Pick me!

[@aegisofhonor]

1% is still not that good. There should be a "knee" point where the THD goes up significantly very quickly, I often consider the "true" power rating to be 10% below the knee or 90% of the power rating of the point where the amplifier starts to increase THD significantly very quickly. It would have been nice if you showed a graph that showed where the THD increases as the power goes up to show where that knee is located. I am guessing the knee is probably somewhere around 23 to 25 watts. so this receiver probably only as a true clean THD of around 20 to 22 watts.

[@FluxCondenser]

No, that’s incorrect.