Kenwood TK-80U Part 5: MPX Stereo Blues

[@banjoperator]

as a strict hobbiest in audio electronics i have been following your channel for about 2 years now and have learned many things but the radio portion of recievers have continued to confound me and give me a headache.. following along on this series i made an important discovery.... as you were going over the schematic i sat up straight and laughed out loud... it all actually made sense.. i could see everything as to where it went ..how the signals got from point A to point B... how the multiplex worked... woohoo.. the sixty yr old brain of mine took long enough to figure it out but i give you the credit for your explainations .... thank you ... cant help any with the current issue but eagerly look forward to the next installment....

[@RobsFixitShop]

This video made my day. I rebuilt one of these stereos and I just could not figure out how this circuit was working. I got close and got it working but your explanation filled in all the blanks for me. Thanks for taking the time to go into the details.

[@jimmissenda6590]

Evacuated thermionic conductance devices. Haven't heard that since my 1971 Penn Tech classroom days. 🤣🤣

[@larryshaver3568]

i replaced a bunch of resistors on my mpx board and the separation improved immensely

[@richardfriedlander8281]

Thanks Tony. I loved this video. The head scratchers are so much fun and we can learn so much while having that fun. It makes us think and if any teacher can get his/her students to think he's doing a great job. Keep safe.

[@johnnytoobad7785]

Kenwood was the first consumer audio company to use an IC (probably an op-amp) and a crystal filter in a FM-stero-Tuner KT-7000 circa 1970 ! I saved a lot of money in high school to purchase one...$270.00 in 1970 dollars (!!) . So much for my college fund. I was pursuing a different form of education. Even though I was living in suburban NY at the time I could pick up stations in south Jersey and Bridgeport Ct using my parents color-tv antenna. "Underground" FM-radio was the "internet" of its day. It had both a signal strength and a center tuning meter and multi-path output jacks. I used that tuner until 1994. :)

[@wedoshotz6645]

Very fascinating. I've acquired a old Sansui TR-707A (mid-1960's) that I'm determined to restore. The innards look a lot like the unit you have there, minus the tubes. I've replaced all e-caps (& yeah, many of them were as bad as the ones you showed). I get a weak AM signal but nothing from the FM. This has helped me understand how to approach this repair. Thanks for posting these vids.

[@ESDI80]

This is very interesting. My Scott 342-B appears to use a similar version of the MPX circuit you just showed. My 342-B's MPX isn't working either and it looks like a transistor has failed as I am seeing the 38khz signal at the two diodes going to the base of the transistor, but nothing past that transistor. Even with just a normal station tuned I can see the similar wave forms. Again, thanks for posting this!

[@fernandogalloso359]

Fascinating!!!!!!With patience everything is possible.

[@gamerpaddy]

this amp is so nice to work on, everything accessible nothing crammed.. i recently had a marantz 4270 on the bench. i closed it up quicker than i took to open it.

[@williamsquires3070]

Also, just to be sure, I’d replace D3-D6 with Schottky diodes, too. And maybe check VR1 and VR2.

[@hestheMaster]

Inching you way to success there Tony. Step back and take deep breath. I know you will get to solving it soon.

[@1959Berre]

Diodes may test "fine" but they can get leaky at certain frequencies over the years. (The same applies to transistors) Replace D3-4-5-6.

[@geirendre]

I would agree with you on that. Just because a diode (or transistor) test ok measuring forward voltages doesn't mean it will function properly in circuit. Maby test them in the Curve tracer?

[@larryshaver3568]

i recapped the stereo fm board on my magnavox astro sonic . I also replaced all but one transistor and the difference was amazing

[@dm-ot1st]

Reverse diodes in the FM detector is usually typical of a ratio detector configuration, and diodes pointing in the same direction characteristic of a Foster-Seeley type.

[@1941lynnslb]

Did you know that the Marantz first tuner was the first phase-lock loop multiplex stereo tuner, and that it was tube type. Also I believe that Harmon Kardon made one called the Citation. I have worked on and aligned an Advent multiplex also. Some of you don't know that Tandberg made one of the best multiplex circuits, it was capable of 40 db of separation at 10khz and very low distortion, I don't remember the number right now. We used a Sound Technolgy FM generator to do the alignments and that was fun to do back in the 70's. Audio has always interested me, and I have worked repairing the best and a lot of the rest. BTW without knowing the real specs of those old units, how can you even know when it is performing to the limits of the design? I do know the ceramic filters that Marantz used in the IF second did cause some of the problems with the 2000 series in the 70's. Thanks for listening.

[@Aswaguespack]

You are very patient! I don’t have your patience. I would have pulled all my hair out if I had any hair!

[@stphinkle]

Wonder if that funky looking waveform shape at time 27:23 indicates something is off or perhaps could be why the channels are deciding unequally. I am no expert but it seems like the odd shape of the waveform may have something to do with how it decides which part goes to which channel later.

[@MrElectrowhiz]

I am working on a Marantz 2220 (non B), and the MPX board has me stumped, too. The circuits are almost identical to this Lafayette. When first turning on the Marantz, everything plays fine, then the left channel cuts out and it is on this board (P300) After replacing everything except the transformers, I found another board, and will just replace the board.

[@JohnAudioTech]

Tony, The board that senses the 19Khz pilot should be sending a control voltage/signal to the MPX decoder circuit that turns the stereo decoding on or off. I'd check this voltage under stereo and mono conditions.

[@isoguy.]

I put a link to a service manual in the last vid, was this of any use? Awesome series, would love to see all the un-edited footage of this and your other repaires, maybe on a second channel? Thank you so much for taking the time to make these vids.

[@larryshaver3568]

shango066 was describing the mpx circuit in one of his videos

[@larryshaver3568]

that looks something like the way the stereo in my astro sonic works

[@stevenzagony6187]

What series of Nichicon audio capacitors did you use "KT" or "KA" series?

[@justincase3880]

Tony, Would a BK Precision 4040A ((up to)20 MHz sweep generator) be suitable for AM & FM alignment? PS. this would be my first gear for this task ..PPS .. this unit also has FM multiplex signaling ...

[@larryshaver3568]

I may use russian transistors on my rf/if board

[@larryshaver3568]

You did a video pertaining to a stereo fm indicator that never stopped glowing, i'm having that problem with my astro-sonic, what is the title of your video pertaining to that problem?

[@geirendre]

This looks to be a time-multiplex kind of stereodecoder. So at collector of Q3 there should be the 38KHz pilot pluss the S-signal. At the right centerpoint of L6 you should have the M-signal with a suppressed 19KHz pilot. These two signals then mix in the transformer, and get decoded over D3 - D6. The audio signal is reconstructed over the four 22K resistors. Given the fact that somebody else has worked on this unit the way they have, could it be that it's just been tweeked so far out of alignment that it stopped decoding?

[@geirendre]

In the old schematics that i find this kind of detector circuit in, those 19KHz frequency rectifying diodes are Germanium diodes, AA119. While the decoder diodes are silicone diodes, 1N4148. BTW that "rectified" 38KHz did not look good to me... sportsbil.com/tandberg/Installation%20instructions%20for%20Tandberg%20Stereo-multiplex%20decoder%20-%20model%203%20in%20Huldra%208.pdf With tuning instructions if that helps.

[@larryshaver3568]

what diode is that which you replaced a1N34 or 1N60?

[@jpopwell1]

hi tony, i would replace the other 1n60's......all of them. in the early 60's diodes had a reputation of going bad over time. i'd use schottky also. you have already PROVED this with the first two you changed i would also change them back in the IF as well. a lot of the SONY stuff had problems with early diodes as well.

[@williamsquires3070]

Hi, maybe you could do a video on FM Stereo encoding? Then, perhaps, decoding it would make more sense. I get the part about a 19 kHz pilot tone, but why do we need the 2nd harmonic? For that matter, the 3rd harmonic, 57 kHz isn’t mentioned, but you made mention of a 67 kHz signal. Where does that come from? Closed Captioning, maybe? Or does it have something to do with the fact that the FM band is close to TV channel 6 (OTA, not CATV). Also, I wouldn’t think there’s much energy in the 4th harmonic @ 76 kHz, so do we really need a trap for that? 🤔

[@geirendre]

SCA is just som extra audio that can be tacked on to the transmission. Not used that much I think. en.wikipedia.org/wiki/Subsidiary_communications_authority kzbin.info/www/bejne/mKHVe6WqpZplidk

[@dimmog]

Just wondering with which ones you replaced the 2sa49?

[@Trex1268]

I notice you have been replacing the filter caps with higher uF caps. How many uF+ is safe to go? TIA

[@xraytonyb]

I am going to be doing a video on this subject in the future.

[@larryshaver3568]

it that thing with the red dot a transistor too?

[@jimomertz]

That transistor isn’t generating the 19kHz is it? Isn’t it a bandpass filter for the pilot carrier? Then it doubles it and demodulates the information on it to get the stereo information. Right? I’m certainly no expert on FM but find this fascinating. It really is FM. You know - F@#$ Magic.

[@xraytonyb]

You are correct. The 19 kHz is part of the MPX signal. The coils on those first two transistors act as 19 kHz filters. Think of them as a type of switch that turns on when it sees a 19 kHz signal. I think I showed what happens when you turn the 19kHz pilot on and off. When 19kHz is present, it will pass through the two stage amplifier and out to the third coil, which is a tapped coil, with the cetnet tap going to ground and the outer windings going to the two diodes. Think of it as a full wave rectifier. The sine wave now becomes a pulsed DC wave with a frequency of 38kHz, or two times the 19kHz sine wave's frequency. This is what is mixed with the MPX carrier signal to decode L and R.

[@larryshaver3568]

maybe the mpx input transistor is weak

[@Digital-Dan]

Therm Eye' On Ic, did you mean? :-)

[@DAVIDGREGORYKERR]

You could have replaced that panel with a MC1350 rf preamp and 455KHZ Murata crystal filters

[@InsideOfMyOwnMind]

C16, 1600p, is this a discreet capacitor or is it built into the transformer? If the later and it is a mica sheet with brass tabs overlaid on it this is possibly the problem. This should be one of the easiest mpx to align since it doesn't have an oscillator that has to sync up to the pilot. Also we like to talk about how the mpx signal is simple. Yes, but the modern implementation of it is anything but, considering all the tricks they pull to get a better modulation index, including but not restricted to chopping off the entire upper sideband from 38k to 53k to give more room for baseband l+r audio and the list goes on. Study an app called Stereo Tools, look at some videos about it, including of the guy that created it and you will gain a new perspective for how the FM baseband signal architecture is used today. It's not the same as it was when FM was meant to be high fidelity.

[@larryshaver3568]

a 2SC401 can be replaced with a 2N3904

[@TrevorsBench]

Replace those 1N60 diodes

[@pdppanelman5889]

The MPX signal is not that complex. The mono audio ( L+R) lies in the range 0-15 Khz of the demodulated FM IF signal. At 19Khz is the pilot tone. At 38 Khz the L-R component is a suppressed carrier double side band signal. The 19Khz signal is doubled and is used as a reference to demodulate the 38Khz SC DSB (L-R) signal. Now we have both L+R and L-R. Two networks one additive and one subtractive yield the final L and R ( Left and Right) Channels.. Note there are no oscillators involved in this. A basic theoretical knowledge of synchronous demodulation of DSB signals and how sum and difference matrix networks operate would help in diagnosing problems in MPX circuits.

[@xraytonyb]

I agree, but this still doesn't tell me what's wrong with this receiver.

[@pdppanelman5889]

@@xraytonyb I have the circuit diagram (Schematic in American). At first glance the decoded L-R gets fed in on the primary of L6 (One end signal is inverted) and the L+R goes by a bit of filtering to the centre tap of the L6 secondary. The L6 phase cancel and add makes L and R appear on the correct half of the secondary and the diodes route the signal. Knowing that it should be possible to find out whats happening

[@PeterMilanovski]

Russian transistors! Oh those Russians! I bet that they put a bit of Rusputins soul into those things LoL. There's something about cold war Russian electronics that totally captivates me, I have some Russian axial capacitors which look like nothing else, they are new old stock and I only purchased them just for their looks. I hope that they actually still work because I would love to see them in circuit... Being Russian they probably are still in tolerance LoL... Make em good or end up in Siberia LoL... Nothing like a bit of incentive to do your job properly... And once again another video that I have no interest in, I really don't like radio broadcast because they compress and limit the music and once you hear it, you can't un hear it! I always make sure that the antenna is not connected for the fear that someone is going to try and switch to the radio, not in my house and car LoL, I I wanted to hear someone talk, I would just turn off the radio and listen to what ever you have to say! And don't get me started on the commercials LoL... But even still! I can't just not watch this video! You claimed that it was getting a bit long but I'm like what? What do you mean long, it's not even two hours long! 30 minutes is only a trailer of the full thing lol.... I want to see that movie!! Sorry that I can't help you with the problem but at least I see a few people chipping in, so maybe a fix is already in the pipeline.. fingers crossed.

[@pibbles-a-plenty1105]

You seem to understand what the circuits do but your explanations are half baked and confusing, particularly for those who WANT to understand how the circuits function. And stop shaking the schematic all about, my eyes are twitching. Onward........

[@xraytonyb]

It would perhaps be more beneficial to post a question of what you don't understand than to post a criticism of someone who said something you don't understand.

[@edd2771]

Here’s a suggestion. Go watch some other channel if you don’t like what you see here.