Any FM section techs here?

[quaddriver]

Question: I picked up not one, but 2 Kenwood KR-4070s for a song (and a short one at that) which I desire to fix up and use as trading stock.

However....neither 'stereo' light comes on. Everything else checks out - it should (im using a 3rd 4070 and the service manual as a 'template' of sorts)

Both use the LA3350 MPX chip that when a certain pin (9?) goes open tells it to run stereo and if it can it sinks 40mA at pin 6? (I may be wrong on the pinout off the top of my head with no paper in front of me)

At any rate, I got a couple LA3350 chips, but day-am - $8 each. I notice the LA3361 is a pin compatible MPX chip for $1 - anyone ever swapped them out?

 

[Maurice442]

Kenwood KR 4070

Hi Quaddriver, I used to be a service depot for Kenwood up to 1986 and I have a fair amount of experience with all kinds of audio problems, I checked in my computer list and I do NOT see those IC's as being similar (close though) because the LA3350 has a NTE 1217 as a sub and the LA3361 has a NTE 1248 as a replacement and the former has nothing connected on pin 9 and the latter has something.... ALSO, do not forget to test the Mpx lamp to make sure it is ok because in those days a burned out lamp would prevent the mpx decoder from working properly as it feeds the IC with a needed voltage for proper operation....
just my .02 cents
Hope this helps
Maurice:nono: :scratch2:

 

[Punker X]

Out of all the tuners I see. I can count on one hand the number of actual bad MPX chips. 99% of the time it a burn out lamp or an alignment issue.

X

 

[quaddriver]

Well thats just it, the tuning section is spot on tracking wise, is well sensitive to weak stations and the main input from the tuner to the MPX section is just the input signal, the LA3350 contains the 19.1mhz oscillator on the chip. I replaced the bulbs in the 2 'bad' ones and they flash when you are detuning a strong station, but only for a second. I did the tri-voltage test to the VCO stop pin, I get forced mono, VCO stop, but never stereo.

You are right tho, 2 have 2 bad in the same way - otherwise these are flawless, great shape, amp section up to snuff, no damage, evidence of shorting or foreign objects etc

But please explain - at datasheet archive they claim the la3350 can be replaced by the la3361 or the HA1127 (which is a very common pioneer unit) (and about 8 others - ECG1217 BA1330 etc)

 

[slow_jazz]

check out the website:

http://fmtunerinfo.com/

they specialize in tuners and help troubleshoot problems.

 

[quaddriver]

Memo to me: never doubt these guys again.

Not listening, I desolder the la3350, put in a socket for easy removal, put in a new chip - no diff.

spend two hours prowling that board (and wondering why the 18K R46 is not implemented on the tail end of D5 - anyone have the SCH to peek?)

turns out VR1 for the VCO voltage was corroded, some deoxit and she works.

live and learn. hopefully the other one is the same.....

 

[backmd]

LA3350 burned out lamp; no stereo

Hi Quaddriver, I used to be a service depot for Kenwood up to 1986 and I have a fair amount of experience with all kinds of audio problems, I checked in my computer list and I do NOT see those IC's as being similar (close though) because the LA3350 has a NTE 1217 as a sub and the LA3361 has a NTE 1248 as a replacement and the former has nothing connected on pin 9 and the latter has something.... ALSO, do not forget to test the Mpx lamp to make sure it is ok because in those days a burned out lamp would prevent the mpx decoder from working properly as it feeds the IC with a needed voltage for proper operation....
just my .02 cents
Hope this helps
Maurice:nono: :scratch2:

Yes, I agree. I have a concept 2.0 receiver and no stereo. Replaced a bad transistor, and a bad electrolytic capacitor close to the LA3350 chip and still no stereo. However after replacing the burned out stereo indicator lamp and adjusting the associated pot, we have STEREO!

 

[onwardjames]

That's a fine little receiver. Sold the one I obtained for peanuts for a nice little profit, and miss its champagne sexiness.

 

[SoCal Sam]

...turns out VR1 for the VCO voltage was corroded, some deoxit and she works.

live and learn. hopefully the other one is the same.....

Most vintage receivers that come through these doors have some sort of FM stereo or reception problem. IME, 3 out of 4 cleared up with trimpot cleaning. The usual culprits were Oscillator and Muting trimmers.

 

[JB8]

Man, this site is so amazing. The amount of knowledge and experience here is ridiculous, and I'm thankful to have access!

Found this thread yesterday while refreshing a Realistic STA-77A that had no stereo separation. Immediately thought it might be the chip or something else, but I kept digging, and reading.

I then tested the dial lights - white light was still functional but the red stereo light was out. I swapped in a new bulb and tweaked a couple pots, BINGO, beautiful FM stereo! Btw, putting blue LED's behind the dial and signal strength meter completely changed the look of this little guy, turned everything green and beige, which looks amazing, to me at least.

Anyhow, thanks, ALL, really appreciate this forum!

 

[johnhoef]

Indeed, this multiplex chip will not produce stereo if the stereo beacon bulb is open.