My first non-BPC receiver: Kenwood KR-4070, and now I have a tuner question

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k_kris

New Member
Hello AK community!

I've been lurking on here for quite a while, and finally have a reason to post. I had been using my junky mid-90s Sony receiver for a living room system, mostly just for radio and my Technics SL-BD2 turntable, driving some equally junky Sony 3-way speakers and/or some 4 Ohm Onkyo bookshelf speakers. The Sony finally completely lost its right channel, so I've been watching local craigslist postings for a good vintage stereo receiver.

I finally found the Kenwood KR-4070 for $40 (not a fantastic deal, but I'm OK with it). It was pretty dusty, but working (mostly). I tore it apart on my bench, checked fuses, cleaned the top shell and bottom plate, dusted all the boards, scrubbed the front plate and glass, did a double dose of DeOxit on all controls, dusted and scrubbed the heatsinks, cleaned all the tuner pulleys, etc.

This thing ABSOLUTELY puts the old Sony to shame! The phono stage sounds great (and now I really need to clean all my old vinyl), the quality of the AM reception/sound rivals the FM on the old Sony, and FM is great... but only in the MONO mode.

Now for the questions...

The KR-4070 has the auto-muting FM Stereo mode, but I get no sound. I'm not really sure how to diagnose it. It almost seems like the reception isn't strong enough to pop it out of the muting mode. Looking at the service manual, it needs to reach a certain threshold to pop out of the muting mode and light the STEREO indicator. I haven't seen the STEREO indicator light turn on, but I'm not sure if it's just a dead bulb, or if it isn't actually getting a strong enough signal.

I'm using a speaker wire antenna for now, hooked up to the 75 Ohm terminals. Maybe that's the problem.

I'm also a little concerned with the tuning meters. The signal meter jumps up to 3 or 4 with strong stations, but the tuning meter is always far left of center. This can be seen in the images below. I'm wondering if it's just a calibration issue? I noticed a couple variable resistors in the tuner section, but I haven't traced the schematic to see what they're for.

There are only a couple other issues. It looks like both the fuse type dial lamps are burned out. I'd like to find replacements locally, but I may have to cough up shipping and order them online. Looks like they're pretty standard 8V 300mA bulbs.

And one of the binding post terminals for the A speakers is somehow stripped. The negative post on the right channel just spins freely and refuses to tighten down. I pulled the bank of binding posts out of the back panel, but I couldn't see any way to disassemble it any further. Any recommendations for repair? or is this going to require a full replacement of the binding post panel?


Sorry for the brain dump, but I guess I have one more question... (this is turning into a ridiculous first post).

I'm afraid to run the 4 Ohm Onkyo speakers with this old receiver, although it says on the back that it will run anything from 4 to 16 Ohms. Is this justified? has anyone else with a 4070 noticed heat issues with 4 Ohm speakers? I suppose I can keep running the junky, boomy 8 Ohm Sonys until I find a decent replacement.

Anyway, thanks to all the other AK posters. This community is a great resource!



 
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Yeah, looks like your first test is with either a dipole antenna hooked to the FM 300 ohm connection or a powered antenna like a TERK unit hooked up to your 75 ohm connection. Check the signal strength first before you go any further into the FM mute switch. You may just need a better signal.

Good luck!
 
Thanks! I suppose my first step should be a simple ladder line dipole from the hardware store. If that doesn't do it, I'll have to look in to a DIY folded dipole or a powered antenna.
 
Greetings;

If the meter isn't moving at all then more diags into the fm are needed like the meter is just stuck. a gentle tap from inside and see if the needle moves. I was wondering if that model had a stereo led lit when locked in and does that work.

edit point. wrong post.. :D
 
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Naptown Rob said:
Or even just move that speaker wire to the 300 ohm connections...
Click to expand...

Actually, yeah, I misspoke. I initially had it on the 75 Ohm connection and realized that it needed to be moved. So it is on the 300 Ohm terminals. I think my next step should be a "real" dipole instead of the temporary speaker wire fix.
 
Binkman said:
Greetings;

If the meter isn't moving at all then more diags into the fm are needed like the meter is just stuck. a gentle tap from inside and see if the needle moves. I was wondering if that model had a stereo led lit when locked in and does that work.

edit point. wrong post.. :D
Click to expand...

Both meters move when tuned. The signal strength meter looks believable--stronger stations will push it up to 4 (out of 5). The tuning meter seems a bit off to me. It does move, but it always stays left of center. I would have guessed that it would center when tuned, moving left or right of center when you move off the center tuned frequency. Perhaps it's just a function of my junky speaker wire antenna. I'll have to see if it behaves better with a real dipole antenna.

It does have a stereo light, but I haven't seen it light up. The service manual says that the muting and stereo light are triggered in the same circuit. Since it hasn't popped out of the muting mode, I don't know if the bulb is burned out or not. I suppose I could pop the top and try hooking a 9V battery up to the leads to see if it's alive.
 
Well, it's good news and bad news.

I hooked my power supply up to the "STEREO" indicator light circuit, and the bulb is good. So that means the FM tuner signal never rises far enough above the threshold to pop it out of "auto muting" mode.

I picked up a twin lead dipole antenna and installed it on the 300 Ohm terminals, but no luck.

Looks like the problem is somewhere in the tuner stage... anyone have any ideas?
 
Could also be FM signal strength as seen at the location of your antenna.
Are you near FM stations; in what direction?
Rember FM is line of sight!
 
woodj said:
Could also be FM signal strength as seen at the location of your antenna.
Are you near FM stations; in what direction?
Rember FM is line of sight!
Click to expand...

I'm in Minneapolis, MN, so there are plenty of strong signals around. I'm getting loud, clear reception in the FM mono mode (without the auto-mute function), so it doesn't seem like it would be a weak signal issue. That's the reason I'm suspecting a problem with the threshold voltage that allows it to come out of the auto-mute mode (and light the stereo indicator bulb).

woodj said:
Also, where on the "meter scale" is 3-4?
Click to expand...

The signal meter has markings from 0 to 5, so 3-4 looks like a relatively strong signal (of course, there are no units, so it's a bit ambiguous). It's easiest to see in the second picture in my first post, which is linked to a high res image. The signal meter is on the left, with the tuning meter on the right. That image was taken with it tuned one of the stronger stations in town. That was using the speaker wire antenna. The signal is stronger now with the twin lead dipole.

The strange one is the right meter, the tuning indicator. I would expect it to be exactly centered when tuned to a strong station, but that isn't the case. When untuned, it's buried all the way to the left. When tuned it moves about half way towards center, but never to the right of center. That makes me suspect some calibration issue in the tuner stage, which may be related to the fact that I can't get it to unmute in auto (stereo) mode.

Here's the description of the muting circuit in the service manual:

FM MUTING CIRCUIT - Detection procedure
Intensity of the input level is detected from a position where a signal equivalent to the quadrature detection input is obtainable (NOISE DETECTOR). The obtained signal is supplied to the MUTE DRIVER CIRCUIT. On the other hand, DC output (discriminator curve) of the quadrature detector is fed to the OV SWITCH and output is generated as shown in Fig. 2-(a). The output and above-mentioned input level detecting signal are put in the OR CIRCUIT, thus generating an OR output as shown in Fig. 2-(c).

Namely, when the input signal is weak or deviates from the discrimination curve of the FM detector, operating voltage for muting is generated exceeding the muting level. (The muting start level corresponds to the antenna input below 8uV.)
Click to expand...

And here's the figure referenced, the important bits of the muting detector:
WTuO597.png


Now that I've given this a little more thought, it seems like the tuning meter pegged off to the left of center may indicate the problem. It seems likely that this is triggering the mute function because it's never falling in line with the discrimination curve. Despite the fact that it's pulling a strong signal, it always appears to be off center. I wonder if one of the variable resistors in the tuning section controls this... hmm. Back to the schematic!

I have it all torn apart on the bench again, so I'll try to find a test point for the threshold voltage that controls the muting. I might have to dust off the scope and see if I can see these signals going in to the OR circuit.
 
That looks promising.
I have never been a fan of muting (AM or FM) because it seemed too intrusive except in the most ideal circumstances.
 
If the tuning target meter is off, this would cause the problem you are having. The service manual says TG4 is the adjustment. Have a look at that. It shows a pic of where it is.

This adjustment must be done with no signal.
 
ManyMoonsAudio said:
If the tuning target meter is off, this would cause the problem you are having. The service manual says TG4 is the adjustment. Have a look at that. It shows a pic of where it is.

This adjustment must be done with no signal.
Click to expand...

Expert diagnosis! Twiddling Tg4 solved the problem. Tuned and playing in auto-muting mode, with dead-center tuning meter and STEREO light ablaze!

Many thanks.

CGSxeM8.jpg
 
ManyMoonsAudio said:
If the tuning target meter is off, this would cause the problem you are having. The service manual says TG4 is the adjustment. Have a look at that. It shows a pic of where it is.

This adjustment must be done with no signal.
Click to expand...

And I just saw the bit about doing this adjustment with no signal. Can you elaborate on that?

I tuned to a known station and twiddled Tg4 until I had the tuning target meter centered when I saw a peak on the tuning signal meter (double checked on multiple known stations). Seems to have worked, but I'm curious about doing the adjustment with no signal.
 
k_kris said:
There are only a couple other issues. It looks like both the fuse type dial lamps are burned out. I'd like to find replacements locally, but I may have to cough up shipping and order them online. Looks like they're pretty standard 8V 300mA bulbs.
Click to expand...

Just collecting the bits and pieces for future reference.

These 8V 300mA fuse type lamps are available for order in single unit quantities at mouser.com, model GF680 by JKL components (link works as of early 2014).

The original bulbs were Stanley model S83 (at least that's what I pulled out of my 4070).
 
k_kris said:
And after a little more searching, some other possibilities:

http://www.audiokarma.org/forums/showthread.php?t=200115

http://www.audiokarma.org/forums/showthread.php?t=105288

Sounds like it may be related to the variable resistors that set the VCO. Plenty of things to look at tonight!
Click to expand...

Another followup for future reference.

I didn't have to bother at all with the VCO referenced in the quoted links (or its associated variable resistors).

My tuning meter was off center, and that killed the auto mute and STEREO lamp. Adjusting Tg4 to center the tuning meter fixed the problem. If you run into a similar problem, don't hesitate to PM me for schematics or the service manual.
 
k_kris said:
And I just saw the bit about doing this adjustment with no signal. Can you elaborate on that?

I tuned to a known station and twiddled Tg4 until I had the tuning target meter centered when I saw a peak on the tuning signal meter (double checked on multiple known stations). Seems to have worked, but I'm curious about doing the adjustment with no signal.
Click to expand...

What you did works just as well. Nice job :thmbsp: With no signal, the meter is supposed to go center. This takes the guesswork out of it.
 
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