Another Brilliant Discovery (Solution later)

Glorious internals of L-1000T

I've long had a serious problem with my most favorite and wonderful sounding Kenwood L-1000T, which was Kenwood's last top-of-the-line FM Supertuner, following their previously renowned L-02T and KT-917 among others.  After building this ultimate FM statement piece in the late 1980's, Kenwood never again tried to build an FM tuner to this level of design, engineering, and construction excellence (they did build more sensitive tuners according to some reports later (6040) and at cheaper prices).  It came out just as Kenwood's competitors Yamaha and Pioneer were introducing their own ultimate supertuners (the TX-2000 and F-93) at a somewhat lesser level of perfection, and Sansui had already perished after producing their last gasp supertuner (the Tu-D99X)--a great but widely unappreciated tuner overshadowed by their earlier Tu-X1.

Or two problems, actually.

Funny the L-1000T has never been as famous or collectible as it's predecessors made by Kenwood.  Only a few well known reviewers, like David A. Rich, and the unrelated anonymous David at FMTunerInfo have even bothered to write about it.  David Rich only had a chance to look at the schematic and said it had the potential to be one of the best tuners ever, except for the digital varactor front end (which simply cannot be as good as old fashioned analog front ends with air capacitor tuning).  In every other way, he said, this tuner was unique in doing everything the best way possible.  Meanwhile David put it among the best 3 tuners ever, the best 5 tuners ever, or something like that.

This lack of more widespread recognition might be related to problem(s) the reviewers failed to observe or describe.  One problem many people have noticed is that it gets hot, very hot, almost like a Class A amplifier.  The second problem that I haven't seen much widespread comment about is that it can seriously drift over the course of 24 or so hours.  It can drift so badly that despite the quartz servo locked oscillator and everything you still have to re-tune the digital tuning after 24 hours for the best reception.  (It could be that only my sample has the drift problem, but I doubt it.  A friend of mine has noticed a similar drift problem with a nearly contemporaneous Kenwood FM tuner, the KT-6040.)

I used to keep 2 memories for every station I liked to listen to, the first at the actual station frequencies like 88.3 Mhz, and the second set for use after 24 hours of thermal drift, like 88.325 Mhz.  The L-1000T lets you tune to any 25kHz increment, which turns out to be very useful for dealing with the drift.  Useful, but the tuner would be much better without any drift.  Most digital tuners don't seem to have any drift that I can detect.  I've never detected any drift in my Sony 730ES, the very tuner that James Bongiorno tested for Sony and told them they needed to fix the drift, which he believed they never did.  But then I don't have anything comparable to Bongiorno's legendary FM tuner distortion measuring rig.  Bongiorno said that before the drift kicked in, the 730ES had the lowest stereo distortion of any tuner he had measured, but not for long.

But unlike my 730ES, which seems to sound continuously good, I could easily tell my L-1000T was drifting in two different ways.  The first was in the sound...it began to sound like there was serious interference on every station that was tuned to the correct nominal frequency.  The second, which I didn't comprehend at first, was on the tuning indicator the L-1000T has.

Even I didn't notice at first that the L-1000T has the kind of tuning indicator I had always lusted for.  It has a tiny LED representation of an tracking RF spectrum analyzer, similar in concept to the big scope display as on a Sequerra Model One with Panoramic Spectrum Analyzer (Panalyzer).

L-1000T with centered tuning indicator (vertical bar on right)

When a station is correctly tuned, you see a vertical bar in the center of the tuning indicator whose height represents the signal strength.  (I took the above photo with twinlead FM antenna resting on a chair, so the vertical bar is short, especially with Wide IF.)  If the tuning is off to one side or another, the vertical bar is on one side or the other side, or there are two vertical bars.  Two vertical or more vertical bars could also occur if there were interfering station(s)--that is what a panalyzer is most praised for showing most clearly.  Only the cheap LED version has the issue that one station could get divided into two rows of LED's if the station is not correctly tuned.  On a true scope display or a higher resolution LED display you might still see one line but more slightly off-center.  The crude LED version on the Kenwood requires more interpretation.

Correctly tuned line in center

Station to right of center, need to tune higher

Station lower than center, need to tune lower

What happened when it was drifting, and there was interference in the sound, was that indicator showed the two-line display instead of just one line (but unlike above two lines, they were above the center, requiring me to tune 25kHz higher).  And when I did tune 25kHz higher, the display went back to one line in the center, proving that drift had occurred and I did in fact need to tune to 83.325 Mhz to receive 83.300 Mhz.

Now maybe all I needed to do was re-tune at the 12 hour point, and in the between-times the servo lock was taking care of everything.  But I was always worried that during the 12 hours of warmup drift, the tuning wasn't right at any of the other times either, but somewhere in-between just too close to show on the tuning indicator or as interference in the sound, just as added distortion that might not be obvious.

And I did sort of feel that after 4 or more hours operation the L-1000T wasn't sounding as good as the first hour.  But that wasn't as clear and obvious and I could have been imagining it.  What was obvious was the big drift that occurred 12 hours or more after being turned on.

One way or another, the drift bothered me horribly.

Meanwhile, I never much worried about the heat.  I no longer stacked anything atop the L-1000T.  I made sure there were a few inches of ventilation space above it also.  And I tried to remember to turn it off every night, so as to keep it from getting too hot (and also having to re-tune every station the next day).  I figured that was the best I could do about the heat and that was that.

Now when James Bongiorno reported (on the FMTuners blog) to Sony about the drift in the Sony 730ES, he added that good tuner designers like his pals at Marantz designing the 10B had this problem solved long ago.  He didn't say more but I had understood it often required pieces that thermally tracked differently made to cancel each other out.  THAT was the kind of serious engineering I might have to learn to solve the drift problem.  But I was finally determined to solve it one way or another.  So after finally swapping the L-1000T out of my Kitchen Tuner slot I put in on my test bench.  That move also required several hours of cleaning the bench which had been accumulating flooby dust like expired or nearly-expired batteries I hadn't had time to re-test.  I was happy to have my test bench back in service for another miracle cure as I have sometimes pulled off before. 

By the time I was moving the Kenwood to my newly-cleaned bench I was figuring...this might not be so bad, all I really need to do is figure out how to keep it from getting so hot.  Since the drift only occurs over hours while it is meanwhile getting very hot, the two are connected.  It's the completely excessive heat of the L-1000T which is making it drift.  Otherwise, it might not.

And that has turned out to be the case.  But how do I keep it from getting so hot???  And especially, to keep it from getting hot near the parts that are responsible for the drift.

It may be the power supply has deteriorated and I should be replacing the power supply capacitors.  The power supply might run cooler then.  But I have observed the hot operation from the first year I acquired it, and it didn't seem overly used then, and many other people have observed the hot operation going way back.  Even if I do sometime replace the power supply capacitors, there's more to it than that.

By it's very design, and pushing everything into the best mode of operation, and with a stiffly regulated power supply which also helps, this tuner consumes a lot of energy for a tuner.

Back in the days when it was common for tuners to consume this much energy because they used tubes, tuners had ventilated chassis.  Many of the tube tuners I have had had no box at all, just a short base of electronics which tubes plugged into.  My first was a Fisher FM-80 with no case whatever.  I put it on a shelf.

By the early 1970's, Kenwood and others were making transistorized tuners but still venting the back of the tuner chassis to prevent heat buildup.  Then, sometime in the mid 1970's, Kenwood removed the ventilation holes from their best tuners, making bigger boxes instead.  That trend continued up through the L-02T.

The cheap and middling tuners in the line didn't generate much heat anyway.  But the big dogs, like the 600T, did generate a lot of heat internally and often show internal heat deterioration in and around the power supply.  I found a thread of people complaining about heat build up in the famous KT-917.  One guy says he wouldn't go to get milk without shutting it off.

Sometime around the mid 70's, a group of engineers left Kenwood and starting making their own Accuphase tuners (originally called Kensonic), which are still among the best, and far better than their Kenwood contemporaries, by many accounts.  Now you might think this was a matter of pioneering thinking engineers feeling they want to try newer and better technology, we in the US believe in such things.  But while Kenwood was busy pioneering things such as Pulse Count Detectors (which they never did very well, though there is still controversy about this) Accuphase was using more traditional approaches, such as ratio detectors, that had been used with tube equipment.  But there was one difference, that's obvious right away, when you just look at an Accuphase T-100, and a Kenwood 600T.  The Accuphase has vent holes in the top, and the Kenwood does not!  So it's clear, at Kenwood the future Accuphase engineers were indeed feeling trapped in the box.  And the heat too.  Not to mention which, the whole pulse counting thing, as deployed by Kenwood, was another way to make a tuner less sensitive to heat induced drift.  So while Kenwood was pushing the envelope of technology to keep from having to use holes in the covers of their tuners, the original engineers left to make tuners the old fashioned way...with holes in the covers.

When Kenwood started making low flat digital tuners, heat wasn't so much a problem anymore, at least with the mainstream models that might just use a few chips and that was it.

Meanwhile, the late 80's L-1000T has many high power consuming chips.  Kenwood didn't stick it into their smallest box, they couldn't, but it's still a fairly flat box, with as much power consumption as a receiver playing bookshelf speakers.

Why?  It seems Kenwood above others was worried about pepsi-syndrome or something.  Keeping tuners in an unvented box keeps them cleaner longer, and generally better working as far as alignment and stuff, except for problems caused by heat.  For the hotter running top of the line tuners, they should have taken more steps to keep power supplies cooler, I think.  And if ever that was true for any Kenwood tuner, the L-1000T is the ultimate worst example.  Many have noted how hot it gets.  The previous L-02T and others had been in a much larger boxes.

So a possible solution might involve something like drilling holes in the case, such as the top cover...

I first tried something much simpler.  I took the top cover off.  There was no evidence of drift or anything, and the power supply heatsinks reached 130F while the front end box was 87F.

The side piece on the power supply side was turned upside down and screwed on the least possible, leaving a 1/4 gap all around top-to-bottom.  Under this condition, the front end box remained as cool as before, and there was no evidence of drift for over 24 hours.  Problem solved!

However the innermost of the two power supply heatsinks was 157F, considered unacceptable to me.

I took the side panel completely off, and heat sink temperature still rose to 153F in 6 hours.  I'm thinking the heat sink should rise no higher than 150F or about 60C, a common design maximum.

The ultimate solution is going to have to involve holes in the top cover, which I will have  a machinist make, and possibly holes in the bottom cover as well.

Meanwhile I may have nudged the basic oscillator voltage adjustment more to spec, but it's still off a bit.  It changes far more during warmup than is ultimately off-by, and there is no change in the X-Y oscilloscope display at all three bandwidths (Wide, Normal, and Narrow) during or after the warmup.

The narrow is visibly off to one side, but this is invariant of warmup.  On an antenna wire laying on a desk.

My belief is the front end side has to be heated up very badly to overcome the servo-system that keeps the tuning locked on.  Otherwise it stays locked on and you notice nothing of the underlying varactor voltage changes because the servo instantly compensates for them (which is what is happening as you make the front end oscillator adjustment, and I've seen the servo jump volts in an instant).

But if the temps rose above 120F on the front end side, which they might do in the fully sealed box left running a long time, it might happen.  That could push RF oscillator to Front End alignment way out of whack.  The voltage is already rising from 23.48 to 23.83 as the RF box temperature rises from 75F to 85F.  Extrapolating on this it could be seriously bad if the front end rose to 120F.

Or likewise with the crystal, I think it would have to be heated up way above 85F to be a problem.

So I believe by letting the cover out 1/4 inch the drift problem is solved.  But the overheating is not completely solved, the power supply gets too hot, and that requires holes in cover, the one thing that Kenwood avoided in tuners after 1975.