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Kenwood L-07M II Issues

Here's a portion of the L-07M II amp schematic showing the biasing. Bias 'drive' current is shown by the red speckled line. The goal is to set up a voltage between the bases of the driver transistors. This is accomplished by current flowing from Qe8, through the multijunction diode, Df9, and through the bias pot and Re23 (Re23 is there to limit the range of adjustment and to protect the output transistors should the wiper of the bias pot open up), and back through Qe9 and to the negative supply.

The voltage across Df9 (an STV-4) will be about 2V. An additional .4V is generated by the voltage drop across the bias pot and Re23. This sets up the 2.4V needed for biasing the bases of the driver transistors (yellow specked line).

As temperatures rise, Df9, which is mounted on the output transistors and heatsink assembly, reacts to this temperature change like all bipolar silicon junctions and the voltage necessary for electrons to diffuse across the P-N junction goes down, thus lowering the voltage dropped across the diode/bias pot combination and finally the voltage applied to the bases of the driver transistor.

No specific reason why the bias pot is 'below' the diode. Could have been above. All we care about is generating the 2.4V necessary to forward bias the bases of the driver transistors and get a trickle of current flowing in the output stage.
 

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EchoWars said:
I call it flux, but in actuality it is likely a substance to help keep the board somewhat isolated from moisture that could swell and degrade the PC board material.
Click to expand...

While I'm no expert on conformal coatings of old, the stuff I removed from my driver boards looked nothing like what we used back at my last job. It dissolved into a pool of goo that looked *exactly* like rosin core flux. The conformal I'm familiar with is a clear silicone-like substance that glows under UV.

I've never seen a piece of hifi gear that was conformal coated. While the goop does have advantages in extreme environments (dirt buildup, high temp, moisture, etc), I doubt any piece of home gear would ever experience such torture (and if it did, another weak link would fail soon thereafter). Besides, when conformal coated PCBs came in for service, they had to be recoated before they were returned to the customer/warehouse. Even if it was just a touchup, it was a pain to get the repaired spot past QC. Did I mention how nasty the chemicals in conformal are? I've got a graveyard full of brain cells that met their demise in our conformal room. :drool:
 
gonzothegreat wrote:
the stuff I removed from my driver boards looked nothing like what we used back at my last job. It dissolved into a pool of goo that looked *exactly* like rosin core flux.
Click to expand...
Yeah I think that I know what you mean. My L-05M has a conformal coating that is thick and irregularly discolored with swirls of a brownish mirangue — the same color as spent flux. Except it isn't. On the other hand, there was some flux crust on the pads which I removed.
 
I have done many of these amps (9) and the coating is flux, whether the flux is applied as a liquid prior to soldering, or whether it was a combination of flux cored solder and liquid flux, who knows, but it is flux. Methylated spirits dissolves it perfectly and using an old soft toothbrush cleans the rest off to leave a brand new board. Note, metho doesn't hurt the silkscreened top printing or the lacquered green trace covering. Essentially the board is left like new- look at the pics on my previous post. (Wash the whole board carefully with detergent and a brush to prevent lifting any dodgy lands) Dry it with a hairdryer and a squirt with CRC2-26 finishes the board and it shines like a new car...then you put the components back in...
The amp boards and the protection boards were partially wave soldered and partially hand soldered- that is obvious by the construction and the solder land structure and I figure back in 1975-6, wave soldering techniques were pretty basic and they may have used too much flux in the interests of fewer dry joints.
Anyway, it is really a moot point as the boards need to be rebuilt totally if you want a reliable amp. Messing around with a few caps and poking at the odd dry joint is not going to do it. The L07's are worth the time, the boards are easy to rebuild (they unplug) and the results are spectacular. Stability is the issue with these amps. By the way, the evil brown goo is also used in a few areas on the driver boards and it results in some pretty nasty corrosion of component leads if not cleaned up.
 
Hello all. New member with a question on an older thread. What a fantastic collection of information!! :thmbsp:

After rediscovering my interest in 2-channel audio when I stumbled upon a Kenwood L-07C II, I had to get a pair of the matching L-07M II amps. Definitely an awesome combination, but unfortunately, one amp is good and the other one is not so good.

After the second or third time using them for 10 or so minutes, I noticed that the bad one would hum and generate excessive heat--so much that I thought "man, something smells really hot" from the other side of the room-- even when played at low volume levels. Subsequent investigation revealed the same 4Mhz output oscillation that EchoWars described.

I've decided to service both units with new driver board caps but I am having difficulty finding the appropriate values (3pF, 4pF, and 8pF) in 1% tolerance, as specified in the service manual. All of the other pico-range caps on the driver board are specified at 10%.

What tolerance of capacitors have been used in the rebuilds discussed in this thread?

Thanks for all of the interesting and helpful info.
 
At least you got it before the amp committed suicide.

All the caps I used (except three 3.9pf caps) are Cornell Dubilier silver-mica caps...I'm pretty sure they are all 5% tolerance, which is fine. As for the 3.9's, as I said in the original post:
I was unable to find either a 4pf or 8pf silver mica cap, and bought 3.9pf 1000V ceramic caps. The 4pf was replaced directly with a 3.9pf ceramic, the 8pf has one cap installed on the front of the board, and one on the rear in parallel.
Click to expand...
These were Vishay/Sprague Cera-Mite 1000V caps, Mouser PN 75-561R10TCCV39, and are also apparently 5% tolerance.

ALL the caps were purchased from Mouser, as Digikey did not have many of the values I needed in silver mica.

You will likely need to replace Rf35 and Rf37 on the mainboard, and the .047µf film cap as well.
 
Echowars,

In your original post you mentioned that these small value caps were mounted approx. 2" off the board. I was thinking (yeah, that usually gets me into trouble) that there had to be a reason why they put them in that way. As I was silently leafing through old and unused brain cells I vaguely remembered that old radio guys used to use specific lead lengths on ceramic (bypass?) capacitors to help regect RF interference, typically in the Mhz range.

I don't know if this applies in any way to the capacitors in question but I thought I'd throw it out there to see if it has any merit. If not please feel free to tell me and I'll go back to my usual lurking mode.
 
The ORIGINAL caps, as installed by Kenwood, were mounted on 2"-long leads. I certainly didn't duplicate their efforts when I reinstalled the caps. Read again:
The stock caps were mounted on very long leads, where the caps themselves were about 2" off the board. I honestly don't know the reasoning behind this, but the replacements are certainly NOT mounted that way. I removed the insulating tubing from the leads of the stock caps, and cut short lead insulators for the replacement caps about 3mm long, so the new caps are just a couple of mm above the board, not 2".
Click to expand...
 
EchoWars said:
The ORIGINAL caps, as installed by Kenwood, were mounted on 2"-long leads. I certainly didn't duplicate their efforts when I reinstalled the caps. Read again:
Click to expand...

Sorry, I guess my post wasn't written very clearly.

I was trying to say that there must have been a reason why Kenwood mounted the ceramics 2" above the board. I was throwing out a possible reason why they might have done that, nothing more, nothing less. It wasn't, in any way, meant to call into question anything other than why Kenwood would use a non-standard technique to mount capacitors on the circuit board of their TOTL unit.

Gary
 
I C... My bad, sorry.

I have not speculated as to why, but have consulted a few other vintage SS repair techs and no one seems to be able to formulate a reasonable theory, other than a few of the caps are in close proximity to some 1W resistors that may get warm (in practice, most resistors on the board seem to run rather cool). But other caps which are NOT in proximity to any power resistors are also mounted on excessively lengthy leads, so protection from heat damage does not seem to be the motivation.

If the long leads were needed for the original caps to work (doubtful...), they do not seem to be needed when replacing with the silver mica caps (and the three ceramic caps) as I did. Works fine, is stable. End result is more than satisfactory. Hard to argue with success, eh?. :D
 
I would imagine there is (or maybe was) an old engineer in Japan that could explain the reasoning behind the design but I'm sure it wouldn't be anything too important. I seriously doubt the engineering behind the amp depended on how long the leads were to a few pF caps. It's just one of those things that you scratch your head and say "why did they do that?".
 
I have the opportunity to buy a pair of L-07MII's. One works and one doesn't, that's all I know. After reading this thread I am not sure that I should. A professional opinion would be nice. If one is total trash, how much should I give for the other one? Thanks in advance.:scratch2:
 
Yes, but I've been preoccupied lately.

$150? What does he want for the dead one? And why would you need only one?
 
EchoWars said:
Yes, but I've been preoccupied lately.

$150? What does he want for the dead one? And why would you need only one?
Click to expand...

I was thinking the one that doesn't work might not be fixable and I would pick another one up some other time. If you think you could fix it I'll try to buy both.
 
Finally replaced the caps in the bad L-07MII...

After several months, I finally got the time to do some work on the bad L-07MII that I have, and it sounds and works great! No more overheating or hum.

Since I was more interested in solving the problem than rebuilding the thing, I decided to purchase only the driver board pF-range caps listed below (list courtesy of EchoWars), which corrected the oscillation problem. All were Cornell-Dubilier Silver Mica caps purchased from Mouser of varying tolerance of up to +/- 0.5 pF for the 3pF, 4pF, and 8pF units. I was a bit concerned about the high tolerance since the manual specifies 1% for for the 3pF, 4pF, and 8pF units, but they work just fine.

C1 - 100pf
C2 - 47pf
C10 - 3pf
C12 - 33pf
C13 - 47pf
C14 - 8pf
C15 - 4pf
C18 - 100pf
C19 - 12pf

Next is to do the same service on the good one, because I'm sure it won't be long before the caps in it go bad too...

Thanks for the helpful info EchoWars!! :thmbsp:
DT
 
I just have a pair of L-07M II amps on my bench from a friend. One is getting very hot and starts smelling...
Guess what, yes it starts oscillating after about 10 minutes usage. I measured something about 4MHz !!!
So I will order the silver micas mentioned above and hope that this cures the problem.
 
Tataaa! In the first attempt, I have installed only the Silver Mica caps and changed nothing else --> The oscillation has gone! Thanks E.W. for the good hint.
I had never thought, that bad ceramic capacitors will degradate a amp like this, since they are normally much more stable the electrolythics.
Now I will continue in changing the electrolythics and then start the soundcheck!
 
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