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Kenwood L-07M II Issues

Ok I got my self a pair of L-07II's replacing my SA-9900... I do beleive it needs a recap job, and I am now thinking of attempting to do it myself - one step at a time. I have got some solder experience now, and since the boards are ISO connected and easy to remove, I thought why not give it a go...?

I cleaned the units thoroughly (they looked like the batcave inside, complete with cobweb and excessive amounts of dust) and after that they are running quite cool. One unite have some slight hum, and they do get a little warm (lukewarm) after 20 mins or so. I have not tested them with an oscilloscope, but judging from other posts & discussions I guess they're oscillating.

So what do you guys think about these caps?

http://www.hifikit.se/show_kat.asp?KatName=KONDENSATORER&KatID=667

(I'm thinking of the last two, the INTERTECHNIKs)

Thanks!
 
lukewarm is good, that is the way they are meant to run. If the amp is oscillating at supersonic frequencies, it will take out the zobel filter cap on the rear panel under the small metal cover (behind the speaker terminals-1 screw to remove). If that cap is not vaporised, you amp is most likely fine. Don't go replacing caps in those amps without ascertaining where any (if) problems exist. Get a cro or high freq fft spectrum analzyer. Make a mistake and you have an expensive paperweight as they can go up like flashbulbs.
 
Thanks for your advice... Perhaps it's too complicated for me to do, but I really would like to learn this stuff if I could.

I'll start a new post asap, bring out the camera, and i'll start snapping some pics. Meanwhile, I'll put my SA-9900 back in service.
 
agger said:
I think your Trio Scope will work. If the amp starts oscillationg,you will see will see a foggy band around the normal music signal.
Click to expand...
That would be a mild case. Certainly a bad thing, but unlikely to destroy the amp. In order for that to happen, the amp has to be swinging wildly with a 500KHz+ frequency at peak-to-peak levels of +/-30V or more (and some may be swinging rail-to-rail at over a MHz).

Replace the 'black flag' caps, and protect those very very rare outputs.
 
I'm resurrecting this old thread because I have a question that I think is relevant. The L-07M II being discussed seems to be problematic WRT stability and oscillation. Is the original 'MK I' version similarly affected? It has a much lower stated bandwidth and several circuit differences. I was wondering if the same oscillation concerns apply.

I've been looking at the documentation for these. Apparently they were intended to be placed close to teh speakers and run with short speaker cables and long interconnects. Special interconnects were provided if you bought the matching pre with them. I wonder if the stability issues are affected by long speaker wires, high capacitance wires or interconnects that might tend to pick up RF and present it at the amp input.
Thx for any info on the 'MK I' version.
 
I'd have to see a driver for a MK I to tell you if it uses the same crappy caps, I have no firsthand experiance with them.

RF is easy to filter at the input of the amp, and pretty much all modern amps do so. No amp likes excessively capacitive loads, and while the L-07MII might be a bit more sensitive to this than many, I suspect that a normal cable run of 10~15' with 'normal' cables (no wierd twisted Cat 5 crap or some such) would not be a problem for a healthy amp (one where the bad ceramics were replaced).
 
What about C11 and C26?

I believe that this thread deserves to be resurrected as more people are discovering how good these amps are. :music:

I am in the process of recapping mine and I just received a copy of the service manual, kind courtesy of AK member "Meggy", aka Don.

I was wondering if anyone knows why C11 and C26 aren't soldered in the power amp board? They are listed as parts in the parts list section but not physically present on the board. :scratch2:
 
The Mk1s are just as susceptible to supersonic oscillation. Of the 7 I owned, I kept the Mk1s (2) and sold all the MkIIs (5). I like the Mk1 but, as EW suggests- replace all the ceramics unless you like having a LW transmitter instead of an amp.
I have one complete set of spare original transistors (6) just in case one of mine goes up one day. I suggest a cap HPF, a diode, resistor and bright red led at the end of your speaker wires- HF oscillation will trigger the LED and you will know you have an issue.
 
cchean said:
I was wondering if anyone knows why C11 and C26 aren't soldered in the power amp board? They are listed as parts in the parts list section but not physically present on the board. :scratch2:
Click to expand...
Beats me. Looks like Kenwood felt they weren't necessary.

You see stuff like this all the time working on electronics. Can't say I lose much sleep over it.
 
Hey? RestorerJohn I think it was me that bought two of those amps from you of ebay.

My Mark 2 just died a hot and terrible death, I was directed to this thread by EchoWars via DIY Audio.

Love the amps and want to keep them but i lack the skills to rebuild them myself, query Do I need to?
 
OK, I lack the skills to do this myself ; I'm throwing myself open to help if there is anyone with the skills willing t help in restoring these classic amps before the oscillate to death.

Problem is I live in Geelong ( Sleepy Hollow) now and most enthusiasts seem to be far away.

Does any-one have a beginning suggestion here??

Amps are in storage until I can repair them as they are too good to use as a paperweight
 
Bias current adjust

I have finished recap both l 07mII follows Echowars instruction and they run cool now vs very hot before. Thanks a thousand time to everybody in AK sharing their tip and helping everybody else. My question is I only have service manual for previously model not MKII and the bias adjustment in manual shown "Turn the pc trimmer potentiometer until the meter indicates 25mV" . I measure my is 580 to 630 mV and I turn all the way out limit but can not reach under 580mV ( new potentiometer installed ) Someone have any idea how to adjust Bias current ? or MKII version need to adjust different bias current value compare to original l-07m ?
Sorry for my terrible English
 
OK, you need to realise the bias current is way too high and the amp is not 'fixed'- it may be working but it is not right - yet.
Also, bias current 'measurement' is only testing standing current for ONE pair of the output transistors- there are three pairs, each driven by their own 10 ohm resistor, with associated .47ohm emitter resistor. You need to check them all which is a complete pain in those amps.
the rated standing current is 25mA. That corresponds to an output stage dissipation of 9.9 watts in total for the 6 outputs.
580mV corresponds to a dissipation of 244 watts at idle for the 6 outputs! They will cook and burn out in no time.
Clearly, you have a problem. You would be unwise to continue until the root cause has been located.
 
the standing current test procedure is the same for both the L07m and the L07mkII- i have just checked the two SMs. The pic above is much easier to read.
 
Bias current adjust

Thanks restore-John ! followed same picture as the one you posted : I clipped the VOM to the third post count from right as #19 it was 580mV. I pulled the amp board and read the printed number on main board it shown the fourth one is #19. Measure from this shown very low current. I can adjust to 25mV now but it is not stay stable, it keep raising up 0.1mV every minute . I leave them at about 12mV and play for 30 minutes both are run very cool. Does the fourth post count from right is the one to measure ??
 
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