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Kenwood L-07M II Issues

I've had the chance to get my hands inside some L07's. Crafty lil puzzles of joy they are! They are fantastic amps!
 
Being a new member of the L-series from Kenwood/Trio, I need to know if the Trio L-05Ms (the Japanese 100V version) have the same reliability (susceptibility) problem that leads finally to oscillation and self destruction. I do not have a service manual (maybe another member of AK is kind to send me a copy via e-mail?).
The outputs of these amps are NEC 2337/1007 (without the A!), the same as in Trio/Kenwood L-07M II:idea:? My amps work flawlessly and they do not generate much heat. Thanks for your reply!
 
STV-4H Diods for Bias regulation in the L-07 M II.

The L-07 M II amplifier has an STV-4H diode to regulate the bias of the amplifier. Due the fact that this amp could get very hot this diodes become brittle and broke down very easy. They are no longer on market, no regular chance to buy them anywhere.

However, if you have an L-07 M II Amp you are lucky, you have two on these Diodes in the clamping circuit (De9 &De10 in the SM). This circuit pulls down the modulation amplitude when the rail Voltage is to low due a big load on the output, so this circuit prevents a clipping of the amp. Replace this diodes with three 1N4248 or similar types because this circuit does not need the voltage regulating effect over a temperature range. Now you can use the cut out STV-4H Diode for bias regulation.

Until this day a have done this diode replacement 3 times in L-07 M II Amps and they all work without any trouble.

Richard
 
Cosmin2000 said:
Being a new member of the L-series from Kenwood/Trio, I need to know if the Trio L-05Ms (the Japanese 100V version) have the same reliability (susceptibility) problem that leads finally to oscillation and self destruction. I do not have a service manual (maybe another member of AK is kind to send me a copy via e-mail?).
The outputs of these amps are NEC 2337/1007 (without the A!), the same as in Trio/Kenwood L-07M II:idea:? My amps work flawlessly and they do not generate much heat. Thanks for your reply!
Click to expand...

I just learned that the L-05M service manual and schematic can be found within one of the L-07M II service manual scans as additional PDF pages.
 
Sorry to bring this old thread back to life, I got pair of Kenwood L-07 MKII and both need to be repair/ recap/ rebuilt. Repair guy said power supply needs to be rebuilt, underling hum, regulator, transistors, all caps replace. Like to know what's the going/ proper rate to rebuilt a pair of these amps? Its got classic symptom of overheating.
 
Old post, but old is gold!

i am starting the black flag replacement as explained in this post:

Here is an updated mouser list. contact me if you need the BOM
upload_2021-1-25_10-9-54.png

nb: this is for one board only
 
I know it is a really old thread but it is really valuable despite that I cannot solder and have no fist clue how to fix electronics.

At any rate I acquired recently a pair of L-07M II and so far tragedy work flawlessly. They are part of bi-amping a set of Infinity Kappa 8 with a set of Marantz MA5. Reading this thread made me wonder if I need to have a full restoration on these amps? Many of you would likely say yes.

My local folks I usually work with may not be abel to do this job justice/can't do it anymore (age and health issues).
If any of the masters here would be open to possibly take these on ( no rush) please PM me.
Thanks in advance!
 
I just did a quick restoration of a recently acquired pair of L-07m ii.
I only changed the black flag caps an the electrolytic in the drive board.

I think that is the bare minimum to enjoy this amp and i agree with @ghamilton .
They are totally worth it!
 
thanks guys! Sorry for the total lack of reply but life happened.
I will try to get them to the shop as I really want to listen to them - I was able to get the matching preamp with original box, manual and the nothing tuner...
 
Back from the shop = made my Sonus Faber Cremona Auditor (first gen.) sound amazing.
Paired with the matching preamp and tuner and a Tjoeb (modified Marantz) CD player
VERY happy!!!
 
I remember working on a few of these a few years back. I accidentally caused a short while I was reading some voltages and I was wondering if the driver amp board can be tested separately from the amp. Or should I have it connected to the amp to test it.
 
When I recapped my brothers L-05M amps they didn't have any black flags in them. And the STV4H can be made. Echowars made me one when he was still on AK.
 
restorer-john said:
by the way, my L07ms performed EXACTLY the same after a rebuild- the distortion was still below 0.003% at full power and the bandwidth was beyond my computer based sweep withing 0.01dB.
Click to expand...
I picked up a pair of L07M Mk1 today. I plugged each one at a time to my 100 watt dbt and they relay just clicked on and off and light is just bright. Should I use a 150w bulb instead? Or do these amps have a soft start circuit and do not like
DBT? Fuses are are intact on both units.
 
dapbmw said:
I picked up a pair of L07M Mk1 today. I plugged each one at a time to my 100 watt dbt and they relay just clicked on and off and light is just bright. Should I use a 150w bulb instead? Or do these amps have a soft start circuit and do not like
DBT? Fuses are are intact on both units.
Click to expand...
if you let me ride that sweet Beemer, I will tell you all you need to know about DBT :biggrin:
:deal:
 
here is another one for the "open primary" statistic, post-mortem complete. the owners not gonna like this outcome. talk about a highly enclosed, no breathing room, amp chassis design. just another parts cooker, like so many others. efficient form factor i like, eventual thermal time bomb not my preference.

EchoWars said:
and this is the fourth one in six months that I've worked on that has oscillated itself to death
Click to expand...
defective by design, #anti-longevity?

txf-1.jpg
LCR meter estimates between 150-200 meg ohm, slightly non-conductive perhaps

txf-2.jpg
.
fuse.jpg
mini control txf fuse measured 39-40 ohms, rather odd for 0.1 amp rating

rear.jpg
somebody installed an extremely long mains cord at some point in time.
 
WOW ... it sure has different guts than the L-05M which is definitely NOT a parts cooker. I can run the daylights out of it, and it may get warm — in the summer ... when it's humid!
 
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