Help with Kelvin Labs Integrated

Hi everybody
Long time since I posted here in the forum but need some help/advice with this amplifier.
It is a Kelvin Labs Integrated (early series) that after a problem in my house wiring (damp in the walls) got hit by a surge after one of the many power cuts.
The problem is that now sounds like the is a car engine running in the background with or without signal in all inputs not changing with the volume settings.
I allready checked the power supply and changed all the caps (after 20+ years was in need) without change to the problem.
Traced the problem to the preamp, if I disconnect the preamp connection to the amp (at volume pot) the amp goes dead silent.
Here is my problem, on the preamp are no elcos just mylar and tantalum capacitors and don't know were to go from here.:confused:
Do I replace the tantalums with same type or can I use other type of caps ?
Any advice is welcome

Ric

PS; Don't know if this is the right forum but the amp is a Class A (?) one
 
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Hi mighty Zen
Going to put a list of the capacitors I need and order them as soon as possible I have around 200 Dubilier caps but the wrong capacity.
It is a pita to work on this amp due to the way it was build and have to make some cable extensions so I can run/test the amp without putting it back together.
After this one I hope to go back an finish some of my projects like the F5 and the Aleph1.7

Will report back
Ric
 
Kelvin Labs

Didn't do much today got stuck at work.:mad:
For the ones that never seen one of these amps here are 2 pics from the inside.
It's a small amp with around 330mm (about the size a midi system).
The small grey box in the back is the preamp.

Ric
 

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Hi,

I tried a Kelvin Labs amp once many moons ago.

CD input was good but I was distinctly unimpressed
with the phono stages compared to my Audiolab.
On CD (I believe directly switched to volume balance)
sound quality was excellent, * until the bass just gave up.

IMO would be a great amp for active bi amping, using
it for mid and treble. As I recall the preamp is op-amp
based, for MC and Phono, cannot recall if there was
a line amplifier or tape buffer, I do recall its pretty
jam packed with components, never a good sign.

Preamp output is top right, around to volume,
then to balance and then to the power amplifier.

Not shown are the knobs and rods for tape
monitor left, and input switching right.

rgds, sreten.

* Really good in a lot of respects. Much
better than the Audiolab for mid/treble.
 
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Hi
I use and have worked on a few Kelvin Labs amplifiers.
On the Kelvin Labs amps I have seen the op amp is ( normally ) only used for the tape output. I think you will find the line amp in the pre amp is transistors.
I would check the transistors 1 by 1. However look carefully as the transistors as the phono pre for mm usually uses a mosfet on the input.
Do you have any dc on the preamp or power amp output?
You could also look at the grounding arrangement at the amp power supply as you will find small diodes on the power supply/amplifier earth connection and these may be damaged.
Don
 
Kelvin labs

Thanks guys
Here is some info on this preamp
1 TL072 opamp (didn't trace it so don't know yet if it is for the tape monitor or buffer output).
4 CA3096E transistors array and some ZTX214 and ZTX384 for input and phono stage.
All caps on the preamp are all tantalum ones and some mylar ones on the phono stage.
Replaced all caps on the preamp for elcos and the opamp for a NE5532.
Result... nothing everything the same, made a small video to record the sound and picture for you guys to have an idea what I'm trying to fix. :headbash:

Again every help is welcome
Ric

PS: I know clean the flux before assembly
Have to change the video estension (mp4) to something I can upload
 

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Kelvin Labs

I Zen Mod :worship:
Yes the sound is the same on both channels.
The funny thing is that it takes a couple of seconds for it to start.
I going to try and make another recording just with sound unless I send it to you (17mb)
Will try CD player direct on the volume has you say but only tomorrow is getting late.

Ric
 
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RS232
The pcb you show in post 10 picture 2 and 3 is the mm and mc pnono pre. I have a pcb just like that as a KL stand alone mm and mc phono preamp.

The ic's on the mm and mc board are just transistor arrays. You do not have a fet input on the mc. If you want to improve the quality of your mm and mc stages you can isolate the pins on the ic which provide the input pairs for each stage of the phono pre. On more expensive models KL used lm394 dual transistors as the input LTP's.

The line amp is all on the main amplifier pcb. The amplifier is designed as an intergrated amplifier with an added mm/mc section at the rear of the amplifier.

Many of the KL amplifiers have slight differences from amplifier to amplifier. On your amplifier the later versions used mosfets as output devices and some had a dac included. These were marked "digital" on the front panel.

Looking at the pictures you have provided seems to show that the rectifier diodes are brown. Have you checked those?

I do not have an overall schematic of your amp. However KL use similar circuits on all amplifiers and if it is of help to you e mail me and I will send you schematics drawings of your mm and mc pre and the two most common pre amp line amps. All of these have been back engineered for my KL amplifiers. ( My hand drawings are too large to add onto diyaudio.)

Don
 
Ok, here we go
Zen Mod,
Connected CD player directly on the volume pot and sound is clear no background noise (listening to Freddy Mercury Best off right now).

AMV8
Yes they are tested, the board under the diodes is discolored do to the heat from them they sit strait on the board without clearance.

So now I'm sure the faulty is on the phono/input stage going to check the 15/0/15 power supply and go from there.

Zen Mod
What are the tests you want me to do ?

Ric

PS; Freddy really sound great (listening with KL (without input stage) and my old philips CD104 (original no mods))
 
SUCCESSS
Found the offending part...
And the winner is the LM79L15 regulator for the phono/input stage.
Now one question:
I have 2 tantalums left in the board just after the signal input and the million dollar question is... do I use same capacity polar cap or bipolar ?
Going to have dinner and think about it...

AMV8
Thanks for the offer on the diagrams/schematics but problem is sorted so have no need for them but thanks anyway

Ric
 
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Hi Zmod
They are the input caps on the amp and they are 2.2uf.
I replaced them with Nichicons with same capacity and polarized.
Is sounding great just a very low level noise that was always there since I got this amp about 6 years ago but I have to be very close to the speakers to ear it.
The other thing is the bass, there is not much of it but it depends on the speakers I tested it with my old Whaferdale Diamonds and was not impressed with it but with my Aurum Cantus speakers the bass is much better.

Ric
 
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