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Old 22nd February 2015, 08:50 PM   #1
RS232 is offline RS232  United Kingdom
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Default 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.
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

Last edited by RS232; 22nd February 2015 at 08:54 PM.
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Old 22nd February 2015, 10:01 PM   #2
Zen Mod is offline Zen Mod  Serbia
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tantalums are certainly for change , so you can start from there

use decent electrolyts or solids if place permits

Elna , Panasonic, etc. .... usual suspects
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Old 23rd February 2015, 09:52 PM   #3
RS232 is offline RS232  United Kingdom
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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
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Old 24th February 2015, 10:13 PM   #4
RS232 is offline RS232  United Kingdom
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Default Kelvin Labs

Didn't do much today got stuck at work.
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
Attached Images
File Type: jpg CAM00041.jpg (472.0 KB, 150 views)
File Type: jpg CAM00042.jpg (519.9 KB, 145 views)
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Old 24th February 2015, 10:27 PM   #5
Zen Mod is offline Zen Mod  Serbia
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interesting to see ..... but now you need to go inside that gray case .....
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Old 24th February 2015, 11:02 PM   #6
sreten is offline sreten  United Kingdom
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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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Last edited by sreten; 24th February 2015 at 11:12 PM.
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Old 25th February 2015, 05:52 PM   #7
RS232 is offline RS232  United Kingdom
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Got inside the preamp box (had to take amp appart to gain access )
Thinking about replacing the output opamp (TL072) on the preamp what do you guys think?
Will come back later with some pictures of the preamp insides.

Ric
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Old 25th February 2015, 06:01 PM   #8
Zen Mod is offline Zen Mod  Serbia
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replace everything what's suspicious

they're cheap
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Old 25th February 2015, 06:34 PM   #9
AMV8 is offline AMV8  United Kingdom
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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
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Old 25th February 2015, 09:05 PM   #10
RS232 is offline RS232  United Kingdom
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Default 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.

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
Attached Images
File Type: jpg CAM00043.jpg (355.4 KB, 103 views)
File Type: jpg CAM00044.jpg (364.0 KB, 96 views)
File Type: jpg CAM00045.jpg (280.4 KB, 92 views)
File Type: jpg CAM00046.jpg (404.4 KB, 41 views)
File Type: jpg CAM00047.jpg (352.3 KB, 41 views)

Last edited by RS232; 25th February 2015 at 09:22 PM.
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