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Lentek Head Amp - Naked!
As part of a prep to create a 9V external supply for this head amp, I thought I'd take a deeper look and see if the circuit could be exposed.
Success.....
http://i132.photobucket.com/albums/q...s/DSCF1456.jpg
The little pot is just a cover and was glued to the sides of the inner tray. The board was also attached via a sticky pad to the base of the tray and the adhesive had worn out -
http://i132.photobucket.com/albums/q...s/DSCF1460.jpg
Here's a closer look at the board -
http://i132.photobucket.com/albums/q...s/DSCF1453.jpg
For copyright (?) reasons, any printing has been removed from the transistors, but hopefully the pics can tell someone the values of the very close tolerance (for the period) transistors used.
As for the caps, the outer tantalums are 6.3V 47 (uF?), the inner ones are 16V 10 (uF?), the two purple topped flat caps are n22's and the three little flat red ones have black and orange dots on (black at top, orange bottom right).
Am I right in thinking that the tants might be the cause od the slight "solid state brightness" this unit exhibits (and has always done from memory?). I don't know enough to know..
I've fitted a PP3 socket to the existing plug, which I've repaired, and taken the leads out through one of the screw holes (screw wrapped and placed in battery tray for posterity). This should make things possible to put back to original easily.
By the way, I've measured the voltage of the extra long life Alkaline PP3 I fitted and on my digital volt-meter it measures 9.14V. This makes me think that the regulator output shouldn't be set to more than 9.5V I think, unless anyone recommends differently :)
http://i132.photobucket.com/albums/q...s/DSCF1460.jpg
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Mmmm, a few things there could be moddified ;)
First up set your regulator to 9 or 10V, it won't be a problem...
If i was you & i owned this head amp i'd change all the resistors for decent ones. The n22 capacitors i'd change to 220pf (which is what they are) polystyrene caps. Pity i can't see the circuit drawn out so we could establish exactly what the tantalum caps are doing precisely :scratch:
The transistors look like TO5 jobbies, i'm guessing that they'll likely be medium power & rated at about 1Amp. These kind as well as TO126 trannies are often used in MC pre amps simply because they have a lower noise floor.
Perhaps if you could get a decent shot of the PCB track side square on i might be able to draw the circuit out & then you can modify to your hearts content :)
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I've put it all back together now and couldn't tilt the board any more without removing some wires :(
I thought the resistors were 1% ones, both metal film and a few carbon ones. I can certainly open it again sometime and try to get better shots (I don't have an enthusiasts camera I'm afraid). Poly caps are fine to change to, but what are the ones with dots on in value?
Am I being picky regarding the tants, or are they really good (I have "memories" of Naim gear stacked to the gunwhales with 10uF tantalums :eek:)
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There are much better caps out there that'd fit in place of the tantalums. Sanyo Os-cons spring to mind & they'd be a similar size with those voltages & capacitances you quoted :)
E2A:- Those 1% resistors you are refering to happen to be 20% tolerance (red band at the end) & they are most likely carbon ;) Sorry i have no idea of the value of the dotted caps...
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Red band at the end is +/- 1% - i.e. 2% I thought - I checked on some charts again to make sure...
I'm not going to go totally mad here, but caps will definitely be looked at at some point. Thanks for advice regarding supply voltage too.
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Hi Dave,
Most interesting, and well done for opening up the old girl! I sense some future 'tweakage' shenanigans :eyebrows:
I'll be following this thread with interest...... :)
Marco.
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Welcome back Marco. It wasn't the same without you, it was like being in class with the Teacher out...:eyebrows:
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Hi Dave,
Gosh, it's nice to be missed :o
And yes, it's good to be back! :cool:
Marco.
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