V72 recapping questions

[adrian]

I've an old V72 wich sound thine with few bass,
so i'm going to recap it to give him a second life!
i found some schematics on google but with a bad definition and i don't have the partlist,

i will replace the 5  8uF 450v caps on the power supply board  by 10uF (as other members here done before me),
but as i'm a newbie i wonder what else need to be replaced?
there are 1,1uF 3v ;  2uF 250V  ; 0,025uF  250v  ; 0,05uF  250v  what do you think about them?

i also found a post where "Jean Clochet" explained : 

"Many years ago I owned a REDD17 desk for a while.
The 1uF cap you're referring to is inbetween the windings and isn't really for blocking DC but is there to act as a rumble filter/low cut.
The cap is brought out to pins 2a & 2b on the rear tuchel connector so you can short it out if you want/need.  In the REDD17 desk, the V72's used as mic amps had an additional cap strapped across pins 2a & b (it was a .25uF for 1.25uF total value) to lower the frequency of cut.  But on the amps used as the mix and output amps, the cap was shorted out."

so maybe i could replace the 1,1uF  by a 0,25uF to lower the frequency of cut?


and i found a schematic where it seemed to be a reaction loop or something like that (see in red), is that a variable gain mod?

here is the original schematic (i guess):

here is the other :

 

[salomonander]

i cant really answer your questions but check this page for all original shematics:
http://audio.kubarth.com/rundfunk/index.cgi

if i'd be you i would try to get rid of any mods (if there are any) i had a few friends with modded v72 (some of them done by very respected techs) and they all sounded bad compared to my stock modules.
especially in terms of fullness of sound. maybe yours is modded? id look into that. re-capping this modules is no fun i imagine. the 8uf caps basically hold the entire power supply together mechanically. so if you plan on simply using modern caps your module will fall apart - literally. i have over 12 v72 preamps myself. none of them is re-capped but all work perfectly. i once had a bad transformer - that lead to thin sound. it might not even be the caps is what im trying to say

 

[adrian]

thank you for the link!
I don't think my unit had been modded because everything seemed untouched..
about the 8uF caps : i gonna keep the enclosure and just put a new cap inside, so it will not change anything mechanically,

this  loop on the schematic is not a mod because it also appear in these old schematics (from you link), i don't find the 50pF and the 40pF on my unit but it maybe cause i didn't disassembled it completely,
what you say about bad transformer scare me... do someone know how to check it?

here is a picture of my disassembled unit:

 

[bezen4uk]

Hi Adrian,
I strongly recommend to not recap V72.
Siemens electrolyte capacitors have very good ESR and ESL, PIOs are good too - no significant leakage.
40/50pF caps prevent HF oscillation.

Firstly, check circuit integrity (I see floating wires at photo).
Then, check DC voltages according to the Siemens V72 circuit diagram.
Short capacitor "30"  (1uf) to avoid bass cut-off.
Check DC resistance of input and output transformers (use 20k range at multimeter (not 200 or 2k) to prevent transformer's breakdown).

Your V72 looks great, it just needs some patience.

 

[adrian]

Thanks for this advice,
i checked every 8uF caps and they seemed to be good with my multimeter so i gonna try to remove the 1uF to avoid the HPF..
i will let you know how it sounds!

 

[Biasrocks]

Thanks for this advice,
i gonna try to remove the 1uF to avoid the HPF..

You don't have to remove the 1uf cap, all you need is a jumper on the back connector to bypass it.

IIRC it's Pin 2 that needs to be jumpered.

Regards,
Mark

 

[adrian]

thank you,
sorry for my ignorance but when i look at the schematic I understand that it's an unbalanced input?(because pin 1b goes to the 0v...) am I right?

 

[Biasrocks]

Not sure what schematic you're looking at, but Pin 1 (a/b) both go to the Primary side of the input transformer (+/-). Pin 2 (a/b) are across the 1uf cap which is attached to the CT on the input transformer.
Pin 3 (a/b) are your output +/-. Pin 4 (a/b) are signal ground. Pin 5 (a/b) are chassis ground. Pin 6 (a/b) are 220vac mains power.

Regards,
Mark

 

[bezen4uk]

Hi Adrian,
V72 has balanced input. Pins 1a/b are shielded and shield is grounded.