Weird problem with Xformer input stage

[volki]

this is a weird little problem, which i actually did get fixed, but would like to hear your thoughts on.

while checking my two telefunken v676a pre's which i got a while ago, i found that one delivered less gain that it should. since the pre's are all modular with little sub boards, i swapped out all the amp and relay boards between the units, but still less gain from the same unit.

after pulling out all the sub boards with nothing else connected to the input xformer but the damping network on the secondary (100k parallel 10k + 100p), i still got a 1:1 ratio instead of the normal 1:3,4 (approx.).

removed the 100k resistor - there was my 1:3,4. :shock: :?:
excuse me?? 100k damping R's don't drop the gain by 10 db or so!!
unsoldering the 10k + 100p didn't make a difference.
so i put the latter back in, and of course now i got a xformer resonance peak at the top end (20k). eventually i found that i also had to put the 100k back, in series with an additional 82k to ground. that gave me virtually the same gain while reducing the reso peak to an amount similar to the working unit (+0.5 db or so)

so obviously something happened to the xformer? these torodial shielded trannies were made in the 1970's, so i don't think any ageing effect of material has taken place?!? any thoughts, anyone?

thanks, cheers,
volker

 

[CJ]

check the voltage step up of the two transformers side by side without any circuits connected and see what you get.

 

[volki]

that's what i did - the working unit measured 1:3,4 with the damping network connected. the "weird" one measured 1:1 with the 100k of the damping netwirk connected, and 1:3,4 with either without it (so remaining were the 10k and 10p in series) or with a total of 182k, which was obviously high enough as not to degrade some weird phenomenon affecting the ratio.
:?: