Vintage Synth Repair

Hi, there! 

Has anybody out there any experience with an Estradin Altair 231?
I just got one, and there are a few strange things:
- filter resonance is sounding strange. There is almost no change when 
you turn the Pot until 3/4, but then only 1 mm more, the Filter is 
going into self oscillation quite loud! 
- The inputs should have 1/4" jacks, but if you insert a standard 1/4" 
jack, you have problems getting it out again... could it be that you 
have to use jacks less than 1/4" (6,3 mm)?? 
- the Keyboard modulation via the LFO (Oscillator 3?) is not working?

Any experience with this unit somebody?

Update.... 

I checked the Pot, it is a linera one, because the Russians used to 
have only this type. 
It has to be a Reverse logarithmic one with 50kOhms, like in the 
Minimoog.

6,3mm plugs are Ok, only a little bit hard to insert them... 

The Keyboard modulation via the dedicated Keyboard LFO is now working, 
there where some problems with a foam rubber under the keyboard...  I 
had to replace this. 

So, the unit is great.... ;-)