Pass Ono oscillation

[patriz]

Hi, I have built quite few years ago the Ono phono with the pcb bought from a group buy here. It has worked flawlessly until I had one of the 2 rcore transformers failure. Changed the trasformers to toroids but in one channel I have an oscillation, the sinusoid in not firm but moving insetself a bit up and down. The other channel is working properly. After much work I localised the problem in the mc preamp part, white background on the attached schematics. I have substitute all the components with newones even made a point to point version of it and implanted without success.
Any idea?

 

[Nelson Pass]

Try a little base resistance of Q24.

 

[patriz]

Q14? I will.

Try a little base resistance of Q24.

 

[Nelson Pass]

If that doesn't work, I have a whole list of things to try.

 

[patriz]

I tried 100 hom in the Q14 base without luck, I forgot to remark that when signal is applied the correct channel after few seconds show the full sino amplitude while the channel in question take considerable longer. This happen even if the phono is on since long time. Already changed power supply and caps.

 

[sottomano]

Q14? I will.

Not Q14, but Q24.

 

[patriz]

Hops, you are right.

Not Q14, but Q24.

 

[patriz]

ok , done it in base Q24, 100R to 1K but nothing has changed.

 

[Nelson Pass]

OK, try operating it with no cartridge and set the loading at 1K.

If that doesn't do it, then put some resistance, say 100 ohms between the
Emitter of Q14 and the drains of the Jfets.

If that doesn't do it, put some resistance, again 100 ohms, in series with
the output capacitor.

 

[patriz]

Hi Nelson, thank you for assistance but unfortunatly I tried all you suggested without success. I took semiconductors from working channel put them in faulty one but that was ineffective too. I took signal from jfets and attached to Q14 correct channel and output was correct. Again, if I attach the correct channel output signal in Q15 to output capacitor C10 faulty channel the output is clean. So problem is in the mc sub regulator.

 

[sottomano]

Try a small resistance in series with C34 (0.5 Ohms or so), and maybe with C35 too.

 

[Nelson Pass]

C35 yes, C34 we already tried.

 

[poli]

Hi Patriz,
not sure if you have already sorted out the oscillation problem, but I was facing once a similar problem on a self-designed Pearl 2 board. Apparently it is also important to bring Q14 transistor and the JFETS (the cascode this is) physically close together on the PCB.
Hope this helps, good luck!
poli

 

[patriz]

thank you for suggestion, I will try it

 

[patriz]

Now Q14 is directly soldered to jfets drain. I checked all voltages, only differences in the two channel are on Q15, in the correct one I have D 9,24V , G 25mV , S 300mV, in the wrong one D 13,82V , G 91mV , S 203mV

 

[wayne]

Try different 170's or remove one for a start. I don't think I have ever seen this although paralleled fets with no gate resistors can be an issue.

 

[poli]

Hi patriz,
just curious how things are with the oscillation problem, since I once had a similar issue. Have you sorted it out? Did the changes give any improvements?
Cheers
poli

 

[patriz]

Hi Poli,
unfortunatly any change I did solved the problem.
I will try some more things, I let you know if I will have any improvment.
Giorgio

 

[poli]

Thanks, Giorgio!