Hello all. I need help with this old preamp. It has a tube MC section, heavily regulated, using two 6DJ8 tubes, one for each channel. MC section is a separate entity, before the MM section, so two separate inputs. MC section started to motorboat. Signal goes thru, and the volume is fine. I checked the plate voltage and it's only 30v, coming from the PS, and after the regulators I get only about 10V on 6DJ8 plates. Knowing Jadis and their idiosyncratic designs I expected something strange but not this. I checked all electrolytes around, and they all check fine, with ESRs in .26 ohm range, and uF within 5%. There is a pot on each plate, to regulate the voltage, I assume. Perhaps they have to be tweaked. Since there is no service manual available, I'm shooting in the dark. Any help is appreciated. Thank you.
No
No, it wasn't. Noise disappeared for a couple of hrs and now came back. Only in the left channel.
No, it wasn't. Noise disappeared for a couple of hrs and now came back. Only in the left channel.
Drawing
Hi. I'm attaching my crude drawing a a few photos. Noise is a combination of high-pitched whine and "hr-hr-hr-hr-hr". I think it's a regulator or any associated components. There is a transistor attached to it but it's unmarked.
Hi. I'm attaching my crude drawing a a few photos. Noise is a combination of high-pitched whine and "hr-hr-hr-hr-hr". I think it's a regulator or any associated components. There is a transistor attached to it but it's unmarked.
Attachments
Hi Mark. Took the liberty to redraw your circuit. It is pretty standard and nothing really points out towards suspicious components. I would think R5/R6 are zeners but unimportant.
The fact that the tube replacement brought a temporary reprieve makes me suspect the socket. Replace it if possible as those are difficult to clean properly if oxidised. Perhaps a metal brush can also help.
If that does not help check/replace zeners and decoupling cap. Next in line are all the transistors. The Fet ccs is likely for 10mA, so a K170BL, selected or with an added resistor will be a good match.
The fact that the tube replacement brought a temporary reprieve makes me suspect the socket. Replace it if possible as those are difficult to clean properly if oxidised. Perhaps a metal brush can also help.
If that does not help check/replace zeners and decoupling cap. Next in line are all the transistors. The Fet ccs is likely for 10mA, so a K170BL, selected or with an added resistor will be a good match.
Attachments
JP
Analog, you did it all great, except each tube is separate, with it's own regulating circuit. I will check the socket, but I tend to think it may be something else. The noise does not start right away, I have to check how long it takes. It may be a cap? Or whatever goes out of whack when it warms up sufficiently. What a pain I got myself into...
Analog, you did it all great, except each tube is separate, with it's own regulating circuit. I will check the socket, but I tend to think it may be something else. The noise does not start right away, I have to check how long it takes. It may be a cap? Or whatever goes out of whack when it warms up sufficiently. What a pain I got myself into...
I think I failed to mention that music signal is fine, but it accompanied by this strong noise, sometimes with motorboating. I turned off my Wi-Fi router and motorboating seemed to go away, but the whine and hrrrr remained.
