Hello everybody,
I am starting a new thread on old project as it's been abandoned for couple of months and nobody will follow back. I am sorry if that is not advisable, I'll correct that if needed.
The original thread is here:
http://www.diyaudio.com/forums/tubes-valves/242064-kuba-serenade-stereo-console-question.html
Sorry dropping the project for a while. Here is the new development and big questions (in red) :scratch1:
Last couple of days I finally spent some time in restoration and debugging the console.
What I have done Ive started rebuilding completely the power supply rail - seems that the power transformer either have been replaced or some gimmicky repair job(s) have been done, extremely dangerous! The transformer have been mounted pressing the 110V cable and the heat ate the insulation to almost running bare 110V to transformers enclosure (see photo 5).
Ive rebuild all the wiring, installed new fuse holder (see how was done before on photo 5), replaced both the can caps (50uF/350V) with Samsung 150uF/400V and made new enclosure for standard 3-pin female power connector, grounding the chassis to power ground (see photos 1, 2 and 3).
I also broke-in all the pots and switches with good contact/silicon spray.
I run standard Line-In (CD-player) with nice jazz CD and power it up. The sound appeared loud and clear all the way! AM with internal ferrite antenna works absolutely clear IN THE BASEMENT, so no complains about AM at all.
The bad I still have FM dead (no hissing noise at all) and I still have one of the channels distinctively quieter than the other one.
I started with troubleshooting the amp as that would be the main use anyways.
Please refer to the imperial_j611k_sch.pdf file from the thread (attached again below) from now on it is NOT the exact schematic, but seems to be close enough to troubleshoot.
What I have checked and done:
- swapping the tubes makes the things even slightly worse (meaning the quieter channel kept being quieter, but louder channel become more loud, very little, but I can tell the difference e.g. tubes are fine.
- if I remove the ground connection from the speakers (pin 1 and 6) and leave speakers in series (fed between pins 3 and 8) the output it equal (the way it should be).
Ive checked the WHOLE passive rail with BASS/TREBLE and some funky corrections (everything west of the triode of ECL86 on the schematic pots, switches and passive components) with signal-generator and scope everything works fine, all controls and balance work.
Ive decided to go to the back end pentodes of ECL86:
- replaced the 470pF caps (C66 and C79) no difference
- replaced the 10nF caps (C65 and C78, actually 3.3nF on the board) no difference
- all surrounding resistors are fine and exact
Ive checked the DC on the anodes seems that the problem is somehow there voltages are as follows:
- main power DC (from rectifier) 221V
- same DC is on the OTs pin rt (rot = red)
- on the quieter channel anode of the pentode (pin 6) has 204V on power up and goes slightly down do 201V after warming up.
- on the louder channel anode of the pentode (pin 6) has 218V on power up and stays 218V, EVEN after warming up.
I may have missed some caps or other components to check on the back-end as the schematic is not exact and the mess on the board is big enough, but I cant see any reasonable explanation why the DC is different. So far, excluding obviously shot big-can electrolytic caps every single passive component Ive checked is dead on the value and I cant see any other electrolytic cap to change...
Ive even swapped the output transformers (see photo 5) the result is the same, but the power voltage rose up to 230V, of course quieter channel still has lower voltage on the anode, but is 3-5V higher before the swap. Go figure now...
I dont have extra tubes to play further, but I feel that I reached dead end for now.
Any ideas would be appreciated, guys, thanks in advance!![Click the image to open in full size.]()
I am starting a new thread on old project as it's been abandoned for couple of months and nobody will follow back. I am sorry if that is not advisable, I'll correct that if needed.
The original thread is here:
http://www.diyaudio.com/forums/tubes-valves/242064-kuba-serenade-stereo-console-question.html
Sorry dropping the project for a while. Here is the new development and big questions (in red) :scratch1:
Last couple of days I finally spent some time in restoration and debugging the console.
What I have done Ive started rebuilding completely the power supply rail - seems that the power transformer either have been replaced or some gimmicky repair job(s) have been done, extremely dangerous! The transformer have been mounted pressing the 110V cable and the heat ate the insulation to almost running bare 110V to transformers enclosure (see photo 5).
Ive rebuild all the wiring, installed new fuse holder (see how was done before on photo 5), replaced both the can caps (50uF/350V) with Samsung 150uF/400V and made new enclosure for standard 3-pin female power connector, grounding the chassis to power ground (see photos 1, 2 and 3).
I also broke-in all the pots and switches with good contact/silicon spray.
I run standard Line-In (CD-player) with nice jazz CD and power it up. The sound appeared loud and clear all the way! AM with internal ferrite antenna works absolutely clear IN THE BASEMENT, so no complains about AM at all.
The bad I still have FM dead (no hissing noise at all) and I still have one of the channels distinctively quieter than the other one.
I started with troubleshooting the amp as that would be the main use anyways.
Please refer to the imperial_j611k_sch.pdf file from the thread (attached again below) from now on it is NOT the exact schematic, but seems to be close enough to troubleshoot.
What I have checked and done:
- swapping the tubes makes the things even slightly worse (meaning the quieter channel kept being quieter, but louder channel become more loud, very little, but I can tell the difference e.g. tubes are fine.
- if I remove the ground connection from the speakers (pin 1 and 6) and leave speakers in series (fed between pins 3 and 8) the output it equal (the way it should be).
Ive checked the WHOLE passive rail with BASS/TREBLE and some funky corrections (everything west of the triode of ECL86 on the schematic pots, switches and passive components) with signal-generator and scope everything works fine, all controls and balance work.
Ive decided to go to the back end pentodes of ECL86:
- replaced the 470pF caps (C66 and C79) no difference
- replaced the 10nF caps (C65 and C78, actually 3.3nF on the board) no difference
- all surrounding resistors are fine and exact
Ive checked the DC on the anodes seems that the problem is somehow there voltages are as follows:
- main power DC (from rectifier) 221V
- same DC is on the OTs pin rt (rot = red)
- on the quieter channel anode of the pentode (pin 6) has 204V on power up and goes slightly down do 201V after warming up.
- on the louder channel anode of the pentode (pin 6) has 218V on power up and stays 218V, EVEN after warming up.
I may have missed some caps or other components to check on the back-end as the schematic is not exact and the mess on the board is big enough, but I cant see any reasonable explanation why the DC is different. So far, excluding obviously shot big-can electrolytic caps every single passive component Ive checked is dead on the value and I cant see any other electrolytic cap to change...
Ive even swapped the output transformers (see photo 5) the result is the same, but the power voltage rose up to 230V, of course quieter channel still has lower voltage on the anode, but is 3-5V higher before the swap. Go figure now...
I dont have extra tubes to play further, but I feel that I reached dead end for now.
Any ideas would be appreciated, guys, thanks in advance!