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Old 16th November 2013, 04:19 PM   #1
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Question Kuba Serenade Stereo Console - Part II - New development - amp debug question

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:
Kuba Serenade Stereo Console question.

Sorry dropping the project for a while.
Here is the new development and big questions (in red)

Last couple of days I finally spent some time in restoration and debugging the console.

What I have done – I’ve 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 transformer’s enclosure (see photo 5).

I’ve 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).

I’ve 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.

I’ve 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

I’ve 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 can’t see any reasonable explanation why the DC is different. So far, excluding obviously shot big-can electrolytic caps – every single passive component I’ve checked is dead on the value and I can’t see any other electrolytic cap to change...

I’ve 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 don’t 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.
Attached Images
File Type: jpg Receiver - New Power - Top.JPG (428.8 KB, 60 views)
File Type: jpg Receiver - New Power - Side.JPG (368.0 KB, 70 views)
File Type: jpg Receiver - New Power - Side 2.JPG (357.4 KB, 60 views)
File Type: jpg Receiver - OTs Before.JPG (499.5 KB, 56 views)
File Type: jpg Power Components - Before.jpg (673.2 KB, 55 views)
Attached Files
File Type: pdf imperial_j611k_sch.pdf (581.1 KB, 23 views)
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Old 16th November 2013, 08:48 PM   #2
Zibi is offline Zibi  Poland
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Did you check a coupling capacitors C64 and C77 22nF?
http://www.geofex.com/ampdbug/coupling.htm
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Old 16th November 2013, 11:48 PM   #3
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Zibi, no, thanks for the suggestion, will do and will post the result!
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Old 21st November 2013, 05:50 PM   #4
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Zibi, the caps are more than fine, both showed 90nF (note the schematic is not exact one). Moreover - Ive checked ALL similar caps in the triode and pentode rails - no difference. I've checked the surrounding caps - all are good.

I am going to replace them all, just to be sure that no leakage occur.

But before doing that - I've made the following observation:
The problem with the amp channel occur about 3-5 minutes AFTER INITIAL WARMUP - when amp tubes are warmed up after 20-30 secs - the sound is CLEAR AND EQUAL on both channels, after another 3-5 minutes - the problem channel start to fade out and stays about 60-70% of the good channel. After cool-down - the procedure repeats always the same way.

What it might be so heat-sensitive - I've already swapped the tubes and output transformers, there is no visual sign of bad/cold solder...

What components might be more sensitive to heat - if I replace all the caps (probably around 15-20 in the amp area) - do you think that resistors may behave this way as well? Or sockets.... I don't know what can be...

Any advice would be appreciated.

Old Hush
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Old 21st November 2013, 06:27 PM   #5
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don't think heat has anything to do with it ...
the ECL tiodes have true grid leakage biasing by means of the very high R 10Meg; that is indeed the primary suspect; any charge slowly building up here - or not building up as intended - will cause a phenomenon as you describe;
if leakage in the coupling C is not the case, it could be the 10Meg either open (check) or leakage across it due to dust or residue from cleaner ...
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Old 21st November 2013, 06:38 PM   #6
jmsent is offline jmsent  United States
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Quote:
Originally Posted by Old Hush View Post
Zibi, the caps are more than fine, both showed 90nF (note the schematic is not exact one). Moreover - Ive checked ALL similar caps in the triode and pentode rails - no difference. I've checked the surrounding caps - all are good.

I am going to replace them all, just to be sure that no leakage occur.

Old Hush
NO, THEY ARE NOT FINE. Those stock caps are notorious for leakage, and if you haven't replaced them, do so now. Checking them on a capacitor checker will often show a high reading. This is characteristic of a leaky cap. ANY and ALL paper capacitors in that set should be changed.
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Old 21st November 2013, 09:27 PM   #7
Zibi is offline Zibi  Poland
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Jmsent is right.
old64goat: "One thing you must remember a modern cap tester like what you will find in some digital multi-meters can read a high capacity reading IF the cap is leaky."
Testing Capacitors Part 2 - YouTube
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Old 24th November 2013, 07:38 PM   #8
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Quote:
Originally Posted by Zibi View Post
Jmsent is right.
old64goat: "One thing you must remember a modern cap tester like what you will find in some digital multi-meters can read a high capacity reading IF the cap is leaky."
Testing Capacitors Part 2 - YouTube
Oh, I didn't know about the leakage in case of higher reading (I have tested them on about 20 years old good digital multimeter).

Thanks a lot, Jmsent, Zibi, guys, that may explain the weird behavior...

OK, that's what I thought needed to be done at the end - replacing ALL caps around triodes and pentodes (around 25 pcs - a little tricky as they are mounted one over another in futuristic mesh with resistors, but at least I'll have nothing more to blame...

I have to go shopping for caps then, but I am a little rusty now a days, haven't shop for audiophile caps for over a decade...so I have few questions:
1. Any particular brand I have to aim for (better spend a little more than do that again)?
2. What type should I use - is modern ceramic good enough?
3. What voltage - on original caps it's written: "1000V DC, 400V AC, 250C AC (b)" - what the (b) stands for and will 400V rating suffice (highest voltage around the console is 235V)?
4. Any link for, let's say, typical 22nF from big vendors like DigiKey will be appreciated!


Thank you very much for the advises!

I'll need some time to get the caps and to replace them so, probably I'll have new update around Christmas time, when the snow will be deep and soldering appears very appealing...

Cheers!
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Old 24th November 2013, 08:59 PM   #9
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Steer clear of ceramic caps., if at all possible. 716P series Orange Drop film parts are a reasonable choice. Another reasonable option is Soviet surplus paper in oil (PIO) parts. You are probably safe in using 400 WVDC rated caps. in the signal path. Parts rated for 600 or 630 WVDC are surely safe.

I have a sneaking suspicion that the 1000 WVDC part(s) you asked about are being used as "death caps.", attached to the AC mains. Get that sort of stuff out of there! Make certain to install a proper, 3 wire, safety grounded, power cord.
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Last edited by Eli Duttman; 24th November 2013 at 09:00 PM. Reason: spelling
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Old 24th November 2013, 09:27 PM   #10
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Eli, thanks for the tip. Whoa, they're expensive - it'll cost me over $50 to replace the whole rail... will keep searching alternatives.
And... no, it is NOT death cap, I'll wrote that I already replaced the whole power rail - all wires, fuses, can caps, installed 3-wire standard 15A female PC-type socket. No death-caps, I've learned my lesson from you, following your advise, when started with another console!
Thanks!
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