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Just picked up a Knight KU-45 Receiver - May need some guidance

Uh Oh! Just ohm'd out the output trannies for both receivers and 3 of the 4 are bad!!!! :(

Measured pin 7 to pin 7 in each pair of P-P tubes which is full winding on the primaries. Three are open-circuit. The one good one measured at 565 ohms - full winding. Ouch. Ouch. Ouch as he hits his head on the wall.
 
Oh man that hurts.

You’ve come this far.......There’s nothing to lose by opening up those trannies and seeing if the open winding is near enough to the outside layer to attempt a repair. I have had successes and failures trying that, but oh man when it works out you want to do backflips. Most open windings are accompanied by a mini “event” and there is often a telltale burn mark that makes it stand out enough to spot it.

And hey, just fixing one of the three OTs will get you a stereo receiver!
 
RWood said:
Oh man that hurts.

You’ve come this far.......There’s nothing to lose by opening up those trannies and seeing if the open winding is near enough to the outside layer to attempt a repair. I have had successes and failures trying that, but oh man when it works out you want to do backflips. Most open windings are accompanied by a mini “event” and there is often a telltale burn mark that makes it stand out enough to spot it.

And hey, just fixing one of the three OTs will get you a stereo receiver!
Click to expand...

RWood,

Been there done that (BTDT). Had to repair a Fisher KX-100 output transformer by pealing back the paper and I was lucky enough to find the break was on top - rather than deep inside the windings. What are the chances that I could hit the jackpot again? (You are right though I have 3 tries to hit that jackpot.)

Here is the thread where I did the output trannie repair starting at post #122 where I found the problem. Found the break by post #133. Had it repaired by post #138.

https://audiokarma.org/forums/index...ube-amp-will-need-some-guidance.765247/page-7

BTW, there were some of the respected guys in the Fisher area helping out. Those guys are wonderful!

So, now I have to find the time to de-solder a first one of the bad ones and start the process to try to locate the break. Wish me luck.
 
Ahh, yes you have. That Fisher thread was a great read...... nice documentation on the repair, too.

I'm liking your odds of fixing one of those OTs!
 
Just confirmed that the Knight KG-400 amp uses the same output transformers because the assembly manual has the same part number as the KU-45/45A. I suspect the Knight KA-40 amp does as well because it is almost the same amp as the KG-400. At least that can broaden the search for original parts.
 
might be worth looking at mono amps too. If one happens to exist you may be able to pick up one or more of those for less money than stereo units.
 
gadget73 said:
might be worth looking at mono amps too. If one happens to exist you may be able to pick up one or more of those for less money than stereo units.
Click to expand...

Super good idea Gadget. Started looking at Knight mono amps. First one is:

Knight KM-15 6BQ5 mono amp - Does not have the screen taps on the transformer. This amp runs at the lower 6BQ5 voltages rather than 7189 voltages. Part number doesn't match either. Close but no dice. Not a match. (If someone were desperate they could mod the KU-45/45A to work, but you'd want a pair of them to do that and it might mess with the feedback network and more.)

EDIT: BTW, I'm finding the part numbers for the output transformers in the assembly manuals for these Knights.
 
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Knight Model (83 YX 797) 18W amp using 6973/6CZ5 outputs - Same as previous. No screen taps. Didn't check the part number as the schematic shows the no screen taps. No go.
 
Checking on the following.....

Knight KA-95 Stereo amp using EL34/6CA7 outputs - It has the screen taps and has the same output taps of 4/8/16 ohms. No sign of a part number yet. Still checking, but I would expect a more robust trannie. I.e. Different part no.

Knight KA-25/KG-250 Stereo amp using 6BM8/ECL82 outputs - No screen taps.

Knight KN-760 Stereo amp using 6L6GC outputs - No screen taps.

EDIT: Knight KN-740 Stereo amp using 6L6GC outputs - No screen taps.

EDIT: Knight KA-55 Stereo amp using 6973/6CZ5 outputs - No screen taps.
 
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prob will have to be something using the same or at least similar tubes to have much chance at the same transformer number I'd think. If this is 6bq5, maybe 6bm8 or 6v6 amps might have the same transformers but I doubt a 6L6 or EL34 amp would.
 
So I'm going to take these off the bench for now as this became a bigger problem with the bad output trannies. I'm not giving up on them but a Fisher project that has been on hold is moving to the front of the queue for a while. When I get back into these Knights I'll post an update. Hopefully that isn't a long time.
 
Nearly a year later but I managed to score a pair of output transformers from a KA-40 amp that was being parted out on the big auction site. Sadly one of them came bad. The seller was very kind in refunding half of the cost which I wasn't expecting. There are still good people in the world.

I've put the transformers with the two units on the shelf. It is down in the queue at the moment but at least I know I can put together one of the receivers now.
 
Scored another KU-45A from the big auction site. This time both trannies are good. Yay!

I can put two together now. The new one doesn't have a case, but I have two cases for 3 units now.
 
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