I recently picked up a vintage Knight KG-400 integrated amp in what appeared to be very good condition complete with schematic and the assembly manual and diagrams. The EBay seller said it "powered up" OK but didn't test it any farther. When I got it I was impressed with how good it looks and proceeded to open it to do some cleaning inside.
What I found was a thin, waxy sort of deposit over all internal surfaces making them feel oily to the touch. The first photo shows it with the cover off so you can see the oily/waxy residue. Stains seem to spread from the OPTs. The components and wiring on the bottom is worse with everything having that sort of oily feeling.
I removed the output transformers since they were the worst looking components and discovered a yellowish wax-like deposit which looks to have come from the frame laminations.
What is this stuff and do I need to worry about it? The OPTs look rather small for the rated 17W per channel - could they have been pushed beyond capacity? Are they safe to use?
I plan on replacing the caps since several test bad. This would be a good time to replace the OPTs if I should.
Michael
What I found was a thin, waxy sort of deposit over all internal surfaces making them feel oily to the touch. The first photo shows it with the cover off so you can see the oily/waxy residue. Stains seem to spread from the OPTs. The components and wiring on the bottom is worse with everything having that sort of oily feeling.
I removed the output transformers since they were the worst looking components and discovered a yellowish wax-like deposit which looks to have come from the frame laminations.
What is this stuff and do I need to worry about it? The OPTs look rather small for the rated 17W per channel - could they have been pushed beyond capacity? Are they safe to use?
I plan on replacing the caps since several test bad. This would be a good time to replace the OPTs if I should.
Michael
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That doesn't look good. Usually that means that these xfmrs are getting so hot that the material used to impregnate them is melting. That definitely should not be happening, and it does the xfmrs no good at all (even if the heating is caused solely by radiation and not I2R heating in the winding(s) ) since that impregnant is there to seal out moisture and dust. Moisture + dust + HV= 
I'd think long and hard before powering up that puppy.
I'd think long and hard before powering up that puppy.
Thanks, Miles. That's what I suspected. I've already started looking for replacements, something with 8K primaries since the output tubes are 7189s. The smallest Hammonds (like the 1608) are larger than what's in the amp, even though they are only rated for 10W, just a little over half this amp's supposed rated power.
Since some of the caps are bad, the xformers may have been stressed as a result. However, they may not be beyond hope. Sme amps (the Leak Stereo 20 for example) have transfoirmers that run hot at the best of times, and a little wax leakage is normal.
I'd replace any suspect coupling caps, then cautiously power up the amp and see how it behaves, in particular looking for 'hot and bothered' output tubes.
I'd replace any suspect coupling caps, then cautiously power up the amp and see how it behaves, in particular looking for 'hot and bothered' output tubes.
I wouldn't be inclined to use Leak's trsnsformers as a standard. They were under-specified and inclined to over-heat. Wax leakage should not happen imder normal conditions.
I wouldn't be inclined to power up anything that leaks, or is called a LEAK, for that matter.
But, that's only me.
Drove past a British Car Festival on Saturday... All I could think of was how bad I felt for the grass that the cars were parked on - all those leaking crankcases... I think I'll stop now before I get into the Lucas, Knight of Darkness jokes... at least they never got into audio gear.
But, that's only me.
Drove past a British Car Festival on Saturday... All I could think of was how bad I felt for the grass that the cars were parked on - all those leaking crankcases... I think I'll stop now before I get into the Lucas, Knight of Darkness jokes... at least they never got into audio gear.
flubug said:
Edcor has some small transformers - 15w and 20w at 8000ohm and 7600ohm for $20 and $30. You may take a look there.
Still, if it were me, this doesn't mean they are bad, and in fact doesn't mean they are even the source of the problem. Something else is likely wrong, though it is possible the transformers are just old the material is failing.
As far as powering it up - if you are looking at new transformers anyway, why not test it as is? You possibly could get it working properly without having to get new iron.
I have seen some old cheap OPT's that were dipped in wax. These are usually found in cheap 5 tube radios from the 50's (AA5's). I have seen radios that never saw much use, and lived most of their lives in the corner of a hot garage (in Florida) that oozed their wax. Some have had the oily slime, often with a good mixture of dirt and dust added. Usually the capacitors are dead, but the transformers still work. How long they would last is a different story, but I have got several old radios that still play just fine after I changed just about every capacitor, and many resistors, but no transformers, yet.
If I was going to replace them, Edcor would be a good start.
If I was going to replace them, Edcor would be a good start.
Tubelab, would you mind suggesting an OPT suitable for a P-P EL84 setup? Or EL34? A power amp is my next project, and those are the only output tubes I've got kicking around.
--k
I've seen this many times over decades in the service industry. Unless the transformer has failed, and it probably will not fail because of this, there is no reason to stop using it.
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