kay 720 short hunt

[ilyaa]

this old kay has a short -

blew a fuse -

with all tubes except rectifier 5u4 pulled, lightbulb shows a short -

the rectifier tube lights up bright on one side only and then goes dim as lightbulb starts to glow bright -

tried diff rectifier tube - same light show

all power resistors, etc look and test OK - could main filter cap be shorted?! or PT rectifier coils....??

no DC anywhere except some negative 14 and 3 Volts on the rectifier plates?!

schematic: http://elektrotanya.com/PREVIEWS/63463243/23432455/egyeb/kay_720.pdf_1.png

[Enzo]

What does your bulb do without even the rectifier tube?

If it still lights bright, you then need to test the transformer while disconnected from the circuit.  If the bulb now stays dim, then what voltages are on the rectifier plate socket pins.

Also power off, and are any of the HV points shorted to ground?  Both on the AC side and on the DC side.

[ilyaa]

no light bulb short without the rectifier -

with rectifier tube removed:
transformer end of the main filter resistor shows short to ground - other end shows 50 ohms to ground....
plates on tube socket also show ~50 ohms to ground....but they're looking through the PT....

ill detach the filter cap from the PT and see what we get....

looking like a bad filter cap?

[J M Fahey]

Post schematic, your transformer secondary may have one end grounded on design.

[Enzo]

http://bmamps.com/Schematics/Kay/kay720.pdf

In this case, I think the transformer end means the 5v winding.  The 50 ohm is between caps.  So one cap is shorted and the other is thus 50 ohms away.

Oops, now that I look, both ends of the 50 go to a transformer.  The rectifier end has the 5v and the other end has the OT CT.

So either one of your caps is shorted, or one of the transformers has a short to ground.

[LateDev]

Use your nose. Transformers tend to whiff if the insulation melts between the windings. this can be a cause of the short.
Best way to check is to lift the ground off the centre tap of the secondary on the mains transformer, the only other way it can short is a melt down between the laminations and coil.

Don't forget to check the caps as these can also short.

[Enzo]

His short is after his rectifier, so that absolves the power transformer.  The output transformer is still a suspect, though I favor a cap.

[LateDev]

 I don't think so, unless I have missed something here.

with rectifier tube removed:
transformer end of the main filter resistor shows short to ground - other end shows 50 ohms to ground....

If the output transformer was blown, that would not account for there being no heater Voltage.

If the Power transformer secondary had melted then there would be a short to ground via either of the 2 centre taps.
Best to start with the Cap anyhow just unhook transformer side and test for short on the cap and the winding, although caps usually show physical signs of a problem, so I doubt it is the cap. Hopefully I am wrong though as it is cheaper.

[Enzo]

I don't see where he said he had no heater voltage.

If the power transformer had melted in any way, then it ought not matter if the rectifier was installed or not.  Shorted turns would draw current and light his bulb limiter with or without the rectifier.

With the rectifier removed, there is no connection between the power transformer and the B+ line.  With the rectifier removed, we have the 50 ohm resistor, one end shorted to ground.  Schematically the only suspects are the two caps, one at either end, or the primary of the OT shorted to frame or to secondary.  And also the possibility of something physically shorting the wiring to ground, like a solder bead or a bend wire or a loose piece of hardware lodged in there.

[LateDev]

I don't see where he said he had no heater voltage.

If the power transformer had melted in any way, then it ought not matter if the rectifier was installed or not.  Shorted turns would draw current and light his bulb limiter with or without the rectifier.

True, I must have misread the heater part, and merely took it that there was no heater, as he had said there were no Voltages around the circuit.

A bulb limiter is a very rough way of determining a fault and the bulb can either light or not light depending on many factors, including the wattage of the bulb. You should never use the bulb method as part of any test, its use is to protect the amplifier under fault conditions, no more than that. Putting a bulb in line with a working amp is decidedly the wrong thing to do as well.

A transformer does not need all its windings melted to cause such a problem, however as you suggested there can be many other things which could cause this problem, even down to a piece of dislodged wire.

At least the area in which the problem lies has been narrowed down.

[g1]

You should never use the bulb method as part of any test

That is just plain wrong.
As Enzo mentioned, the bulb in this case clearly shows the PT is not the short in question.
The bulb limiter is used extensively for rough fault finding on this forum.  If you are that opposed to it, you're gonna be real disappointed around here  .

[ilyaa]

unhooked the PT from the RC filter - and no tubes in the amp:

transformer side of the main filter cap shows short to ground.....boooo

its the can cap, for sure. all the other leads of the can (its 20/20 and 40/40) seem fine. would i be okay to just interject a separate large filter cap where that first section of the can cap is supposed to go?? think im gonna try that - seems a shame to replace the whole can if only of the sections is shorted, unless that means its time to toss it.....

[LateDev]

You should never use the bulb method as part of any test

That is just plain wrong.
 you're gonna be real disappointed around here  .

I guess I will just have to be disappointed and you will have to carry on in the mistaken belief it is a test device.

unhooked the PT from the RC filter - and no tubes in the amp:

transformer side of the main filter cap shows short to ground.....boooo

its the can cap, for sure. all the other leads of the can (its 20/20 and 40/40) seem fine. would i be okay to just interject a separate large filter cap where that first section of the can cap is supposed to go?? think im gonna try that - seems a shame to replace the whole can if only of the sections is shorted, unless that means its time to toss it.....

That is a relief then. Am I to understand this is a single filter unit, or is it a double cap in a single can. Either way you should replace the entire filter, getting 2 caps and a resistor.
I would opt for larger caps anyhow as the DC side of the PSU is prone to droop at certain times which in itself causes current limiting and poor sound quality.

[ilyaa]

its four caps in one can!

a 40/40 for the first filter and 20/20 for later on.....

a light bulb limiter is a great device for finding shorts! and for saving fuses! hellooo!!

[Enzo]

I think it false economy to replace one section of the can and leave the others.  All the sections are the same age - ancient - and all have suffered the same conditions.

Let's not get in a tussle over the bulb limiter.  It is an effective tool, but it has limitations like any other tool.  My screwdriver is very handy but sucks at driving nails.  When an amp blows fuses, the bulb limiter is used to prevent further damage to the amp, as well as save all the extra blown fuses at each step of the troubleshooting.  Every time it glows bright when powering up the amp, that is another blown fuse we saved, plus the amp did not take a shot of full mains power before the fuse opened.

Once the blowing fuse issue is corrected, then the bulb is in the way.  And in fact will confuse things because it then will reduce the mains voltage to the unit.  That is an important limitation.