Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Tubes and Hybrids => Topic started by: ilyaa on June 23, 2015, 04:42:51 AM

Title: kay 720 short hunt
Post by: ilyaa on June 23, 2015, 04:42:51 AM
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
Title: Re: kay 720 short hunt
Post by: Enzo on June 23, 2015, 11:48:10 AM
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.
Title: Re: kay 720 short hunt
Post by: ilyaa on June 23, 2015, 09:32:45 PM
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?

Title: Re: kay 720 short hunt
Post by: J M Fahey on June 28, 2015, 06:56:00 PM
Post schematic, your transformer secondary may have one end grounded on design.
Title: Re: kay 720 short hunt
Post by: Enzo on June 28, 2015, 10:59:58 PM
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.
Title: Re: kay 720 short hunt
Post by: LateDev on June 29, 2015, 11:30:46 AM
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.
Title: Re: kay 720 short hunt
Post by: Enzo on June 29, 2015, 04:15:28 PM
His short is after his rectifier, so that absolves the power transformer.  The output transformer is still a suspect, though I favor a cap.
Title: Re: kay 720 short hunt
Post by: LateDev on June 29, 2015, 06:30:56 PM
 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.

Title: Re: kay 720 short hunt
Post by: Enzo on June 29, 2015, 11:55:55 PM
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.
Title: Re: kay 720 short hunt
Post by: LateDev on June 30, 2015, 11:23:21 AM
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. :)

Title: Re: kay 720 short hunt
Post by: g1 on June 30, 2015, 11:51:38 AM
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  ;).
Title: Re: kay 720 short hunt
Post by: ilyaa on June 30, 2015, 03:28:04 PM
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.....
Title: Re: kay 720 short hunt
Post by: LateDev on June 30, 2015, 04:00:09 PM
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.
Title: Re: kay 720 short hunt
Post by: ilyaa on June 30, 2015, 04:08:11 PM
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!!
Title: Re: kay 720 short hunt
Post by: Enzo on June 30, 2015, 05:27:53 PM
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.
Title: Re: kay 720 short hunt
Post by: g1 on July 01, 2015, 12:45:25 AM
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. ;)
I realize it is not intended to be a test device.  But when something acts as such on occasion, incidentally, why not use it to your advantage.
Consider the example I was referring to.  The bulb lit up with the rectifier tube in place, but did not when the rectifier tube was removed.  Had he used fuses, the conclusion would have been the same:  fuse blows with rect. tube in, does not blow with rect. tube removed.  By logic we conclude that the fault is after the power transformer.
The bulb limiter has served an unintended function, and has in fact been part of a test.
Yes, as Enzo pointed out, it has limitations.  But that is not to say it can't serve more than one function.  A fuse can serve as a "go/no-go" current measurement, in the case above, the bulb did the same.
  Your unequivocal statement was too sweeping and misleading in my view.  Had it not been absolute, I would not have been so opposed to it.
  We must make the most of the tools we have at hand, and sometimes that means using them in ways they may not have been intended for.
  I don't believe the bulb limiter is a test device, but my belief can not prevent it from occasionally being so.
Title: Re: kay 720 short hunt
Post by: DrGonz78 on July 01, 2015, 02:06:01 AM
Of course the dim bulb tester has limitations. It limits current for one...  :lmao:
Title: Re: kay 720 short hunt
Post by: J M Fahey on July 01, 2015, 05:24:27 AM
Agree that not a measurement instrument by itself, although lamp blinking and later stable brightness and colour give me a reasonable idea about how much the amp is trying to pull, at least I can distinguish between 4 broad areas:
* not even blinking : something open, maybe a fuse but sometimes a blown open rectifier or pulled connector, etc.
* blinking, then stabilizing red or dull orange: what we expect to see.
* blinking, then orange or yellow: not a full short but sensibly higher than normal consumption: maybe an overbiased but otherwise normal amp or there is some DC (say a couple Volts)  present at the output (load connected) or blown (shorted) transistors but also blown (open) emitter resistors so drivers are carrying the load, in that case I expect to see some smoke after a minute or two, or hear driver transistors exploding or at least cracking, hopefully the bulb limiter (which won't prevent that for long)  at least tells me to turn amp off befor other parts getb damaged .
* white lamp, full power: gross short.
Not bad for such a simple device.

But the main function is to let me measure *something*  , impossible with a fuse which blows in milliseconds.

At least I can check whether power rails are reasonably normal (say +/- 30V where I'd expect +/- 40) or dangerously low (say +/- 12 or 15V in the same situation) or indicating a shorted semiconductor  (say +/- 2V or thereabouts) or a GROSS short , +/- 0V (or a few mV).
Or whether there is DC at the output or not.

Admittedly not a "measuring instrumant" but they give me  a very broad indication and let the multimeter provide the actual numbers.
Title: Re: kay 720 short hunt
Post by: LateDev on July 01, 2015, 06:29:39 AM
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. ;)
I realize it is not intended to be a test device.  But when something acts as such on occasion, incidentally, why not use it to your advantage.

Yes, as Enzo pointed out, it has limitations.  But that is not to say it can't serve more than one function. 

Both statements are of course correct, but from my reading on here, the limitations of the bulb method of testing are not pointed out.

The bulb is there to stop you blowing fuses and in some cases components, however it must never be thought of either as a protection device for everyday use or a test device for the many that don't understand its limitations or even why it is used.

 Yes it does have its uses in testing,providing you use it correctly, which many people don't. I even use the light bulb method for a load when testing, but I know its limitations so am aware of what to use it with.

I have seen others say that their amp distorts and the bulb lights up when they crank it up and play. They take this as some kind of fault.

For most people the light bulb must only be considered as a fuse saver, As Enzo stated earlier "My screwdriver is very handy but sucks at driving nails."
Title: Re: kay 720 short hunt
Post by: Enzo on July 01, 2015, 07:50:18 AM
You have to keep in mind that in the context of this forum, it has been explained many times over.
Title: Re: kay 720 short hunt
Post by: ilyaa on July 01, 2015, 11:17:49 PM
well, bulb-limiter aside -

i popped in a separate 47uF cap for the time being instead of the first section of the can and things are looking and sounding good -

looks like it was a shorted filter cap - didnt realize part of the big multi-section can could short....but i guess it can!
Title: Re: kay 720 short hunt
Post by: Enzo on July 02, 2015, 01:28:22 AM
Why would a section of the can cap not be able to short?  ANY cap can short.
Title: Re: kay 720 short hunt
Post by: LateDev on July 02, 2015, 06:09:25 AM
This is just a number of caps in the same can for convenience and cost.

Having a look at how a cap is constructed may help you to understand more about these things. Just do a search for Electrolytic Cap Construction. then imagine an insulator around each cap. In a filter can, they just connect all the cathodes together for convenience.

Some companies specialise in different filters for different applications and may present you with 2 tabs and a ground, yet inside there will be 2 caps and an inductor between the caps, any of which can individually blow.

I would imagine Enzo knows the construction of these cans, just as I do, which is why we have both said replace the whole filter circuit. With one cap inside already going short, the likely outcome would be that the other cap may go as well.
Title: Re: kay 720 short hunt
Post by: J M Fahey on July 02, 2015, 09:56:07 AM
Some companies specialise in different filters for different applications and may present you with 2 tabs and a ground, yet inside there will be 2 caps and an inductor between the caps, any of which can individually blow.
Sorry but you are confusing ilyaa .
You are mixing this kind of multi electrolytic cap, which ilyaa has in his 50 years old amp:
(https://images.campyent.com/sites/default/files/uc_products/c-ec50x4-350.png)
where capacitance is measured in microFarads, is electrolytic and so is polarized (DC rated only)... and has NO internal inductor of any kind, with this modern EMI/RF/Digital interference filter which does have picoFarad measured caps, joined by internal inductors.
(http://img138.imageshack.us/img138/637/f0ib.jpg)
The caps are anything but electrolytics, and are meant for direct AC mains use.

A very different thing.

A typical schematic of such filters is:
(http://www.walshcomptech.com/ohlandl/9595/9595_PSU_EMI.GIF)

Quote
I would imagine Enzo knows the construction of these cans, just as I do
I bet Enzo does ;)
Title: Re: kay 720 short hunt
Post by: LateDev on July 02, 2015, 05:26:48 PM
 You are mistaken. Just because you have no experience with these, don't automatically think someone else is talking of something different.

As we are talking about power supply smoothing, why on earth would you assume that a mains filter is suddenly being talked about.

 A long time ago Pi filters could be had in a single can for filtering and smoothing power, I did think that Enzo may have come across these at some point in the past.

Title: Re: kay 720 short hunt
Post by: Enzo on July 02, 2015, 08:51:45 PM
Quote
Some companies specialize in different filters for different applications and may present you with 2 tabs and a ground, yet inside there will be 2 caps and an inductor between the caps, any of which can individually blow.

When you said that, that is why Juan said this:

Quote
You are mixing this kind of multi electrolytic cap, which ilyaa has in his 50 years old amp
:

I will happily believe you didn't mean the things Juan mentioned, but I can see why he would say that.  I was confused by it myself.

I have never seen a multisection can cap in a guitar amp that included a choke inside.  Especially from the era of the amp we are discussing.  There are or have been specialized parts, but guitar amps use real common stuff, especially basic amps from that era.  I have seen resistor/capacitor combination parts in real old amps, like the 1930s.  And in my work in RF I see various parts that would make no sense in audio.

Juan won't toot his own horn, but he is a design engineer and a commercial manufacturer of professional audio equipment, as well as a guy just interested in amps, so I would hesitate to assume he is ignorant of the parts we use.  it is one thing to not understand something you say, but another thing to think him inexperienced.
Title: Re: kay 720 short hunt
Post by: LateDev on July 03, 2015, 07:26:34 AM
 I didn't say he was inexperienced at all, I merely stated he had no experience of something.
Quote
You are mistaken. Just because you have no experience with these, don't automatically think someone else is talking of something different.

By stating as a fact I had said something wrong, is incorrect and should never have been put in the fashion it was.
Quote
You are mixing this kind of multi electrolytic cap,
A statement of fact.

He could just as easily have stated, that his experience was of the first type only, and he had no experience of the type I had stated, or in fact warned ilyaa not to confuse a mains filter with the type of cap he showed first, which would have been more helpful, Instead he decided to state I was wrong and he was right.

As far as someone not blowing their own horn, it matters little to me what his pedigree is, just how he conducts himself. I can obviously see he wants to be helpful, but it should never be his aim to put someone else down, because of his own misconceptions.
Suffice to say I have had experience in all the areas you have stated he has, plus quite a few more in both pro audio and commercial electronics.
Title: Re: kay 720 short hunt
Post by: nosaj on July 03, 2015, 08:50:31 AM
Quote"Instead he decided to state I was wrong and he was right.

As far as someone not blowing their own horn, it matters little to me what his pedigree is, just how he conducts himself. I can obviously see he wants to be helpful, but it should never be his aim to put someone else down, because of his own misconceptions.
Suffice to say I have had experience in all the areas you have stated he has, plus quite a few more in both pro audio and commercial electronics."Quote

Sounds like someone's ego got bruised.  maybe you need to go back and re read to see what your goal really is. That your right and he's wrong?  It's kind of time to let this go.


nosaj
Title: Re: kay 720 short hunt
Post by: g1 on July 03, 2015, 12:16:55 PM
  Please keep in mind that this is a guitar amp forum, and we speak in that context here.
  No one has ever seen a cap can with built in coil in any amp similar to the one in question.  So we don't talk about it as an option because of the context.
  To speak of esoterica it should to be mentioned as such.
Title: Re: kay 720 short hunt
Post by: LateDev on July 03, 2015, 01:03:46 PM
  Please keep in mind that this is a guitar amp forum, and we speak in that context here.
  No one has ever seen a cap can with built in coil in any amp similar to the one in question. 
That is a fair enough statement, which I can accept, and I was speaking in terms of guitar amps, but if as you say no one has seen one, then that is acceptable, to tell me I have not seen one or am mistaken is just a misconception.

Sorry I did not mean to bruise anyone's egos, but apparently have.

I found your statement that you have not seen such a device acceptable, just accept that I have seen one, and let it go at that.
Title: Re: kay 720 short hunt
Post by: g1 on July 03, 2015, 01:21:10 PM
  The statement that you have seen such a cap can is perfectly acceptable.
What was not acceptable was the statement that in this particular cap can, he could not disable part of it and use the rest because it might have a coil in there.