Kay 752 amp - does this sound normal?

[famous beagle]

It may also be a good idea to fit a suitable fuse before moving on.

I'd like to do that. Do you know how I determine what a suitable fuse would be? And where in the circuit would I place it?

Sorry for all the questions. Still learning a lot.

 

[arcaxis]

Turned on the amp and immediately saw a spark, heard a pop, and saw some smoke. Turned off the amp and, after determining with my meter that everything was safe, took a closer look.

Long story short, I'm pretty certain that I fried the output transformer, because that's the side from which the smoke emanated, and it smelled burnt. That smell has faded since then, but something was smoking, and I can't see any other signs of charring. So I'm assuming it has to be that.

Then I remembered the ribbing system garww mentioned and took a look at the old 2-prong cord. I noticed that it was ribbed, and the ribbed lead (i.e., the neutral wire) was wired to the switch --- not the hot wire.

So it looks as though I had the power cord wired the wrong way. Would this have done it? I'm 99.99% sure I had the speaker (which is 4 ohms, just like the Kay speaker) wired correctly, and I didn't touch anything else in the amp.

Also ... does anyone know how I can determine what a suitable replacement for the OT would be?

Inherently the output transformer shouldn't have taken a hit from mis-wiring the power cord. One side of the primary winding is connected to the plate of the 50C5 which is essentially an open circuit until the tubes warm up and a failure wouldn't be immediate, as for current to flow through the output transformer the 35W4 would need to warm up to supply B+ through the transformer to the 50C5. If the amp worked prior to the cord change it's unlikely the 50C5 is bad, as shorted or gassy. Wiring of the temporary speaker may have caused an output transformer failure if the output transformer had been open circuited and a signal applied through the amp, but it doesn't sound like this is the case also because the tubes would need to have warmed up. When the speaker was hot wired did you also connect the ground wire to the other speaker or just the voice coil leads (I don't think this would have made a difference if you did though)?

The schematic that was previously posted in the thread doesn't show the power switch and AC lead connection clearly. Could you post another image that shows the switch and the incoming AC (lower right where says 117vac 60cyc). In this amp there is a 'chassis ground' and a 'common return'. The common return would have been either hot or neutral depending on the plug orientation with the old cord. If I'm seeing part of the power switch in the schematic, the switch is connected to the common return. The chassis ground appears to be 'floated' off the common return by a 100kohm resistor and .005uf cap. Did you connect the green wire of the 3 wire cord to the chassis? If you have an DMM, check with the ohmmeter the metal chassis (or ground prong on the plug if the green ground wire connects to the chassis) to both the hot and neutral of the cord with the switch on and off.

Not 100% sure, but I don't think the output transformer will be cooked. Your wiring of the new cord would have to be way off to toast it. Old transformers sometimes can have an almost burnt odor from what they were impregnated with. On the circuit board there is a .005uf cap in parallel with a 100kohm resistor the ties the common return with chassis ground. See if that looks wonky.

 

[famous beagle]

Thanks for all the information. To answer some of your questions:

"Wiring of the temporary speaker may have caused an output transformer failure if the output transformer had been open circuited and a signal applied through the amp, but it doesn't sound like this is the case also because the tubes would need to have warmed up."
Correct. The amp had zero time to warm up. The spark happened a split second after I turned the amp on.

"When the speaker was hot wired did you also connect the ground wire to the other speaker or just the voice coil leads (I don't think this would have made a difference if you did though)?"
I did connect the ground wire, but I had to use a jumper wire to do it because it wouldn't reach the center tab on the Weber (8-inch) speaker. I did test for continuity with the meter, though, and the jumper wire was making solid connection to the center speaker tab.

"The schematic that was previously posted in the thread doesn't show the power switch and AC lead connection clearly. Could you post another image that shows the switch and the incoming AC (lower right where says 117vac 60cyc)."
Yes, I've attached a picture of the schematic in my amp. It should be pretty clear.

"Did you connect the green wire of the 3 wire cord to the chassis?"
I connected the green wire to, I guess, the common return on the circuit board. See attached photo. I thought they were both ground (the common return and the chassis ground). I guess not?

"On the circuit board there is a .005uf cap in parallel with a 100kohm resistor the ties the common return with chassis ground. See if that looks wonky."

It looks ok to me (see photo), but I don't know what it's supposed to look like if it's been fried.

I've attached two pictures of the three-prong wiring. In one of them, you can see how the white wire (neutral) leads to pin 4 of the rectifier tube. This, I think, was one of my mistakes.

Was the connection of the green wire my other mistake? See "green wire mistake" photo. Should I have connected it to the blue circle instead? This terminal connects to the hexagonal spacer, which connects to the chassis.

Hopefully, all of this makes sense. I really appreciate everyone's help!

 

[CoolCat]

testing transformers isnt so easy because you are testing a copper wire and the resistance is small.
the insulation can breakdown and the wires short or short to the iron core but to test with a ohmmeter it requires very small resistance measurements and can be tricky. sometimes the short wont show up unless the transformers hot. ive replaced these before and speaking to a transformer repair tech he said Lighting often hits stuff and breaks it down and the parts wont fail for weeks or months later but the hit of lightening is what took out the transformer in one way or another.

I have a huge 100-150lb transformer now that I believe is tripping the panel 60amp breaker. But the only way to test it was removng the output and the C/B still tripped so I assume nothing on the output side of the transformer is at fault or at least not the initial fault.

then removing the primary side voltage the C/Breaker didnt trip so I found only when I hooked up the primary side voltage to the transformer the C/B trips! even with nothing on the secondary/output side.

but when I put my multimeter on the transformer primary coil..... well...its a tiny bit of resistance and I dont know whats normal or if my meter even is working at this small a value.....and again theres no voltage/current to arc, resistance meters cant capture arcing ....so basically I dont know where the short is.

I think the pro's use a tool that can rampup the voltage/current and tell when it might short or if its a healthy transformer. A "meggar" or something.

but anyway, transformers arent so easy to test. I guess if it was an open it would be easy, but a piece of wire thats wrapped and shorting to other pieces of wire is harder. imo. you know the wire coils are coated in a thin insulator, its not just a wire wrapped, it has a very thin coating of insulator that breaks down and starts shorting sometimes.

interesting thread though!!! one of the best Ive read in a long time!

good luck on that amp

 

[Steenamaroo]

Your green wire chassis connection certainly looks to be in the wrong place, as you say.
Yes, It should go to where you've circled blue but it really should go to the physical chassis - not a trace or point that leads to the chassis.

Fit a solid O-ring terminal to your green chassis wire and fit that to a solid bolt in the chassis, having roughed up the surface.
If there's an existing mounting bolt that you could use, that would do...as long as it's rock solid.

Am I correct in saying that common return should always be neutral, now that you're using a third pin to chassis? I.E. Live and neutral orientation very much matters now?

If so, it looks to me like you have neutral and live reversed too, although I'd urge you to prove that to yourself and not take my word!

 

[garww]

Often, they will punch out that transformer foot and anchor the green there

 

[famous beagle]

Your green wire chassis connection certainly looks to be in the wrong place, as you say.
Yes, It should go to where you've circled blue but it really should go to the physical chassis - not a trace or point that leads to the chassis.

Fit a solid O-ring terminal to your green chassis wire and fit that to a solid bolt in the chassis, having roughed up the surface.
If there's an existing mounting bolt that you could use, that would do...as long as it's rock solid.

Am I correct in saying that common return should always be neutral, now that you're using a third pin to chassis? I.E. Live and neutral orientation very much matters now?

If so, it looks to me like you have neutral and live reversed too, although I'd urge you to prove that to yourself and not take my word!

The only spot that I could reach with an O-ring on the green wire (without extending it) would probably be the bolt on that hex spacer that's connected to the terminal I circled in blue. See new photo "green wire O-ring."

And yes, I think I do have the white and black wires reversed too. See the photo "Original power cord wire connections." When I wired the 3-prong, I didn't know about the ribbed neutral wire system (which garww pointed out to me). But after the amp blew, I looked more closely at the original cord, and I noticed the the side going to the switch was indeed ribbed (neutral). And I have my black (hot) wire going to the switch. So I think those (hot and neutral) are reversed.

It looks as though I did make two mistakes. That's not easy to do with one three-conductor power cord!

 

[famous beagle]

testing transformers isnt so easy because you are testing a copper wire and the resistance is small.
the insulation can breakdown and the wires short or short to the iron core but to test with a ohmmeter it requires very small resistance measurements and can be tricky. sometimes the short wont show up unless the transformers hot. ive replaced these before and speaking to a transformer repair tech he said Lighting often hits stuff and breaks it down and the parts wont fail for weeks or months later but the hit of lightening is what took out the transformer in one way or another.

I have a huge 100-150lb transformer now that I believe is tripping the panel 60amp breaker. But the only way to test it was removng the output and the C/B still tripped so I assume nothing on the output side of the transformer is at fault or at least not the initial fault.

then removing the primary side voltage the C/Breaker didnt trip so I found only when I hooked up the primary side voltage to the transformer the C/B trips! even with nothing on the secondary/output side.

but when I put my multimeter on the transformer primary coil..... well...its a tiny bit of resistance and I dont know whats normal or if my meter even is working at this small a value.....and again theres no voltage/current to arc, resistance meters cant capture arcing ....so basically I dont know where the short is.

I think the pro's use a tool that can rampup the voltage/current and tell when it might short or if its a healthy transformer. A "meggar" or something.

but anyway, transformers arent so easy to test. I guess if it was an open it would be easy, but a piece of wire thats wrapped and shorting to other pieces of wire is harder. imo. you know the wire coils are coated in a thin insulator, its not just a wire wrapped, it has a very thin coating of insulator that breaks down and starts shorting sometimes.

interesting thread though!!! one of the best Ive read in a long time!

good luck on that amp

Thanks for the info! This is all a learning process for me!

 

[arcaxis]

The spark you saw I believe was from the switch (this sounds familiar ) If the 'hot' side was on the switch, when you put the switch to 'ON', it put 120vac to the 'common return' which it sounds like where you had the green wire attached to which would create an instant short. The burnt smell was likely the switch as it's close to the transformer. The switch contacts might be toast.

Ideally the AC and switch would be rewired so the one side of the switch would be hot and the other side of the switch would go to the opposite side of the resistor that goes to the plate (pin 5) and the filament (pin 3) of the 35W4. Neutral would tie to the common return to take the place of the switch. The green ground of the AC cord would connect to a convenient point on the chassis or drill hole where there is space and with a Stakon type ring terminal on the green wire attach it with a screw and nut. I can doctor up the schematic if you want with the modification. It should be simple I think to move a few wires around.

Edit........ Just saw your post with the idea for green ground. Might be OK but would rely on the outside nut being socked down real tight, which I don't think would be best with the type of nut used on those threads.

If the pot/switch needs replacement, there are ground lugs that fit on the threaded shaft of the pot/switch that are soldered to. Not sure how easy those would be to find for you.

 

[famous beagle]

The spark you saw I believe was from the switch (this sounds familiar ) If the 'hot' side was on the switch, when you put the switch to 'ON', it put 120vac to the 'common return' which it sounds like where you had the green wire attached to which would create an instant short. The burnt smell was likely the switch as it's close to the transformer. The switch contacts might be toast.

Ideally the AC and switch would be rewired so the one side of the switch would be hot and the other side of the switch would go to the opposite side of the resistor that goes to the plate (pin 5) and the filament (pin 3) of the 35W4. Neutral would tie to the common return to take the place of the switch. The green ground of the AC cord would connect to a convenient point on the chassis or drill hole where there is space and with a Stakon type ring terminal on the green wire attach it with a screw and nut. I can doctor up the schematic if you want with the modification. It should be simple I think to move a few wires around.

It certainly does sound familiar, doesn't it? Just call me "Switchkilla."

I don't know if the switch is fried or not. The mechanism didn't break (like it did on my Champ). In other words, it still "clicks" both ways.

I just did a quick continuity test, and with the switch clicked in the "on" position (the amp is unplugged from the wall), I'm not getting any continuity between the terminals.

When I perform the same test on my Champ (again, unplugged), I do get continuity. I'm guessing this means the switch is indeed toast?

Regarding your suggestion for the mod, I'm very interested in that. However, I'm wondering what your opinion on adding an isolation transformer is. I've heard lots of people say that adding a 3-prong is a good start, but in order to be entirely safe, you need an isolation transformer.

As I mentioned earlier in a response to Einstein Magoo, I don't understand why this is because, if 3-prong plugs and outlets are standardized now, and I've added a 3-prong cord to the amp, how could it still be unsafe?

Thanks!

 

[famous beagle]

I must add, it's really great that you all have been so helpful and patient when you're likely thinking, "Oh god .... what has this dipshit done now?"

 

[garww]

hah If you want to be safe, don't plug it in : ) You might screw in a ground fault box and plug your new power cord into that

 

[Steenamaroo]

That sounds like you may have gotten off lightly! I still can't quite see where the original switch out wire goes but it looks like it's going to the point that you wired the chassis wire to (by mistake).
That would mean, as said, you fried the switch.
Your continuity test would support that.

I'm not 100% sure about the reason for an isolation transformer being considered safer.
I always assumed it was because it would isolate you from any cap-stored DC in the circuit if you were to touch the mains plug pins but what I think and what's true rarely line up.

 

[arcaxis]

It certainly does sound familiar, doesn't it? Just call me "Switchkilla."

I don't know if the switch is fried or not. The mechanism didn't break (like it did on my Champ). In other words, it still "clicks" both ways.

I just did a quick continuity test, and with the switch clicked in the "on" position (the amp is unplugged from the wall), I'm not getting any continuity between the terminals.

When I perform the same test on my Champ (again, unplugged), I do get continuity. I'm guessing this means the switch is indeed toast?

Regarding your suggestion for the mod, I'm very interested in that. However, I'm wondering what your opinion on adding an isolation transformer is. I've heard lots of people say that adding a 3-prong is a good start, but in order to be entirely safe, you need an isolation transformer.

As I mentioned earlier in a response to Einstein Magoo, I don't understand why this is because, if 3-prong plugs and outlets are standardized now, and I've added a 3-prong cord to the amp, how could it still be unsafe?

Thanks!

Sounds like the contacts of the switch vaporized The switch housing looks like it has metal tabs around the edge that retain the brown 'bakelite'. Those tabs sometimes can be bent back to remove the switch part so you could take a look at it and confirm its destruction. Questionable if it would go back together after this surgery, but a sure way to know.

If the chassis is grounded properly through a 3 wire power cord, there shouldn't be a need for an isolation transformer. The caveat would be an outlet that is wired incorrectly or if a ground lift adapter were used which may result in the chassis not going back a real ground.

 

[garww]

With proper tools, those switch/pot caps are not that much of a problem - those older ones. I just posted a pick of a open L-Pad the other day

 

[famous beagle]

Sounds like the contacts of the switch vaporized The switch housing looks like it has metal tabs around the edge that retain the brown 'bakelite'. Those tabs sometimes can be bent back to remove the switch part so you you could take a look at it and confirm its destruction. Questionable if it would go back together after this surgery, but a sure way to know.

If the chassis is grounded properly through a 3 wire power cord, there shouldn't be a need for an isolation transformer. The caveat would be an outlet that is wired incorrectly or if a ground lift adapter were used which may result in the chassis not going back a real ground.

Ok, considering that (regarding the iso transformer), I think I'll just proceed with your mod. I don't ever intend to take this amp out of the house (it's strictly a recording amp), so I won't ever be plugging it into any unknown/untested outlets.

If you could draw up the mod, I would highly appreciate it!

 

[famous beagle]

So to replace the switch ... I know the pot is 500K, but I'm having trouble finding a similar one with a built-in rotary switch that fits the other specs. The shaft (knurled) height is 1/4" (see photo).

I know they've gotta be out there, right?

 

[arcaxis]

Attached is how I would likely mod the AC and switch circuit. It's kind of just removing one switch wire and moving it and replacing where it was with the white wire from the cord. If not sure I'll try to help further.

 

[arcaxis]

Might work.....
Uxcell a14071400ux0773 2 Piece B500K 500K Ohm Single Linear Rotary Switch Carbon Potentiometers: Amazon.com: Industrial & Scientific
Linear taper, rather than audio taper, but for the amp won't make much difference. Measure the shaft length on yours.

Seller/manufacturer site...
Buy Cheap 2Pcs B500K 500K Ohm Single Linear Rotary Switch Carbon Potentiometers

 

[garww]

A combo switch like that will be more common in radio parts. You can bypass the on/off as a temp measure if you have to.