Kenwood SM-220 project

[Radio Tech]

Back in 2012 late year I was working on a TS 830. One of my favorite piece of test gear was my SM-220. Really helps in making sure the transmitter is linier. I really loved that thing!

I left the shop for lunch and returned 30 minutes later to find the shop full of smoke. After a few minutes of searching the source of the smoke was the SM 220. After a quick tear down I found that one of the high voltage caps had failed. Bad part was this shorted the high voltage (500 V) from the transformer to ground. The put the transformer into meltdown.
And the .5 amp fuse did not blow. Well not until the transformer completely fused every winding. :headbang

The transformer is 120 volt primary.
Secondary's are:
6.3 volt @ 0.3 A
15 volt with center tap @ 0.12 A
150 volt @ 0.1 A
500 volt @ 6 MA

After searching the web for a year I come up with no replacements. Did talk with several companies on re-winding or building this little transformer. To the tune of 150 bucks but they say it will not be a direct replacement and may not fit.

Today I thinking about mounting several transformers in a box. Hoping to get all the voltages I need to bring this piece back to life. I will install a fuse on both sides of the mains input. I feel like if it had this to start with the transformer would has never melted.

I really miss my SM-220

 

[Captain Kilowatt]

Just use a common mains fuse and individual secondary fuses. It's not uncommon for a small transformer to go into meltdown when shorted and not blow the mains fuse. I remember one time many years ago we still had a tube type audio amplifier/AGC unit in our master studio at work. It was not used on air but rather just as a utility line amplifier when doing remote broadcasts from a client's and was patched into the telco line as required. One day a filter cap failed shorted and the thing overheated with copious amounts of smoke pouring out of the equipment rack. The announcer called in a panic state and I took off for the studio. By the time I got the rack screws out and the wires cut on the back the entire chassis was getting almost too hot to handle and the studio was full of acrid smoke.The shellac on the transformer windings and core lamentations was dripping out of the unit. It was an exciting couple of minutes to say the least.

 

[2RT307]

Cool. I've got an OLD Heathkit HO-10 that decided to smoke, too. Haven't fiddled with it in a long time, but it's funny I was just talking to a club member about reviving it about two weeks ago. I hope you'll post some pics and step-by-step description of your repair. There might be some common things for me to look out for.

73,
Brett

 

[Radio Tech]

You are most likly right on the fuse. I think putting a fuse on the secondaries is a great idea! And I know that smell all to good lol. Almost did not think I would get the irons apart on this transformer. Looks like the fuse did pop before it started to drip.

Here is the transformer torn apart. You may or may not see the core is completly melted. And the money shot is that someone in the past replaced one of the HV caps with the incorrect part. This is what caused the meltdown.

 

[Radio Tech]

Cool. I've got an OLD Heathkit HO-10 that decided to smoke, too. Haven't fiddled with it in a long time, but it's funny I was just talking to a club member about reviving it about two weeks ago. I hope you'll post some pics and step-by-step description of your repair. There might be some common things for me to look out for.

73,
Brett

Sure, post of some findings. Maybe we can get it back running.
I will try and add as much info as I can on this repair.

 

[Captain Kilowatt]

Usually what happens is that in the case of a HV winding shorting out, the winding resistance tends to limit the current draw from the primary and the thing just sits there frying without blowing the mains fuse. If it was a filament winding then usually a tremendous amount of secondary current has to be drawn before the mains fuse pops.A typical 6.3 volt winding will draw 20 amps or more before blowing a small 1 amp AC primary fuse and most filament transformers or windings will go into meltdown long before then.

 

[Radio Tech]

Agreed

You see the ratings of these secondary outputs?
500 v is only 6ma.
Guess there is some small wire on that thing. Can see why it took so long to burn lol.

Anywway at the moment I am going through a stack of transformers to see what I can find. have plenty of 6 volt ones. It is the 150 and 500 I have not ran into yet.
Still searching.:headbang

 

[Radio Tech]

Spent the better part of the time searching through a ton of transformers. So far, nothing even close. Got to keep digging.

 

[Radio Tech]

Well after searching the shop and the internet I was a little dissappointed. Nothing close to what I need. What is close is too high in amps.

So time to restudy re-winding transformers. Been a long time since I have done one. And never did one with this many taps before. Always a first time

Got to figure out all the windings and wire sizes now.

 

[Radio Tech]

Well after spending the week investigating what I need to do I think I have it figured out. Seems the manufacture of this transformer for kenwood used an nonstandard wire size. It is smaller than the normal stuff used. By about one size. They also did a funny trick on the primary. There is a 120 volt and a 100 volt coil and were connected together using double wire and jumpers.

Now the problem with using bigger wire.
You run out of space on the bobbin. :headbang

Solution:
Omit the 100 volt tap. Not needed. This will give extra room for the bigger wire. I hope....
So I calculated the turns needed for this transformer.

Primary = 783..3. turns
500 volt secondary = 3264 turns
150 volt secondary = 979.2 turns
15 volt secondary = 97.2 times 2 turns center tapped (15 +15=30 volts)
6.3 volt secondary = 41.1 turns

If anyone wants to wind their own use this spreadsheet and info from this page:
Practical Transformer Winding
Down half way the page there is a green and orange screen shot of the spreadsheet. Just click it and enter you data.

 

[Captain Kilowatt]

100 volt primary? Japan uses 100 volts as normal household voltage. It must have been made with that market in mind as well instead of a different transformer.

 

[Radio Tech]

100 volt primary? Japan uses 100 volts as normal household voltage. It must have been made with that market in mind as well instead of a different transformer.

Think you are right about that. Would make sence to me.

Anyway working on construction a former to hold and spin the bobbin on. Will be all hand powered. No fancy winding stuff here.
just may get the primary wound today!.

 

[Captain Kilowatt]

Beats winding everything by hand like I did with an old TV transformer when I needed 6.3 volts for filaments and something around 560-600 volts for my first home brew sweep tube amp.I was about 17 at the time and that HV winding was two turns per volt. Do the math. That's a crap load of turns. The 6KD6's ran well on it however.

 

[Radio Tech]

Beats winding everything by hand like I did with an old TV transformer when I needed 6.3 volts for filaments and something around 560-600 volts for my first home brew sweep tube amp.I was about 17 at the time and that HV winding was two turns per volt. Do the math. That's a crap load of turns. The 6KD6's ran well on it however.

Man that is a lot of turns by hand. What folks dont know is it takes a long time to do this. I started this morning at 6am working on this. I now have the first layer of the primary wound. 57 turns. Only 733 turns to go!

Here is the home made winder.

 

[Captain Kilowatt]

Yeah almost 1200 turns will tax your patience. I used something between each layer as I wound it. I think I used fish paper IIRC and then once done wound it all with fiberglass reinforced filament tape before assembling all the E I sections. In the end it all worked quite well giving me 800 volts of filtered DC. Just about right to make a 6KD6 scream. It was screaming because on a couple occasions I had the plates the same colour as the filaments.