The AM Forum

Well, it’s the second weekend of going thru the HB transmitter and I'm starting on the modulator deck. It's still a little dirty, but I thought I show some pics. It’s a pair of 810's in ordinary class B push-pull configuration and it has all Peter Dahl transformers. The mod transformer is a tailored Desk KW/KW1 transformer. I basically had him modify the windings slightly for the exact ratio I needed. The small driver transformer is the KW1 transformer with no modifications and the bias transformer is a Desk KW transformer. The other is for the 810 filaments.

One thing I never had the chance to do was use insulators under the mod transformer’s legs to isolate the iron from ground. I plan to do that before placing it back on the air. At the time I never could find any heavy enough ceramic standoffs. I need to go thru the boxes to see if I ever did find any, I just can’t remember. Maybe some wood blocks if necessary. Any ideas?

PVC should work for the spacers/stand-offs.

In my original high-power homebrew AM rig of circa 1978, I used a stained and varnished oak "sled" to float the RCA 1KW-class modulation transformer way above ground. I still have that xfmr and sled arrangement sitting in the W2XR heavy metal pile. It was stained and varnished for appearance purposes only, and not to improve the insulation rating, etc.

In my current homebrew rig, where the modulation xfmr is out of a Gates BC-1H, I use (4) ceramic stand-offs to accomplish the same thing. In fact, the mod xfrmr, the mod reactor, the HV swinging choke, and the HV smoothing choke, are all mounted way above frame ground on ceramic insulators. Very cheap insurance to protect against a winding-to-core short. The only high power magnetic in the whole rig not floating above ground is the 6200 VCT plate xfmr.

I also protect both the modulation xfmr primary and secondary windings with spark gaps, and I do the same thing with the HV swinging choke. The spark gaps are set to 0.05" if I remember correctly. I'm sure that I documented this somewhere, in the event you want me to confirm this. Again, it's very cheap insurance, especially since these high voltage or high-power magnetics are becoming hard to come by and expensive and/or difficult to replace.

The layout of your 810 modulator looks very clean, and I'm sure it will sound beautiful once she's on the air.

73,

Bruce

gotta love the clean chassis and matching iron. 8)

How about a Fibergalss plate or some sort of thick plastic then you can use metal stand off. Check out Home Depot. Wood looks nice Bruce, but I have had leakage problems at high voltage.

Somewhare in the teck area thar is a pic of plexiglass spacers being used as standoffs..........  klc


like here

http://amfone.net/Amforum/index.php?topic=11648.20

Hi Brian,

Beautiful craftsmanship. Looks great!

What length standoffs do you need? I have some from old tuning coils from a BC TX site that may work. I think they are threaded and are about 3/4-1" dia. and 1.5 -2" or longer. I know some are quite long. Would be nice to have ceramic in there and keep the proper look to it.

Let me know.

Paul

[quote ]
I wonder if the varnish you used could have been a slight conductor? I was thinking of using painted wood, but I'm worried about the paint and the wood too. Wood not a very good insulator, but I guess it would be better than nothing at all.
[/quote]


Try baking the moisture out of the wood first, at VERY low temperature.

You can also boil the wood in paraffin (sp) after, but I've always wondered about baking the moisture out, then boiling it again.

--Shane

Dupe post. see below.

HI Brian,

Okay, I'll go look for them this weekend and I'll try to find 4 all the same. I'll get them in the mail to you on Monday if I can locate them... ::)  I know I have them, just need to locate the box they are in. I don't want anything for them, glad they can go into a nice rig that can use them.

Cheers

Paul

MFJ sells some feed thru insulators that might work. They also have standoffs, but the feed thrus already have bolts going through them and hold the ceramic in compression and therefore could tie down a heavy transformer.

They also sell come ceramic standoff, but those might not be strong enough???

Do a search on Glastic insulators. they have 5/8" threaded bosses at each end.

http://www.stormcopper.com/Glastic-Standoff-Insulators.htm (http://www.stormcopper.com/Glastic-Standoff-Insulators.htm)

http://store.electrical-insulators-and-copper-ground-bars.com/standoff-insulator.html (http://store.electrical-insulators-and-copper-ground-bars.com/standoff-insulator.html)

These things can handle any voltage or weight you want to use on them. I used them on my ground busses coming into the shack and distributing the ground wiring.

Pricey a bit, but it's a lifetime part and they will do this job. If I had 4 I'd give you mine, but my last 2 are already mounted and ready to wire with #4.

Brian, Consider a thick nylon cutting board and cut holes in the chassis where the bolts are. I use a Greenlee tapered bit for nice clean holes.

Beautiful homebrew you have there.

Ur working too hard on this one...

For starters you can buy heavy Nylon bolts and washers at the local hardware store. They even use them for holding on toilet seats too...

But my solution, to get slightly fancy would be to hold the iron to a sheet of some insulator, like polycarbonate, PCB material (no copper), HDPE (high-density polyethylene) that they sell for various home and industrial uses, CORIAN or other solid surface counter top material, Delrin, or Nylon, and then mount the xfrmr with the bolts UP from the bottom, and the assembly down to the chassis. If the sheet is thick enough it is sufficient to counterbore the xfmr mtg hole for clearance (pot it with epoxy or silicone rubber if you like), otherwise make it out of two thinner sheets...

But the simplest is to mount the tranny to a plate, mount the plate to the chassis with 4 heavy nylon bolts, and 8 nuts, one set goes between the plate and the chassis and is the standoff... cheap, fast, effective.   ;D

I'd skip the ceramic, too brittle for heavy iron imho.


             _-_-bear

Nylon isn't very good for holding heavy weight. Derb's solution is a good one.
A material with fiber will be a lot stronger.

Glastics ( sized to the need) can hold OBSCENE amounts of dead weight. I know that 4 of the ones I have would support 500 lbs of dead vertical weight and not even blink. I wish I had 4 more, but I only have 1 not needed.

I'll go snap a pic or two with wifeys camera.  Would be downright instructional, it would.

Brian W5HRO

I have 6 of these I will part with very reasonably. They are unused from a cardboard box that got moist and corroded the brass inserts. I think the inserts could be easily removed by filing. Heavy gasket material could be used between them and the xfmr and chassis.

They are 8 1/2" long x 1 7/8" wide x 1" thick. The hole center are 6". They are old brown porcelain. The crud on them is the old box.

If interested PM me. All 6 could be shipped in a USPS priority mail $9.80 flat rate box.

Is this a mechanical issue or a breakdown/insultation requirement?

Brian, if your good in QRZ I'll send ya some glastic standoffs.

Buddly, from personal knowledge 'you is a perfect gennelman.'  heh, heh.
Kinda wondered if you'd break in.
Brian, Buddly's insulators are outstanding.

The Glastics are want you want. I'd send you mine but I only have 1 left, that wont help much.

It looks too nice to bodge it up with some sort of insulator, just run it as is, it worked before, didnt it?
Its very rare in homebrew to find matching anything, screws, iron or meters.
Its amazing some of the stuff I have run into, you would think it would not be hard to come up with the same hardware, but no, many things look like someone took half the screws out of all their apliances and lawn mowers and used them to assemble their pride and joy homebrew rig.
Square nuts, hex nuts, philips, and streight screws, different sizes, etc.

If you have to insulate it, use some short fat ceramic insulators.

Brett
N2DTS 

they wont take it. a serious bump moving it an it'll break. Glastics are the ticket.

PS - Buddly, I got my 30 volt battery and the Triplett is FB again!!!!!!  TY!

I have a bunch of red plastic type things, insulators for power buss, with metal tapped inserts for bolts, the plastic is some sort of fiberglass plus plastic stuff I think, I dont think you could break them.
They came in various sizes, from really big 3 inch high ones that take 5/8 inch bolts, to small 1 inch high units that take 1/4 inch bolts.

The power buss is for battery plant in phone company equipment, the voltage is about 50 volts, but they would likely take thousands of volts even on the little ones.

I also have sheets of insulating stuff they mount racks on to insulate them from the floor, its also red, 1/32 thick plastic and fiberglass stuff.

Let me know if you want any of this stuff, its free...

Brett
N2DTS

yep those are Glastic insulators. use 4 of the big ones and you are set 4 life.

Yep, thats what I have.
I have a bunch, I should collect them at work, a lot must have went out in the trash...

Brett

Run a high value resistor from the core to ground if you decide to insulate the transformer. There may be a charge developed that can get fairly high if you don't. N4VMY (RIP) & W9OTN clued me in to this many moons ago. I personally don't understand all of the hoopla about transformer insulating. In the 30 or so commercial /military transmitters I have owned I have yet to see one mounted this way. I admit I have done it, but mainly to cover up what's under the tranny. The new Peter Dahl iron is FB stuff and doesn't need the babying. A wimpy SNC or used iron of unknown pedigree might be a different story.
2 cent Skippy

This doesn’t apply to your situation exactly. A source for thick cheap insulator is those plastic cutting boards for veggies. I have cut them up for tesla coil projects. Also when working on dielectric processing equipment. We use a lot of stand off’s and insulators mainly Teflon. Any insulators we use near RF go for a ride in a microwave. If they get hot or even warm they go in the trash. I have found that ceramic insulators have a tendency to explode at 10 KV and up. This has happend when there is RF on the DC. and  is usually on the order of 20kw. We used 30MHZ of RF to dialectically bond fabric for inflatable buildings. Also for bonding and embossing the vinyl door panels on cars.
73 Johnathan N8QPC

Derb gets my vote. Why JS a beautiful rig

Those Glastic things look like lightning protection stuff we used in our microwave tower sites. The old Halo ground concept..
Leave it up to DERB to come up with "heavy duty"

Phred

We use them at work. First rate looking.

thats what hey are used for. they isolate ground buss bars with them. I used 6 here, got one left I gotta match up this hmfest season. you put 5/8" grade 8 SS hardware in those holes and nothing goes anywhere.  You size them to the need, you could tip a 150 pound plate xformer on its side and mount it that way in the cab if you really wanted to.  ???

We have a box that has welding cable feeding it at 650 DC. They used them all over the inside. It is a beautiful thing.

Brian

I had a lot of audio feedback from my KW-1... caused by the mod transformer, resonating with the KW-1 cabinet.

To solve the problem, I removed the bolts that held the mod transformer to the cabinet, and I placed a 1/4" thick computer mouse pad under it. It is held in place by gravity... and it has worked fine (drastically reduced the acoustic feedback problem) since I did that several years ago

Your home brew modulator is beautiful!

Best regards
Stu

Stu, AB2EZ has come up with one of the few good reasons to "insulate" the mod transformer. KW1, BC 610, etc.all have the talking problem--mouse pad is a great fix.
Skip

4 insulated shock isolators would be cool in that application.

I'd just put those wide neoprene washers on the Glastics, top and bottom.

Na, you want a $200 mil isolator on each corner.

www.mcmastercarr.com (http://www.mcmastercarr.com)

check out page 1350 ... Vibration-Damping Mounts

Derb has a good idea. Just use a longer screw so it bottoms out and the washers are not compressed. A metal washer between the transformer and top soft washer. 

they'll never break or fail. go to home depot and get the wide neoprene washers and SS hardware (flat split & screw) to match and you R set for life. How much did they cost you? I got mine for 50 cents each at a fester, so I dont know what they go for new.