The AM Forum

 Picked up a SB-220 last weekend, looks good inside after cleaning and inspecting, I have copy of the manual but the schematic is of very poor quality. I want to bring it up on a 120 volt variac.

 It has a 220 plug and the 4 terminal strip that configures 120 or 220 is jumpered for 220 operation, see picture. I measure NO resistance across the plug when I turn the amp on no matter what position the cw-tune/ssb switch is in.

 If I change the jumpers for 120 operation I now measure 1.7 ohms in SSB and 3.3 ohms in cw-tune with the amp on.

  I plan to bring it up on a 120, that is what my variac is but this doesn't seem right, I could use some advice. Thanks!!

Probably one of the primary windings is open.  Normally with a dual voltage xfmr there are two primary windings.  They are wired in parallel for 120 volts and in series for 240 volts.  So, in series you're not measuring any resistance across the plug and in parallel you're measuring only one of the primary windings.

I would suggest you check the resistance of each primary winding directly from the wires that come out of the xfmr.  If both are OK there, then back trace all wiring and connections from those xfmr wires to the plug.

Fred

Attached is a decent copy of the schematic.  Make sure the circuit breakers are both closed/good.

Here's the schematic all on one sheet.

  Thanks!! A GOOD schematic was what I needed.

  Having a schematic I can print and then read makes all the difference, thanks again!!

 Good news is that the HV transformer looks to be OK, bad news is that the Blue/Green to Black/Red winding for the Filament/Bias transformer is open.

 I gave the amp a good look over, it looked very clean inside but had been stored for years from the look of it. Didn't see anything that was burnt or obviously over heated, is this a common problem?

That kind of failure isn't common as far as I know.  I wonder if a previous owner wired it wrong and applied the full 240 to a single winding or had one winding disconnected on 120 causing excess current flow which would pop the light weight primary winding pretty quickly.  But measure right where it enters the transformer to be sure there isn't an interconnecting wiring error and that there isn't a repairable mechanical wiring break.

There is a SB-220 carcass on ebay now that appears to have the transformer.  You might contact the seller to see if you can have him check for continuity and see if you can make a deal for it:

 http://www.ebay.com/itm/Heathkit-SB-220-HF-Linear-Amplifer-Amp-99-99-C-MY-OTHER-HAM-RADIO-GEAR-AMATEUR-/191143925853?pt=US_Ham_Radio_Amplifiers&hash=item2c8110d85d

   I have never seen that xfmr fail, do the tubes light?
I always replace that 20uF cap on the bias supply.
If the dual primaries are in series for 220 operation
maybe just one is open. as Fred suggested.

   I made up a power cord with 2 alligator clips and
hook my variac to the primary winding of the power
supply I'm working/checking. It allows trouble shooting
without HV on all the time.

GL

/Dan

Schematic says it's Black/Green. If you can't find a replacement for the bias part of the transformer, disconnect the leads for that section and replace it with a small pair of filament transformers connected back to back (secondary to secondary) or find a small 110 volt line isolation transformer.

Tom
I may have a spare filament transformer leftover from a parts cadaver. I used the HV trans to build
the power supply for my ART-13. If you're going to the AMI BBQ on the 3rd, I can bring it with me.
73 de Billy N6YW

  Billy,

            Thanks for the offer but it looks like I found the problem, I carefully rechecked what I had looked at the other day, the colors on those cloth insulated  transformer wires are not very distinct, may also fade over the years. All the transformers look good!! The problem is the double pole double throw ON/OFF switch, one side is bad, that also happens to be the side only used when the amp is wired for 220.

             I plan to make the AMI meet and will be there with Vern who told me he was buying a radio from you.
 
             Thanks to the others here for the advice and the good schematic!!

                                    Tom, W6TOM

Tom,

You could use the existing good switch section to control a relay for power switching and this should allow the remaining side to live indefinitely given the very small current it would see.

Tom
FB on the troubleshooting. If need be, I may have the exact replacement DPDT switch. I will look, and if you need it I can bring it with me to Georges. BTW, Vern bought my complete Eico transmitter setup and boy is it a beauty. He's going to be stoked!
73 de Billy N6YW

  This is an odd ball switch, it doesn't set into a hole in the chassis but has it own carrier, when I take it apart I can see if there are any numbers and maybe a manufacturer but I have a feeling it may be unobtainium. Looking at Mouser I see rocker switches all of which are made off shore and are metric.

 I did think of using the good contact to control a relay, a bit more hassle but it does keep the original look.

 I'm going to be too busy for the next few weeks to play radio repair, back to work tomorrow!!

                            Tom, W6TOM

I've taken that type switch apart and made some repairs to it.  You back off those taps and the switch will come apart.  I just did this a few months ago and used the switch in a project.  I don't remember were I pulled the switch from, probably some appliance I took apart.

Fred

Yes yes,
Been there done that, also with a Kenwood TL-922 switch. Same thing. De-carbonize the contacts unless they were welded "away" then you have to replace.
You'll get it Tom. Otherwise, my offer stands.
73 de Billy N6YW

The lights in my shack don't blink nary a bit with my SB220 wired at 240V, 1 phase. The receptacle is mounted directly below the shack power panel.

That's how I check to see if I'm modulating. It's even more exciting when you modulate the neighbor's lights. Hang some fluorescent tubes at the ends of your dipole and scare the heck out of your neighbors at night.

Unless you like to keep everything stock I would definitely put in a soft-start circuit and consider a relay controlled by the switch. Replacements are hard to get! Also a fuse instead of the unreliable circuit breakers.

Here is how I did it on my SB200 -

http://crompton.com/hamradio/heath/sb200/sb200.html

My SB200 runs fine on a shack dedicated 20A 120V circuit. It is fused for 10A and has never blown.  The SB220 would also work on a 120V dedicated 20A circuit but you probably would not want to have much else on that circuit.