Heathkit SB221 Inspection

[357]

Just bought this and it got here yesterday. Prognosis, blown R3.
So far everything looks ok, looks like new actually.
The plate transformer looks tweaked a bit though.

Anyone have advice on actions to take?
Both meters work, haven't plugged the amp in yet.

The input section looks like the 11M hater version I think.
Anyone see anything I should investigate?

Thanks!

 

[357]

 

[nomadradio]

Blown R3 can mean several possible things. Or maybe even more than one of them. It is close to 40 years old, after all.

First is a short inside one or both tubes between the cathode and grid. This can be an intermittent problem if just one end of a grid wire comes loose from the wire ring that holds them all in parallel away from the filament. It can curl up when it gets hot, and touch across that gap. Cools down, and won't actually touch when the tube is pulled to check for continuity grid-to-cathode.

Turning the tube on its side, and rotating it around the long axis while gently slapping the side can reveal this.

But only if your continuity tester responds really fast. Not all of them are fast enough,

Second possibilty is massively excessive drive power. First clue can be the coax from the input side of the relay, clipped where it would go through a hole in the chassis upstairs to the front deck of the band selector. If that coax leads directly from the relay to the tube sockets this permits more drive without melting the tiny 10-meter input-matching coil. Also raises the amplifier's input-side SWR to around 3 or higher.

Likely sign of excess drive is melted solder inside the ends of the tubes' cathode pins and badly-discolored spring contacts on pins 1 and 5 of the tube sockets. Loose contacts on the sockets' filament pin contacts occur when overheating ruins the temper of the 'V'-shaped tension spring in the sockets' contacts. As it loosens, the connection to the tube pins degrades. Resistance of the circuit rises, tube pins get hotter still until enough solder drips out and the tube's filament goes dark. Overheated tube pins nearly always indicate the need for two new tube sockets. Loose socket pins will ruin a perfectly-good new tube.

Third and most likely possibility is the worst case, more or less.

Shorted HV transformer. Usually the first thing to go bad inside the big, heavy, expensive HV transformer is the insulation between the HV winding and the grounded frame of the transformer. Can't see this from the outside, short of taking the two secondary wires loose and feeding a few thousand Volts DC from an insulation tester. That test will reveal leakage current that indicates broken-down insulation.

When the HV winding shorts to ground, the fault circuit gets completed through R3. Typically releases enough energy to blow half the plastic body off of the part.

That fault surge also goes through the coil of the right-hand meter, if the switch is set to grid current. But not for long. Probably takes only microseconds to pop the coil in that meter. If the needle moves with the leads to your meter on it set to "ohms" and the right-hand meter switch set to "HV" then it's still good. It's good luck for that switch NOT to be set for grid current when R3 explodes. And if that's where the switch was set, an exploded R3 will guarantee a blown right-hand meter.

Don't remember ever seeing this problem until the oldest SB-220 amplifiers reached the 30-year age mark in the early 90s.

By Y2K it had become a common problem.

By since about ten years ago, we just expect the factory-original HV transformer to be bad, or to fail after a year or less of regular use. A barn find that was kept scrupulously dry (like in a desert) and has 500 original miles might check perfectly okay.

Haven't seen a SB221 in that condition for a long time. No telling how long the transformer would perform okay before "remembering" how old it is and "POW!"

73

 

[kopcicle]

STOCKED PART - HAMMOND Part Number: PWDP13034
STOCKED PART - HAMMOND Part Number: PWDP13026
http://www.pwdahl.com/PWD_HamCatalog.html

...and been there done that.

 

[357]

Hey thanks for the replies.
This amp came with no tubes, but I have a set left from when GSP stole my 220.
https://www.worldwidedx.com/threads/just-got-f-ked-by-the-global-shipping-program.230837/

I got this one from the same seller, Hamhifi.

Today I plugged it into 120v while being tapped for 220.
I have 2.7vac on the filaments and 1000+ on the plate, no tubes.

The amp looks literally like brand new.
Now I've confirmed its ok, I'm going to defiantly recap it but I'm on the fence with the hv board, it looks mint. I'll probably get the harbach kits but I did notice the boards on ebay, w7***.

The band switch looks good, the load and tune are great, no signs of arcs.

 

[357]

 

[nomadradio]

There is one other way to check the HV transformer for insulation failure inside.

Take both the red and red/yellow wires loose, one from the rectifier/meter board, the other one from the midpoint of the filter-cap stack.

Ground one of these wires. A gator-clip lead will do the job. Tape off the end of the other wire, so it can't accidentally touch anything.

Insert a small breaker or fuse, 2 or 3 Amps in line with one side of the AC line cord. If you have a variac, turn it up slowly. If not, flip the power switch. The fuse should hold if the HV transformer is okay. After trying this with one of the two HV winding wires grounded, take that one loose, tape it off and try it again with the other wire grounded.

If the fuse holds both ways, the transformer is probably okay.

If it blows with either of those wires grounded, this indicates the insulation is breaking down inside the transformer's HV secondary winding.

This is not a conclusive test, but it can be done with simple tools.

73

 

[357]

You are saying I'm not out of the woods yet?

 

[kopcicle]

If a tree falls on a transformer and only your imagination is there to see it ,
does it still make a fuse blow ?

 

[357]

no...look at the image, 1500v at 1/2 line voltage.
I'll go do 240 right now brb

 

[357]

 

[kopcicle]

 

[357]

still worry?

 

[kopcicle]

Seriously ? Yeah , a little bit . Needs meter protection. Things can happen under load that a static test won't show as Nomad explained up thread. I've never seen R3 go POP! w/o some reason .I'm not saying I always found the reason but ...

 

[nomadradio]

Before you flip the meter switch to "Grid", it would be wise to to solder a pair of cross-connected parallel diodes across the lugs of the right-hand meter. We use 6-Amp rectifiers for this. 3-Amp rectifiers might be considered overkill, but the price of the meter makes the price difference look small.

The voltage rating is not an issue. That rating only applies to using a diode as a rectifier. In this application, the diodes will never see any significant voltage on them. The 50-Volt version is probably better than the 1000-Volt part, but not by much.

We learned to add those diodes to both meters in the SB220/221 before we apply any power to it. Learned to do this when a customer's amplifier broke down the first time we keyed it. Poofed the right-hand meter first key of the mike.

Not good for customer relations when that happens.

An exploded R3 is always a bad sign. Had one customer whose line voltage turned out to be a bit higher than ours. This was 30 years ago when the very-oldest SB220 amplifiers were reaching the "awkward age". Until then, that HV transformer was known as a pretty reliable part. This guy was the first one we saw with this fault in his SB220.

His amplifier kept blowing that resistor. We couldn't duplicate the fault no matter what we did. The symptom was that R3 would explode when the power was switched on. If you turned in on and it held, the amplifier would work fine until you turned it off.

Just about wore out his power switch trying to duplicate this fault. Try as we might, that part never went bad when we had it hooked up here. The customer would take it home, and "CRACK!". R3 would explode.

Eventually the transformer's internal insulation continued to deteriorate and finally it broke down here while we were watching. The "ground one side" test proved the HV transformer was bad. The customer was already unhappy. The news didn't improve his mood. The only explanation I could see for the difference between how it behaved here and how it did for him at home was a difference in AC-line voltage.

That was when I built a HV breakdown tester. Figured out we really needed one of those on hand. We don't use it every day, but it's the go-to tool any time a high-voltage "mystery" problem comes in the door.

73