Heath Sb-220, ON/Off Rocker Switch

[W6TOM]

 Any one know where I might get a replacement for the DPST Rocker Switch used for ON/OFF on the Heath SB-220 AMP?? Looked around on the net and they seem to be unobtainiun, I have another switch that I can make do with but it doesn't match.

[WQ9E]

If either section of your current switch is at least somewhat good use it to control a relay which will then switch the power.  Even if the contacts aren't in good shape they are probably good enough to reliably switch the small amount of current used for a relay.  A solid state relay is even easier to switch but you need a pair along with heat sinks and given the fairly limited space in a SB-220 a mechanical relay would be easier to implement.

[N8ETQ]

   I had that exact same problem with an MLA-2500 last spring.
I used SSR's and a wall wart guts.. Works great. The metal box
tacked on to the rear of the chassis. The 9V wall wart fires the
SSR's.  Amp was converted over to 4CX400's..

GL

/Dan

[w1vtp]

I'm surprised that Harbach doesn't offer a rocker switch.  It might be worth a call.  I hate to do drastic mods on such classic equipment.

GL es 73, Al

[W6TOM]

  I got the amp last summer at a swap and I did contact Harbach last summer but no luck there. I do have a switch that will work, right voltage and current rating but it has solder lugs instead of spade lug connectors and the the rocker is white but it will work without drastic modification.

[N8ETQ]

I got these if u want em' Not sure they fit but similar.
3PST.. Very solid.

  I got the amp last summer at a swap and I did contact Harbach last summer but no luck there. I do have a switch that will work, right voltage and current rating but it has solder lugs instead of spade lug connectors and the the rocker is white but it will work without drastic modification.

[N8ETQ]

I'm surprised that Harbach doesn't offer a rocker switch.  It might be worth a call.  I hate to do drastic mods on such classic equipment.

GL es 73, Al

  Hey Al,

      well if it wasn't for "Drastic" Mods, all the
MLA-2500's would be "Landfill". Tubes are gone. This
is really a pretty sweet mod and lends new life to a
Vintage piece. (all be it, kind of "Low End").

     The SSR's are Zero Cross devices that keep the
HV supply cool at turn on. Commander Amps used em'.

     Taking advantage of "Newer" technology to pump
life into a an otherwise POS is almost an obligation
IMHO for future operaters that don't believe we had
to walk 10 miles uphill both ways to skool.

     The thing needed the blower for the new Jugs
anyway. Also it is somewhat "Safer" as Dentron
only switched one side of the AC line. OK for 110
operation, "Farmer" for 220. Same with the fusing.

     The list of gear that will require "Drastic"
modification is growing daily, I'm gud with it.

73

/Dan

[W6TOM]

  The switch might work, I'll check the current rating and get back to you on it, thanks.

  Your work around is actually pretty clever, more and more that type of thing is going to be required if want to keep this stuff running with parts not being available. I've had guys give me grief for using a D-104 with my ART-13 or T-368 instead of the "correct" carbon mic. The carbon mic sounds CRAPPY and on my ART-13 also doesn't modulate well. I do use the T-17 carbon mic on my BC-1306, works well and sounds well.

  The Plate Relay in my T-368 sticks, when it does the plate breaker trips, I've had to tap the relay cover to free it a few times. The day is coming when I will need to replace it, pulling that 300 lb power supply shelf is something I'm not looking forward to at all. It may also be sooner than later. To do the job will also require a major rearrangement of the (garage) shack.

  Looks like my washer is leaking water, that I need address before it becomes a disaster too. Since the washer is in the garage too and getting at it will require a major rearrangement, sooner may have just become sooner yet!

  I could replace the Plate Relay with an open frame type or maybe a solid state one. I wonder, do the solid state relays cause RFI ??

[Pete, WA2CWA]

Carling and several other switch manufacturers still make high current 120/240 V AC type rocker switches. Generally distributed by Newark, Mouser, etc. Obviously, the dimensions need to be verified.

[N4zed]

Dan, don't be slapping around the MLA-2500...Mine still has original unobtainable toobs. Only mod was to up-grade the filter caps and etch a new board....Old Skool !

[N8ETQ]

Dan, don't be slapping around the MLA-2500...Mine still has original unobtainable toobs. Only mod was to up-grade the filter caps and etch a new board....Old Skool !

  Wow!


       You should list those jugs on Epay! I bet you
could get $350 a piece. Maybe more. Then call these
guys.

http://www.rfparts.com/catalogsearch/result/?q=4CX400


       While your in there grab one of these and rip
out whatever your amp has for an "Input" Circuit. IIRC
the modified MLA only required about 40 watts to drive
to full power and presented a pretty good load to the
exciter, even on 10M.  Something they didn't do very
well even from Birth.

GL

/Dan

[WQ9E]

 
  I could replace the Plate Relay with an open frame type or maybe a solid state one. I wonder, do the solid state relays cause RFI ??

I have never experienced any RFI from a SSR but they are subject to RFI.  I used a pair to replace the original plate relay in my Johnson Desk KW because of pitted contacts.  Until I added bypass caps and RF chokes to the relay control line the relay tried to modulate the HV output with RF which made bad noises. 

It turns out that zero crossing relays actually lead to greater inrush current so you will want to use either NTC inrush limiters (I use these in the Desk) to limit turn-on inrush current or a relay shorted resistor.  See this link for zero crossing relay issues:  http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3206_AppNote&DocType=CS&DocLang=EN

If you use a relay to temporarily put a resistor in series with the transformer primary for current limiting be sure and use a relay that has a spare set of contacts which can be put in series with the PTT line so in case your relay turn-on circuit  fails the PTT line cannot be enabled.  Otherwise your resistor is going to turn into a crispy critter when transmission is attempted.  Some commercial amps that use the relay based inrush limiting don't take this simple precaution and the control board gets fried when the relay fails.

And this link from Omron provides good info for basic implementation of SSRs:  http://www.omron.com/ecb/products/pdf/precautions_ssr.pdf

I really like solid state relays and they have performed perfectly in my homebrew amp, in a couple of Drake amps (also to save the power switch), and in my Desk KW.  The relays do require some heat sinking depending upon how much current they must handle, in the case of my Desk KW the 872A rectifiers were replaced by solid state plug in units before I got it so I removed the rectifier filament transformer and used its location and mounting holes to mount a heat sink for the relays.  In this case I could have used a smaller sink because that location puts in directly in the air flow path from the lower pedestal cooling fan.

The attached photo shows a pair of SSRs mounted in my Desk KW and the place occupied by the original mechanical relay is now occupied by a barrier strip which provides a point for mounting inrush current limiters.

[WQ9E]

  Hey Al,

      well if it wasn't for "Drastic" Mods, all the
MLA-2500's would be "Landfill". Tubes are gone. This
is really a pretty sweet mod and lends new life to a
Vintage piece. (all be it, kind of "Low End").

Dan,

My Alpha 274 has a good pair of 8874 tubes in it so it will stay stock until they die.  But last year I bought a very nice looking Alpha 374 (the no tune 3 tube model) without tubes or transformer for $40.  It will be getting a pretty major rework using a G3SEK tetrode board set.  My current plan is to use 3 8072 tubes (I have many spares from when I bought my Signal One CX7 a few years ago) which will get anode coolers from some defunct 4CX250 tubes.  I have a couple of suitable power supplies so it should be an interesting experiment.  I also bought the RF deck from a Henry 4K2 at the same time for $30 that will be rebuilt with a 4CX1600 and another set of Tetrode boards. 

[KJ4OLL]

Hi,
That switch looks similar in design to the Collins 30L-1.
Mine eventually welded itself "ON".
It was easy to take apart and fix.
But the basic switch design seems to be overloaded, so even though I restored the contacts, the switch
is now left in the "ON" position, and amp power is operated from a normal wall on/off "light switch",
connecting a standard NEMA 6-15R receptacle
(seen to the Right of the bucking transformer for the other amp, a 30S-1)







 
73
Frank
KJ4OLL

[WQ9E]

Any of these high current switches at the least need some help from inrush current limiting.  At the initial point of turn-on they will see a surge basically limited only by the resistance of the circuit wiring and this occurs just as the contacts are starting to make contact.  It is no surprise to see them pit and the pitting leads to localized arc welds which will lock them together.  On opening the switch they may see arcing from the collapsing magnetic field of the transformer windings.

Relays do a little better job of resisting damage since the actuation is always swift and sure unlike a human operated mechanical switch. 

[N8ETQ]

  Hey Al,

      well if it wasn't for "Drastic" Mods, all the
MLA-2500's would be "Landfill". Tubes are gone. This
is really a pretty sweet mod and lends new life to a
Vintage piece. (all be it, kind of "Low End").

Dan,

My Alpha 274 has a good pair of 8874 tubes in it so it will stay stock until they die.  But last year I bought a very nice looking Alpha 374 (the no tune 3 tube model) without tubes or transformer for $40.  It will be getting a pretty major rework using a G3SEK tetrode board set.  My current plan is to use 3 8072 tubes (I have many spares from when I bought my Signal One CX7 a few years ago) which will get anode coolers from some defunct 4CX250 tubes.  I have a couple of suitable power supplies so it should be an interesting experiment.  I also bought the RF deck from a Henry 4K2 at the same time for $30 that will be rebuilt with a 4CX1600 and another set of Tetrode boards. 

       Very Nice Rodger, Yeah the Alpha Amps are SWEET! Just
goes to show what a little "Overload" protection can do for tube
life.  The Dentron, "Let err' rip tater chip" philosophy  cooked
thousands of those tubes.  

        My experience with the SSR's has been all good as well, some
equipment that used to "Bang" when switched on no longer did
that after going to SSR's. I guess were all going to need to go
with Thermistors, (Which I Hate) but fact is, AC line voltage won't
be going lower anytime soon.

73 and thanks for the link

/Dan


      

[WQ9E]

Dan,

You are welcome and I like the SSRs a lot too.  I was surprised when I first ran across the information about zero crossing switching but after spending some time with it the paper does make sense.

Dentron made some very nice looking gear and I am glad you have your MLA-2500 in working order again.  A lot of them just got junked which is a shame.

I remember Alpha from my early days when they used to come to a lot of hamfests and to their brick on the key demonstrations.  Alpha and the Signal One CX-7 share a common engineering heritage and clearly were designed to work and look nice together.  I picked up my 274 with a set of 6 month old tubes (receipt in the manual) for $150 at a small hamfest so I expect they will last a very long time.

Another amp which will get some modification when the tubes die is this twin 8873 powered Henry that was clearly designed to match the popular Kenwood TS-520.

Too bad the smaller members of the 887X family weren't popular outside of ham radio.  A friend in the field tells me that medical gear using the 8877 and variants is pretty much obsolete so the future isn't good there either.  I saw that Max Gain systems has some NOS 8877 tubes at a very good price so anyone with one of those amps should probably buy a spare.  I imagine he acquired the stock of one of the med service shops.

[Opcom]

On the heathkit switch, if it is no longer good enough to control a relay, it could possibly be modified to actuate a microswitch that could perform that duty. It would be worth it to keep the front panel appearance intact. There should be some kind of matte black paint that will stick to the plastic rocker of a new switch of the wrong color.

The carbon mic sounds CRAPPY and on my ART-13 also doesn't modulate well.

Have you tried tapping or rapping the mike on the desk? The ol' carbon granule shake-up.

Taking advantage of "Newer" technology to pump
life into a an otherwise POS is almost an obligation

The poor MLA-2500 is a victim of its own light weight, compactness and marketing competitiveness.

Lots of amps had 'AM' ratings and tune-up instructions stated in the manual, but the manufacturers often omitted to state that a duty cycle even existed, or even give one paragraph to the concepts of CCS, ICAS, or IVS.
I'd bet if you ask 100 hams to write a page on those concepts and what they mean for a given amp, half would not do it properly without researching.

Those mods on the 2500 are great. Now it is good for 40 more years.


On solid state relays I 100% agree they can go nuts when subjected to RF. That should be expected and planned for and not be a new or unusual concept for any ham. Sadly, the blank stares commence.