[Heathkit] How about the SB-220?

Sat Aug 22 09:58:19 EDT 2009

My view based on close to 40 years experience with servicing the SB-220 
family pus converting over 200 to 6M.

See below for my interjected comments.

----- Original Message ----- 
From: "Mike Hyder --N4NT--" <Mike_N4NT at embarqmail.com>
To: <mmoss at mindspring.com>; "Heathkit Reflector" <heathkit at mailman.qth.net>
Sent: Saturday, August 22, 2009 12:40 AM
Subject: Re: [Heathkit] How about the SB-220?

> Marvin, you're welcome.  I'm sending this to the reflector so as to expose
> my ideas to flying barbs.
>
> I'll give you my experiences briefly.  When the FCC ordered that amps no
> longer could cover 10 meters (approximately 1977), I ordered an SB-220 
> kit.
>
> Once, there was a big bang in the amp -- some say it would have been 
> caused
> by a parasitic oscillation.  Rich Measures belongs to this school.

Where he is a very small minority and has been proven to be wrong time and 
again.

Common engineering sense as well as tube manufacturers realize that 
transmitting tubes will arc internally. This is due to either outgassing, 
grid alignment, or sharp points not caught in inspection. The cure is to 
first accept this as a normal occurance and do what is necessary to minimize 
the damage.

The most important step here is to remove RFC-2 and replace with a 15-20 Ohm 
25W real wirewound resistor. Do not use cement or aluminum encased versions. 
This resistor acts as a HV current limiter and is not sacrificial...it 
should survive an arc.
RFC-2 is also self resonant close to the tubes parasitic frequency and adds 
to instability. The resistors resistance wire also kills any VHF that 
reaches it.

 I
> installed new tubes (one was ruined by the bang) and installed parasitic
> suppressors -- NiChrome coils.  Measures sells them.  Look here and 
> specify
> your amp model when ordering: http://www.somis.org/Price-Info.html

That site is good reading and mostly to see what not to do.

>
> I use Ten-Tec rigs and run 35-45 WPM CW.  My main consideration is that 
> the
> transmit relay in the SB-220 presents over 100 VDC across the back-panel
> jack and current sufficient to cause pitting to the contacts in a
> transceiver's relay.

And totally destroys a SS switching transistor. The voltage is approximately 
+125-130VDC.

You can buy a small circuit called the SK-220 Soft-Key
> for the Heathkit SB-220 or SB-221 from this website:
> http://www.harbachelectronics.com/main/page_products_sb220__sb221.html
> This is a very worthwhile modification.

I agree 100%

>
> They also sell a module called the SS-221 Soft-Start which I believe is 
> NOT
> NEEDED AT ALL, don't waste your money (read about that on Measures' site).

Here we go again with the king of junk science. First of all the power 
switch is not a common replacement item any longer. Secondly the inrush 
currents effect on the switch depends upon where in the AC cycle it happens. 
Since this is not a zero crossing controlled circuit it can be anywhere.
The Harbach circuit is good conceptually but uses cheap cement power 
resistors which are known to fail. Replace them with real wirewounds.

>
> I have a full QSK amplifier but greatly prefer my SB-220.  Here's why: 
> The
> Ten-Tec rigs have a drop-out delay on the relay which switches an 
> amplifier
> to transmit.  That delay is to prevent the rapid clatter of the 
> amplifier's
> relay.  But there is a most salutory effect to that.  When the amplifier
> switches to transmit, there is sufficient signal coupled from the antenna
> through the antenna relay and into the transceiver.  So when I'm working 
> QSK
> at perhaps 45 WPM and start to transmit, my receiver becomes much 
> quieter --
> but enough of the other guy's signal comes through for me to hear him, 
> even
> if his signal is perhaps S4 or so (maybe much less) because the signal is
> still well above the noise level.  I keep my transceiver's drop-out delay
> set to maximum.

The stock relay is way too slow for QSK and can even hot switch using VOX on 
CW or SSB. This is a major cause of arced bandswitches....not voodoo 
parasitics. A speed step-up circuit could be used or use PTT or a foot 
switch. If controlling with an old tube transmitter the sequencing is OK for 
VOX. Some modern rigs have adjustable delays but you really need a scope to 
verifiy sequencing. Many rice boxes come on with a full power spike which 
raise havoc with many amps, not just a SB-220.

>
> With the age of the your amp, here's what I'd consider doing to it:
> First, remember that the 3-500 tubes must be operated vertically!  I'd
> remove the front spacers on the feet to make the amp sit level on the 
> desk.

Since the fan also cools the filament pins below the chassis this may not be 
a good idea. Pins have been known to overheat and melt their solder which 
causes an open filament until repaired, and destroys the socket temper 
resulting in poor tension.
Its the old cause and effect of making changes that have not been completely 
thought out.

> a. I'd oil the fan motor as suggested by Rich Measures

Take it out, take it all apart, clean both the front and rear bearing 
surfaces and relube with a good grade machine/electric motor oil. Its 
extremely simple to do. Read the manual as to exactly where to mount the 
blade. If using graphite tubes go for the Harbach high speed fan and learn 
to live with the noise unless you want to install a 2 speed modification. 
Leave the amp on for at least a minute after transmitting before powering 
down.

> b. I'd install that SK-220 Soft-Key circuit.

See comments above

> c. If the electrolytics are original, I'd replace them.

The Cornell-Dublier 381LX series 330 uF 450V 105*C fits the original blocks 
perfectly and adds a bit of dynamic regulation.

> d. I'd REPLACE the equalization / bleeder resistors across each 
> electrolytic
> capacitor per Measures, whether or not I replaced the electrolytics!

I suggest 75K 3W MOX across new caps. Do not reduce the value for original 
caps as their age and even initial tolerance could seriously unequalize 
voltages.

All parts that Ive mentioned are regular stock at Mouser.

> e. I'd make sure I had installed the NiChrome parasitic suppressors from
> Measures.

Pure garbage. Instead use a pair of the same style 100 Ohm resistors in 
parallel using the original coil if the original resistors have increased 
more than 10% above 47 Ohms. RF and heat cooks carbon composition resistors, 
when the value increases too much they no longer absorb the parasitic RF.
Ive also used a pair of 100 Ohm 5W MOX resistors (Mouser again) and had no 
problem with the slight added inductance.

> f. I had trouble with the big zener diode used for PA bias.  The Harbach
> RM-220 Rectifier/Metering Board fixes this problem.

I prefer a string of robust 1N5408's for the bias. The 1N4007's on the 
Harbach blow almost as easy as the zener. Haven heard of a 1N5408 blowing in 
the 25 years Ive used and recommended them.

The diodes in the stock board should be replaced by 1N5408's also. Drill out 
the holes slightly to fit the thicker leads.

> g. I'd take Measures' advice about removing the interlock from inside the
> final cage.

Ive been doing that since the amp was first sold!

 and I'd use "Stove Black" or as he said, "black liquid shoe
> polish" to paint the inside of the final compartment (including the cover 
> to
> same).

More pure voodo science.

> h. If I had lotsa money, I'd buy a spare fan & motor and a spare antenna
> relay (just in case).  I was stationed in Ethiopia for some three years 
> and
> developed a great fondness for spares.

Fan motors are too low torque to fail but yes on the relay. Many have 
already lost their flexible arm temper from use and RF heat.

>
> But my main points are these:
> Don't convert the amp to QSK.
> Do install an SK-220 circuit (or build your own circuit).
> Otherwise consider ALL of Measures' ideas.
>
> The SB-220 is a great amp.

Absolutely.

One other modification that may be a bit hard to source. Besides the reasons 
already mentioned the bandswitch can arc due to high circulating RF due to 
the shorted turn effect of the design. Again, no voodoo science parasitics 
are involved. This can be snubbed by installing a 10-12pf 1000V silver mica 
between the 20 and 80M contacts directly at the switch. It will affect the 
Tune control by that much capacity at 10M so the coil spacing may have to be 
stretched a very small amount. A 5Kv RF doorknob can also be used.  Fair 
Radio, SSN and others sources may have suitable caps.

Carl
KM1H
Amplifier Repairs and 6M conversions since 1964
National Radio 1963-69
Member of NCL-2000 Design Team

>
> 73, Mike N4NT
>
> ----- Original Message ----- 
> From: "Marvin Moss" <mmoss at mindspring.com>
> To: "Mike Hyder --N4NT--" <Mike_N4NT at embarqmail.com>
> Sent: Friday, August 21, 2009 04:59 PM
> Subject: Re: [Heathkit] How about the SB-220?
>
>
>> That is a good one.
>> Thanks.
>> Marvin
>>
>>
>>> [Original Message]
>>> From: Mike Hyder --N4NT-- <Mike_N4NT at embarqmail.com>
>>> To: <mmoss at mindspring.com>
>>> Date: 8/21/2009 03:43:24 PM
>>> Subject: Re: [Heathkit] How about the SB-220?
>>>
>>> Look here: http://www.somis.org/
>>>
>>> particularly here: http://www.somis.org/SB220ci.html
>>>
>>> 73, Mike N4NT
>
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