[AMPS] More SB-220 amusement(!)

[Rich Measures]

>
>On Thu, 18 Sep 1997 10:39:52 Pete Smith <n4zr@contesting.com> writes:
>>
>>Recently, I installed a Harbach RM-100 metering/diode board in my 
>>SB-220.
>>It fixed a low HV meter reading and has performed admirably for a 
>>couple of months.
>>
>>This evening, I went to tune up on 80 meters and noticed below-normal
>>output.  While tuning, key-down, in high-voltage (SSB) position, I noticed
>>declining RF output (measured on an outboard meter), coupled with climbing 
plate
 >>current over the key-down period, followed by smoke from the area of 
the metering 
>>board.
>
>Probably a grid choke frying and trying to tell you a problem was
>happening. 

 Check the tube with a high-Pot.  If you don't see over 6 kV breakdown 
between the grid and the cold filament, the tube may have been damaged by 
a VHF parasitic.  

>It could also be a grid bypass capacitor, particularly if it
>was one of the old style rectangular ones.  

However, a shorted grid bypass C does nothing overt.

>At that point you should have
>shut off and investigated.
>
I would change the 220 from V-cutoff bias to R-cutoff bias.  In the stock 
config. a shorted tube will destroy the unfused, filament transformer. 
>>
>>Trying to figure out what was happening, I tried the amplifier 
>>key-down on
>>20 and 40 meter, and all meter readings seemed normal.  I then went back to
>>80, and verified that the same combination of symptoms was still happening,
>>both in HV and LV (CW) position.  At this point, there was a popping 
>>sound,the plate current meter pinned...

Check to see if the anode is bent toward the grid cage.

>> (with the amplifier unkeyed), something 
>>went white-hot on the metering board, and I shut things off.
>
>About time!  You probably lost a tube...which was damaged from the prior
>big bang; 

... which is typically caused by an transitory vhf oscillation condition.

>...and maybe the meter also unless there were protection diodes
>across it.
>Other things probably blown are one or more of the bias diodes on the
>Harbach board.
>
>Private E mail me and I'll give you a list of other things to check 
>
>>
>>Inspecting the metering board, it appears that the lower of the two 
>>large
>>resistors on the right side of the circuit board (viewed from the 
>>front)
>>went white-hot and then failed open.  There's a burned spot under the
>>resistor.  The one above it also appears to have an overheated band 
>>around
>>its center, which suggests that I'd better think about replacing it 
>>too.  I
>>don't have a schematic of the replacement metering board, but it 
>>appears
>>these are the multiplier and shunt resistors associated with the plate 
>>meter.
>
>>They are the same values and in the same position as the original Heath. 
>>From top to bottom:
>
>.82 Ohms,  3600 Ohms,  1 Ohm
>
>
>> I'm presuming that replacing the resistors will restore the plate metering
>>function, and have contacted Harbach about those, but am concerned that
>>there's some underlying cause.  Does anyone have any idea what 
>>happened?
yes
>>Except for the plate current and power output changes on 80m., there 
>>were no signs of trouble.  I was operating the amp into an antenna SWR of 
>>less than 1.5 to 1.  Is there some underlying cause I need to chase down?
>
Typically, troubles start when the amplifier intermittently oscillates at 
roughly 110MHz.  
[see the 9/90, 10,90, 11/90 and 12/90 issues of *QST* for additional 
information.]
>
>Probably time to think about replacing the tubes!
>
Not unless the grid/filament breakdown voltage is abnormal.  
>
>>Any advice greatly appreciated.
>
>Here is some free advice. When something STARTS to happen, shut down
>immediately to investigate. The SB-220 is a very forgiving amp and often
>gives warning signs....just heed them.

amen, Carl.  .....and if you hear arcing and spitting, it's probably not 
from a squadron of fruit flies crusing through the tank circuitry. 
Rich---

R. L. Measures, 805-386-3734, AG6K   


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