[Heath] SB-200 problems...again...

[Bill Cromwell]

Hi Bill,

I cannot believe you would do something so utterly stooopid. I think you 
are just saying this to make people like me feel better when we do 
things like this. I do appreciate the thought even though I know you 
didn't really do that.

73,

Bill  KU8H

On 05/19/2016 10:59 PM, Bill Bear wrote:
> Ken,
>
> I’m embarrassed to tell you how I solved the “no idle plate current” problem.  Nothing wrong with the SB200.  All mods wired correctly and nothing else wrong.  It was the “operator”…..  I was trying to read idle plate current with the meter switch in the “grid” position.  Made all the difference in the world when I realized my mistake and switched to the “plate” position… 90 mils as advertised !
>
> But I’m sure I have not been the only one to do that.
>
> 73
>
> Bill, KL7YB
>
>
> On May 6, 2016, at 8:21, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:
>
>> On 6 May 2016 at 7:46, Bill Bear wrote:
>>
>>> Ken,
>>>
>>> One of the first things I did after an initial visual inspection was replace C19
>>> ( 2uF) and C3 (20 uF).  They just looked too old and my experience with old
>>> electrolytics is that they are no longer up to specs.  So, I must have something
>>> else to investigate.
>> OK. Let me see if I can remember exactly what I ran into: for one thing, I
>> began having that problem after I had installed a Harbach Soft-Key interface
>> to help my more modern transceivers key the amp.
>>
>> It turned out that the high 60 Hz AC ripple in the DC to the grids was causing
>> the Soft-Key to partially key on, increasing the bias voltage to below ("above"
>> i.e. more negative)  -2 volts.
>>
>> In order for the 572Bs to draw 90 mA of idle current, the grid bias must be
>> -2VDC. If the voltage increases "above" that (becomes more negative), the
>> idle current will drop, and the tubes will possibly be operating nearer a
>> non-linear region of their operating curve.
>>
>> Secondly, there is also the fact that in order to read that 90 mA, the metering
>> circuits have to be working properly. That 90 mA shows at the very bottom of
>> the scale and in fact is very hard to read there, and is not very accurate.
>>
>> What one really should do is to add another section or rewire a seldom-used
>> section, of the meter switch to read idle current at a point at least half-scale
>> on the meter.
>>
>> Thirdly, idle current will also fall if plate voltage is not close to where it should
>> be. I get 2150 VDC under load.
>>
>>> After you changed C19, did you get a nominal idle plate current reading near 90
>>> ma?
>> Yes.
>>
>>> I am also wondering what resistance check results you had after the mods:
>>> plate to gnd and tube pin 3 to gnd ?
>> I have not checked those, but will as soon as I can.
>>
>> My most-used SB-200 has recently come up with a problem very similar to
>> yours in that both idle current has pretty much disappeared, and it is requiring
>> a lot more drive to get the grid current and power output to be where it should
>> be.
>>
>> Tubes are brand-new RF Parts jobs too. And this problem suddenly occurred.
>> This indicates to me that some component in the grid circuit has suddenly
>> failed.
>>
>> Due to our youngest daughter's upcoming wedding, I have not been able to
>> do much radio operating for the past several months, but hope to be able to
>> get back to that after May 21. :-)
>>
>> Therefore, I have not been able to dig into the SB-200
>>
>> vy 73 for now,
>>
>> Ken W7EKB
>>
>
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