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

[Bill Bear]

Hi Bill,

Yup…. really did it.  Glad it was a “simple fix”; I wasn’t looking forward to removing the chassis again to figure out what might be wrong with the bias supply.

73,

Bill KL7YB


On May 20, 2016, at 13:30, Bill Cromwell <wrcromwell at gmail.com> wrote:

> 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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