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

[wittfa19 at earthlink.net]

Bill, my story:

In my reckless youth (the 1970's), I was an electronics tech in Silicon Valley.  Like any respectable tech, I HAD to have a high-end stereo system, including a SOTA $200+ phono cartridge.  The system sounded great, but I wasn't getting much volume from the cartridge.  After weeks of tweaking and diddling with no success, I decided to take the effing thing back to the store.  Fire in my eyes, lawyer's card in my pocket, right?  Well, I take it in, the kid behind the counter looks at it, pushes the needle the rest of the way in, everything works fine! Stooopid...  So don't feel bad.

BTW, kudos for having the courage to admit it. 

-----Original Message-----
>From: Bill Cromwell <wrcromwell at gmail.com>
>Sent: May 20, 2016 2:30 PM
>To: heath at puck.nether.net
>Subject: Re: [Heath] SB-200 problems...again...
>
>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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