[Swan] Sensitivity and meter on 500

[Jeff]

Wanted to post an update to the problem for posterity -

It would appear that I had a leaky diode in the AGC circuit -- D-1603: 
TS-2 diode.  I replaced it with a 1N4007 and now the AGC and sensitivity 
is completely stable and solid.

Thanks to Steve - K7DNA for the advice and the follow up post that just 
confirmed that what I did should have fixed the problem!

And since I had the bag of diodes out already, I also replaced the three 
other TS-2 diodes with the 1N4007.  All looks good, and got rave reviews 
of audio quality on 40 meters the other night.


Thanks to all on the list that offered suggestions.  Now, if I could 
just get 80 meters to work on my 400...


Jeff

WB3JIH



On 7/4/2017 4:41 PM, Jeff wrote:
> Thanks Steve .
>
> I have a copy of the 4.0 compendium.  It's proven invaluable in the 
> 350, 400, and two 500Cs that I've rebuilt.
>
> I'm having a look at the IF and AGC bus.  I've replaced a few of the 2 
> watt and 1 watt resistors, even though they tested OK cold. I bought 
> triple the requirements for the 500C, so I figured I'd give it a try 
> since I've got the components.
>
> I'm pretty sure I've got a leaky cap on the AGC as someone suggested.  
> Now it's a needle in a haystack though.
>
> Jeff
>
>
>
> On 7/4/2017 1:30 PM, Steve Sutfin wrote:
>> Jeff, I just realized that I should have said the 2nd IF was a 12BA6, 
>> not 12BE6. Bit of an eyesight problem.
>> 73,
>> Steve k7dna
>>
>> On Jul 3, 2017, at 11:43 AM, Jeff wrote:
>>
>>> Thanks Steve!
>>>
>>> C202 has been replaced by discrete caps under the chassis, so that 
>>> should rule out one.  I'll replace the 12BE6 as you suggest.
>>>
>>> I've also removed the grid, plate and load variables and sent them 
>>> to the dishwasher.  I'm getting intermittent faults with them, so 
>>> might be another variable.
>>>
>>>
>>> Jeff
>>>
>>>
>>>
>>> On 7/3/2017 2:29 PM, Steve Sutfin wrote:
>>>> Hi Jeff,
>>>>  This is in case my response didn't get thru the 1st time.
>>>> Both the 12BE6 2nd IF and the C202 CanCap have an effect on the 
>>>> S-Meter AND the AGC system which could result in low receiver 
>>>> sensitivity. I can see that happening with the 12BE6 after warm up. 
>>>> Something to look into.
>>>> 73,
>>>> Steve k7dna
>>>>
>>>> On Jul 3, 2017, at 7:11 AM, Jeff wrote:
>>>>
>>>>> Hi all -
>>>>>
>>>>> Seeking some advice on a 500.  This seems temperature related, and 
>>>>> I'm having a hard time tracking it down.  I picked up a basket 
>>>>> case, and have been slowly rebuilding it.  I have it transmitting 
>>>>> and receiving OK now, but only when it's cold.
>>>>>
>>>>> From a cold start, the s-meter and receive function works as 
>>>>> expected, with good meter swings on signal peaks. However, as the 
>>>>> rig heats up, the s-meter deflects less and less until it 
>>>>> basically remains stationary.  I also begin to lose receive 
>>>>> sensitivity.  Compressed air blown at spots underneath the chassis 
>>>>> doesn't seem t have any effect, nor does air applied to the air 
>>>>> variables.  It just... dies out.
>>>>>
>>>>> Short of just starting to shotgun everything, does anyone have any 
>>>>> suggestions on where to start looking?
>>>>>
>>>>> Thanks...
>>>>>
>>>>> Jeff
>>>>>
>>>>> WB3JIH
>>>>>
>>>>>
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