[R-390] Gain drift

[dog]

Roy,

At the moment, I'm just feeding 455 RF into the IF module. That appears 
to be where I'm getting the gain drift. It's even doing it with the AGC 
jumper disconnected. Need to fix this before I get into the RF section 
which actually may be OK. Thinking about measuring along the 455 RF line 
through the 4 tubes with the Boonton to see if I can find the area it's 
changing. Slow process since I have to let the RX cool off between 
sessions. I may start at V502 which breaks the IF section in half.

I did replace the recommended caps in the RF section, 3 of them. The IF 
has all the recommended ones replaced, but not C551, it seems OK at 
least measuring, I don't have a ESR tester.

The test equipment I have is a Moto 2005D sig gen/calibrated rx/spectrum 
analyzer/scope. All purpose monitor, it will put out +13dBm into 50Ω and 
a step attenuator with adjust pot down to -120dBm. Also a Boonton 
milivoltmeter and a Fluke 198 along with an old HP427A, and a Tec 545A 
that's still working.


On 10/19/2018 11:24, Roy Morgan wrote:
>
>> On Oct 19, 2018, at 11:15 AM, dog <agfa at hughes.net 
>> <mailto:agfa at hughes.net>> wrote:
>>
>> Interesting. I disconnect TB102, 3-4, ground 4 and 3mV into J513 is 
>> way too much RF for the carrier meter in AGC mode. Reduce the input 
>> to about 250uV and that gives me 50 on the carrier. 0V on TB102-3. 
>> About 5 minutes later the carrier drops to 0. I have to increase the 
>> RF input to 1.3mV to get 50 on the carrier again.
>
> I suspect you have an intermittent capacitor or coil in the RF system.
>
> Insert the RF signal into the various test points in the RF deck. 
>  Look for the table of voltages (they will be APPROXIMATE) for the 
> various stages (and appropriate frequencies).
>
> I assume you have ruled out the IF deck with an IF signal at it’s 
> input - and perhaps set the IF gain while doing that.
>
> Roy
>
> Roy Morgan
> k1lky68 at gmail.com <mailto:k1lky68 at gmail.com>
>
>
>