[R-390] Gain drift

[hamfish at comcast.net]

On C551:

I've never found a good one. Test equipment is a Sprague TO-6A. C551 is a paper oil filled cap, value 2uF @ 500VDC. Sometimes the value is close to 2uF, but at rated voltage it is a dead short. Insulation resistance is a big fat zero. This is one of the caps I no longer test, it gets replaced with all the other shotgun caps. A good replacement is a NTE MLR205K630. This NTE will fit inside the old metal can or fit on the bottom side of the IF module. 

Regards,
Craig

-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On Behalf Of dog
Sent: Friday, October 19, 2018 1:47 PM
To: Roy Morgan <k1lky68 at gmail.com>
Cc: r-390 at mailman.qth.net
Subject: Re: [R-390] Gain drift

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>
>
>
>

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