[R-390] Gain drift

[dog]

Jacques,

Yes, those T502 resistors are still in there. So far I haven't heard the 
gain drift since I resoldered the joints but I need to give it some 
hours. I also replaced the detector tube for the noise and it didn't do 
anything, tried the Limiter V507 and that may have done it. I need to 
give it some time now.

T502 can looked from the top like it had been messed with. Inside it 
appeared that the input circuit had been tampered with, but what looks 
like original parts. I took a big iron, heated it up good, used a little 
rosin solder paste and re-tinned the joints. That's all I did. They did 
look a little cold. I doubt the joints were original.

I notice the tuning on T502 changes with different tubes? I was looking 
in my Navships book for the tuning of the transformers in the IF stage 
but can't find it. Anyone know where that is? I thought I'd been through 
that years ago. I know I did the first variable IF and I seem to 
remember going through the IF.

Dave N3DT


On 10/20/2018 14:11, Jacques Fortin wrote:
> Dave, speaking about the "funky solder joints" in T502...
> Does someone previously removed the internal damping resistors within ?
> Meaning R553 and R554 that can be seen in the schematic ?
> Both are 47k ohms, 5%, 1/2W.
> R511 and R512 in T501 are the same (and should be present also !).
>
> Removal of those resistors was a "modification" recommended by some people
> 30 years ago !
> But the end effect was - in some cases- self-oscillation of the IF stages.
> Like having a regenerative receiver within the IF strip...
> No wonder then if the gain is very high at turn-on and the global operation
> unstable.
>
> Have a check !
>
> 73, Jacques, VE2JFE
>
>