[R-390] AGC

[dog]

Guido,

Seems to me it was very difficult to measure the AGC leakage resistance 
of the filters without taking off the output and ground. Otherwise all 
you are measuring is the 50Ω coil. What you want to measure is the 
resistance of the coil to the shield which should be in the 100's of 
MegOhms with nothing on the connectors. I used the microSiemens 
measurement with my Fluke189 after pulling the filters out since I had 
to fix them anyhow. Since one end is grounded, it's difficult without 
removing wires. I would save that for last as if the filters were 
working before, it's probably not the problem until you've eliminated 
everything else. It's a real mess to remove the filter wiring. Plus it's 
hard to get to and you have to do each side. Go through the AGC line and 
start disconnecting sections out until you find some low resistance 
first. BTDT. Also trouble is the schematic is not an exact 
representation of how it is hooked up in the RX.

Good luck.

Dave N3DT


On 10/5/2018 07:51, Guido Santacana wrote:
> Gents
>
> Following up on the AGC problem I measured the resistances at the AGC jumper points 3 & 4 with jumper removed. They are 500k at 3 and 100k at 4. The result at 4 could indicate problems in the IF or RF bus so I went to the IF deck, did resistance measurements at J512 and found 400-200k at pin 6. It should be infinite. Proceeded to check C502, C512 and C519. All tested ok. Then I disconnected R507 from the mechanical filters and the AGC bus as measured in pin 6 went to infinite resistance. The filters were working properly and each one measured 44 ohms except FL502 at 51 ohms.
> This IF deck is an early version without trimmers in the mechanical filters, just fixed mica caps all at 110pF.
>
> I can isolate each filter and check for leakage but first I wanted to ask if pin 6 at J512 gives a infinite resistance in all versions of the IF deck or it may change with some mods? This is a Collins IF deck and it has a mod indicated but the number wore off.
>
> 73s
>
> Guido KP4FAR