[R-390] AGC

Fri Oct 5 07:51:36 EDT 2018

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

Sent from my iPhone

> On Sep 11, 2018, at 7:14 AM, Larry H <larry41gm at gmail.com> wrote:
> 
> Hi Guido,  You got lots of good advice.  Hope it's easy to fix.  
> 
> Are there any mods on the agc line, possibly around R546 or R547?
> Did you do the prescribed resistance checks?  I usually disconnect R545 to be sure.
> Is the antenna trimmer insulator pushing good?  Sometimes it gets oiled and is bad.
> 
> Also, since it was sitting for a while, did the C553 get updated to protect the filters?
> 
> Good luck.
> 
> Regards, Larry
> 
>> On Mon, Sep 10, 2018 at 4:22 PM, Guido Santacana <gsantacanav at gmail.com> wrote:
>> Gents,
>> 
>>   I will follow through the recommendations starting with the easiest and
>> building up to the most difficult. That will be removing the RF deck again.
>> I should say it's tedious not difficult. A report on findings and final
>> solution will be forthcoming too.
>> 
>>  Twenty years ago one of the specimens in my R390 group included a nonA
>> too. I will always regret getting rid of that one. Each one of my R390s had
>> been overhauled and realigned. The only one that came up with an AGC
>> problem was also an EAC unit. This was the first one I ever got. It was
>> from Fair and came as repairable. Z503 had an open coil. Before I got a
>> replacement a common 455kHz IF coil was used as a substitute and it worked
>> until an original came in. I don't know if anyone has tried this before as
>> a temporary solution to a bad Z503.
>> 
>> One recommendation I have to save our aching backs from hauling these BAs
>> is to use a scissor table. I bought one that is used for motorcycles and it
>> works great for radios this size.
>> 
>> Thank you very much guys and if anything else pops up let me know. It's
>> been a while since I worked in one of these but I am warming up.
>> 
>> Best 73s
>> 
>> Guido KP4FAR
>> 
>> On Mon, Sep 10, 2018 at 4:25 PM dog <agfa at hughes.net> wrote:
>> 
>> > Another thing that is often overlooked is the mechanical filters. If the
>> > coils develop resistance to ground it will mess with the AGC big time.
>> > But the only way to figure this out is to disconnect the filters and
>> > measure them, I suppose you could disconnect the AGC line from the
>> > filters and measure the filters resistance to ground individually. It
>> > should be very high, I measure mine using the nanoSeimens position with
>> > the fluke 189. But I'd go everywhere else first.
>> >
>> >
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