R390A AGC

[flintstone mop]

Hello everyone,
Gotta EAC R390A that gets low AGC voltage after being on for over an hour.
This low AGC voltage will overload the RX on big signals. I looked on Chuck Rippel's old website and there are some tips, but no help for "it's good when first turned on then turn to crap after 1hr of use"..................I know someone will recommend turing off the radio after 1hr and wait till it cools down.
I wish the cables were longer sos I can operate the radio with the IF out of the chassis and troubleshoot.
Any body been in this situation?
Fred

[WV Hoopie]

Fred,

Couple of things come to mind. The Y2K manuals give voltages expected at the toob pins, easier with toob extenders to measure. Any old BBOD's still in the IF section?

I've never played with an EAC, but the Motorola & Amelco both had bad 2mF oil filled caps. Its that rectangular thing about 1X1X2 inches. The value was close to 2mF, but the insulation resistance sucked wind.

Craig,

[ke7trp]

PUT EM A FAN ON DA TOOBS MISTA

C

[n2bc]

There is plenty of length in the cables to tip the IF chassis up.  Pull the filter and BFO shafts out of the way, tip the chassis up and stuff something under the end - I use a piece of 2x4.

Hang a voltmeter on the AGC line on TB102 and watch it as you rotate the chassis up, just in case the problem is in the wiring harness / plugs.  I had one where P112 was full of crud.

[flintstone mop]

There is plenty of length in the cables to tip the IF chassis up.  Pull the filter and BFO shafts out of the way, tip the chassis up and stuff something under the end - I use a piece of 2x4.

Hang a voltmeter on the AGC line on TB102 and watch it as you rotate the chassis up, just in case the problem is in the wiring harness / plugs.  I had one where P112 was full of crud.

Thanks for the good replies and N2BC!!!!!!!! There were at least 5 green lights that went off in my head..........."Why didn't I think of that?"
It's a heat problem for sure.
Fans will just blow dust and dirt into the chassis. Look inside a computer and you'll know what I means.
Fred

[flintstone mop]

Good news, hopefully!
Time will tell. With some reasonable info from Chuck Rippel's website and a little "re-engineering"........I found my problem with low AGC in my EAC R390A.
Unfortunately it turns out that something in the mechanical filters circuit is sucking down my -11 volts AGC tuned into a local 5kw A.M. station to -5 volts and distorted audio from an overloaded I.F.
The culprit was the AGC takeoff point going through R507(22K) into the mech filters. For some reason the engineers wanted this AGC voltage routed through the filters to get AGC to the control grid of V502, if one of the filters is getting a little aged it could suck the AGC down, according to Chuck's postings. In my case the filters do take my AGC voltage down, but they still work. I can hear the change in audio switching to narrow bandwidths.
So, I figgered they used the ever famous .01 (C553) to bring the I.F. into the filters, what would be wrong with the same value to bring the I.F. out to the grid of the 2nd I.F. amp V502, and THEN connect R507 (22K) to the same point to get AGC to that stage???
It woiks FB OM!!!!!
This took a few days, several hours, and my "Optivisor around my bald head.

Phred