The AM Forum

Rebuilding the Globe Scout 40A today. So far, so good thanks to inputs from WQ9E and WB2GCR. Replaced the electrolytics, mic jack and Filament Gain / power switch.

There are a pair of couplates installed, one on the oscillator, the other on the modulator. True confessions time: I never heard of a couplate and had to Google it to find out what it is! Both couplates (a PC81 and a PC91) look to be very delicate with small gauge wire connections. I found a site (http://www.oldradioparts.com/2a7fl.txt) that apparently still has them for sale, much to my amazement.

Do I leave well enough alone or should I assume they need to be replaced?

I have had a couple fail, one in a Gonset G76 and another in a Hammarlund receiver, but those are the only two instances in personal experience.  They seem to be pretty reliable so I would assume it is good until proven bad. 

I can't remember if WRL provides the component values, Gonset did and I just built one out of discrete components.

I think most people replace them to get better audio, not because they go bad.
The coupling caps inside tend to be small limiting lows, and maybe the bypass caps tend to be large, limiting the highs.

Here's what's in them:

http://www.amwindow.org/tech/htm/couplate.htm

Wow, that was fast!

WRL didn't show the values but I was able to find them on the AM Window. I think I will take a chance on them for now and see what happens when I apply power to the rig and try to modulate it. The $20 minimum order makes them rather expensive and maybe crossing my fingers will work. The audio may not be broadcast quality, but that's OK. My overall objective is to recreate my first station as it was.

Thanks, Pete. I think we are typing simultaneously!

Thanks, guys

73
Jack
WA2OLZ

You can measure the resistors with an ohm meter and the capacitors with a cap checker.  If  good don't worry about them.  If you want wider passband then parallel the coupling cap with another to suit your needs. 

Should the resistors or caps be bad, replace them with individual components.

Hammarlund shows the discrete components in their schematics.

Pretty rugged little guys.  I imagine that the resistor portions fail more than the ceramic capacitor portions, but can't substantiate that.

PC-80   500k   500k  0.01 uF   250 pF
 
PC-84   500k   500k  0.01 uF   250 pF

Pc 80 and 84 appear to be identical: wonder about the rest of the data.

The plate load resistors are usually the first components to fail inside the couplate.

In essence, couplates were some of the first integrated circuits.