Globe Scout 680 "Couplates"?

[KB5JO]

Continuing my Scout 680 saga, trying to get it to work on AM.  After replacing the (broken) AM/CW switch, wiring it correctly, correcting wiring for one of the couplates, still have no modulation.  Checking voltages, I have no plate voltage on the microphone amplifier 6U8.  The tube tested good ( no shorts ), but I'm suspecting the PC-91 Couplate.  Resistance between pin 2 & 5 ought to be 1Meg, I measure 250 K.  Are these things prone to failure?  Should I replace both with descrete components?

[KC4VWU]

Steve, K4HX, has a mod circuit worked out in the tech section for the Scout. If you want to try to improve on the audio, you'll have to bag the couplates anyway. I have one that will probably get the treatment sometime in the future.

Phil   

[KM1H]

Couplates are extremely failure prone and I replace them with discreet components in every repair job.

Carl

[k4kyv]

Couplats (couplets? couplettes?) were designed to give the rig space-shuttle audio.

[Pete, WA2CWA]

Like Carl, I've found these things to be extremely failure prone or way out of spec after many "X" years in service. I just replaced two in a Heath SB-610 with discrete components. Get rid of them.

[k4kyv]

Look up the R and C values for resistance coupled amplifiers in the charts in a 50s-60s vintage handbook. Multiply the recommended coupling capacitance by a factor of 3 or more to get good low frequency response down to 50~ or lower, since IIRC those charts were designed for a low frequency rolloff of 100~ or higher.

Even if you don't plan to run that much bass, you will still have less phase shift distortion if you maintain flat response at least one octave above and below the intended frequency range of your actual audio, and do the response shaping right after the microphone.

[Steve - K4HX]

Info on what's inside the couplate is at the link below.

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

[Pete, WA2CWA]

Here's a good brochure many of the couplates marketed by Centralab: http://www.k5mo.com/Documents/CouplateData.pdf

[KM1H]

That little catalog has saved my butt many times over the past 50 years or so. Many of the competitions were not marked but using a Centralab circuit worked every time.

Carl

[KB5JO]

Just to finish this out, I replaced the "couplates" with individual components.  I didn't modify the circuit to use the 12AX7 as suggested in some posts, rather installed components per the info about what is inside the PC81 and PC91 devices, found at "The AM Window Tech General Couplate Info.htm".

That restored plate supply to the 6U8 audio amplifier, and with a D104 microphone, the rig is now making an AM signal.  I wouldn't say the audio is the finest have ever heard, but sufficient for communication.  Next task is to put it back into the case, connect up the Eico 722 VFO and try for an AM QSO.

Thanks for the help, 73 KB5JO

[Steve - K4HX]

Congratulations. The  680 is a nice little rig. Have fun.