GK500C modulator

[w4fms]

I've been working on a GK500C here and there for a few months that came from Jim, K9RJ. 

After rebuilding the various supplies and replacing a number of other parts, the transmittter is making RF fine however I'm having some trouble with the modulator.  I've got idling current but no audio.  I believe all the mod iron, tubes and voltages to be good so I'm guessing my problem is in one or both of the couplets.  I won't know for sure until I start poking around with a scope probe.

Could someone point me to the mods for coupling the 6SJ7 preamp to 6C5 and 6L6 stages.  I've seen some discussion on this but haven't actually found any instructions for doing the mod since it's more than just inserting a few capacitors.

Any help appreciated..thanks in advance.

Frank, W4FMS

[Jim, W5JO]

Where can I find your email address?

[w4fms]

Thanks Jim...

w4fms@aol.com

[Jim, W5JO]

check your email, it should be there.  If it is unsatisfactory, I can re-scan at 600 dpi.  Just let me know.

[k4kyv]

Regardless of how well the G-K works,  get rid of the couplets!  Replace them with conventional R-C coupling circuits.  The couplets were designed to give the rig crappy sounding slopbucket quality "communications" audio by severely restricting frequency response.  Info on R-C coupling constants can be found in the RCA receiving tube manual as well as in the (older) ARRL handbooks.

[w4fms]

Thanks Don.

I initally was going get the GK goiing with the stock audio just so I could hear how bad it really was for myself.  As you know, it's rare anymore to hear a GK with the couplets.  I've heard a few Globe Champs out there with them but not that often.

The mods Jim sent me look pretty straight forward and will require just a few new parts and a couple termainal strips.

The only other parts of this rig not working are the little green light on the RF deck and the PTT.  Some of the very late 500Cs had the light and I don't even see it on the schematic so I'm not sure yet what it's supposed to do.  The PTT is simple with 6VDC going through relay coil and PTT switch to ground so I'm hoping it's not the relay coil.  I've already replaced the seleninum rectifier while I was working on the modulator supply so not sure yet what's the problem there.


Frank

[ke7trp]

Jim. Can you send me the file also please? 

Clark at Clarkturner dot com

THanks!


My Globe Champion 300 has the couplates. I bypassed the main one though. It really sounded much better with it bypassed with a cap. 

On my 500C, I reworked the mod deck.  Solid state rectifiers, I also ran the 500V supply up through a BNC connector from the bottom deck. My replacment Trans did not have the 5oo volt tap. 

Mine also has the Couplates removed.  With almost 1600 volts on the 811s now, I get 125% peak easy. I ran the phone patch jack directly to the 6C5 for outboard audio.  IN this config it passes 20 to 10,000HZ.  Past that it rolls off.  If I use a D104, I go to the first stage 6JS6 tube. 

If you want the best audio, You are going to have to change the bypass caps on the 4-400 also.  Easy job. I used big micas. 

See my post on my restoration of the GK500. Maybe something there will help you.

Clark

[ke7trp]

I meant 6JS7 in the above post. Not 6js6.  THis is the first preamp tube.

Another thought....

The GK500 mod deck is known for being on the weak side. Most cant reach 100% mod in stock form.  Solid stating supplies and upgrading the oil cap like I did really helps out.

I also learned that the 6JS7 tube is a big limiting factor.  With the D104 or EV mike plugged in to the front jack, I can now get 100% mod with mine but thats IT.  However, If I bypas this tube and run a mixer or preamp with studio type mic into the 6c5 (second stage) I can easily get 125% positive out of the rig.  Its much cleaner on its output this way. The difference is very noticable.  If you are looking for performance then this should be addressed.

Clark

[Steve - WB3HUZ]

In case you haven't seen it - couplate info.

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

[ke7trp]

On the PTT...  MIne never worked.  I found that the cap that is near the old solenium unit was the wrong value from the factory. It was clearly the old paper style.  This was causing it to not produce the voltage to latch the relay. Once replaced with a near factory value the PTT started working again. 


Clark