[Central-Electronics] 20A Serial 4544

[Jim Candela]

 Curt,
   Sounds like TR-1 got pretty warm, and bled out some tar. Not a good symptom to find.
As to the consensus of solid stating a rig like this, there will not be a consensus. Ask 10 people the same question and you will get a variety of answers, and every one may have some valid points.
Here is my take. Put a variac on the rig, and measure the filament voltage. See what the incoming AC is to get 6.3V on the filaments. I think you will find the filaments at 7v or a little more with 125vac going into the rig. Something like 6.6v is probably OK. Also measure the unloaded DC voltage, and keyed too at say 10 watts RF output.

Now, if you want to go solid state, that frees up the 5v 3A filament winding which goes to the 5U4. This can be used to "buck" 5 volts from the AC incoming voltage if the 5V winding is put in series with the AC line winding. It must be properly phased to reduce the incoming by 5v. That might be enough reduction without the need for a dedicated variac. After solid stating the HV supply, add enough series resistance on the B+ line to get the voltage down to something like measured before. Might start with a 330 Ohm 10 watt resistor. I'd follow that resistor with another capacitor (say 40uf 450v in parallel with a 0.1uf 630v polypropylene) to ground. Sometimes adding diodes will lead to switching spikes that get into the low level audio and Rf stuff. If there, you will hear a buzz in receiver when nulling the balanced modulator. This will also reduce the depth of the carrier null.
Another thing, 1/2 of the 6AL5 is used as a bias rectifier. You lose the bias, and the rig cooks until it either smokes, or blows the fuse. The 6AL5 pins and socket pins are prone to bad connection due to age and corrosion. It is simple to solder in a 1N4007 under the tube socket. That way, no Chernobyl effect can occur.
Good Luck,JimWd5JKO

    On Tuesday, February 11, 2020, 10:29:52 AM CST, Curt <radio.ku8l at gmail.com> wrote:  
 
 Just received a very nice 20A in the rack mount configuration. Includes 
the QT-1.   I have never seen a picture of one of these before and 
wonder if they all came with rack handles?

Also, on receiving inspection, I see potting pitch around the bottom of 
TR-1.  It is a bead about 3/16 dia around 1/2 of the diameter of the 
can.  Is this a common thing or should I plan on finding a replacement?

What is the consensus on changing to SS rectifier  and resistor  to 
replace the 5U4?  I like reducing the load on the transformer and the 
exposure of the HV to the filament winding.   Are theses transformers a 
problem area like the ones in the old Hallicrafters HT-37s?

Thanks.  I look forward to putting this 20A on the air.

Curt,  KU8L
______________________________________________________________
Central-Electronics mailing list
Home: http://mailman.qth.net/mailman/listinfo/central-electronics
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Central-Electronics at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html