Well, after a month or so of operation with the SB-200, my first “oops” happened. I went to turn on the amplifier and I thought I had everything correct and when I went to TX I heard a loud “Pop” sound. Quickly shut off the amp and disconnected the power. Opened the case to find that one of the resistors and the parasitic suppressors popped. I knew something was going to happen because the resistors were turning black.
Since I had to fix the suppressors, I figured this was the time to do some other “Modifications” to the amp. When I first rebuilt the amp, I ordered all the parts to do almost all the mods that you see on KL7FM’s SB-200 Page. I figured since it’s opened up, this would be the time to do SOME of those mods listed on his site. Are these mods needed?.. Nope! If they were then I would assume that they would have been incorporated a long time ago. I just figured if it makes it run a fraction of a percent better, why not invest the extra couple of dollars.
First thing I did is replaced the Parasitic Suppressors and I also replaced the .001uF 1kV ceramic Disk cap with a .0025uF 10kV disk cap (as seen in orange in the picture above),
Apparently it allow less RF to leak back into the power supply on the lower bands. Cap was around $2.00US
In this photo I replaced the two 33ohm carbon resistors that I had with some higher wattage metal oxide resistors. I then replaced the 3300Ohm resistor with a high wattage metal oxide type as well. I also replaced the 2 200pF caps with 4 470pf 1kV. I guess by replacing the caps, there will be decrease the grid-to-ground reactance for better improvement on the lower bands.
In this photo I replaced the nylon protected wire for the input to the cathode and replaced it with a 10Ohm 3W resistor. I guess this will help dampen the VHF oscillations as well as help by prevent over driving the amplifier.
After hooking everything up, The amp turned on and sprung back to life. After messing around with it for a couple house, I haven’t really notice really any improvements. doesn’t mean nothing is happening which is WAY better than having it break on me. So it was a success.
Since the AMP has been operational, I’ve notice that it gets very hot with little use. My gut is telling me that the Pancake AC fans are not doing the job efficiently. The fins are quite small and most of the room is taken up by the motor in the center. I am seriously considering the purchase and installation of a replacement of the type of fan originally used in the SB200. The company that sells the fan kit want over $60 and I am still tossing it around in my head. So far I keep my eye on the temp and I’m using a bigger fan that sits on top of the case to exhaust the heat. |
When the time comes to install the new fan, I am also going to do more modifications that were on that website that have to do with protecting the power supply and the meter. I might even go as far as replacing the meter light with an LED that turns Red when I’m TX. I also might change the “REL Power Sense” POT with a Switched POT so I can put the AMP in standby mode. Hopefully after this, I will never have to go in that amp ever again!
Ever since I moved to my house I always had noise in my receiver. It wasn’t loud enough for me to care until now. For the past couple of months the noise has been real bad. Bad to the point where I didn’t even go past 20m (14mhz). I Figured when Field day comes around I will be able to see if its something with the radio. I turned on the radio at field day and the noise was still there. I was upset that I was going to have to ship my radio to YAESU for repairs. That is until someone turned off the light that was 3ft near the radio. THE NOISE IS GONE!!! YES!!! Found out that the cheap-o CFL Bulb (or it’s ballast ) was causing the RFI. I thought that this might be the same cause at my QTH. So after field day I setup back home and tried the radio without the lights. Noise was still there. Today I finally got off my ass and started to track down what was causing the RFI. I went to cellar and turned off all the circuit breakers except for the room which the FT-950 is in. The noise is still there. I unplugged everything in the room except the power supply and the Noise was still there. At this point I took a TV car battery (sealed) and hooked it up inside. The noise is GONE! With the process of elimination, I found out that the power supply was to blame. Back when I purchases the FT-100, I Moved the power supply close to the radio so I could power both the radios.
The power supply that was causing the RFI is the SAMLEX SEC-1223. I googled the model number and there were numerous sites that mention the RFI that power supply generates.
ZL2DF used ferrite beads and ceramic disk capacitors (about US$10 for part modification) placed right before the output terminals and OZ1DB used two 1.5uf 100V Capacitors ( US$ 3) on the output terminals