This is the 3rd and update to my HeathKit SB-200 Amplifier Project.
See the orginal post here
See update #1 here
The amp has been running fine since I’ve got it going. I had a resistor pop in the parasitic suppressors and I think that the amp is running too hot in temperature for little time I use it. At this point I am placing the blame on the two muffin fans that are currently “Cooling” the tubes. I am thinking that the fans are not providing enough air to cool the tubes fast enough. When I was replacing the parasitic suppressors, I performed some modifications (see update #1) and ordered a new fan from Harbach Electronics to see if it would make a difference.
I also held out on performing a couple more modifications because I had to fabricate some parts to encase the glitch resistor so if it were to pop, most of the resistor mess would be contained. The fan was placed on back order so It gave me time mark up and cut the lexon glass to sandwich the resistor with.
The new fan came in and I just had to install it.
The fan kit does come with installation instructions which at first were a little difficult for me to understand. This was due to the fact that the new fan is a modified replacement of the original fan that came from Heathkit and I had purchased the amp with PC type fans. There is pre-assembly you have to do to the fan before installing. It involves removing a couple of screws on the fan that stick out and replacing them with shortened screws. Then you have attach aluminum spacer blocks to the fan which the block also has holes spaced out to fit in the orignal pattern of the fan from Heathkit.
Aluminum Spacers Used On Installtion
The installation is pretty simple and straightforward. The kit comes with new rubber grommets to absorb any minor vibration that the fan causes.
You can either cut the wires near the old fan and tie into them or what I did was un-solder the old wires and wire the new ones in place.
The Installation is complete and we’ll see what happens!
I initially wrote this article in early may. I wanted to do some other upgrades at the same time but the New England QSO party was really close so I decided just to do the fan. The install went easy and would like to thank those at Harbach for rushing out the part. I made sure the fan was balanced as much as possible and I was able to use the amp during the entire contest. It’s a tad bit louder than the PC Muffin type fans that were in there but you can actually feel more air being pushed with the replacement fan. Over the past months the amp has performed very well. I should find more screws to secure the hood and the sheet metal covering the tubes/rf deck as it tends to rattle during operation.
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!