The UTB-220 (Use To Be -220) started life as a Heathkit SB-220. At some point is was badly abused physically and operationally. Someone attempted to wire in some Harbach upgrades and did it incorrectly.
When I got the amp, it had a shorted transformer, fried rectifier board, would not key, no filament voltage and looked like it was on Mount Saint Helens when it blew up.
First I stripped it down to bare metal, ran new wire, then began replacing components. A Harbach/Peter Dahl transformer (actually made by LLC) was put into the amp. This transformer is a huge upgrade to the factory transformer with a secondary of 1171 volts at 1.3A ccs. This results in about 3400 no load plate. It is higher than the original transformer and does take some metal work on the inside screen but once the case is on, you do not even know it was done. Next the I tackled the filament problem, which turned out to be the previous mis-wiring of the on-off switch and softstart. Once I got it working I measured 4.97vac which is just about perfect.
Next I replaced both the metering and rectifying board with Harbachs. Along with that I put all new Electrolytic Caps with the new bleeder resistor board.
I verified proper meter movement and measurement before moving on to the tank circuit.
I started with the normal parasitic upgrades which I wound myself using nichrome wire, then I replaced the tank coil with one I wound to replicate the original. Bias Diode was removed in favor of the harbach bias system on the metering board. DC blocking caps were replaced with higher breakdown voltages and finally the fan was modified to accept oil to the bearing area. Finally I added a HV glitch resistor to protect everything.
Flipping the amp over, I finished the re-wiring of the lower side. I fixed the inherent high key voltage with my own design. I then removed the rf coils and caps from the Grid and increased the capacitance along with using 28ohm resistors.
Softstart is provided by an external relay using two bypass resistors and a timer set to a little less than a second.
After putting it roughly back together, I was so impressed with its performance and ease of tuning I decided to tackle the cosmetic issues. After doing some math, the original not quite 2kw input amplifier was now a true 2.5kw amp. So with that as a starting point I decided to design my on screen for printing a new panel.
The panel was designed with Adobe Illustrator and while I am certainly no expert in that software; I found it saved me valuable time in the positioning of all characters and symbols. Starting with grid paper I drew the panel, then reproduced it with the software system. I probably printed 20 or 30 versions before I was happy with the alignment.
If you have never been around screen printing, the above preparation takes hours and hours of patient work. It is the absolute hardest part of the process. I then printed to a clear overlay in preparation for exposing the screen. I used a DCM type emulsion in the exposure process and was quite please with the results.
After stripping the original panel with a environmentally safe product (never again, what a pain) I put on 3 thin coats of enamel and allowed it to dry for 3 days.
I screen printed the panel using a white enamel thinned about 5%. Everything looked good so I allowed it to dry for a full 36 hours under a heated lamp. I then went over it with 3 thin coats of Polyurethane. I then baked it all for 2 hours at 200 degrees.
I thoroughly enjoyed the process and Will do it again.
