[David G4EBT]

Replacement of Sentercel selenium rectifier:

Whatever other faults might exist, the first task was to replace the defective rectifier and get the Voltages up to spec.

I removed the rectifier and in its place I fitted a small tag-board (actually a ‘turret board’), similar in size to the selenium rectifier, with a 1N4007 silicon rectifier and a series resistor chosen to drop the higher voltage produced by the 1N4007 down to the desired 160V. By select on test, I found that a 560 Ohm resistor met this requirement. I made a small aluminium stand-off bracket to mount the tag-board on the chassis, using the same mounting holes as the original rectifier.

The reservoir & smoothing capacitors consists of three 20uF electrolytic sections in one can. One is used as the reservoir, and the other two are wired in parallel giving 40uF for smoothing. The outer capacitor for the reservoir is identified is marked red. The three sections reformed well, with little leakage, but had they needed replacement, the old can could have been re-stuffed with modern Rubycon miniature electrolytics which have a high ripple rating. One 22uF for the reservoir and one 47uF in place of the two 20uF sections used for smoothing would have sufficed.

Of the 21 carbon composition resistors, nine were more than 20% high - some by as much as 40%, so while I had the generator dismantled I decided to replace those. Most of the capacitors were either disc ceramic or silver mica, and didn’t pose a problem. There were just two tubular paper ones, which I replaced as a precaution. The actual voltages attained will vary to some extent as compared to those shown on the circuitry depending on the mains voltage to the RF1U, which - at the time of testing - was 245 Volts.

After the remedial work, the voltages attained were close to those stipulated:

V1:
Pin 1: Spec 70V. Actual: 74V
Pin 6: Spec 65V. Actual: 63V
Pin 7: Spec -4.0V Actual: - 3.5V

V2:
Pin 1: Spec 55V. Actual: 57V
Pin 2: Spec 125V. Actual 128V
Pin 6: Spec 120V. Actual 124V
Pin 7: Spec 2V. Actual 2V.
Pin 8 Spec 1V. Actual 1V.

Accuracy:

On completion of the remedial work I checked the accuracy of the frequencies at the extremities of each band using my PW ‘Robin’ home-built frequency counter (designed by the late Mike Rowe, (forum name ‘Swordholder’), designer of the ‘Sussex’ Valve Tester. Back when the RF1U was originally marketed, few amateur constructors would have had a frequency counter at their disposal so the dial accuracy was important. Nowadays, we have the luxury of being able to set the frequency by reference to a frequency counter rather than the dial, so the dial accuracy is of much less significance. Nevertheless, the results showed that on all bands, apart from Band F, the RF1U was well within the stated +/- 2% range. Band E, as minus 3% at 10 Mhz and 4% at 30 MHz was perfectly acceptable:

Range A: 100 KHz - 300 KHz: Exactly to spec.
Range B: 300 KHz - 1 MHz: At 300kHz was 304 KHz (+1.3%). At 1 MHz was 980KHZ ( - 2%).
Range C: 1 MHz - 3 MHz: At 1 MHz was 990 KHz (-1%). At 3MHz was 2.98 MHz (- 0.7%)
Range D: 3 MHz - 10 MHz: At 3 MHz was exactly to spec. At 10 MHz was 9.75MHz (-2.5%)
Range E: 10 MHz - 30 MHz: At 10 MHz was 9.7 MHz (- 3%). At 30 MHz was 28.8MHz (- 4%)
Range F: 30 MHz - 100 MHz: Exactly to spec across the band.

More a tribute to the Heathkit design engineers than my handiwork!

Though replacing the rectifier was a simple task, I've gone into some detail with a couple of sketches which might help less experienced forum members should they have an RF1U which isn't performing as well as it should.

Pic 1: Old knotted 2 core mains flex replaced with 3-core and cable restraint.
Pic 2: Faulty Sentercel rectifier
Pic 3: Diagram of old and new rectifier circuit
Pic 4: Drawing of the new tag-board and mounting bracket.
Pic 5: The replacement rectifier board in place.

In the next post, I’ll include to pics of the waveforms and frequency readouts obtained.