Customers Repairs; Communications receivers, etc.

Heathkit SB220, 2KW Linear Amplifier, Repair E207


 This piece of equipment uses a couple of directly heated high power triode valves in grounded grid configuration to develop a lot of RF watts output. Input is via the cathodes of the valves, which are decoupled from the filament supply via large coils. HT is for some reason derived from a voltage doubler connected to a 1000volt transformer. There are a number of possible reasons for the use of a voltage doubler rather than using a simple full-wave 2000 volt transformer. I suppose the transformer insulation and wiring need not be as great, and safety-wise it's marginally better to use a 1000volt transformer. However I don't suppose the regulation is as good with a doubler though and I guess the 1000 volt winding needs to have a higher current rating than a 2000 volt one, therefore heavier wire.

 The original transformer had developed a fault because the input fuse expired even when it was disconnected from its load. It's not that easy diagnosing a fault, when high power circuits have to be powered up to decide on the reason for the problem, but a variac helps as this can be used to drop the mains voltage to safer levels. In this situation the variac was overloaded as soon as the smallest setting was used and by judicious measurements I discovered that the primary and secondary windings fused together at some point. Originally the short was not present when no power was applied, only developing as the input voltage was increased, but after several tests the short went hard.
A new transformer needed to be built as an exact replacement was not obtainable, and anyway, most if not all American equipments, although specifying use on 240 volt (or 230 volt) mains, use transformers better suited for use at 60Hz rather than the UK standard of 50Hz. This results in a hum, which in the case of large equipments, can be very objectionable. A new transformer, correctly specified, would overcome this problem. We chose to have Majestic Transformers of Poole carry out the work. The old transformer was used as a pattern for physical size and rating, and given this size the rated power input was specified as 750 watts at 240 volts. This roughly relates to the Heathkit Amplifier rating of 2000 watts which is a "PEP" rating, generally understood to be 2 root2 times the "DC" level, i.e. 2000 divided by 2.8 is 700 watts. From the weight of the original transformer that rating was thought to be marginal, and probably only good for intermittent use (probably because it was designed to run at 60Hz).
The amplifier can be set to a lower power input for CW, or tuning-up, and the original design uses overwindings on its pair of 115 volt primary windings, as a safer method of switching the high voltage, i.e. rather than switching 1000 volts they chose to switch at mains level. This means that a standard power switch can be used. To cater for this feature the new primary was specified to have an additional winding of 100 volts. Using the overwinding, if 240 volts is applied across the whole primary the output voltage will drop from 1000 to about 700 volts.
Because of the design of the voltage doubler, the secondary winding must be completely insulated from ground, unlike a transformer for a microwave oven, which has one of its secondary connections wired to ground. On test, with the new transformer, the voltage doubler supplied a DC output, across a bleeder network of eight series-connected 30 Kohm resistors, of about 2800 volts. This represents 1000 x 2 x 1.4 volts, or the peak output from the doubler. The doubler circuit comprises two chains each of seven diodes and two blocks of four series-connected electrolytic capacitors having a working potential of about 450 volts each. The original diodes were in the 1N4000 series, rated at 1000 volts 1 Amp but on several occasions some of these had failed and when inspected, all those fitted showed signs of distress, so I fitted two sets of BY255 diodes, rated at 1300 volts 3 amps. Chris, the owner tells me that now, under load, the high voltage stays up whereas before, it used to drop. Probably under normal SSB conditions the voltage doesn't have time to drop much anyway and the drop noticed is presumably when tuning up or whistling into the mike.

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