Thanks, Randy, for letting me evaluate the transformer from your failed HT-37!? I found that all windings are isolated from each other by at least 100 megohms, as measured by a Keithley 2100 DMM.? However, when applying 2000 volts from a Bertan HV power supply (5 mA limit), I found arcing between the HV winding and the filament winding marked ¡°YEL¡± on the schematic.? These wires are pale yellow.? The other 5V winding, ¡°BRN¡± now has pale red or orangish insulation.? It seems possible that these were once brown.? All other winding combinations withstood the 2000 volts.? Not wishing to destroy insulation elsewhere in the transformer, I did not test the other windings with more than 2000 V.
I measured AC voltage and winding resistance on the transformer which I¡¯ll post later.
I still wish to understand how turning the transmitter off without a pause in STBY, or turning it fully off and immediately on again, can cause a voltage spike between these windings that¡¯s substantially in excess of what occurs during operation, and could cause an arc between windings and thus deteriorate the insulation at the arc site.? If the transmitter is turned on before the HV B+ bleeds off (RC is 3 seconds) from having been on just before, then the 5V winding could still be at about 1000 V relative to the center of the HV secondary.? If the rectifier filament has cooled enough such that the tube is no longer conducting, then each end of the HV winding will alternately reach about -790 volts, for a total of about 1800 V that the insulation must withstand.? During operation (but not transmitting), the HV winding is loaded by the bleeder resistors via the rectifier, drawing 3.3 mA.? Each side of the HV secondary has 113 ohms of resistance, dropping less than a volt in the winding.? So that¡¯s not it; ~1800 V appears during standby operation, too.
What voltage condition occurs during turn-on with charged HV capacitors that is not also present when the transmitter is on?? I understand that heavy inrush current is typically present at the primary, but I'm not aware of that current causing a voltage spike at the secondary.
Or, what voltage situation is present at turn-off that¡¯s different when turning off from STBY compared to turning off from MOX?
Cheers
Halden VE7UTS
