| Electronics > Repair |
| Fixing Tektronix RM503 power supply (Fixed) |
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| Henri:
I have lifted one side of all the diodes so they are no longer connected and removed V634A. So the circuit is pretty much isolated. The primary of the transformer is only 2Ω which seems very low for 500 volts. still no oscillations. |
| BlownUpCapacitor:
2 \$\Omega\$ isn't unreasonably small as the oscillations should be at 30kHz. Inductive reactance should bring up the impedance of the primary. |
| Henri:
Do you know if the oscillations are the full 500 volts? Odd it seems that the oscillations should start but no feedback is getting through the secondary of the transformer there so it does not start. |
| BlownUpCapacitor:
Remember that vacuum tubes offer fairly high impedance when they are on. They can't quite be close to a full short like a regular transistor. So in order to get the required current flowing, they use a very high voltage of say 500v. This should apply to the 503 as well. In my tube transceiver, I use 250v B+ voltage. My 6AQ5 tube drives the antenna coupling transformer with the coil connected directly to 250v, and the cathode of the 6AQ5 is connected directly to GND. The coil has less than an ohm of resistance and offers only 50 ohms of impedance at 3.57MHz; yet the circuit draws only a few milliamperes from the 250v. If you can, hook up a function generator to the transformer primary and set it to 30kHz. See if the voltage ratios match up with the input RMS voltage of the transformer. Be careful, if the transformer is good, there can be lethal voltages on the secondary! |
| George Edmonds:
How that is a blast from the past, I last worked on a RM503 when I was about eighteen, and I am now eighty. Having had a very quick glance at the service manual I am not surprised that you are having problems as this is an early valve/tube switch mode supply. The oscillator AC feedback path is via C621 and the DC regulation control path is via V624A. In my experience by now most of the resistors will have gone high in value and the capacitors failing. Please be aware that this power supply can be lethal, Strange way of drawing the valves/tubes G Edmonds |
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