15-01-2019, 09:49 AM
Strange, the D3800 power supply is short -circuit protected. I use one on my workbench all of the time and wouldn't like to guess how many times my flyleads have accidentally "met". No damage happens, just un-short the leads and the power supply is back to normal. Unless you disabled the over-current protection when you last 'fixed' it ????
I have my overload current pot behind the small black round panel-filler just below the voltage control knob and it needs to be adjusted cautiously. I use long nose pliers to adjust it so that it protects at short-circuit but still allows you to power a 100W HF transceiver at close to full power - a fine line. Once set, forget. No extending the knob to accidentally change the setting either, mine ends level with the plastic front panel to avoid this.
The presence of 21V on the output sounds like a short-circuit pass transistor. If it is, you should replace all of them with the same batch code on the case so that they are all likely to have essentially the same hFE and thus share the DC load currents equally , and that is despite having equalisation resistors in the emitter leads. I found my schematic for the D-3800 and it reveals 5 x 2N3055 transistors, bases and collectors in parallel, driven by a 723 regulator through a TIP31C. It could be all of the 3055s are shorted or maybe just one.
There should always be a properly rated in-line fuse in the DC power leads to any 12V radios so that it blows if the over-voltage protection diode (virtual crowbar) needs to short-circuit. No fuse, no protection.
Advise by email or PM if you need the schematic : 1083KB PDF.
I have my overload current pot behind the small black round panel-filler just below the voltage control knob and it needs to be adjusted cautiously. I use long nose pliers to adjust it so that it protects at short-circuit but still allows you to power a 100W HF transceiver at close to full power - a fine line. Once set, forget. No extending the knob to accidentally change the setting either, mine ends level with the plastic front panel to avoid this.
The presence of 21V on the output sounds like a short-circuit pass transistor. If it is, you should replace all of them with the same batch code on the case so that they are all likely to have essentially the same hFE and thus share the DC load currents equally , and that is despite having equalisation resistors in the emitter leads. I found my schematic for the D-3800 and it reveals 5 x 2N3055 transistors, bases and collectors in parallel, driven by a 723 regulator through a TIP31C. It could be all of the 3055s are shorted or maybe just one.
There should always be a properly rated in-line fuse in the DC power leads to any 12V radios so that it blows if the over-voltage protection diode (virtual crowbar) needs to short-circuit. No fuse, no protection.
Advise by email or PM if you need the schematic : 1083KB PDF.
Doug VK4ADC @ QG62LG51
http://www.vk4adc.com
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http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.