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Robyn 520D issues.
- Thread starter Staybolt
- Start date
Okay. If I were looking at this I would start in this area. The 5 supply ( 1.89 or 1.90 v ) is what feeds the channel display LEDs. Follow the 5 supply left to TR36. TR36'S base is fed by TR37.
Now.... follow TR36's collector supply down.... to R113 (68 ohm) and drop down to the rotary switch S402-1. Notice that is your mode switch (AM, USB and LSB).
It seems to me that is something is wonky down in the power supply in the area of the mode switch.. it might be able to pull down hard on the voltage feeding TR36.
if it pulls down a little (AM)... would cause your display to dim.
If it pulls down harder (SSB)... THAT would cause the display LED to blank out.
I would snoop around this area with a DMM while keying in AM or SSB.... and measure voltages around TR36.
If you key up and supply5 drops...
Look at the TR36 collector and base...
If the base does something funky... that could be turning off TR36 and dimming the display.
If the TR36 collector gets pulled down.... then something further down near the modes switch is pulling that voltage down and THAT is causing the dimming.
Good luck..... let me know.
Bob
Adding a little more thought....
TR36 is pretty much a pass transistor for providing power to the channel display.
TR37 switches the base of TR36 and I believe that the intention is to kill the display when the signal on the base of TR37 does something. That signal comes from somewhere in a reference oscillator. I haven't chased the schematic far enough...... but just guessing..... it would seem reasonable to detect some kind of failure in an oscillator and shut off the display as a clue to the operator.
So .. .. still..... I would be measuring voltages around TR36 and TR37.... and look for changes when you key on AM.... AND when you key on SSB.... and see if the 5 supply drops a little in AM .... and a lot on SSB.
Bob
TR36 is pretty much a pass transistor for providing power to the channel display.
TR37 switches the base of TR36 and I believe that the intention is to kill the display when the signal on the base of TR37 does something. That signal comes from somewhere in a reference oscillator. I haven't chased the schematic far enough...... but just guessing..... it would seem reasonable to detect some kind of failure in an oscillator and shut off the display as a clue to the operator.
So .. .. still..... I would be measuring voltages around TR36 and TR37.... and look for changes when you key on AM.... AND when you key on SSB.... and see if the 5 supply drops a little in AM .... and a lot on SSB.
Bob
Try unplugging the two bias-test jumper wires.
If this stops the power-supply overload, testing the driver and final transistors would be next.
73
If this stops the power-supply overload, testing the driver and final transistors would be next.
73
Nomad, I'm still out here in the weeds trying to learn various facets of troubleshooting these things.
If you don't mind... how did you peg that so quickly/efficiently??
My thoughts I posted ... I knew it HAD to be something "loading" the power supply and pulling it down...and it had the "net effect" of pulling down the LED display voltage.... thus causing the dimming.What led you to the final shorting so quickly?
Just thinking about it.... the original post said AM was DIM but SSB was BLANK! I am thinking that, in AM you have the AM regulator holding the final's Vcc at about half the usual... while the two sidebands would bypass the AM regulator... but you would see the entire 13.8v supply loaded down which would cause the display to go darker/dark.
Does that sound reasonable? Or was it "just one of those itches" that experience gives you??? Thanks inadvance.... Bob
Yup when id key down on SSB the display would go dark! A shorted final was the cause!
Bingo! This is the first clue for this type radio. Doesn't tell you exactly where the short is, but narrows it down a lot.
Practice is an unfair advantage. That circuit board has been around almost 45 years. After you see the first hundred, patterns emerge.
73
Thank you so much for sharing your thoughts on this. I appreciate it.
One other thought to what Nomad so perfectly explained. There are a number of radios that have the ability to remove the jumpers (or shorting boards in the case of RCI/Galaxy) in order to disconnect the volatge feeds to the driver and final. This troubleshooting method will work in all of those cases. I was repairing a Galaxy 959 with a shorted final and was able to narrow the issue down quickly by removing the shorting bar.
Well done Nomad!!!
Well done Nomad!!!