Got an old school Mirage 88 on the desk today. Basically a Galaxy DX-88HL. This used to be the personal radio of a local "tech", ehhh, from years ago. I knew the man personally, and he was good at antenna building and amp building. Well, after looking over this radio, I can't say the same on his radio work, lol...yuk...
Issues:
Low output
Variable key unstable
Receive low
Meter light out
Meter stuck
Running hot
Upon evaluation:
Low output is due to someone tinkering around with the bias diodes and such in the driver/final section. I'm not impressed in the least with what I'm seeing.
Variable key unstable is due to the variable pot being overkill in all aspects. Too small in resistance, but way to large in wattage rating. And the wires from the pot to the board is overkill for this circuit, wire is solid and big. The variable was also installed improperly. R238 was changed to a 10k ohm.
Receive low is due to alignment. The radio is in desperate need of a complete alignment. I also noticed L6, L8, L11 is cracked and stuck.
Running hot is due to a 560 ohm resistor tied to a zener diode strapped across the passthrough regulator (2SB754), TR53 (and R249, but why?) yanked from the top side of the board, some legs are still standing up, plus the bias mess in the driver/final section.
Got everything taken care of, but still has an issue with output. The issue being, after reworking that section back to specs, resetting all three bias adjustments VR's (which I replaced all 3 with new ones) back to specs. All I can get out of it is 37w PEP, about 25w peak and around 16w average. I lifted the collector of the first final in the chain (the one in between the driver and the last final) and the output dropped to around 22w pep. I replaced that final with a known good working final because I thought it was weak, but the end result is the same at 37w PEP.
I didn't like the way the guy did the bias diodes. He just snipped one lead of each one and folded it back a little. That is unsafe due to creating a future possible short from that snipped piece shifting in the event the finals have to be replaced by using the soldering iron. Why not take the extra time to remove the entire diode along with the wafer it's attached to and then unsolder the leads completely?
The only things in the driver/final section I replaced with new parts are the driver and finals VR's, VR11, VR10, VR20. I also replaced the bias diodes with bands in the correct direction for a BJT, and I corrected a few of his mistakes in this area along with cleaning up some of that soldering mess. I had to replace the VR's because VR20 was giving no ma, no matter where you turned it, and after replacing that one, then the driver bias acted funny by having 50ma with it turned all the way down and would increase dramatically up to 1 amp before reaching a quarter turn. Plus I couldn't get a driver bias reading from TP9 to the driver TP, only from TP8 to the driver TP. Then the other VR wouldn't come off of 0ma regardless of where you turned it, and while replacing this one I noticed the collector pad and base pad seemed awfully close, looked like they were touching. After replacing all three, and fixing some mess, then all that worked out.
Still have somewhat of a low output and some heating issues, driver/final section slight heating, passthrough some heating.
I did intentionally short out the collector and base of the passthrough while Transmitting on AM to see if the passthrough was weak or the finals were weak, there was no difference in output, so I guess the regulator is fine and the low output issue must be in the RF section, am I correct on this?
Could the driver and other final just be weak from the abuse is why I'm only getting 37w PEP? SSB is 37w also.
Any suggestions guys/gals?
Issues:
Low output
Variable key unstable
Receive low
Meter light out
Meter stuck
Running hot
Upon evaluation:
Low output is due to someone tinkering around with the bias diodes and such in the driver/final section. I'm not impressed in the least with what I'm seeing.
Variable key unstable is due to the variable pot being overkill in all aspects. Too small in resistance, but way to large in wattage rating. And the wires from the pot to the board is overkill for this circuit, wire is solid and big. The variable was also installed improperly. R238 was changed to a 10k ohm.
Receive low is due to alignment. The radio is in desperate need of a complete alignment. I also noticed L6, L8, L11 is cracked and stuck.
Running hot is due to a 560 ohm resistor tied to a zener diode strapped across the passthrough regulator (2SB754), TR53 (and R249, but why?) yanked from the top side of the board, some legs are still standing up, plus the bias mess in the driver/final section.
Got everything taken care of, but still has an issue with output. The issue being, after reworking that section back to specs, resetting all three bias adjustments VR's (which I replaced all 3 with new ones) back to specs. All I can get out of it is 37w PEP, about 25w peak and around 16w average. I lifted the collector of the first final in the chain (the one in between the driver and the last final) and the output dropped to around 22w pep. I replaced that final with a known good working final because I thought it was weak, but the end result is the same at 37w PEP.
I didn't like the way the guy did the bias diodes. He just snipped one lead of each one and folded it back a little. That is unsafe due to creating a future possible short from that snipped piece shifting in the event the finals have to be replaced by using the soldering iron. Why not take the extra time to remove the entire diode along with the wafer it's attached to and then unsolder the leads completely?
The only things in the driver/final section I replaced with new parts are the driver and finals VR's, VR11, VR10, VR20. I also replaced the bias diodes with bands in the correct direction for a BJT, and I corrected a few of his mistakes in this area along with cleaning up some of that soldering mess. I had to replace the VR's because VR20 was giving no ma, no matter where you turned it, and after replacing that one, then the driver bias acted funny by having 50ma with it turned all the way down and would increase dramatically up to 1 amp before reaching a quarter turn. Plus I couldn't get a driver bias reading from TP9 to the driver TP, only from TP8 to the driver TP. Then the other VR wouldn't come off of 0ma regardless of where you turned it, and while replacing this one I noticed the collector pad and base pad seemed awfully close, looked like they were touching. After replacing all three, and fixing some mess, then all that worked out.
Still have somewhat of a low output and some heating issues, driver/final section slight heating, passthrough some heating.
I did intentionally short out the collector and base of the passthrough while Transmitting on AM to see if the passthrough was weak or the finals were weak, there was no difference in output, so I guess the regulator is fine and the low output issue must be in the RF section, am I correct on this?
Could the driver and other final just be weak from the abuse is why I'm only getting 37w PEP? SSB is 37w also.
Any suggestions guys/gals?
