Hi Everyone,
I'll make this as short as possible as I have some grandkids to play with..
A little over 2 years ago I picked up a 1st edition (2 button) President Washington very cheap as it did not work. The reason I got it was because it was in VERY good shape. As is with most of these radios, someone had been in it and well, it needed help. When I put it on the bench and spent a few hours on it I just gave up. I realized that it had multiple issues and it was beyond my troubleshooting skills (and time) at that point. I have been very busy at my real job as well so time has not been on my side. I have repaired about 10 radios in the last month or so and I have learned a TON thanks to all of your reply's to my questions or to previous posts on here!!!
Yesterday I put the Washington back on the bench and decided to try again. This time I just looked at it as one problem at a time and not as "multiple" problems. The 1st was the freq alignment. I replaced a missing varactor diode that I was able to get from an eBay seller recommended to me on here. That allowed me to be able to align it all the way to the last step in the freq section. I could not get the USB 7.8025MHz even close. I tested a bunch of the parts in that section as shown in the schematic and I discovered the the vari-cap was bad. installed a new one and bam...it's on frequency.
Next was the very low AM carrier. One interesting thing here was that VR did not have any affect on the carrier at all. Again, back to the schematic and testing parts, While I did test a few parts the one that I thought would be my issue ended up being bad. The AM regulator control transistor was bad (2sc945). Installed a new 945 and whala it works but........ then it didn't. I'll be back to this point.
As I was working on the radio it blew its 3 amp fuse and I could not figure it out as it was random. I hooked the radio up to my current limiting PS, set it for 2 amps, and occasionally it would kick out. I'm about 3 hours in now but this took most of the time. Why would it keep spiking on high current? After some time I figured out that it was happening when my test gear was connected. It could be my scope, counter, sig gen or even the ancient sinad meter I use. I figured out that the radio essentially had an internal short/ground loop. Any time the ground plane of the board and the case were "shorted" the current spike happened. I could take a jumper and touch the shield of the EXT SPKR jack and touch it to the case and it would trigger this high current draw. I thought it must be a shorted cap that goes between the board and case. Nope, I removed all one hundred (ok not that many) of them and still had the issue. I could read about 70 ohms between the case and the ground plane. I tried that on another radio with the 858 board and it was open so I knew I was on to something.... I decided to next try to see if one of the transistors was shorting out to the case. Now back to the AM regulator working then not...... I pulled the regulator off the heatsink and don't you know it, the short went away. The plastic insulator was cracked and in bad shape. Somehow, there was about 70 ohms resistance between the transistor and the heatsink and that was causing the "ground loop. I replaced the plastic insulator with a thermal pad, reinstalled the transistor and AM came back as well as the short is now gone. I completed a proper alignment, cleaned the radio, got an on air positive check and now its time for a new home......
I did a lot of research on hear to help me overcome those various issues so the truth is, WE fixed it. Thanks so much to each of you that give this forum your time!!!!
Now its time to sit on the floor and play with Match Box cars until the grand kids leave.
I'll make this as short as possible as I have some grandkids to play with..
A little over 2 years ago I picked up a 1st edition (2 button) President Washington very cheap as it did not work. The reason I got it was because it was in VERY good shape. As is with most of these radios, someone had been in it and well, it needed help. When I put it on the bench and spent a few hours on it I just gave up. I realized that it had multiple issues and it was beyond my troubleshooting skills (and time) at that point. I have been very busy at my real job as well so time has not been on my side. I have repaired about 10 radios in the last month or so and I have learned a TON thanks to all of your reply's to my questions or to previous posts on here!!!
Yesterday I put the Washington back on the bench and decided to try again. This time I just looked at it as one problem at a time and not as "multiple" problems. The 1st was the freq alignment. I replaced a missing varactor diode that I was able to get from an eBay seller recommended to me on here. That allowed me to be able to align it all the way to the last step in the freq section. I could not get the USB 7.8025MHz even close. I tested a bunch of the parts in that section as shown in the schematic and I discovered the the vari-cap was bad. installed a new one and bam...it's on frequency.
Next was the very low AM carrier. One interesting thing here was that VR did not have any affect on the carrier at all. Again, back to the schematic and testing parts, While I did test a few parts the one that I thought would be my issue ended up being bad. The AM regulator control transistor was bad (2sc945). Installed a new 945 and whala it works but........ then it didn't. I'll be back to this point.
As I was working on the radio it blew its 3 amp fuse and I could not figure it out as it was random. I hooked the radio up to my current limiting PS, set it for 2 amps, and occasionally it would kick out. I'm about 3 hours in now but this took most of the time. Why would it keep spiking on high current? After some time I figured out that it was happening when my test gear was connected. It could be my scope, counter, sig gen or even the ancient sinad meter I use. I figured out that the radio essentially had an internal short/ground loop. Any time the ground plane of the board and the case were "shorted" the current spike happened. I could take a jumper and touch the shield of the EXT SPKR jack and touch it to the case and it would trigger this high current draw. I thought it must be a shorted cap that goes between the board and case. Nope, I removed all one hundred (ok not that many) of them and still had the issue. I could read about 70 ohms between the case and the ground plane. I tried that on another radio with the 858 board and it was open so I knew I was on to something.... I decided to next try to see if one of the transistors was shorting out to the case. Now back to the AM regulator working then not...... I pulled the regulator off the heatsink and don't you know it, the short went away. The plastic insulator was cracked and in bad shape. Somehow, there was about 70 ohms resistance between the transistor and the heatsink and that was causing the "ground loop. I replaced the plastic insulator with a thermal pad, reinstalled the transistor and AM came back as well as the short is now gone. I completed a proper alignment, cleaned the radio, got an on air positive check and now its time for a new home......
I did a lot of research on hear to help me overcome those various issues so the truth is, WE fixed it. Thanks so much to each of you that give this forum your time!!!!
Now its time to sit on the floor and play with Match Box cars until the grand kids leave.
