08-20-2015, 12:03 PM
http://www.nostalgiaair.org/pagesbymodel...006713.pdf
This one looked to be a simple recap, but ended up frustrating me so far. I replaced all caps and most of the resistors by lifting one end, soldering in the replacement, then lifting the other end and soldering in the other end of the replacement. The filter cap arrangement puzzled me, but I ended up with the low voltage 22 uf running with the positive end on the CT and the negative end on the chassis. The FC measures correctly, the output transformer as well, as do the IF coils. When I applied power, the set failed the dim bulb test with a 40 watt bulb just at the 105v mark.
I rather stupidly tried to take no-dim-bulb voltages as quickly as possible at 110v line voltage on my varian and got the following more or less (the 6A7 voltages are suspect to me because I was moving too fast): 80: F=5.2, P=623; 41: heaters=6.5 (I was working quickly, so perhaps the 80 warmed up as I went along. I got the same on the rest of the heaters), 2=275, 3=226, 4= -72, 5=0; 75: 2=106, 3=104, 4= -396 mv, 5=not sure, 6=.9v; 6D6: 2=89.2, the rest are zero; 6A7: 2=216, 3=88, 4=88, 5= -2.2, 6=0; 76: did not measure (remember before when I said "stupid?"). The small voltage cap popped just at that moment. I had used a 20 uf @ 50v for that one and it blew. Don't want to do that again.
The power transformer and other windings throughout seem to have survived without issue, so I then replaced all of the filter caps again with a 22uf 450v, 10uf 500v, and 22 uf 35v (what I had on hand), and I put a small terminal strip in place as a junction for all the filter caps' negative side and the CT so they lie in the chassis with a bit more stability.
I remeasured all my previous work and found a couple of things to set right. I had replaced R42 & R43 (the closest I had for R43 was a 300 ohm resistor), and I resoldered a couple of places where things seemed a bit close to me.There was originally a .05uf Motorola branded cap connecting pin 3 of the 75 to pin 4 of the 6D6. It looked to be factory installed, but I can't seem to find it on the schematic. I replaced it with another before the first catastrophe. I remember the original of this cap was quite burned by the 270 ohm resistor that runs along side it in the chassis (R42) making its value hard to read. I wonder if that is significant to the current problem?
With the changes in place, the set lights up my dim bulb just at about 75v or higher. It seems to dim somewhat after lighting up, but remains lit and gets brighter if I increase the voltage, so I haven't gone any further on voltages.
Originally, the set had an old repair, an 8 uf cap running from the chassis and pin 3 of the 41 tube (which has a red wire that goes to the .1 cap that connects to pin 5 of the 6D6 which I have left out of my repairs. I’m not sure why the 8uf cap was there since the original cap was still in place...hum management, and an indication that something else was wrong?
I also am going to wait until I get a couple of new tubes before trying to power up again since the 41 is marginal at best and the 6A7 could be better. Everything else tests strong including the 75 (all three tests).
So, can any of you recommend a methodology to help me track down the short? This is still not my long suit.
This one looked to be a simple recap, but ended up frustrating me so far. I replaced all caps and most of the resistors by lifting one end, soldering in the replacement, then lifting the other end and soldering in the other end of the replacement. The filter cap arrangement puzzled me, but I ended up with the low voltage 22 uf running with the positive end on the CT and the negative end on the chassis. The FC measures correctly, the output transformer as well, as do the IF coils. When I applied power, the set failed the dim bulb test with a 40 watt bulb just at the 105v mark.
I rather stupidly tried to take no-dim-bulb voltages as quickly as possible at 110v line voltage on my varian and got the following more or less (the 6A7 voltages are suspect to me because I was moving too fast): 80: F=5.2, P=623; 41: heaters=6.5 (I was working quickly, so perhaps the 80 warmed up as I went along. I got the same on the rest of the heaters), 2=275, 3=226, 4= -72, 5=0; 75: 2=106, 3=104, 4= -396 mv, 5=not sure, 6=.9v; 6D6: 2=89.2, the rest are zero; 6A7: 2=216, 3=88, 4=88, 5= -2.2, 6=0; 76: did not measure (remember before when I said "stupid?"). The small voltage cap popped just at that moment. I had used a 20 uf @ 50v for that one and it blew. Don't want to do that again.
The power transformer and other windings throughout seem to have survived without issue, so I then replaced all of the filter caps again with a 22uf 450v, 10uf 500v, and 22 uf 35v (what I had on hand), and I put a small terminal strip in place as a junction for all the filter caps' negative side and the CT so they lie in the chassis with a bit more stability.
I remeasured all my previous work and found a couple of things to set right. I had replaced R42 & R43 (the closest I had for R43 was a 300 ohm resistor), and I resoldered a couple of places where things seemed a bit close to me.There was originally a .05uf Motorola branded cap connecting pin 3 of the 75 to pin 4 of the 6D6. It looked to be factory installed, but I can't seem to find it on the schematic. I replaced it with another before the first catastrophe. I remember the original of this cap was quite burned by the 270 ohm resistor that runs along side it in the chassis (R42) making its value hard to read. I wonder if that is significant to the current problem?
With the changes in place, the set lights up my dim bulb just at about 75v or higher. It seems to dim somewhat after lighting up, but remains lit and gets brighter if I increase the voltage, so I haven't gone any further on voltages.
Originally, the set had an old repair, an 8 uf cap running from the chassis and pin 3 of the 41 tube (which has a red wire that goes to the .1 cap that connects to pin 5 of the 6D6 which I have left out of my repairs. I’m not sure why the 8uf cap was there since the original cap was still in place...hum management, and an indication that something else was wrong?
I also am going to wait until I get a couple of new tubes before trying to power up again since the 41 is marginal at best and the 6A7 could be better. Everything else tests strong including the 75 (all three tests).
So, can any of you recommend a methodology to help me track down the short? This is still not my long suit.
Charlie in San Antonio
