05-05-2023, 06:41 PM
Well sometimes when it rains, it pours. No this time it wasn't the wrong DeOxit that caused the problem....that lesson has been learned!
I finished up a Bendix 55L2 back in March. Total recap, all new resistors, micas, everything. It has a beautiful white Plaskon cabinet but the dial scale was badly chipped. So I ordered a repro from Radio Daze on March 6th, and finally received it on Monday.
In the meantime, I had the radio tucked aside on a shelf awaiting the dial cover so I could put it together. I had it all back together, looking sharp, plugged it in, and no reception. Just a hum. Not even a hint of reception.
So, back out of the cabinet, checked all the tubes, swapped out tubes with known good tested tubes, and still no change. Then I took the bottom cover off and quickly discovered what happened.
The last time I tried it, I didn't have the metal bottom cover on the chassis, but cut a thin piece of cardstock and glued it to the inside of the cover to prevent anything from shorting against it (I hate metal covers on the bottom of radio chassis for that reason). I bolted the cover on before I set the radio aside to wait for the dial cover and thought nothing of it.
Well there was an original 2.2K, 2 watt resistor that tested fine so I left it in the chassis. I didn't disturb it. Apparently, I should have replaced it if only because of the difference in size to a modern 2 watt resistor. Apparently the bottom cover pushed against the resistor, and its lead, in turn, touched against one of the leads on the OSC coil. The 2.2K resistor goes between the positive of one of the two electrolytics, and the output transformer, so it sent quite a voltage spike to the OSC coil. It's black color gave away the damage.
I always install a 3.5 mm cable to my radios, just to have the option of listening to something other than what's found on AM radio these days and for better fidelity than I can get from my AM transmitter. Just a simple connection from the high side of the vol. pot to ground. Yes, I know this is a hot chassis set, but I've never gotten a shock using this simple methog, being careful what I'm touching when using a device the cord is plugged into. So I was able to test the audio output section this way and it's perfectly fine. Plays great. So the problem is definitely in the RF stage past the volume control.
I had one of those AES Universal OSC coils on hand, which always worked great in the past, but I'm still not getting any hint of reception with it. I tried reversing the leads on the primary and secondary sides and still nothing. I rechecked all the resistors, and the capacitors and everything is still good. The IF cans both ohm out as they should on the primary and secondary. The tuning capacitor isn't grounding anywhere that I checked with my test light to ground. So I am at a loss as to what other damage that voltage spike through the OSC coil could have caused. Voltage readings for the most part are right where they should be, except for these:
Pin 5 of the 12SA7 reads - .2 VDC, and should be 5 VDC
Pin 8 of the 12SA7 reads -9.8 VDC and should be - .5 VDC
Pin 4 of the 12SK7 reads -5/76 VDC and should be -.5 VDC
Pin 4 of the 12SQ7 reads -.59 and should be -2.3 VDC
These are all measured to B-
Does anyone have any thoughts as to what may be at fault?
I finished up a Bendix 55L2 back in March. Total recap, all new resistors, micas, everything. It has a beautiful white Plaskon cabinet but the dial scale was badly chipped. So I ordered a repro from Radio Daze on March 6th, and finally received it on Monday.
In the meantime, I had the radio tucked aside on a shelf awaiting the dial cover so I could put it together. I had it all back together, looking sharp, plugged it in, and no reception. Just a hum. Not even a hint of reception.
So, back out of the cabinet, checked all the tubes, swapped out tubes with known good tested tubes, and still no change. Then I took the bottom cover off and quickly discovered what happened.
The last time I tried it, I didn't have the metal bottom cover on the chassis, but cut a thin piece of cardstock and glued it to the inside of the cover to prevent anything from shorting against it (I hate metal covers on the bottom of radio chassis for that reason). I bolted the cover on before I set the radio aside to wait for the dial cover and thought nothing of it.
Well there was an original 2.2K, 2 watt resistor that tested fine so I left it in the chassis. I didn't disturb it. Apparently, I should have replaced it if only because of the difference in size to a modern 2 watt resistor. Apparently the bottom cover pushed against the resistor, and its lead, in turn, touched against one of the leads on the OSC coil. The 2.2K resistor goes between the positive of one of the two electrolytics, and the output transformer, so it sent quite a voltage spike to the OSC coil. It's black color gave away the damage.
I always install a 3.5 mm cable to my radios, just to have the option of listening to something other than what's found on AM radio these days and for better fidelity than I can get from my AM transmitter. Just a simple connection from the high side of the vol. pot to ground. Yes, I know this is a hot chassis set, but I've never gotten a shock using this simple methog, being careful what I'm touching when using a device the cord is plugged into. So I was able to test the audio output section this way and it's perfectly fine. Plays great. So the problem is definitely in the RF stage past the volume control.
I had one of those AES Universal OSC coils on hand, which always worked great in the past, but I'm still not getting any hint of reception with it. I tried reversing the leads on the primary and secondary sides and still nothing. I rechecked all the resistors, and the capacitors and everything is still good. The IF cans both ohm out as they should on the primary and secondary. The tuning capacitor isn't grounding anywhere that I checked with my test light to ground. So I am at a loss as to what other damage that voltage spike through the OSC coil could have caused. Voltage readings for the most part are right where they should be, except for these:
Pin 5 of the 12SA7 reads - .2 VDC, and should be 5 VDC
Pin 8 of the 12SA7 reads -9.8 VDC and should be - .5 VDC
Pin 4 of the 12SK7 reads -5/76 VDC and should be -.5 VDC
Pin 4 of the 12SQ7 reads -.59 and should be -2.3 VDC
These are all measured to B-
Does anyone have any thoughts as to what may be at fault?
Greg V.
West Bend, WI
Member WARCI.org
