07-26-2015, 11:44 AM
Sean, So did you pickup that HRO on epay, the green one??
Sam, On your RCA I don't think it has an AC power supply so you'll need a good strong 6v or 12v battery (like car battery).
Paul, I've gotten to the place where I don't move much of anything unless I'm going to work on it or trip over!
So the L76 has been calling me by name since I moved it to the dining room table about 2 weeks ago. So I put on my bib overalls (this means I mean business). Grabbed my free SG-7 (Heathkit signal generator) that a friend gave me. and started at fixing L76. Well found why I got it for free, it makes rf but no modulation (400cy tone) which makes it a pain to use. After a half hour of looking up the info on it, replaced the filters and found a leaky paper cap that was the issue with the modulation. Now it's up and running properly!
Hooked up the power supply and powered up L76. As I posted before when I power it up seemed like the audio stages where working but the rest was unknown. Connected the generator to the second IF stage to see if it would pass a signal at the IF frequency (456KC). No dice! Then I checked the resistance of the IF transformer. Bummer secondary is open. Popped the cover off the transformer to investigate further. Both winding checked OK. Hmmm. Turns out I miscounted the pins on the detector tube socket where the secondary is connected. Cleaned it up a bit and back together it went.
But why won't pass a signal at 456KC?? Well I let that problem go for the time being and went to the 1st IF stage. Hooked the generator to the input (control grid) and then hook a signal tracer (it's an audio amp with a diode connected to the input so it can turn a RF signal to an audio signal) to grid of the second IF stage. I was rewarded with a weak signal from the tracer. So what do you do when you have a weak signal? You make it stronger, in this case by repeaking the transformer.
After that I went to the plate circuit of the mixer tube. This set has a crystal filter which is used to give this receiver very good selectivity it's connected to the plate of the mixer. Sets without a filter the mixer would feed the 1st IF transformer. I wanted to see of it would pass a 456 KC signal. It did!
So what about the last IF not working but the transformer is OK? Well What I found was the adjusting condensers had oxidized so they where way out of tune (resonance). These IF transformers use small air variables not compression trimmers. The oxidization had formed at the rotor bushing. Out came the De-Oxit and a quick squirt into all the IF adjustments. Repeaked the IF stages and the last one peaked right up like it should (before it wouldn't peak at all). Funny thing they use the same tool to adjust as the Philcos a 1/4 hex nut.
Now it's picking up the generator thru the whole IF strip, detecting and making audio. The audio is distorted I'm thinking there's a bad mica cap in the detector stage.
More later! Stay tuned.
Terry
ps If any are interested a manual for the HRO senior can be found here:
http://bama.edebris.com/manuals/national/hro1935/
Sam, On your RCA I don't think it has an AC power supply so you'll need a good strong 6v or 12v battery (like car battery).
Paul, I've gotten to the place where I don't move much of anything unless I'm going to work on it or trip over!
So the L76 has been calling me by name since I moved it to the dining room table about 2 weeks ago. So I put on my bib overalls (this means I mean business). Grabbed my free SG-7 (Heathkit signal generator) that a friend gave me. and started at fixing L76. Well found why I got it for free, it makes rf but no modulation (400cy tone) which makes it a pain to use. After a half hour of looking up the info on it, replaced the filters and found a leaky paper cap that was the issue with the modulation. Now it's up and running properly!
Hooked up the power supply and powered up L76. As I posted before when I power it up seemed like the audio stages where working but the rest was unknown. Connected the generator to the second IF stage to see if it would pass a signal at the IF frequency (456KC). No dice! Then I checked the resistance of the IF transformer. Bummer secondary is open. Popped the cover off the transformer to investigate further. Both winding checked OK. Hmmm. Turns out I miscounted the pins on the detector tube socket where the secondary is connected. Cleaned it up a bit and back together it went.
But why won't pass a signal at 456KC?? Well I let that problem go for the time being and went to the 1st IF stage. Hooked the generator to the input (control grid) and then hook a signal tracer (it's an audio amp with a diode connected to the input so it can turn a RF signal to an audio signal) to grid of the second IF stage. I was rewarded with a weak signal from the tracer. So what do you do when you have a weak signal? You make it stronger, in this case by repeaking the transformer.
After that I went to the plate circuit of the mixer tube. This set has a crystal filter which is used to give this receiver very good selectivity it's connected to the plate of the mixer. Sets without a filter the mixer would feed the 1st IF transformer. I wanted to see of it would pass a 456 KC signal. It did!
So what about the last IF not working but the transformer is OK? Well What I found was the adjusting condensers had oxidized so they where way out of tune (resonance). These IF transformers use small air variables not compression trimmers. The oxidization had formed at the rotor bushing. Out came the De-Oxit and a quick squirt into all the IF adjustments. Repeaked the IF stages and the last one peaked right up like it should (before it wouldn't peak at all). Funny thing they use the same tool to adjust as the Philcos a 1/4 hex nut.
Now it's picking up the generator thru the whole IF strip, detecting and making audio. The audio is distorted I'm thinking there's a bad mica cap in the detector stage.
More later! Stay tuned.
Terry
ps If any are interested a manual for the HRO senior can be found here:
http://bama.edebris.com/manuals/national/hro1935/
