I have replaced the mech. synchronous vibrator with a AAR 1015N and inserted two NTE506 1400 V PIV diodes on the xformer secondary, high to low, and grounded the center tap. In order to increase the setting time of the hv supply I inserted a 150k resistor to ground on the output of the diodes. Without the resistor the startup overshoot of the supply was greater than 500 V and did not settle for over a minute. With the resistor the supply voltage overshoots to about 335 volts and settles in 3-4 seconds (I have 350 WV 10 mfd filter caps). With the supply now modified I measure about 217 volts and have about 0.7 volts of 115 Hz ripple, the Rider spec is 220 volts, and draws about 6.5 amps total warmed up.
Next I aligned the IF and tuned the antenna, seems only minor adjustments were needed. I then hooked up a copper 40 ft antenna wire. The radio does tune however here are the issues: (All paper and electrolytic caps replaced)
1) There is noticable hum and most significantly at the lower end of the frequency band.
2) Below about 1 MHz I cannot tune any station to intelligable level.
3) The sensitivity seems a little mushy, but this could be normal.
I seem to recall reading on a website somewhere that the above is a symptom of a specific component problem (weak rf amp ?). Can someone give me a hint if this is the case, the FAQ did not have a discussion. I have not used a tube tester but my impression is that it may not help if the problem is cathode related.
Also is there a brief general description on a website somewhere of how to methodically analyze each section of the radio, using an oscope, to find the source of hum.
One last question, what resistance values would one
use to build a suitable oscope voltage divider to probe
the hv rails. Seems like a 10 meg series and 500 k
parallel should do it.
As always your help is most appreciated.
Regards, Steve Schreppler
A radio like that should have a padder cap to adjust for the low end of the band. It will look like a trimmer but larger, mounted on the chassis. Tune in a station at the low end, adjust the padder while rocking tuning back and forth. Adjust for strongest signal.
You shouldn't need 40 feet of antenna. Most car radios have a trimmer to match antenna to the radio. Peak it up if you haven't already.
Don't think you have a tube problem. Do you have hum with volume turned down? Not sure how the solid state vibrator was made but it may have fast switching speeds that need an oil/paper cap to filter. A .1 mfd across electrolytics may help. Did you replace the buffer cap?
Since you aren't working with high frequencies when checking the supply, a voltage divider, like you mention, will work.
Norm
: I am back, again, for some more advice and wisdom. Same project, I am restoring a United Motors model 985400. This radio was installed in a 1935 Chevrolet Master Sedan and is powered by a 6 volt battery system.
: I have replaced the mech. synchronous vibrator with a AAR 1015N and inserted two NTE506 1400 V PIV diodes on the xformer secondary, high to low, and grounded the center tap. In order to increase the setting time of the hv supply I inserted a 150k resistor to ground on the output of the diodes. Without the resistor the startup overshoot of the supply was greater than 500 V and did not settle for over a minute. With the resistor the supply voltage overshoots to about 335 volts and settles in 3-4 seconds (I have 350 WV 10 mfd filter caps). With the supply now modified I measure about 217 volts and have about 0.7 volts of 115 Hz ripple, the Rider spec is 220 volts, and draws about 6.5 amps total warmed up.
: Next I aligned the IF and tuned the antenna, seems only minor adjustments were needed. I then hooked up a copper 40 ft antenna wire. The radio does tune however here are the issues: (All paper and electrolytic caps replaced)
: 1) There is noticable hum and most significantly at the lower end of the frequency band.
: 2) Below about 1 MHz I cannot tune any station to intelligable level.
: 3) The sensitivity seems a little mushy, but this could be normal.
: I seem to recall reading on a website somewhere that the above is a symptom of a specific component problem (weak rf amp ?). Can someone give me a hint if this is the case, the FAQ did not have a discussion. I have not used a tube tester but my impression is that it may not help if the problem is cathode related.
: Also is there a brief general description on a website somewhere of how to methodically analyze each section of the radio, using an oscope, to find the source of hum.
: One last question, what resistance values would one
: use to build a suitable oscope voltage divider to probe
: the hv rails. Seems like a 10 meg series and 500 k
: parallel should do it.
: As always your help is most appreciated.
: Regards, Steve Schreppler