Frank, I'm by no means an expert in restoration of antiques but keep in mind that a silicon diode is far more efficient than a selenium stack. Consequently you may end up with much higher B+ than was originally intended. Also I've run into trouble trying to solder a diode across the terminals of an apparently open selenium stack. The old rectifier can still pass AC if it has any leakage.
I'm probably way in left field on this one but it is interesting. I do know that excessive B+ will cause hum with tubes. It stands to reason that as the volume is increased the B+ to all the tubes would decrease as the audio output tube conducts and pulls down the unregulated B+ line. I would re-check the plate voltages of the tubes in the tuner and preamp sections with the volume turned down. Too much B+ is my first guess. Just my 2 cents, Mark Wentowski Perry Hill Electronics 3836 Harrison Rd. Montgomery AL 36109 phone: (334)279-5622 fax: (334)273-9050 [EMAIL PROTECTED] -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Frank Burnham Sent: Thursday, January 22, 2004 9:28 PM To: [EMAIL PROTECTED] Subject: [Techs] Zenith help needed- Transoceanic H500 Make: Zenith Model H500 In Sams but didn't bring home the # with me tonite. Problem Weird hum Hum like 60 cy- varies with volume. Tune radio on local starion on AM ( Yes there is a problem with AM tuning too, only gets 1 station the local one); When volume control at min massive hum- almost identical to bad filter- in audio. As volume control increased, hum gradually fades and station comes in clear. Several stations on SW come in also. Can inject hum with finger on slider of vol control, and hear the change between it and the hum present. Replaced the main filter cap in the radio. Triple checked the wiring and grounding of the filter against the schematic in Sams. Verified 2x all hooked up right. Checked the filter caps prior to installing. Note- previously the selinium rect was replaced with a silicon diode(- & subbed the diode too). Checked the B+ for ripple- maybe 4 VAC on the input cap and 2V or less max on the output filter. The only thing I can think of is an open gnd, but have ohm'd all of them and even run a jumper wire as a test. Have also replaced all the caps in the volume control/ tone control network and 1 resistor in the IF, which had almost doubled in value. Tubes check ok on store tester Plate and screen voltages appear almost proper on the tubes. What the heck am I missing??? Could this radio be in some strange oscillation? Any help appreciated, Frank Dr. Digital _________________________________________ Please include original text when replying to this email. Once you get your unit repaired, please post the fix to the group. Order your tech repair CD today, Version 3.0 only $49.00-more monitor tips-user friendly program. [EMAIL PROTECTED] www.tech-repair.net ______ Techs mailing list [EMAIL PROTECTED] http://intrepid-video.com/mailman/listinfo/techs_intrepid-video.com Go to the link above and scroll to the bottom to enter your email address _________________________________________ Please include original text when replying to this email. Once you get your unit repaired, please post the fix to the group. Order your tech repair CD today, Version 3.0 only $49.00-more monitor tips-user friendly program. [EMAIL PROTECTED] www.tech-repair.net ______ Techs mailing list [EMAIL PROTECTED] http://intrepid-video.com/mailman/listinfo/techs_intrepid-video.com Go to the link above and scroll to the bottom to enter your email address
