Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Sep Sun 22, 2019 7:40 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 7 posts ] 
Author Message
 Post subject: sylvania 302 polymeter with Hot chassis
PostPosted: May Wed 29, 2019 11:52 pm 
New Member

Joined: Jan Thu 03, 2019 3:17 am
Posts: 15
I have a sylvania 302 polymeter with a hot chassis (62 Volt AC to earth ground). No matter how I plug it in and even with an Isolation transformer I have the same problem. Any help will be greatly appreciated


Top
 Profile  
 
 Post subject: Re: sylvania 302 polymeter with Hot chassis
PostPosted: May Thu 30, 2019 2:27 am 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 37478
Location: Livermore, CA
A capacitor from each side of the AC line to chassis will give that reading.

_________________
Norm


Top
 Profile  
 
 Post subject: Re: sylvania 302 polymeter with Hot chassis
PostPosted: May Thu 30, 2019 6:25 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20214
Location: Warner Robins, GA
To test if that voltage is a problem put a low value resistor across your meter probes then measure from chassis to ground.

If the voltage is zero then all is good. If however the resistor starts to get warm or smokes then there's a problem.


Top
 Profile  
 
 Post subject: Re: sylvania 302 polymeter with Hot chassis
PostPosted: May Fri 31, 2019 1:07 am 
Member

Joined: Jan Tue 16, 2007 7:02 am
Posts: 3092
Location: Lexington, KY USA
A manual:
http://bama.edebris.com/manuals/sylvania/302

It might be useful to measure the available AC current from chassis to ground with the power plug inserted each way.

Be careful in case there is a fault. The acceptable value might be as much as a few mA.

The schematic in the manual on BAMA shows no capacitors on the AC power connections, but production changes or later mods may have resulted in your example having them.

With no separate AC line capacitors, you still have the power transformer's internal capacitance, and strays, so a sensitive AC voltmeter might well still show about 1/2 the line voltage from chassis to ground.

If there are actual capacitors, they might be just regular paper capacitors, which often fail in very unsafe ways.

Has this unit been serviced? There might be several bad caps and drifty resistors in there, given the age.

If there are capacitors from the AC line to the chassis, they can often be removed without causing problems.

If you need or want such capacitors, use safety rated caps. Put a larger Class X cap across the power transformer primary, and a smaller Class Y cap from the unswitched side of the line to the chassis. If you add a polarized cord, this side of the line should connect to the wider N prong on the plug. That should minimize the AC on the chassis.

Ted


Top
 Profile  
 
 Post subject: Re: sylvania 302 polymeter with Hot chassis
PostPosted: Jun Sat 01, 2019 8:57 pm 
New Member

Joined: Jan Thu 03, 2019 3:17 am
Posts: 15
Thank you for the help, all of you. The meter has not be service and does not have a cap on the power lines. The meter does not zero on any function or range. Could this be part of the hot chassis problem?


Top
 Profile  
 
 Post subject: Re: sylvania 302 polymeter with Hot chassis
PostPosted: Jun Tue 04, 2019 1:38 pm 
Member
User avatar

Joined: Jul Mon 08, 2013 2:48 pm
Posts: 360
Airwavesantiques wrote:
Thank you for the help, all of you. The meter has not be service and does not have a cap on the power lines. The meter does not zero on any function or range. Could this be part of the hot chassis problem?


Hard to tell, but a hot chassis may introduce hum in the circuit which could result in a residual indication on the meter which can't be nulled out (zeroed).


Top
 Profile  
 
 Post subject: Re: sylvania 302 polymeter with Hot chassis
PostPosted: Jun Tue 04, 2019 3:31 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 4110
Location: Littleton, MA
Airwavesantiques wrote:
Thank you for the help, all of you. The meter has not be service and does not have a cap on the power lines. The meter does not zero on any function or range. Could this be part of the hot chassis problem?

Before trying to troubleshoot any further, you should replace the electrolytic and paper capacitors. Not replacing the electrolytic capacitor risks burning out the irreplaceable power transformer. Not replacing the paper capacitors means that problem diagnosis will be confounded by multiple faults caused by electrical leakage in the capacitors.

How are you measuring the chassis voltage? With a DMM or VTVM? The high input impedance of those kinds of meters will give false indications of "phantom" AC voltages caused by small capacitances, either parasitic capacitances like that between the power transformer primary winding and the chassis-grounded electrostatic shield, or any line filter capacitance that might be in the circuit.

As mentioned before, you should measure the chassis voltage either with a VOM with a 5K Ω/volt AC rating, or with a DMM or VTVM by placing a 100K Ω resistor in parallel with the meter. Until you've done that measurement, there is no reason to believe that there is an actual hot chassis problem.

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
Post New Topic Post Reply  [ 7 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 3 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB