Resistors masquerading as capacitors
Apr 22, 2020 16:54:56 GMT 12
Steve, elorens, and 1 more like this
Post by trombone on Apr 22, 2020 16:54:56 GMT 12
I've been working on a Stewart Warner Model 136 "Ferro dyne" from about 1936.It goes well but the hum level was getting high so I thought I'd better take a look.
In this set the centre tap of the H.T. winding is earthed through a chain of three resistors, from the junctions of which three tappings are taken to provide bias for the 6F6 , the A.V.C. line and the 6F5 which is the first audio valve.The two old wet electrolytics were still mounted on the chassis,the tag of one still serving as a tag, the capacity long since having departed.Using these tags on old electrolytics is bad practice and I made a note to myself to put in a proper tag. Using the tag on the electrolytic was and is standard practice but it seems Stewart Warner weren't keen on using tag strips in general so vacant valve socket pins were often employed as tags.
The more immediate concern was the H.T. centre tap wire from the transformer. As mentioned above it doesn't go direct to earth. It was soldered to a vacant tag on the 5Z4 rectifier socket, as was the tapping wire leading away to the bottom of the 6F6 bias resistor and the "earth " lead of the first electrolytic. Also from this vacant tag on the rectifier socket was the first of the three resistors leading to earth , in this case a 270 ohm porcelain and metal ended beauty which measured correctly.So far so good.
And the resistance measured from the top of the 270 ohm to earth was the correct 313 ohms. (270 + 18 +25 = 313 ).
And here was where the fun started.
From the bottom of this 270 ohm resistor the resistance to earth read 43 ohms, again exactly correct. But the bottom end of this 270 ohm resistor was soldered to a tag from which one wire led away to the A.V.C. line -and there was no visible chain of two resistors leading from there to earth. All there was, attached to this tag, was a brown moulded mica capacitor,presumably also being earthed through this non existent chain of two resistors.Where were these other two resistors ?
Exasperation is a very good word so I'll use it to sum up my mood here.Exasperation is a word with its origins in latin and is so much more genteel than some of the anglo saxon words which were imminently suggesting themselves. Eventually I cut the wire which went to the A.V.C. line thinking that may have led mysteriously to the 43 ohms of resistance to earth. No change.
Okay, get serious. Unsolder that brown mica capacitor.Maybe it is leaky. Guess what - it measures 18 ohms .The correct 18 ohms . And from the tag to which the other end of this brown we- are- beginning- to- suspect "resistor in capacitor clothing " was soldered is one of those quaint flexible resistors reading a commendably stable and correct 25 ohms.
What was pretending to be a mica plastic bodied capacitor was in fact a "Micamold " resistor made in the U.S.A. It is Type 800 and the company who made it were obviously so pleased with it they patented it.I have it by the computer as I type this. It is slightly smaller than a normal plastic mica cap and slightly longer and narrower and has the usual enigmatic coloured dot code (which normal people associate with a value in microfarads !) with an arrow moulded in to tell us in which direction to read it.I have never come across one before.
Perhaps I should get out more. But ,you know, Level 4 and the bubble. But,yes , this particular style of resistor was a first for me, something else to remember the lockdown by.
Cheers everyone.
Clearly others know about this . Google "micamold " resistors.The values of resistors mentioned in this post have been rounded to make the story clear. Overall they were sufficiently close to not warrant mentioning the difference.
In this set the centre tap of the H.T. winding is earthed through a chain of three resistors, from the junctions of which three tappings are taken to provide bias for the 6F6 , the A.V.C. line and the 6F5 which is the first audio valve.The two old wet electrolytics were still mounted on the chassis,the tag of one still serving as a tag, the capacity long since having departed.Using these tags on old electrolytics is bad practice and I made a note to myself to put in a proper tag. Using the tag on the electrolytic was and is standard practice but it seems Stewart Warner weren't keen on using tag strips in general so vacant valve socket pins were often employed as tags.
The more immediate concern was the H.T. centre tap wire from the transformer. As mentioned above it doesn't go direct to earth. It was soldered to a vacant tag on the 5Z4 rectifier socket, as was the tapping wire leading away to the bottom of the 6F6 bias resistor and the "earth " lead of the first electrolytic. Also from this vacant tag on the rectifier socket was the first of the three resistors leading to earth , in this case a 270 ohm porcelain and metal ended beauty which measured correctly.So far so good.
And the resistance measured from the top of the 270 ohm to earth was the correct 313 ohms. (270 + 18 +25 = 313 ).
And here was where the fun started.
From the bottom of this 270 ohm resistor the resistance to earth read 43 ohms, again exactly correct. But the bottom end of this 270 ohm resistor was soldered to a tag from which one wire led away to the A.V.C. line -and there was no visible chain of two resistors leading from there to earth. All there was, attached to this tag, was a brown moulded mica capacitor,presumably also being earthed through this non existent chain of two resistors.Where were these other two resistors ?
Exasperation is a very good word so I'll use it to sum up my mood here.Exasperation is a word with its origins in latin and is so much more genteel than some of the anglo saxon words which were imminently suggesting themselves. Eventually I cut the wire which went to the A.V.C. line thinking that may have led mysteriously to the 43 ohms of resistance to earth. No change.
Okay, get serious. Unsolder that brown mica capacitor.Maybe it is leaky. Guess what - it measures 18 ohms .The correct 18 ohms . And from the tag to which the other end of this brown we- are- beginning- to- suspect "resistor in capacitor clothing " was soldered is one of those quaint flexible resistors reading a commendably stable and correct 25 ohms.
What was pretending to be a mica plastic bodied capacitor was in fact a "Micamold " resistor made in the U.S.A. It is Type 800 and the company who made it were obviously so pleased with it they patented it.I have it by the computer as I type this. It is slightly smaller than a normal plastic mica cap and slightly longer and narrower and has the usual enigmatic coloured dot code (which normal people associate with a value in microfarads !) with an arrow moulded in to tell us in which direction to read it.I have never come across one before.
Perhaps I should get out more. But ,you know, Level 4 and the bubble. But,yes , this particular style of resistor was a first for me, something else to remember the lockdown by.
Cheers everyone.
Clearly others know about this . Google "micamold " resistors.The values of resistors mentioned in this post have been rounded to make the story clear. Overall they were sufficiently close to not warrant mentioning the difference.
