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


It is currently Dec Sun 15, 2019 7:36 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 29 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: Another VTVM question... rectifier
PostPosted: Nov Sat 16, 2019 6:18 pm 
Member

Joined: Jul Sun 21, 2019 3:32 pm
Posts: 118
Location: Upstate NY
Ahhhh.. so the higher capacitance simply produced a smaller ripple, and higher voltage..Thank you!


Top
 Profile  
 
 Post subject: Re: Another VTVM question... rectifier
PostPosted: Nov Sat 16, 2019 8:19 pm 
Member

Joined: Jul Sun 21, 2019 3:32 pm
Posts: 118
Location: Upstate NY
I also measured the voltages at pins 1 and 6 of the 12AU7A tube, and they are at 88 VDC within a few tenths of a volt of each other, The only thing I can find is the R4 9.75Ohm measures 11.5 in circuit..

Before I lift one end, I am trying to understand the bridge circuit to see if a higher resistance would cause the meter to favor one side of the dial and max out the zero and ohm knobs to compensate.


Top
 Profile  
 
 Post subject: Re: Another VTVM question... rectifier
PostPosted: Nov Wed 20, 2019 9:41 am 
Member

Joined: Jul Sun 21, 2019 3:32 pm
Posts: 118
Location: Upstate NY
Still unable to get full right deflection of the needle on ohms without maxing out zero and ohms knobs. Could a bad cap be causing this issue?
I think I checked all the resistors in the circuit, the megohms are within 5 % and my meter is probably only good to 5% at best. Running out of Ideas at this point..
Thanks
Mike


Top
 Profile  
 
 Post subject: Re: Another VTVM question... rectifier
PostPosted: Nov Wed 20, 2019 12:54 pm 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10680
Location: Victoria, Australia
Vacuum tube voltmeters & vacuum tube anything, use the same caps & resistors as radios but normally better quality and tighter specs. If this thing has paper caps toss the lot. A leaking cap is a resistor: It will have a substantial effect on accuracy as it can have a substantial & unpredictable effect on bias.

The better quality stuff uses 1% resistors & the odd ball values may be either specially made, or some Bunny got a bucket full of duds to sift through. The generalisation for rough checking resistors in tube circuits; Is that if it is "high" in circuit it's 99.9% likely its a dud. From my position R4 has to go, its possibly been cooked.

If its a Wheatstone bridge (? Circuit) then if parts are substantially out of tolerance it will not balance.

Marc


Top
 Profile  
 
 Post subject: Re: Another VTVM question... rectifier
PostPosted: Nov Sat 23, 2019 12:30 am 
Member

Joined: Jul Sun 21, 2019 3:32 pm
Posts: 118
Location: Upstate NY
First, Thanks for the help! I have made some progress...(I think..) I did not find any other bad resistors in the Ohms circuit, and replaced C5 as it was dark brown. Whatever I disturbed, was good! I now get full deflection left to right when moving from dc+ to Ohms! Now the needle has NO REACTION when I short the probe and ground together. I checked the probe switch, and it seems good, switchable to 1MEG. I'm thinking it might be a missing connection between chassis and common? anybody???
Thanks
Mike


Top
 Profile  
 
 Post subject: Re: Another VTVM question... rectifier
PostPosted: Nov Sat 23, 2019 2:19 am 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10680
Location: Victoria, Australia
Now if one replaced C5 and that which was once awry is now good: Then there is a high probability that C5 was a dud.

There are with capacitors two tests. One is a capacitor tester that can read the capacity value to see if its in spec. More important with all but electrolytic type caps is leakage. When DC is applied no current should flow after it is charged. The only accurate assessment with tube apparatus caps in my book, is to use an insulation type meter (or cap tester) that can measure leakage at the test or operating volts of the cap.

At this point in time and due to their construction, it is extremely rare to find a waxed paper & some oil filled types that do not leak & that's why many of us just toss them on sight. As I was suggesting before, if you can identify a cap as being waxed paper: "Get rid of it".

The conventional electrolytic is chemical & relies on leakage to maintain its polarity (Form). There is such a thing as too much leakage and that is exacerbated by long periods without use; To the point they can present as a short circuit. This often manifests as expensive smoke as something unchecked is powered to see if it goes.

So as just another phase of repair, one needs to get an understanding of the components and their function.

Marc


Top
 Profile  
 
 Post subject: Re: Another VTVM question... rectifier
PostPosted: Nov Sat 23, 2019 2:40 am 
Member

Joined: Jul Sun 21, 2019 3:32 pm
Posts: 118
Location: Upstate NY
Thanks Marc,
C5 was a ceramic cap as are all the rest. I went thru the circuit until all the obvious culprits were checked ok. Not sure if the ceramic was bad, or I jostled something around, but, I'll take my small victory.. I am hoping to find no continuity when I look again, but I doubt it is that simple...
Mike


Top
 Profile  
 
 Post subject: Re: Another VTVM question... rectifier
PostPosted: Nov Sat 23, 2019 8:59 am 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10680
Location: Victoria, Australia
Here, from experience, one learns from statistics that there are caps like Mica and Ceramic that are rare failures and certain types of construction, waxed paper, oil filled, Micamold anything and electrolytic, plus a few like NPO and some axials around 3/8" & 1/2"or so used in transitorised stuff, that just cannot be trusted & there is no component that can't fail. I have actually replaced two sockets recently: Even they can fail.

Never, has the "dry joint" (Corrosive, badly soldered, or not at all) fallen into disuse in any electrical, or electronic device. Its still really popular.

Marc


Top
 Profile  
 
 Post subject: Re: Another VTVM question... rectifier
PostPosted: Nov Sat 23, 2019 4:14 pm 
Member

Joined: Jul Sun 21, 2019 3:32 pm
Posts: 118
Location: Upstate NY
WOOT! WOOT! I found it! Working like a champ now! (Uncalibrated) but responsive on all functions! I must have twisted the probe, and it would not release the 1 meg resistor... :oops: thanks everyone!
What a trial! 4 resistors, one cap, and one tube... 8)


Top
 Profile  
 
Post New Topic Post Reply  [ 29 posts ]  Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: Pierrot du 82, w3jn and 8 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB