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 Post subject: Zenith MH 80 B+ short?
PostPosted: Feb Fri 01, 2013 9:37 pm 
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I have a zenith MH benched with a Short from B+ to ground. With the 80 out of socket and both electrolytics[new] disconnected, I still get a short. See schematic here-

http://www.nostalgiaair.org/PagesByModel/369/M0043369.pdf

That leaves a short to ground from the transformer or the socket. Are these older 80 sockets prone to ground shorts? [The transformer is a new one that I installed and has been working satisfactorily for months. ] EDIT - also with the unit on a dim bulb, vibration can turn the radio on or off - sound wise, or alternately make the resistance from B+ to ground go to zero.]


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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Fri 01, 2013 10:34 pm 
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Shut it down and get a new tube socket and a sure 80 as well. No need to blow up transformer.


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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Fri 01, 2013 10:46 pm 
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codefox wrote:
Shut it down and get a new tube socket and a sure 80 as well. No need to blow up transformer.

Disconnected the 5v transformer leads. No path to ground with leads. So that leaves the socket. NICE.....Now who sells 80 type sockets?

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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Fri 01, 2013 11:02 pm 
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Try radio classified forum for socket. Sounds like you ay not tube too, but same place will get you both. I don't sell parts at present time.


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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Fri 01, 2013 11:13 pm 
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As long as I have to change out the socket - is there any reason not to convert this radio to use a 5Y3 while I am at it?

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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Fri 01, 2013 11:18 pm 
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The 5Y3 and 80 are electrically identical, so the 5Y3 should work fine.

:wink: Larry

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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 1:17 am 
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JohnOBrien wrote:
With the 80 out of socket and both electrolytics[new] disconnected, I still get a short.
That leaves a short to ground from the transformer or the socket.
???

How did you get from sentence one to sentence two???

A short with the rectifier and filter caps removed does not in any way suggest a problem on the transformer side.
The short could be anywhere in the entire B+ distribution line in the radio.

- Leigh

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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 1:28 am 
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...


Last edited by init4fun on Jun Mon 17, 2013 3:10 am, edited 1 time in total.

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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 2:38 am 
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I had a similar issue with a radio and found it was a somewhat hidden rubber coated wire in the B+ circuit I'd missed had shorted to ground.

I assume your meant you are sill finding the short via resistance check to ground/chassis with the 80 removed.


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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 2:47 am 
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init4fun wrote:
Um , Leigh , with all due respect , how can a rectifier that's been pulled from it's socket power up the "entire B+ distribution line in the radio" ?
Once that Type 80 has been pulled , I'd darned well expect not a whiff of B+ anywhere in the radio ?
So how is it possible that , for instance , an IF transformer primary shorted to ground , it would still load down the transformer with the rectifier not even in the circuit ?
Huh??? What are you talking about?

The question is about a short from the B+ line to ground. My point is that the short could be any place.

Somehow the OP got focused on replacing the rectifier socket, but I see no evidence that the socket is bad.
The problem could be anywhere on the B+ distribution line in the set.

Nowhere did I suggest "powering up" the radio, nor did I use that phrase.

- Leigh

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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 3:12 am 
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B+ extends from transformer high voltage winding to plate of the output tube.
A short anywhere along this path will appear as a short along the whole path.
A short at the socket is possible but is less likely then elsewhere along the chain.

Exactly where are you taking the measurement along the B+ buss ?
What is the nature of the short ? Low B+ volts or low Ohms ?


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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 5:08 am 
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Disconnected the 5v transformer leads. No path to ground with leads. So that leaves the socket. NICE.....Now who sells 80 type sockets?[/quote]

John, that pretty much says it all, the socket. Might be fun to check the pins for the filament line to ground while it is in that state as a confirmation. Got to be there.
Jerry

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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 5:34 am 
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If this is still an issue, you can easily replace the socket with a good quality ceramc octal socket and 5Y3 tube if you want to Just make sure the underlying causes for the initial failure i.e. bad filter capacitors in votage dividers or down hill made the intitial failure. Best of luck.


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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 6:07 am 
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John, I might add that those pesky cheap sockets especially on rectifiers are very sensitive to the intrusion of cleaning stuff while getting the chassis looking nice. After a couple of failures on rectifier sockets I learned to really cover them well (and all sockets) with tape prior to going after chassis cleaning. Never had a dead short but they will certainly emit the magic smoke nicely if they get anything like navel jelly on them. No matter how much you clean them out. Steel wool typically will give you some nice sparks and pops.
Just my limited experience.

Jerry

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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 7:20 am 
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Jerry,

What makes you think there's a problem with the socket?

Have you disconnected the lead from the socket to the first filter capacitor to confirm a socket problem?

- Leigh

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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 4:56 pm 
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Dale Saukerson wrote:
B+ extends from transformer high voltage winding to plate of the output tube.
A short anywhere along this path will appear as a short along the whole path.
A short at the socket is possible but is less likely then elsewhere along the chain.

Exactly where are you taking the measurement along the B+ buss ?
What is the nature of the short ? Low B+ volts or low Ohms ?



:wink: Dale has said it perfectly . It is not clear from the OP's description exactly how and where he is measuring this short circuit , so any further ideas will have to be based on the next info from the op .

PS , were this mine , my first checks would be ;

With ALL tubes out of their sockets , does it pass the dim bulb test ? If not , the only two possibilities I see could be a shorted transformer or possibly shorted rectifier socket . If it passes the no tube dim bulb test , put all the tubes except the rectifier back in and assure the dim bulb is still ok . Then , I would remove the B+ lead coming from the rectifier's filament (the one that leads into the B+ filtering network) and with the rectifier in place again try the dim bulb . If it doesn't draw excessive current , your short will be further downstream of the B+ path . If it does draw excessive current , again I'd suspect the transformer or socket . So , moving along , if it passes the rectifier in place and B+ feed line to the filters removed test , it's time to troubleshoot further down the B+ line . A "divide and conquer" approach could be applied here in that the next step would be to put the B+ lead back onto the rectifier and , leaving the B+ connected to the audio output stage , open the B+ going to the rest of the circuits and again dim bulb it . If it passes the dim bulb here , your filters and final audio stage are fine and the short is again still further down the B+ line . By successivly removing tube stages from the circuit , working from the final audio all the way back to the receiver's "Front end" , you'll eventually isolate the stage where the short resides . I mention the dim bulb because I see that the OP has mentioned it's use in post one , and so figuring the Op is comfortable using the dim bulb method , the approach I've listed above will isolate the defective area of the circuit ....


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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 7:06 pm 
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Thanks for all the replies. I am measuring 1-2 ohms from the filament lead on the 80 [B+] And Leigh is correct - the short can be anywhere on the B+ line. Before I order a socket, I am piecemeal going to disconnect all B+ connections to see if I can narrow this down. The short goes in and out with thumping the chassis. I still tend to suspect the socket, because one time I put pressure on the 80 filament tab and caused it to go into short mode, but I need to nail this down before I go any further.

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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 8:41 pm 
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You need to follow the short, like a string of Christmas tree lights, starting with the B+ lead on the 80 socket.

Lift a wire and check its open end to chassis. If the short exists, go to the other end of that wire, lift all the wires on that node, and check each, following the one that has the short, etc.

We just went through this same exact exercise last week. It turned out to be a shorted .047 bypass cap.

- Leigh

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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sat 02, 2013 11:21 pm 
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I'm sorry Leigh is correct. When you said you lifted the 5V lines from the socket I assumed you also lifted the line from the filament to the first cap. See if the short exists on that line (the B+). Then if it is not there, measure the empty socket for the 80, filaments to ground. It has to be either in the tube socket or along the B+ line you lifted from the socket.
Good luck.

Jerry

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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Feb Sun 03, 2013 12:50 am 
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It may pay to browse the latter part of last weeks "short circuit" exercise. there are lessons to be learned.from it. One being in respect of a systematic approach.

As was trying to be communicated: The only way to to be absolutely sure there are no shorts on a suspect socket, (ie to ground or between pins) is to not have anything connected too. or in it. As was pointed out here, and in the last related posting, one method is to systematically disconnect everything attached to B+ either at once. or preferably one item at a time until the short dissappears on a particular rail. It could be any form of wire short, a component short, miswire, even a wire strand, bit of stray solder, or an under rated cap punching through. New parts can fail.

With a systematic approach the cause area should be located in minutes. While it may only have a minor, or insignificant, effect at LV, or none with a DVM (perhaps), do apply the right polarity to the rails. Anologue meters on ohms range, in most cases reverse the terminal polarity.

It may pay to pull all of the tubes, on the remote, but not unheard of chance that one of them is shorted /shorting

Marc.


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