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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Oct Sun 09, 2016 10:00 pm 
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Joined: Oct Wed 22, 2014 6:52 pm
Posts: 84
Location: Alexandria, VA
Hi Folks -

Well, sad to report but John passed away Feb 14th this year. I'm his business/tech partner that now has the chassis on my bench. He did replace the transformer with something "close" but it has more secondary windings than Hector has pups. Right now it's just a heater for my shop (lights are on - gets hot - nobody's home) and in the process of repair, he modified the radio to support a PM speaker. No documentation on why - but that's beside the point.

I did find a broken tube socket for the 51 tube and replaced it. As was stated, when you turned the chassis on it's side it would work (reportedly). When it was in the normal position for playing in the cabinet, the broken socket pin would fall off the tube pin and stop working. I used a ceramic socket from China - it's almost identical. You have to clip the mounting tabs open all the way and use machine screws with a washer. Photos included for amusement/enlightenment (or something like that)

So now, there is no sound coming out - audio dead - and there's only about 12 mv on the primary of the audio transformer. My guess is that there is still a missing B+ issue. At least - after reading all this - that's my best guess.

The transformer puts out 317 v to the 80. But that doesn't seem to propagate anywhere. After reading this post, it looks like I'm back to square one with a lot more work to find out whats going on.

Suggestions are always appreciated.

Steve


Attachments:
Old 51 Tube Socket.jpg
Old 51 Tube Socket.jpg [ 12.57 KiB | Viewed 100 times ]
New 51 Tube Socket - Installed.jpg
New 51 Tube Socket - Installed.jpg [ 26.77 KiB | Viewed 100 times ]
New 51 Tube Socket - Top of Chassis view.jpg
New 51 Tube Socket - Top of Chassis view.jpg [ 23.74 KiB | Viewed 100 times ]
Extra one - just in case.jpg
Extra one - just in case.jpg [ 6.33 KiB | Viewed 100 times ]
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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Oct Sun 09, 2016 11:46 pm 
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Joined: Feb Sun 01, 2009 2:56 pm
Posts: 8209
Location: Victoria, Australia
I can see that a socket like that is of no assistance, based on it, it may not be alone? As I wrote earlier, B+ shorts can be tricky and it is a case of a systematic approach that does not risk cooking something. I normally start a refurbished set with analogue meter on the "B" supply. Should it not go according to plan, its hit the "kill" switch.

You do get faults where as soon as a specific voltage is attained all hell breaks loose (as noted previously). Two tricky ones were a wire cut on a filament wire (flashover) and a recent effort where a filter cap got to around 470V and then started conducting at a hyperbolic rate (500v type).

I would, if it looks like a dead short, perhaps consider lifting the ends of the filter caps, do check if they are correct? lowering the value of the first one on a tube rectifier will reduce the B+ developed. Also check that the speaker is OK (9V batt method) if the secondary / Voice coil are open the transformer will "ring" and generate EHT then flash-over. After that I would, with tubes removed power B+ with a 6 or 12V battery in series with a very low watt (minature) globe/ lamp. If it glows, systematically, from the far end disconnect the B+ legs until it extinguishes (no mains power)

In situations like this I also use my home made reformer, not only to check the ecaps but to power B+ only, in steps (goes from 25V to 400V) to see if there is a breakdown, or flash over. NP caps I use an insulation tester.

It may turn out being something as simple as a miss wire.

Marc


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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Jan Wed 11, 2017 6:55 pm 
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Joined: Oct Wed 22, 2014 6:52 pm
Posts: 84
Location: Alexandria, VA
Hi All -

Well, I'm back at it again and have had no joy. B+ not propagating and still can't figure out why. To complicate things, John replaced the ED Speaker with a PM and added the requisite load for the missing choke and a replacement audio xfmr. At least I think he put it in correctly. What bothers me is that the new xfmr CT on the HV secondary is going to chassis ground - at least that's the way John wired it. According to the schematic it's supposed to go to one end of the choke then to ground through the choke. Across the choke there are two resistors in series with a tap between them that goes to pin 3 of the 47 output tube. There's no plate voltage on the 47 tube so no output. Since the e-caps were replaced long ago, and the schematic doesn't show the polarity, I'm only assuming that the bottom of the ecaps on schematic are the negative sides.

Also, the old ED speaker is long gone so I can't check the choke resistance. One schematic shows a 100K value for the choke but don't know if that's inductance or resistance.

Any thoughts?

Steve


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 Post subject: Re: Zenith MH 80 B+ short?
PostPosted: Jan Thu 12, 2017 12:09 am 
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Joined: Feb Sun 01, 2009 2:56 pm
Posts: 8209
Location: Victoria, Australia
A choke / field cab be found in ether of the B+ or B-. It is the latter that causes problems. There is a set I have seen posted recently with the same back bias setup ( Field & parallel resistance in B-).

I cannot see a field being below 500 Ohms? I think that may be 1000 Ohms field and if there is a back bias resistor to ground that might be 100 Ohms. The back bias if connected to the control grid of the OP tube is normally CT to chassis (Chassis positive) the grid bias voltage of the OP tube. If you add all of the screen & plate currents, which you may have to take from the tube data (unless data is on a schematic) You should get an approximation of the sets current through the back bias. The voltage across the back bias resistor /resistance will tell you the current draw.

It is unwise to turn a set with a 5Y3 sideways And that may also apply to an #80: They can short. In a B- back bias system the norm is both caps B+ to filament / cathode; First cap Neg to CT, second to ground. It is not unusual with particularly Philips to not ground the second cap & have a 25uF cap across back bias: Positive to chassis.

Essentially; This qualifies for analysis purposes as an unknown set (without intending disrespect) that has been messed with. You have no option but to go back to square one and check everything. A 9V battery "cracked" across the primary of the OP transformer will give an indication to it functioning. No plate voltage does not necessarily mean its open, I have seen ISO types short to frame.

One of the things I have done with an obscure "dead shorts" is to put a small wattage (say 12V) "grain of wheat globe" in series (with an appropriate battery) to replace B+ & not cook anything. You then, if your lucky, & it glows, "systematically" starting at the end of the supply chain, (RF) you disconnect B+ rails until the light extinguishes. That can ping the short to an area.

If you attack this in a systematic way you will fix it. Clearly is is wiring & Power supply errors first.

I note that the original circuit bit posted shows a CT Heater transformer. That means that all of the heaters & likely dial globes float above the chassis make sure that is so & is one reason I suggested removing tubes to make sure there are no heater cathode shorts. Where possible I run them through a tube tester.

Marc


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