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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Fri 01, 2017 11:40 pm 
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There's a remote possibility that a Candohm section could be shorting through the fish paper insulation to chassis? The time delay after turn on makes me think thermal breakdown of insulation. Considering there was the other b+ short, that Candohm has likely gotten pretty well cooked.


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Sat 02, 2017 1:22 am 
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Location: alameda,CA
I'm about ready to throw this thing down a well... So I fixed the issue with the dropping voltage. There is a large 1500 ohm dropping resistor coming off the rectifier and that thing was shorting out slightly as it warmed up. Now with that fixed I realize there is NO radio reception. I'm putting aside for the evening. Stupid set better sell for $1,000 at this point...


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Sat 02, 2017 1:40 am 
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Even with a heater rectifier nothing will be stable until the heaters have reached their operating point. I have had situations where a dud electrolytic cap will get to "a voltage" and then start to conduct at a hyperbolic rate. This as I have said several times , is why I built a reformer that has provision for current measurement, into the bench power supply: Its to catch them.

Candohms do not have a good reputation when it comes to reliability. Having it overloaded it no help. If I suspect an insulation fail, in that sort of resistor, it is a case of taking all of the wires off it so that it's resitor/s float, then using an insulation tester at say 500V to see if its gone to ground.

In many cases it is not a good idea to run without the speaker. Often the screen is left powered, & that can damage a Pentode. In others, where the VC or secondary of the OP transformer are open, that can cause the transformer to "ring" which generates EHT, which will flash over.

I note the comment re resistor. I would review how that happened & perhaps replace, or shift it. Now that you have B+ there is actually a better chance of getting radio reception. Starting by measuring element voltages to see if they have any and they are right?

Marc


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Sat 02, 2017 1:50 am 
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I too wondered about the Candohm but even after it heats up it seems to test perfectly within spec. The electrolytics as well as all other caps have been replaced. I believe the voltage drop was a defect because I could hear a distinct crackle and then fade in the audio. Now the voltages seem to act accordingly with an initial surge then a leveling off. I did not think about having removed the speaker from the set and its effects on the tubes. At that point I wanted to confirm the transformer itself wasn't shorting.

But I will re-test my tubes. One I recall was marginal at best. Perhaps yes- one or more was compromised. Will proceed now...


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Sat 02, 2017 4:10 am 
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If I think I have a transformer short AC side the first thing out is the rectifier as it is implicated HV side. With a filament rectifier B+ & CT is disconnected. Then you can use an insulation tester. The 5V winding should not ground, but I have seen it with a cut filament wire: That why one tests it. It has B+ on it when running.

I always test winding to winding as well as to ground to ensure one winding is not leaking to another (cooked).

Logical sequence after voltages, is to then move to the audio. Fixing it backwards is the way to move forward. Invariably the signal has to move from antenna to speaker: Never going to happen if something in the middle is dead. Once the voltages are sorted, I use a Signal Generator & Oscilloscope at IF frequency, as I have found that a fast way of finding a problem area. If its dead on the signal grid of the Pentagrid I move to the last IF. That gives a quick indication of it being it or the audio. If it works I calibrate. The oscilloscope is great for spotting signal & whee it stops, plus spotting distortion.

Marc


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Mon 04, 2017 7:18 pm 
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Well... It now is getting closer to being a working radio.
The reasons for it not working were a mixture of my tube tester now catching an issue with the 6F6 output tube a well as my own stupidity.

So the B+ voltage drop was caused by the 6F6 having a sort of internal short that my tube tester for whatever reason didn't indicate at all. Replacing that with a known tested good one solved that issue.

As far as lack of radio reception? That was caused by me. When I replaced some of the rotten wires going to one of the RF cans I'd somehow soldered the new lead to the wrong lead. How I managed to do that is beyond me. But anyway soldering that to the correct lead solved that problem and now the radio works fine.

I still have issues to deal with: The antenna loop has all of its wiring as nasty rotten wiring and needs replacement and the tuning eye, which was working stopped because I managed to jumble up the leads to that when I took the socket apart to replace rotten wires.


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Mon 04, 2017 8:39 pm 
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Excellent Bob it sounds like it will once again be a nice playing old radio. Caution when replacing the wires from the antenna to the plug. Be sure you don't change the length of the wires. On the eye tube socket don't forget you looking at the top of the socket when you think about getting the right ones back into the right little holes in the socket.

John k9uwa

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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Mon 04, 2017 10:20 pm 
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A word of caution for all:

One of the things that can, and will happen (seen several examples), in particular with Tetrodes & Pentodes used in the Output, is damage from the loss of Plate voltage (open). This can be caused to happen when a speaker plug is left out and is wired such that the screen remains powered.

Invariably the screen then becomes the plate: The screen is not designed to handle plate current and will fail.

One of the reasons we take photo's before messing with a set (even if it is wrong) is for "Senior moments". These photos must be of a high quality and "auto focus" is not always your friend.

These good photos are your layout diagram for referring back too, to see where things were, before you put them somewhere else. I sometimes put a paint marker dot on the node I took something from, before the (frequent) interruption / distraction.

Good to see progress.

Marc


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Tue 05, 2017 1:56 am 
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Well, its back together now. Replaced the rest of the crappy wiring, fixed eye tube wiring, ran a line-in for audio and a switched power AC supply off switch for addition of bluetooth module, re-strung dial, oiled all the bearings and put it back together. Surprised at how bassy it is for having what appears to be a small speaker. Must be good cabinet design.

Image
Image


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Tue 05, 2017 6:03 am 
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Looks good. That should bring enough $$$ to the club that all your efforts were worth it!

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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Tue 05, 2017 12:46 pm 
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The size of the speaker cone was often to get efficiency and the cubic volume of the area surrounding it was rarely a happening : They were matched.

Marc


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Tue 05, 2017 11:03 pm 
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Quote:
Looks good. That should bring enough $$$ to the club that all your efforts were worth it!


Ha ha! Doubt it on this one. Oh well, every once in awhile a problem set comes in and takes way more time than intended. Sadly the market for these 40's sets has fallen out and so we won't get much for it to speak of. But at least it won't be tossed into the garbage.


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Tue 05, 2017 11:05 pm 
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I think a Zenith console will still bring in some $$$. Lots of other brands -- not so much.

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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Wed 06, 2017 1:33 am 
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Too bad I live across the country. I'd be interested just because of the cabinet. My radio works great, but the cabinet is missing a lot of the photo finish.

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People may not remember how fast you did a job, but they will remember how well you did it.


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Wed 06, 2017 6:42 pm 
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I'm not sure why but these days we have a really difficult time moving these. This one might bring mayybbbeeee $150 on a good day, bluetooth and all and only because at least it has a sort of cool deco style. I really like this model though so anytime one gets donated I generally jump on it. Usually they're cake but this one was a real bear.


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Wed 06, 2017 11:05 pm 
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One of the big issues with Consoles and large Radio Grams is not so much what they are. Those who are going "Retro" may grab them, but thereafter their physical size, relative to the dwelling's size, renders them too big.

Marc


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Thu 07, 2017 12:00 am 
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Its the era more than anything. What is REALLY hot now are mid century consoles... the flat elongated rectangular ones. I spent around a month overhauling a large 1959 Grundig console. This thing weighed over 200 pounds. A very nice unit. I priced it at $800 and the thing sold immediately at the antique show last week. The same with most any other similarly styled console, even the lower quality ones. OTOH I also brought along a very nice 1941 Zenith console-again with bluetooth- and I wound up getting $140 for it. If it didn't have bluetooth? It probably wouldn't have sold at all... Kind of sad in a way.


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Thu 07, 2017 12:24 am 
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From what I see (haven't looked) there is probably a market for a module that can recombine stereo and have provision for impedance matching, for WiFi & Bluetooth & straight out transfer.

Marc


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Thu 07, 2017 4:08 pm 
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It's been some times since I'm looking for a black dial Zenith console. To bad I'm sooo far away. This one looks great.


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