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 Post subject: 35Z5... Strange...
PostPosted: Feb Fri 02, 2018 5:11 am 
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Location: Springdale, Arkansas
I just got a little AA5 radio from the late 40's that was completely dead. As it turned out, the 35Z5 had an open filament. Pretty common. But, when I pulled the bad tube out I noticed that it had only 5 pins and the replacement that I installed had 6. What's up with THAT? The pin wasn't broken or anything, it was just never there.

I also noticed that the dial light bulb was burned out. I read somewhere that using a radio with a burned out bulb when the dial light is tapped off the rectifier tube can shorten the life of the tube. Is there any truth to that or is that mere fiction?

Thanks.


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 Post subject: Re: 35Z5... Strange...
PostPosted: Feb Fri 02, 2018 5:23 am 
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Quite often, not all the pins are used in an octal tube. Leaving out unused pins results in a cost savings for the manufacturer. There's the occasional customer who complains that he wants all the pins.

One section of the 35Z5, and 35W4 filament is made to run at a slightly lower current. The pilot light is connected across this section. Sometimes a resistor is connected across the light in case the light burns out.

Many newer sets were built without pilot lights, which is why the 35Z5/35W4 always burns out the same section. Best to have a working dial light in place.

There was an article showing how to use the good sections of a bunch of tubes in series adding up to 110 volts (old article) to make them useful.....

Bob

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 Post subject: Re: 35Z5... Strange...
PostPosted: Feb Fri 02, 2018 4:00 pm 
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Elvirafan wrote:
Quite often, not all the pins are used in an octal tube. Leaving out unused pins results in a cost savings for the manufacturer. There's the occasional customer who complains that he wants all the pins.

One section of the 35Z5, and 35W4 filament is made to run at a slightly lower current. The pilot light is connected across this section. Sometimes a resistor is connected across the light in case the light burns out.

Many newer sets were built without pilot lights, which is why the 35Z5/35W4 always burns out the same section. Best to have a working dial light in place.


There was an article showing how to use the good sections of a bunch of tubes in series adding up to 110 volts (old article) to make them useful.....

Bob

Not true, full heater including tap on 35Z5, 35W4 etc is 150ma... Extra current is drawn because the tap is also supping voltage to plate for rectification of B+ voltage... Sooo without a dial lamp there is approx 200-210ma being drawn across the 150ma tap(actually its higher due to constant charging of filter cap)... Adding a #47 dial lamp in parallel with tap raises current capability to 300ma, which results in approx 5 to 5.4v across the tap and bulb combo... So in theory the tap & bulb should have a easy life... Problem is any issue in B+ that raises current draw or burned out dial lamp overheats the tap so it soon fails... Also turning set off and back on within 15-20 seconds results in huge surge across tap due to recharging "dead" filter caps(a discharged electrolytic cap resistance is very low)...

The whole design of this circuit was to support a dial lamp at a reasonable brightness... Before release of 35Z5 there were sets that had a parallel resistor across dial lamp to feed the 35Z4(or 35Z3) plate... Result is similar but required at least a 2 watt resistor in parallel with bulb...

As far a no dial lamp sets, the tap generally wasn't used to supply B+, most of those feed the plate voltage directly from the AC line... Usually there is a surge resistor in either plate or cathode circuit... But in later sets beginning approx 1952, and phased in over next few years, the tap isn't used as heater at all... The tap solely supplies AC to plate for rectification, in this configuration its a current limiter/fuse... This results in total voltage of string being reduced to approx 115v, meaning at today's voltage, some type voltage rating increase for string is desirable...

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 Post subject: Re: 35Z5... Strange...
PostPosted: Feb Sat 03, 2018 5:53 am 
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35Z5 makes a good point about his namesake. I stand, rather, sit corrected!
Obviously the source I used so many years ago was misleading, and that was before the internet.....
Looking at the RCA manual again, it's obvious why that section would burn out so easily, even with the 150 mA rating.

Bob

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 Post subject: Re: 35Z5... Strange...
PostPosted: Feb Sat 03, 2018 6:05 pm 
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When I joined, 35Z5 was more or less was the first thing that popped into my head, soon realized I knew little about the actual conditions that led to their demise... I'd last I worked on any of these radios in late '60s but I knew as far back as '64-ish, without dial lamp a portion of heater burned brighter(clearly visible in some tubes)... Figured this can't be good... Anyway I decided to learn as much as possible about this maligned and misunderstood piece of electronic history...


BTW the 35Z5 will be celebrating it's 79th birthday Feb 17...

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 Post subject: Re: 35Z5... Strange...
PostPosted: Feb Sat 03, 2018 9:39 pm 
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Sometimes manufacturers omitted unneeded pins to save money.

I have had the problem of the 35Z5 that had an open filament in an old Emerson AA5. It turned out to be a shorted cap - the foil cap that bypasses line to the chassis.

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 Post subject: Re: 35Z5... Strange...
PostPosted: Feb Sun 04, 2018 6:53 pm 
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For testing you can put a jumper across the open part of the heater. Just wrap some wire around those two pins. Solder it if you want to. Once you verify that the set works, put in a new tube and pilot light. Keep the bad tube for testing in other sets.

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 Post subject: Re: 35Z5... Strange...
PostPosted: Feb Mon 05, 2018 1:53 am 
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If you get an AA5 with a popped pilot light and an open 35Z5, beware. It often means that something in the radio was drawing excessive B+ current or perhaps shorted out. If you just change the tube and the pilot light they could burn out again quickly.

With household radios of this age, it's almost certain that the filter capacitors will be bad, as will be most of the wax-impregnated paper caps. There could be other problems besides but the capacitors are the usual suspects.

One capacitor you have to pay particular attention to is the one connected to the grid of the 50L6 or other output tube. If that cap is even a little bit leaky it will throw the grid bias off, causing the tube to draw more current than it should. This produces a characteristic garbled or distorted sound. If allowed to go on it can cause the output tube to overheat and become gassy, then you have to change it too.

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 Post subject: Re: 35Z5... Strange...
PostPosted: Feb Mon 05, 2018 5:28 pm 
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Probably four of five 35Z5 failures were due to turning set off & on again within a few seconds... This will blow the dial lamp causing excessive current through tap... Continuing this operation and especially additional hot starts will open the tap...

A pair of 6 to 8v zeners back to back across bulb & tap will eliminate surge that does the damage...With zener addition, even a tube with open tap will function normally...

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 Post subject: Re: 35Z5... Strange...
PostPosted: Feb Mon 05, 2018 7:41 pm 
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Chris108 wrote:
If you get an AA5 with a popped pilot light and an open 35Z5, beware. It often means that something in the radio was drawing excessive B+ current or perhaps shorted out. If you just change the tube and the pilot light they could burn out again quickly.

That is why I always put a jumper across the open heater and get everything working before replacing the 35Z5 and pilot light.
Everything else Chris108 said is right on too. Not only the plate to grid cap in radios, but any plate to grid caps in TVs, should you start to work on those too.

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