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 Post subject: Pilot tv 37 HV Rect. Filament Power
PostPosted: Feb Tue 04, 2020 4:01 am 
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Joined: Feb Tue 04, 2020 3:43 am
Posts: 3
Hi everybody, All my tubes light up on my pilot tv 37 except V18 1B3GT.I pulled the tube and cleaned the socket. Got a low resistance reading on the filament and it looks intact. Looking at the sams and ryders schematic I am having trouble figuring where the current is coming from to supply the filament and why it is coupled to T2. Any help is greatly appreciated! Regards, Patrick


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 Post subject: Re: Pilot tv 37 HV Rect. Filament Power
PostPosted: Feb Tue 04, 2020 5:08 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 759
Location: Crystal Bay, NV
The 1B3 does not "light up" like most tubes. It can be seen to glow if the ambient light is dark enough. Since the 1B3 gets its filament power off the high voltage transformer, any fault here will shut the 1B3 down completely.
=====
Ron


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 Post subject: Re: Pilot tv 37 HV Rect. Filament Power
PostPosted: Feb Tue 04, 2020 8:04 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 573
I can provide some advice, which can help if you give it some thought.

When a question is posed about some circuit for another to help, its much easier if the schematic fragment at least is posted, so anyone wanting to help can see what you and they are dealing with. I have attached the TV-37 EHT supply that I found on the net (assuming it is correct).

If you are working on a circuit trying to fix it, you need to have a fundamental understanding of how it works, to have much of a hope of success. For vintage TV work therefore, you should get the book Basic Television, by Grob, 2nd Edition, McGraw Hill. It explains the circuits you will see in vintage TV's, including the type of power supply, that generates the EHT in your TV-37, known as an "RF power supply"

These supplies are like a large power sine wave oscillator. Typically they run at around 100kHz. They generate a lot of interference which is why in TV's they are normally in a shielded box.

To sustain oscillations, there is sometimes a metal clip around the glass bulb of the 1B3 rectifier, which provides the feedback to the tube's G1 grid (25L6) driving the primary. When it is oscillating normally this induces voltages in all the windings of the RF transformer, including the winding that powers the filament of the 1B3. So if the oscillation frequency or amplitude is wrong, there won't be enough energy to power the 1B3's heater. (though as noted, it might be powered but difficult for you to see it).

These RF power supplies can malfunction for a number of reasons. The tuning capacitor (0.002uF) can fail. The large overwinds can absorb moisture over time and damp the oscillations. Sometimes to fix these requires more than a meter; a scope with high voltage probes might be needed. Or you might get lucky if its a single component failure. It pays to have an EHT probe , there are many that are not too expensive and the ultimate test of course is to measure the EHT that the supply is supposed to be generating.

But like I say it really does pay to read up on things like this first, especially if its a supply that generates very high voltages.


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 Post subject: Re: Pilot tv 37 HV Rect. Filament Power
PostPosted: Feb Tue 04, 2020 6:46 pm 
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Joined: Mar Sun 01, 2009 10:27 pm
Posts: 5277
Location: Ann Arbor, MI
ACORNVALVE wrote:
To sustain oscillations, there is sometimes a metal clip around the glass bulb of the 1B3 rectifier, which provides the feedback to the tube's G1 grid (25L6) driving the primary.

Sometimes a metal clip and sometimes a spring wrapped around the tube. The spring or clip and the plate of the 1B3 form a capacitor. A capacitor is just two conductors separated by an insulator. In this case the plate of the 1B3 and the clip or spring are the two conductors. And the glass and vacuum in the tube are the insulator.

The spring or clip has to be positioned near the plate of the 1B3, typically a little above the bottom end of the plate.

_________________
Tom


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 Post subject: Re: Pilot tv 37 HV Rect. Filament Power
PostPosted: Mar Sat 28, 2020 3:36 pm 
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Joined: Feb Tue 04, 2020 3:43 am
Posts: 3
Thanks for the replies. I am going to recap it and then I will go from there. I have had this set for at least 20 years and seem to remember it did work. I will update you when I get it working. Thanks.


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 Post subject: Re: Pilot tv 37 HV Rect. Filament Power
PostPosted: Apr Wed 01, 2020 4:58 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1687
Location: Phoenix, Arizona
Respecting all that has been said above . . .

Most 1B3 rectifiers do show a very faint filament glow if you look at the lower shield below the plate cup in the 1B3, you will see the reflected filament glow.

The high voltage oscillator has to be operating so keep your nose away when looking.

The 1 volt filiment voltage is taken from a couple of turns of HV wire
on the oscillator coil, You can't measure it.

If the high voltage capacitors are old, you can assume they are failed and they will load down the high voltage oscillator so it can't run.

I tried several 1B3's in the last set I worked on to find one that worked the best measuring the high voltage with a HV probe.

I usually touch an insulated screwdriver to the cap of the 1Bs to see the tiny arc and know it is making high voltage.

The metal can on the HV supply will affect the tuning a little.

Jim


Last edited by jimmc on Apr Wed 01, 2020 5:03 pm, edited 1 time in total.

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 Post subject: Re: Pilot tv 37 HV Rect. Filament Power
PostPosted: Apr Wed 01, 2020 5:00 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1687
Location: Phoenix, Arizona
Respecting all that has been said above . . .

Can't get rid of this echo.


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 Post subject: Re: Pilot tv 37 HV Rect. Filament Power
PostPosted: Apr Thu 02, 2020 4:54 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5342
Location: Woodinville, WA USA
This Pilot TV-37 restoration article has some info that may be helpful in completing your project:

https://antiqueradio.org/PilotTV-37.htm

Have fun!

Regards,

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: Pilot tv 37 HV Rect. Filament Power
PostPosted: Apr Thu 02, 2020 10:42 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 573
philsoldradios wrote:
This Pilot TV-37 restoration article has some info that may be helpful in completing your project:

https://antiqueradio.org/PilotTV-37.htm

Have fun!

Regards,

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


I definitely agree with the remarks in the article about removing the 3kp4's heater from the series heater chain and running it from a small auxiliary transformer. There are 3 risks that threaten the irreplaceable 3kP4:

1) the initial turn on current surge with cold heaters (unless the power NTC resistor is added)
2) If the vintage RF bypass caps along the chain short out, can be a disaster
3) The heater cathode voltage may result in failure of the 3kP4 too.

Generally what I do for my vintage CRT's in my TV's, if required I add a line to 6.3V transformer, it only requires a small core cross section, typically 1/2" by 1/2" (so its a compact transformer normally) , then I connect one of the heater connections , via a 1Meg resistor to the CRT cathode, so that the heater element in he CRT assumes the same average voltage as the cathode. I thave seen some make a direct connection, but if the video drive is to the cathode, that raises the capacitance and degrades the high requency response of the video, so its better to use the resistor.

One other thing that can be done to protect vintage CRT's is to add a pair of low capacitance series diodes across their cathode and grid. Of a polarity that prevents the grid from being driven positive with respect tho the cathode.I did it for example on the VCR97 crt in this article, note the two BAV20 diodes shown on page 31:

http://worldphaco.com/uploads/ARGUS.pdf

Finally, if in the particular TV there is a turn off bright spot, a spot killer needs to be added, takes the form of an added capacitor, resistor and a diode, so that at turn off, the grid goes negative for a while with respect to the cathode, killing the beam current. Early non-aluminized CRT phosphors were fragile and easy to burn with high beam current on a small area. When the CRT is being scanned the energy is spread over the whole screen. This article shows one version, and the pulse it produces, see figure 6:

http://worldphaco.com/uploads/CONVERSIO ... P4_CRT.pdf


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 Post subject: Re: Pilot tv 37 HV Rect. Filament Power
PostPosted: Apr Fri 03, 2020 3:36 pm 
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Joined: Nov Thu 11, 2010 6:03 pm
Posts: 1256
Location: Pewaukee, WI
A few years ago someone on this forum figured out that a TVS (transient voltage suppression) Diode in of the correct voltage in parallel with CRT heater will prevent anything above 6.3V RMS from appearing across the CRT heater. The diode will transfer the surge to other tubes in the string but all the other tubes in the string are MUCH easier to find and cheaper than a 3KP4 so it is a good trade off IMO. It would also be possible to use a TVs in parallel with the whole string and have a series resistor between that and the line (you'd want to keep the one in parallel with the CRT so if other tubes warm up at different rates it doesn't strain the CRT).
It's easier to hide a TVS than a transformer.


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 Post subject: Re: Pilot tv 37 HV Rect. Filament Power
PostPosted: Apr Fri 03, 2020 4:14 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1687
Location: Phoenix, Arizona
Some folks solder in a Zener Diode and I don't recall the part number. The filiment connection to the 3KP4 happens to connect to a convieniant terminal strip so it is easy to stuff one in.

The little transformer will require a couple of holes drilled in the chassis.

The Pilot tv has such a small screen, I hardly ever fire mine up, but sofar I use a variac and should probably do one of the modifications.

Jim


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 Post subject: Re: Pilot tv 37 HV Rect. Filament Power
PostPosted: Apr Fri 03, 2020 5:12 pm 
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Posts: 1256
Location: Pewaukee, WI
jimmc wrote:
Some folks solder in a Zener Diode and I don't recall the part number. The filiment connection to the 3KP4 happens to connect to a convieniant terminal strip so it is easy to stuff one in.

The little transformer will require a couple of holes drilled in the chassis.

The Pilot tv has such a small screen, I hardly ever fire mine up, but sofar I use a variac and should probably do one of the modifications.

Jim

Since the heater is AC you can't use a single straight up Zener since zeners effectively act as a short when forward biased and clamp voltage when reverse biased (thus a single Zener PN junction can only clamp DC). A TVS diode is essentially an AC Zener which is made of 2 Zeners in in a single package wired in series such that both cathodes or both anodes are tied together. Basically if voltage across it is forward bias for one series diode then it is reverse bias for the other diode...thus one diode will act a short and the other a voltage clamp and they will both switch jobs each time polarity reverses.


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