Scored #5 on list: Pilot TV-37
 

I only posted a "wanted" list a month or so ago ... and today I got
#5, a Pilot TV-37. This is the first auction I ever bid at (I was at the ETF
convention last month but did not bid, as I bought a set at the outside flea
market.) And it was only 12 mile from my house.

I and one other guy bid on it, selling for $40. I had to sit through
an hour of dishes and dolls, then 75 minutes of model trains until
they got to the electronics.

I had tested the line continuity and it was 115 ohms, which sounded
reasonable for such a set with two heater strings. So I hoped the
3KP4 at lead had a filament.

When I got home I got the Sam's and Rider's .. and they disagree
on the filament chain wiring! Both have all the current
going through the 3KP4, but one has it at one power line
end, the other in the middle. Apparently mine is in the
middle. Removing the tubes on either side shows continuity
for the 3KP4. I hope this is real!

This is not my front-line project ..
today is hopefully the day I get my first Atwater-Kent 44 working.
Its close! It gets audio out, and gets RF to the detector.

Doug McDonald

Congrats! Something that I've always wanted!

Excellent! Good 3KP4 CRT's are becoming more and more scarce with each year that passes. I had sold my TV-37 years ago (with the octagon mag.) and regretted it. I recently came across another nice one with an exceptionally good CRT and jumped at the chance. I paid quite a bit more than you did though - to say the least!
Enjoy it.

Make sure you put a TVS (transient voltage suppressor) diode across the 3KP4 filament to limit cold start filament voltage.

Part number 1.5KE8.2CA


tubesrule (Darryl Hock) did a wonderful post on the subject: Post subject: Pilot TV37 crt filament modification

Over at ARF.

TV-37s are cute, & still relatively easy to come by. Guess I've had mine for a good 10 years now. Unfortunately, it has the green CRT.

Here is a link to Darryl's thread over at ARF.

http://antiqueradios.com/forums/view...p?f=3&t=132513

For me, the photos are missing.... ?

Got lots of zener diodes but no TVS. Does anyone know if two 8.2 volt 1 amp zeners connected back-to-back will work? (I'm guessing the difference between the 1 amp rating on the zeners and the 1500 watt rating on the TVS is continuous vs. transient current, but how do they compare?)

No, the zeners will also conduct in their forward direction as well as their reverse direction Zener point. You will only see about 0.7 volts across the pair.

jr

Yes, it should work. Back to back (i.e. in series, back to back or head to head)
is OK. Its in parallel, opposite directions, that clamps at 0.7 V. The
other guy was wrong.

The peak current, during warmup, will be about an amp, maybe a little more,
but only during a small part of the cycle. At the peak the filament
all also conduct about that, total twice that.

Doug McDonald

Slaps Forehead! In Series it should indeed work... I always used the term back-to-back to indicate parallel (connected + to -) for suppressing surges.

jr

Why not just buy the proper TVS diode, they are readily available and inexpensive. Problem solved.

Just the usual things that slow me down: minimum orders, shipping charges, general laziness and inertia.

But seriously, if using two zeners works just as well, why not use what I've already got in stock?

I've been looking at my TV-37.
On the schematic there are oodles of 0.0015 and 0.005 uF and 0.01 uF bypass
caps. The pictures I have look like they are "dogbone" ones (cylinders with
ridges at the ends where the leads are wrapped an come out laterally). The
parts list does not show a voltage. Are these ceramics? Do they need to be
replaced?

Doug McDonald

If they look just like dogbone resistors then they are ceramics. They usually are good, but I think I have heard of them failing so it may be a good idea to test them.

The vast majority of the "dog bone" ceramic capacitors are still good. However, it seems that mica caps which were given a clean bill of health have slowly been going bad. I would leave them alone for now unless you have a reason to suspect them!