Restoration Idea 21
Restoring a 19" Portable 1960 Hoffman K1919 Television
This was my first serious venture into TV restoration. The nice thing about this TV was when I got it it was completely intact and well cared for. The well engineered layout and quality design of this set by Hoffman Electronics, makes this is a great starter set for those interested in TV restoration aside from the weight. The thin profile using 110 degree deflection picture tubes was a very popular design pushing the big picture with a small foot print. The Philco "Seventeen" is a popular example of a set pushing this design. What thought to be simple restoration became a dragged out search for problems that caused the restoration not to be as straight forward as I thought. When I restored it some seven years earlier and a small update three years later, things didn't quite work right. It wasn't until July 2018 that I figured out what were the problems. This is an example of the saying "experience is a good teacher"!
Quality set! Nice straight forward layout makes the Hoffman K1919 an ideal starter set for those wanting to restore a TV. Notice the power transformer at the bottom of the TV, a rare sight to see in a portable TV set.
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2011 Original Restoration:
2011 Original Restoration:
I bought this TV from George Fowler from Fallbrook, Cailfornia (Avacado growing capital in USA). This TV has always been in the family and was well cared for, complete with instruction manual (very rare). Hoffman (originally Mission Bell Radio) is a regional consumer manufacturer, headquartered in Los Angeles. In later years designed military and space electronics. Two of Hoffman's significant designs are the solar panels for the early US satellites Explorer and Vanguard. The other is TACAN, a electronic guidance system to track and guide planes for landing and takeoff until GPS became the primary system.
Restoration
I read a lot of TV collector websites on how to approach my first TV restoration, the following photos are the restoration phases.
The 110 degree deflection 19 inch picture tube gives the TV a thinner profile than previous portable set, just a few years earlier. Notice in the photos above and below how easy access to components are. The electronic layout is very logical and one looks around the set
HV Power Supply and Horizontal Sweep section
Video IF Strip
Synch, Sound IF and Power Supply Section (above)
Restoration Process:
Restuffing Electrolytics: (started 2011 completed July 2015)
The K1919 has 3 multisection capacitors, making this task shorter. However one, located near the Horizontal Sweep was very difficult to remove. As shown below mark the wires, unsolder.
The K1919 has 3 multisection capacitors, making this task shorter. However one, located near the Horizontal Sweep was very difficult to remove. As shown below mark the wires, unsolder.
The can is cut at the base with a hacksaw. Some like to unfold the aluminum wrap at the base, and pull out the old part. I never like that because, not only is it tedious, but run the risk of deforming the base as you keep prying and damaging the terminals in the process.
The new stuff, to be covered after TV is checked out.
Remove and replace all molded and paper capacitors. Check resistors, and replace those defective and out of tolerance.
Above: Synch, Sound IF and Power Supply Section.
I decided to not restuff capacitors, because most of them are molded types and will be destroyed if I tried to take the "guts" out.
I decided to not restuff capacitors, because most of them are molded types and will be destroyed if I tried to take the "guts" out.
Above: Video IF Section
Above: Horizontal Sweep and HV Section
Above: Vertical Osc/Sweep and Audio Section
Turning the Set On:
My first TV restoration is working. After it warmed up the picture started to become unstable, had vertical and horizontal rolling. I would get it to stop then in a minute started to have picture instability. It could be tubes, since I didn't test them, but unfortunately I didn't have any spares available.
Vertical Linearity: Another problem was that the picture was distorted. If there is a closeup of a person, their head started to have a cone shape. Notice the horizontal scan lines at the top versus the bottom. They are farther apart at the top. I could not adjust the distortion out.
I couldn't figure out the cause, but I still like what I had, my first vintage TV. Maybe in the future I can get back to this and figure the problems out.
I couldn't figure out the cause, but I still like what I had, my first vintage TV. Maybe in the future I can get back to this and figure the problems out.
July 2018 Restoration Update:
July 2018 Restoration Update:
Had some time to revisit this set since last restoration (about 7 years ago). The unstable picture was still an issue. I decided to pull the set out and check the synchronization tubes and sound IF tubes. In these sets most of the tubes do two or three functions in one glass envelope.
Used my Hickok 600A Gm tester that I finally understand how to read the tube chart and the meter reading for mutual conductance (Gm). Found that the 6U8A and the 6DT6A tubes were questionable. I replaced those with new ones. I check the others on the IF Strip and found a weak 6AM8 and 6BZ6 tubes, so I replaced them as well.
Turn the set on and got a "rock solid" picture, no more adjustments. However the non linear vertical problem was still there at the top of the screen. Everyone in the picture still had the "cone head" look.
I checked resistors again, wiring errors I made and solder connection. Everything seems good. Then I said to myself, maybe some of those capacitors I installed were out tolerance or the wrong type. I did a scope check on the vertical signal at the 6DE7 input. Shown below is the results:
The red curve is what I saw on the scope before I found the problem and compare that to the other trace showing the proper waveform after I corrected the problem.
When I originally restored the TV, I had to find a 0.022 uF 1600 V capacitor (the original was a molded type). I found a 0.022 uF 2000V ceramic that should work for the area shown below:
When I originally restored the TV, I had to find a 0.022 uF 1600 V capacitor (the original was a molded type). I found a 0.022 uF 2000V ceramic that should work for the area shown below:
It is the signal couple return to form the vertical oscillator network which feeds into the timing circuit for the saw tooth waveform. Somehow this ceramic capacitor response to the fast transition interfered with the timing circuit causing a non linear response. The proper waveform is shown on the lower left of the photo above.
Didn't have have a 0.022 uF 1600 V capacitor, so I took two 0.047 uF 630 V mylar capacitors and connected them in series as shown below. This will be a 1260 V capacitor, but that is more than enough safety at this part of the circuit.
Pattern Generator Results:
Finally I was able to adjust the Vertical Height and Linearity controls to get a pretty good linearity. I did replace the vertical height potentiometer with a linear taper type, making adjustments less cranky.
Replacing a capacitor went from:
THESE
TO THIS
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