Hoffman 7M112 tabletop

[Eric H]

Looks like a Hoffmuntz.

[Phil Nelson]

This is a little different. These two items protrude through the rear panel as adjusters. The lower one looks like it would lever the core (?) up and down. The upper one terminates in something like a switch at the base of the coil. If these are HV adjusters, it's a new type to me.



Today's puzzle: I see this little empty socket near the 3rd IF section. Wonder what goes here?



There are no empty sockets elsewhere on the chassis, so presumably it's for some plug-in component, not a cable. Underneath, one terminal of the socket goes to ground and the other two are wired to a little network of resistors and (peaking?) coils that go to the 3rd IF stage and (through the big pink .1 cap) to the video amp.



The TV has loud and clear audio and the sound takeoff appears to be from pin 5 of the video amp, so presumably a signal is getting to that tube with nothing plugged into the mystery socket.

I don't know why you'd have a plug-in component here unless it's something optional or variable. Could this be some kind of local/distance thingie plugged in by the dealer in certain locales?

Phil Nelson

[jr_tech]

Originally Posted by Phil Nelson

This is a little different. These two items protrude through the rear panel as adjusters. The lower one looks like it would lever the core (?) up and down; there is a locknut on the screw at the rightmost end. The upper one terminates in something like a switch at the base of the coil. If these are HV adjusters, it's a new type to me.

From the pix of the back of the set, it looks like the switch is a coarse position control and the funny screw thing is "width"... odd!

Originally Posted by Phil Nelson

Today's puzzle: I see this little empty socket near the 3rd IF section. Wonder what goes here
Phil Nelson

I have seen plugs like that used as test points for alignment.

Did the green glow return after sitting overnight?

jr

[dieseljeep]

Originally Posted by Phil Nelson

Here is a quick peek under the 7M112 chassis. It's uncluttered and it should be easy to work on.



The set has two selenium rectifiers and a sturdy (and clean!) turret-style tuner with 6J6 and 6BK7 tubes. The HV output section is also under the chassis and I removed its (solid steel) cage for this photo. The TV actually has 18 tubes including the CRT, even though only 15 are shown in the back-cover diagram.

Someone serviced this TV not too long ago. Notice the new resistor under the 1B3GT HV rectifier socket. I have exactly the same type of resistor in my parts boxes.



Here are a couple of other recent replacements:



The electrolytic is piggybacked in parallel with a can electrolytic rather than taking the old cap out of circuit. I found another new electrolytic wired between two pins of the 6AH6 video amp. The red Big Chief cap looks like an older replacement. The original paper caps are a brand that I hadn't seen before: "Good-All Marbelite."

I made a couple of cursory checks and the B+ voltage seems reasonable. HV measures only 8.5KV, where that CRT normally wants something like 14KV.

The TV is still full of ancient electrolytics and paper caps, so I'm not going to burn a lot of time diagnosing specifics until it has been recapped. Lacking a good place to work, who knows when that will happen?

It's interesting to peek underneath, anyhow. Yes, it's an inexpensive, series-string set, but the factory wiring is tidy and the overall build quality looks decent. It's surprisingly sensitive, all things considered. When I first powered it up, it made very clear audio receiving a signal from my in-house transmitter, without anything connected to its antenna terminals.

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

I would check the resistor that was replaced under the 1B3 socket. Maybe the wrong value was chosen and is starving the 1B3 filament.
The shaft the adjusts the flyback core gap is the width control. I would turn it fully clockwise, for now. The other shaft is for the horizontal centering.
Regarding the tuner: It could a Standard Coil. If the strips are in one piece, someone else made it!
BTW, the set is a parallel wired set. IDK, if the transformer has a winding for the damper tube or a secondary tap.

[WISCOJIM]

Your unidentified socket is between the 3rd Video IF tube and the video output tube. From Sam's 205-5 (see point A):

[Phil Nelson]

Looks like a test point to me. I will quit fretting over it. The schematic shows their 3-position range switch in the same neighborhood, so perhaps my local/distance guess wasn't completely insane.

Unfortunately, the green glow seems to be gone for good, no matter where you put the ion trap. It was fun to see, anyhow, even it seems a little gimmicky. I can see using it for a coarse setting, but I'd want to make the final adjustment looking at the screen.

The CRT is indeed a Rauland and its label has some instructions about using the green marker. Sooner or later, I'll loosen the retaining strap so that I can read the whole thing.



The tuner appears to have two strips per channel; it looks like the one in my Admiral console.

Thanks for the other advice. I'll check out that replacement resistor (and the replacement caps, for that matter).

Regards,

Phil Nelson

[Phil Nelson]

OK, I'm going to quit dissing this poor little $20 craigslist Hoffman.





After replacing paper caps in the horizontal and sync sections, I decided to sub the 1B3GT and 6BQ6 tubes, out of superstition. (The others tested good, but you know.) I had also scrubbed the 1B3GT area with alcohol and given that resistor a good hard look.

When I went to play with the ion trap one last time -- bingo! -- the green glow was back, and within seconds I had a bright picture with terrific contrast and sharp focus. Don't ask me why someone put a brightener on this CRT. It looks as strong as a new one.

The second photo is receiving a cable broadcast with rabbit ears from my in-house transmitter, a decent real-world test of any old TV. A little blurry, but that's how all of my photos of live broadcasts seem to turn out.

Tomorrow I'll do some grown-up stuff like clean all of the pots and adjust the screen geometry, etc. It's fun to see it wake up, anyhow.

Phil Nelson

[dieseljeep]

Originally Posted by Phil Nelson

OK, I'm going to quit dissing this poor little $20 craigslist Hoffman.





After replacing paper caps in the horizontal and sync sections, I decided to sub the 1B3GT and 6BQ6 tubes, out of superstition. (The others tested good, but you know.) I had also scrubbed the 1B3GT area with alcohol and given that resistor a good hard look.

When I went to play with the ion trap one last time -- bingo! -- the green glow was back, and within seconds I had a bright picture with terrific contrast and sharp focus. Don't ask me why someone put a brightener on this CRT. It looks as strong as a new one.

The second photo is receiving a cable broadcast with rabbit ears from my in-house transmitter, a decent real-world test of any old TV. A little blurry, but that's how all of my photos of live broadcasts seem to turn out.

Tomorrow I'll do some grown-up stuff like clean all of the pots and adjust the screen geometry, etc. It's fun to see it wake up, anyhow.

Phil Nelson

It looks like the CRT might be original. It sure looks good.
I worked on a Hoffman set, that was a year or two newer. It was real impressive, for a 21" table model.
A 6X9 speaker on one side, an 8" speaker on the other and push-pull audio output. It had that crazy Standard-Coil UHF-VHF tuner, with the two coil turrets, four stages of IF and large power transformer with two 5U4's.
Hoffman was really a high-end line at the time. This model had the easy-vision tinted safety glass.

[ChrisW6ATV]

Nice picture for such a basic, low-end-appearing TV set.

This is the first non-tinted Hoffman I have seen, not counting the one color set I have been in contact with.

[Phil Nelson]

The safety glass is tinted. I thought it was just grubby until I removed the glass for cleaning, but if you photograph it against white, the color jumps out. It's sort of a greenish amber.



After recapping, the TV is working nicely except for a slight horizontal non-linearity and lack of width (see previous test pattern). The air-gap "width" adjuster mentioned earlier adjusts the HV output directly; I can get anything from 8KV to 10KV by turning it in or out, which makes the entire screen image grow or shrink. At 10KV, it's much too small and at 8KV (as low as I can go), it doesn't quite fill the mask horizontally.

I have the feeling that it would fill the screen if I could just correct the linearity, but the TV has no horizontal linearity adjuster. Here's the schematic for anyone who's curious.

http://antiqueradio.org/art/Hoffman7M112Schematic1.jpg
http://antiqueradio.org/art/Hoffman7M112Schematic2.jpg
http://antiqueradio.org/art/Hoffman7M112Schematic3.jpg

Perhaps there's a solution, but I'm not sure how much I can expect from this set. I wouldn't mind the minor non-linearity if it would just fill the dinged-danged mask.

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

[kvflyer]

Originally Posted by Phil Nelson

The safety glass is tinted. I thought it was just grubby until I removed the glass for cleaning, but if you photograph it against white, the color jumps out. It's sort of a greenish amber.



...

After recapping, the TV is working nicely except for a slight horizontal
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

What timing. I am reading an article in our local radio club magazine and it has reprinted an article written by an Australian collector. It talks about the Hoffman company and specifically says that Hoffman used surplus WWII Plexiglas for the safety glass screen. He also mentions the colour as you have described it.

Interesting!

[dieseljeep]

Originally Posted by kvflyer

What timing. I am reading an article in our local radio club magazine and it has reprinted an article written by an Australian collector. It talks about the Hoffman company and specifically says that Hoffman used surplus WWII Plexiglas for the safety glass screen. He also mentions the colour as you have described it.

Interesting!

You can see it was a lighter tint, where the sun didn't hit it. It must turn greener with age.

[Eric H]

They didn't use Plexiglass.
I have a 49 Hoffman glass right in front of me and it consists of a sheet of 3/32" clear bonded to a sheet of 9/32" green glass.
I have seen other 49 Hoffman sets that just had thick green glass but I have never seen one made of plastic.

I have read that the green glass was surplus glass from a building project in Los Angeles but I have my doubts.

If the glass Phil has is bonded I suspect the plastic material may have just discolored with age.

[Phil Nelson]

Yes, it's bonded like other safety glasses I have seen. If it was originally clear, maybe I'll replace it. This color isn't very appealing.

Any ideas about the linearity issue?

Phil Nelson

[rrrhre2s]

Horizontal linearity could be...

Problems with these

.1 across L21

120mmf across Damper tube 12ax4gt

47mmf across 1/2 of Horiz Yoke winding

the 12ax4gt itself...

I would guess you already tested such...

Try about a 75mmf across the damper tube to see if there is an improvement.

Have a nice afternoon...

rrrhre2s