Hoffman 7M112 tabletop - Page 4 - Videokarma.org TV - Video

Phil Nelson · #46 03-18-2014, 12:10 PM

Thanks for the ideas. My set does not have the .1 cap across L21. Maybe that's why they show the coil and cap inside dotted lines. I could try adding that to see what happens. The other caps are easy to reach except for C73 (47pf), which is crammed in the yoke.

Phil Nelson

Electronic M · #47 03-18-2014, 02:15 PM

You may also want to try adding capacitance across the damper to increase the width if rrrhre2s' suggestions don't pan out...

Robb · #48 03-18-2014, 02:42 PM

Very nice little set there !

Phil Nelson · #49 03-18-2014, 03:27 PM

Increasing capacitance across the damper made a difference. That cap (C72) is listed at 120pf/3KV, and I don't keep many high-voltage caps around, but I found a couple of 82pf/5KV ceramics and connected one across C72. That bumped out the width to nearly fill the mask. Wiring a second one in parallel increased the width to overscan the mask slightly.

The tradeoff is that with lower HV, the screen is less bright. With two 82-pf caps in parallel, the HV sinks to only 7.5KV and the screen is noticeably dimmer than before. Maybe I'll go with one cap and call it done. I'd rather have a bright screen with slivers of black on the edges than a dim one. There is still slight non-linearity, but not enough to be annoying when watching a program.

For the record, I also tried adding a .1 cap across L21. It improved the linearity very slightly, but at the expense of width.

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

***Eric H*** · #50 03-18-2014, 03:48 PM

Phil did you try subbing the Horiz Output tube? Even if it checks good it might not work as well as another one.

As for the linearity I think some sets just aren't as good as others by design.

If you've ever seen old copies of Consumer Reports they would test them for linearity and overscan and show test patterns, some models, especially low end models, had lousy linearity right out of the box.

old_coot88 · #51 03-18-2014, 03:59 PM

Phil,
Bear in mind that when you add C across the damper to gain width, the cathode current of the HO tube drops (along with HV). You can bump the G2 voltage up a tad to bring the current back up.
But insert a milliameter in the K leg so you can see what's going on.

For a 'quick & dirty' test of the HO tube, shut the set off for 7 seconds or so, and then back on. If the width is slow to come back to where it was, the tube is borderline weak.

Phil Nelson · #52 03-18-2014, 05:18 PM

This HO tube passes the 7-second test with flying colors. I tried subbing the other 6BQ6 that I have on hand, which seems slightly worse.

Maybe I have arrived at the land of tradeoffs with this issue. I can make things slightly different, but not significantly better.

Phil Nelson

Electronic M · #53 03-18-2014, 05:27 PM

Here is a potentially dumb idea...Maybe you could get more HV by adding turn(s) to the HV rect. filament winding on the fly...If the fly filament winding was made assuming the higher HV condition then at reduced HV the filament may be starved for power and not emitting electrons as efficiently as it ought to, thus reducing HV.

Phil Nelson · #54 03-21-2014, 01:06 AM

Adding a 39pf cap across the damper (in addition to the added 82pf) did the trick. Now I can fill the screen horizontally and the brightness is just fine.

The second photo is a live broadcast received with rabbit ears.

I suspect the screen geometry will never be perfect, but I'm calling this one good enough for now. Thanks again for the advice.

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

bandersen · #55 03-21-2014, 11:52 AM

I've come across a few sets with caps tacked in to increase the width.

old_coot88 · #56 03-21-2014, 12:07 PM

Quote:

Originally Posted by Phil Nelson

...I suspect the screen geometry will never be perfect, but I'm calling this one good enough for now.

Dang, Phil. That's as close to perfect geometry as i've ever seen on any vintage set, bar none.

With added C across the damper, the HO tube will run a little cooler, albeit with slightly reduced HV.. per the old sayin' "there is no free lunch".

jr_tech · #57 03-21-2014, 12:31 PM

Quote:

Originally Posted by bandersen

I've come across a few sets with caps tacked in to increase the width.

So what is the reason? I assume that the sets made full width at reasonable HV when new and that relevant caps, tubes, resistors and supply voltages in the Horizontal osc and output stages have been checked/replaced. Does the "q" of the flyback change as it ages? How about the yoke? Are the caps in the yoke a problem that may be overlooked?

Just curious,

jr

dieseljeep · #58 03-21-2014, 12:47 PM

Quote:

Originally Posted by bandersen

I've come across a few sets with caps tacked in to increase the width.

IIRC, Workman made those width correctors that would fit on the damper tube, that had a small value HV disc cap, molded into it. They worked rather slick.

bandersen · #59 03-21-2014, 01:16 PM

Quote:

Originally Posted by jr_tech

So what is the reason? I assume that the sets made full width at reasonable HV when new and that relevant caps, tubes, resistors and supply voltages in the Horizontal osc and output stages have been checked/replaced. Does the "q" of the flyback change as it ages? How about the yoke? Are the caps in the yoke a problem that may be overlooked?

Just curious,

jr

I think they were a quick fix for aging components. After a recap and fresh tubes, I ended up removing them because they had too much width.

rrrhre2s · #60 03-21-2014, 03:44 PM

I think sometimes we forget that when this equipment was newer that people usually wanted the service man to repair the set in the home.

I had a book from RCA that an in home service call would have been about $15.00.

So the guy doing the repairs had to get the set working as fast and as cheaply as possible to maximize his income.

This meant that he could service more sets in one day.

I will digress here getting off topic.

rrrhre2s