Zenith porthole poor width question....

[Findm-Keepm]

Originally Posted by vts1134

I was more asking for education. What does putting a cap across the cathode and plate of the damper accomplish (besides increasing width )?

More coupling of the horizontal sweep to the yoke, I'd imagine. Weak drive, a weak flyback, poor B+ regulation, improper impedance match/LC between yoke and flyback, variances in replacement yoke/flyback - you name it, it can produce reduced horizontal sweep. I remember one Emerson portable that had a shorted IF tube, and gave us no horizontal sweep, just a vertical line. The lack of sound and reduced B+ gave it away.

[old_coot88]

Originally Posted by vts1134

I was more asking for education. What does putting a cap across the cathode and plate of the damper accomplish (besides increasing width )?

Just to expand a bit (pun), adding the cap reduces the HV comensurately with the width increase. It also reduces cathode current a tad in the H out. tube. So it's a trade-off, no free lunch. Much higher than 100pf, the HV drops to where you start getting blooming.

If you recall, this was discussed near the end of the thread on your little Majestic 'Mighty Monarch' set. It had not quite enuff width to completely fill the screen. Using a damper cap was discussed. And since the set also had had issues with the HV rectifier arcing, it woulda benefited from slightly reduced HV also.
I didn't know then if the damper cap trick work on a pre-standardization setup, so it was never tried. But we now know it will work.

We usta keep 6KV disc ceramic caps of 47, 68, 82 and 100 pf for just this purpose. Also a 120, but never ended up using it that I recall.
4KV caps would be OK for just B&W sets, but 6KV covered color sets as well. (Certain Admiral color sets had horrendous issues with HV regulation and H out. tube overcurrent. It took some serious juggling of H out. sceengrid voltage and damper cap values to keep the the cathode current within reason.

[Findm-Keepm]

Originally Posted by old_coot88

Just to expand a bit (pun), adding the cap reduces the HV comensurately with the width increase. It also reduces cathode current a tad in the H out. tube. So it's a trade-off, no free lunch. Much higher than 100pf, the HV drops to where you start getting blooming.

If you recall, this was discussed near the end of the thread on your little Majestic 'Mighty Monarch' set. It had not quite enuff width to completely fill the screen. Using a damper cap was discussed. And since the set also had had issues with the HV rectifier arcing, it woulda benefited from slightly reduced HV also.
I didn't know then if the damper cap trick work on a pre-standardization setup, so it was never tried. But we now know it will work.

We usta keep 6KV disc ceramic caps of 47, 68, 82 and 100 pf for just this purpose. Also a 120, but never ended up using it that I recall.
4KV caps would be OK for just B&W sets, but 6KV covered color sets as well. (Certain Admiral color sets had horrendous issues with HV regulation and H out. tube overcurrent. It took some serious juggling of H out. sceengrid voltage and damper cap values to keep the the cathode current within reason.

You and Zeno, GURUs !! I lost my Dad (and his 50yrs experience in TV repair!) to cancer in 2011, but got new mentors between you and Zeno.

Thanks!

[kramden66]

Riders Tv troubles and cures helps too. They are on line at etf I think. A lot of modifications and cures listed for various models. I know in the portholes some say put a 1 mfd across two connections on the fly or a .1 its been a while but it helped

[old_coot88]

It's not kludging, but corrective reengineering.

[DavGoodlin]

Originally Posted by old_coot88

It's not kludging, but corrective reengineering.

Some of us former techs got started on the bench in the late 70s when many of the tube Gurus were getting out of this busuiness, some bitter and treating it as a bad dream and not recalling much.

Electronic Technician/Dealer magazine was helpful to take us back to the design stage, allowing such improvements. It became very important with keeping the HV in check with Solid State Sweep.

Just imagine the pressure to get these designs into showrooms, before all the "burn-in" testing is done.

Thanks Old Coot!