RCA 621ts Restoration

[old_coot88]

By golly now that we got more of the schematic to eyeball, there's one more CRT element, the focus electrode or "G3" that hasn't been shunted with the .1 (or .25) cap.
Try clamping it, with the cap going from B+ to "G3" (pin 6). If the bar still remains, Mr. Earlyfilm has an idea i hadn't thought of yet:

Originally Posted by earlyfilm

VTS1134,

I see the vertical section attempting to run at 120 cps and it is badly speed modulating the sweep resulting in dual white bands. In other words, the normal 60 cps sawtooth is being speed modulated by a two backwards steps at a frequency of 120 cps.

Hah. "Speed modulation" of the v sweep waveform itself. That's an idea worth exploring.
IIRC, VTS1134 has a variac. While troubleshooting a vert problem in his vintage Majestic set, the idea was floated of driving the vert. yoke winding directly with the variac. Even though it's a pure sine wave, if it were tried on this set, it would at least reveal whether the bar is sweep derived (ie., 'speed modulated' sweep) vs. a rogue pulse on one of the CRT elements.

[tubesrule]

Originally Posted by old_coot88

By golly now that we got more of the schematic to eyeball, there's one more CRT element, the focus electrode or "G3" that hasn't been shunted with the .1 (or .25) cap.
Try clamping it, with the cap going from B+ to "G3" (pin 6).

The focus anode is sitting at around 1500V so even bypassing to B+ is still way over 1000V. Better have a cap rated for this.

[old_coot88]

I wuz just goin' by the schematic which calls for approx. 495V (varies with focus control setting).

[tubesrule]

I'm not sure why they show that. The voltage on the focus anode is going to vary between 1475V and 2250V based on the resistor values shown. The RCA spec sheet for the 7DP4 says it should average around 1800V for 7500V operation. Definitely need to use cation and a high voltage cap.

Darryl

[old_coot88]

Originally Posted by tubesrule

I'm not sure why they show that. The voltage on the focus anode is going to vary between 1475V and 2250V based on the resistor values shown. The RCA spec sheet for the 7DP4 says it should average around 1800V for 7500V operation. Definitely need to use cation and a high voltage cap.

Darryl

Good call. Maybe it's a misprint of 1495V.

[earlyfilm]

Re: +495 volts on focus anode on schematic

Originally Posted by tubesrule

I'm not sure why they show that. The voltage on the focus anode is going to vary between 1475V and 2250V based on the resistor values shown. The RCA spec sheet for the 7DP4 says it should average around 1800V for 7500V operation. Definitely need to use caution and a high voltage cap.

Darryl

Darryl,

RCA specified that all voltage readings were to be read with a Voltohmyst. Those meters were very sensitive service meters for the era and only have a 11 meg ohm per volt circuit load on DC. The WWII era Voltohmyst was limited to 1,000 volts DC, without the high voltage attachment. (Confirmed by the manual for my RCA 195-A, although the meter scale goes higher. Duh ! )

Note the little asterisk beside the +495 volts on the schematic.

The asterisk means they specified that pin 6, the focus anode, was to be measured only with the brightness control turned fully counterclockwise. This would bias the Kinescope to cutoff.

I suspect cutoff would drop the focus voltage to within the range of the Voltohmyst. It also would increase the 2nd anode voltage to 7,500 volts, which most 1946 service shops could not measure.

This has nothing to do with curing the present vertical problem, but once the set is operational, it would be an interesting experiment to perform, since the focusing anode voltage dropping with beam cutoff, goes against conventional logic.

James.

[tubesrule]

Originally Posted by earlyfilm

Re: +495 volts on focus anode on schematic

Darryl,

RCA specified that all voltage readings were to be read with a Voltohmyst. Those meters were very sensitive service meters for the era and only have a 11 meg ohm per volt circuit load on DC. The WWII era Voltohmyst was limited to 1,000 volts DC, without the high voltage attachment. (Confirmed by the manual for my RCA 195-A, although the meter scale goes higher. Duh ! )

Note the little asterisk beside the +495 volts on the schematic.

The asterisk means they specified that pin 6, the focus anode, was to be measured only with the brightness control turned fully counterclockwise. This would bias the Kinescope to cutoff.

I suspect cutoff would drop the focus voltage to within the range of the Voltohmyst. It also would increase the 2nd anode voltage to 7,500 volts, which most 1946 service shops could not measure.

James.

James,
Something still doesn't add up. The focus voltage does not change much over the full operating range of the crt as the load is negligible. The RCA manual does indeed say that besides the Junior Voltohmyst you need the optional high voltage probe which allows measurements to 10KV. That must have been used only for the 7500V measurement as the Voltohmyst was capable of 1000V on it's own, so if the 495V is correct, the meter was used without the HV probe.
The loading of the meter is a fixed 11meg, not 11meg/volt, but even with this extra load, and with the focus control turned to the low side you still end up with 966V. With the control to the high side you get 1250V. I'm still guessing this was a misprint in the original RCA manual and just got duplicated in Riders and elsewhere.

I suppose one of us could go measure it on a working set. Since modern meters are 10M input impedance, any digital or VTVM should work.

Darryl

[vts1134]

Well I did have some time today to apply the decals to the set. There were a few coats of lacquer applied to the set to smooth out the surface. There will be many more coats applied now to bury the decals into the finish. I was surprised to find out that the finisher never uses grain filler, instead he simply shoots lacquer and sands until he gets a glass smooth finish (if that is the goal). I'm very happy with the progress so far and can't wait until I see the finished product.





The sides of the set are a bit darker than the front and top. I would have been happier if they were the same color. The lighting accentuates this fact a bit more than what it looks like in person.





The decals are not 100% where they were before, but I think unless you had a picture next to it you'll never notice.







Now to get back to the chassis!