RCA 621ts Restoration

[vts1134]

Originally Posted by old_coot88

Sounds like a heavy power supply ripple is getting into the vert circuit. Possibly one of the filters is miswired(?).

Checked and rechecked the filters for a miswire but came up empty. I've gone through the resistors from the sync tube through the vertical output tube and they are all where they need to be. This one's a real .

Originally Posted by old_coot88

What does it look like with a regular picture?

I recently acquired a B&K 1077 so I can show what a regular picture looks like now. I had prevously thought that the vertical was running too slowly but you can see now that is not the case. The horizontal white bands do not roll with this signal, instead they are steady right where you see them.



Any thoughts?

[compucat]

I know you have probably checked this but going by just the picture it looks like a power supply cap has let go.

[old_coot88]

Originally Posted by vts1134

...I recently acquired a B&K 1077 so I can show what a regular picture looks like now. I had prevously thought that the vertical was running too slowly but you can see now that is not the case. The horizontal white bands do not roll with this signal, instead they are steady right where you see them.

It sure looks like a huge power supply ripple. The bars not rolling would be the case if the B&K's vert sweep is locked to the powerline (whereas a TV station's vert is offset slightly, which would cause the bars to roll). Possibly one of the replacement filters has gone open.

If you've got an extra 'lytic cap of equivalent rating as the main filters, you can jumper each one and see if the ripple disappears.

Or alternately if you have an analog AC voltmeter, you can measure ripple. Just put a cap of .1mf or so in series with the meter's hot lead (to block the DC component). I'm not sure i would trust a digital meter in that application.

[vts1134]

Originally Posted by old_coot88

It sure looks like a huge power supply ripple. The bars not rolling would be the case if the B&K's vert sweep is locked to the powerline (whereas a TV station's vert is offset slightly, which would cause the bars to roll). Possibly one of the replacement filters has gone open.

If you've got an extra 'lytic cap of equivalent rating as the main filters, you can jumper each one and see if the ripple disappears.

Or alternately if you have an analog AC voltmeter, you can measure ripple. Just put a cap of .1mf or so in series with the meter's hot lead (to block the DC component). I'm not sure i would trust a digital meter in that application.

I did try adding a cap to each of the electrolytics except for the 1,000mfd cap as I do not have a replacement for it.
How would I measure ripple with an analog meter? I have an analog VTVM and a .1mfd cap. That is to say what would I look for?

[old_coot88]

Originally Posted by vts1134

I did try adding a cap to each of the electrolytics except for the 1,000mfd cap as I do not have a replacement for it. How would I measure ripple with an analog meter? I have an analog VTVM and a .1mfd cap. That is to say what would I look for?

First i think it'd be a good idea to see perzackly what's connected between the center tap of the power xfmr and ground. Some of those filters' negative sides are insulated from ground and are connected to the CT. Just for the heck of it, take a resistance reading from that CT to ground. How much resistance does it show? If by chance it's inadvertantly shorted to ground, there would be some hairy ripple and hum bar effects.
In that pic of the power supply area that you posted earlier, it doesn't show where the center tap goes to (kinda fades into the twilight zone). Is it possible you could post a pic showing more of that area?

[vts1134]

I've got some readings on the electrolytics and a better picture of the schematic.

There are 5 multi stage electrolytic capacitors in the set. Towards the top of the picture, just above, and just to the right of the vertical centering control you can see two sections of C116. The negative on this cap reads shorted to ground which according to the schematic is correct. The other four caps are isolated from ground. Coming right off the center tap of the power transformer is the negative leads of C127, C128, and C131. Further along the line you have C132. Rather than give it away, how would I read the schematic and figure out what resistance reading those four caps should have to ground?

[old_coot88]

Originally Posted by vts1134

I've got some readings on the electrolytics and a better picture of the schematic.

There are 5 multi stage electrolytic capacitors in the set. Towards the top of the picture, just above, and just to the right of the vertical centering control you can see two sections of C116. The negative on this cap reads shorted to ground which according to the schematic is correct. The other four caps are isolated from ground. Coming right off the center tap of the power transformer is the negative leads of C127, C128, and C131. Further along the line you have C132. Rather than give it away, how would I read the schematic and figure out what resistance reading those four caps should have to ground?

Hmm.. so the power xfmr CT is the negative voltage source*, supplying 110V negative for stuff elsewhere in the chassis. And the main filter for it is the 40mf.cap (square symbol). Can't read it too good, but i b'lieve it's C128A). The positive side of this cap should go to ground. Is this wired correctly? If it's not, the negative supply would have a large ripple on it. So check this first, and then check for ripple. And verify that there's 110V negative on it (+/- 10% or so).

To check resistance of the -110V source to ground, just look for a DC path to the nearest ground symbol in the schematic (hint: it goes thru the ion trap coils), and add up the values of the resistances in that path. And see if the sum agrees with the ohms reading you get on the meter (+/- 10% or so).
------------------------------------------
*This technique is called "back biasing". You'll find it used in old equipment quite often, particularly in 30s radios. The CT of the power xfmr winding, instead of going directly to ground, goes through a chain of resistors or a large tapped resistor to form a voltage divider, supplying negative voltages for biasing other tubes in the set. A Google under "Back bias voltage" should turn up some good info on it.

[vts1134]

Originally Posted by old_coot88

To check resistance of the -110V source to ground, just look for a DC path to the nearest ground symbol in the schematic (hint: it goes thru the ion trap coils), and add up the values of the resistances in that path. And see if the sum agrees with the ohms reading you get on the meter (+/- 10% or so).

Such an easy answer to my question. Seems silly that I had to ask, but now I know. All of the resistance readings on the negative sides of the capacitors read correctly.

Originally Posted by old_coot88

Hmm.. so the power xfmr CT is the negative voltage source*, supplying 110V negative for stuff elsewhere in the chassis. And the main filter for it is the 40mf.cap (square symbol). Can't read it too good, but i b'lieve it's C128A). The positive side of this cap should go to ground. Is this wired correctly? If it's not, the negative supply would have a large ripple on it. So check this first, and then check for ripple. And verify that there's 110V negative on it (+/- 10% or so).

This is wired correctly, and there is -110v present.
I believe that I am checking for ripple, but perhaps I have the test wrong. I have put my meter on an AC 4v scale. I've connected the ground lead to chassis ground and the probe through a .1mfd cap to negative on C128. Is my test correct, and am I seeing ripple on the meter?
http://www.youtube.com/watch?v=b5BBxQlqp1o

[old_coot88]

By golly it looks like power supply ripple has been pretty decisively ruled out. I went back and reread the thread and looked more closely at the pics. Post#43 shows the raster as having nice straight sides (no hourglassing), indicating no appreciable ripple in the B+ feeding the H output stage (good). And post#58 shows the CRT displaying strong video which would rule out a heater-cathode short in the CRT (which would kill the video).

So.. it kinda looks like the hum is arising somewhere in the video chain 'upstream' of the CRT. And since the sync is affected too, the hum must be occuring upstream of the sync take off point. (Just guessing, but the sync may be picked off at the vid. output which was common practice in the old stuff.) It could possibly be nothing more than a H-K short in a tube. Sometimes tapping the tubes will show up an intermittent that won't show up on a tester.
If it ain't a tube, maybe you could post a pic showing the video amp and sync areas of the schematic.

[earlyfilm]

Originally Posted by vts1134

. . . . . . I had prevously thought that the vertical was running too slowly but you can see now that is not the case. The horizontal white bands do not roll with this signal, instead they are steady right where you see them.

VTS1134,

OldCoot is suspecting one problem, and he very well may be correct.

Let's stop and think about the problem.

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.

The wrong speed operation could be caused by either a 120 cycle hum being fed to it from the power supply or from some component in the vert osc/output circuit being badly off value. (A heater cathode short would introduce a single band. Only the 120 cps hum can create a double band and 120 usually is only found in a full wave power supply.)

1) I assume that you do not have a service scope. If you do, the first thing to check is the B+ and B- power supply sources for hum. Then check to see if the waveforms from the sync separator through the vertical output are correct.

2) Substitute the vertical tube. If you don't have a spare, switch the video output 6SN7 with the vertical output 6SN7 tube. If this changes your problem, then you probably have a bad vertical output tube.

3) Since you restuffed the electrolytics, you might check to confirm that no electrolytic can became accidentally grounded. None of the original electrolytic cans in either the power supply or the vertical section were grounded when RCA built the set.

4) Check every electrolytic in the vertical to make sure you have a close to correct value and it is wired in with the correct polarity. On the Riders on the ETF website, which I've copied in an attachment, the positive side of the electrolytic is always marked with the section marker (ie, the triangle, square or inverted cup). Especially, check C127B between the height control wiper and the 100K going to the wired end of the vertical hold. Another possible source is an incorrect electrolytic across the cathode resister of the vert output. Also check C131B at the vert output transformer.

5) If you had a real mess up in the front end of the power supply you would hear a rather annoying buzz from the speaker, as the field coil of same is used as the filter choke. (If you have a loud hum, then pull the audio output tube and if it continues, then there is a problem in the power supply.)

Hope something here helps.
James

[vts1134]

Thank you for your thorough and thoughtful insights. I can answer a couple of things right away.

Originally Posted by earlyfilm

1) I assume that you do not have a service scope. If you do, the first thing to check is the B+ and B- power supply sources for hum. Then check to see if the waveforms from the sync separator through the vertical output are correct.

I have inherited a scope but it is badly out of calibration, and the probe is questionable at best. After reading a few period TV servicing books from front to back two or three times, and with several more on the way I have strengthened my resolve to acquire a working scope and learn how to use it. Unfortunately this doesn't help me currently.
One thing to note about the vertical sync pulses: I performed the following test. I disable the vertical osc by shorting the the grid to cathode and injected vertical grid pulses directly at the grid of the vertical output from my B&K television analyst. My thoughts were to isolate the output from the oscillator to see if the problems came from the sync/osc stage, or elsewhere. I'm not sure if I would have totally isolated the problem as being sweep derived (as OC suggested with the variac test), but the two white bars remained.

Originally Posted by earlyfilm

2) Substitute the vertical tube. If you don't have a spare, switch the video output 6SN7 with the vertical output 6SN7 tube. If this changes your problem, then you probably have a bad vertical output tube.

I had previously swapped the 6SN7s around in the set and the problem was unchanged.

Originally Posted by earlyfilm

3) Since you restuffed the electrolytics, you might check to confirm that no electrolytic can became accidentally grounded. None of the original electrolytic cans in either the power supply or the vertical section were grounded when RCA built the set.

I'll check on this.

Originally Posted by earlyfilm

4) Check every electrolytic in the vertical to make sure you have a close to correct value and it is wired in with the correct polarity. On the Riders on the ETF website, which I've copied in an attachment, the positive side of the electrolytic is always marked with the section marker (ie, the triangle, square or inverted cup). Especially, check C127B between the height control wiper and the 100K going to the wired end of the vertical hold. Another possible source is an incorrect electrolytic across the cathode resister of the vert output. Also check C131B at the vert output transformer.

I'll recheck all of the lytics, but I can tell you with certainty that 127A and B is polarity and value correct (in so far as the markings on the cap itself). It was one of the cans that I uninstalled and checked twice. Also R145, the 100K resistor going to the wired end of the vertical hold, checks ok. I'll recheck the other two capacitors tomorrow just to be sure.

Originally Posted by earlyfilm

5) If you had a real mess up in the front end of the power supply you would hear a rather annoying buzz from the speaker, as the field coil of same is used as the filter choke. (If you have a loud hum, then pull the audio output tube and if it continues, then there is a problem in the power supply.)

There is no buzz or hum in the speaker. I get audio output on channel 2 through my television analyst and what sounds like FM on channel 1.

I'll report the other findings tomorrow. Also if I have time in the day tomorrow I'll have some pretty pictures of the cabinet to share as I put some decals on.