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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Jan Mon 28, 2019 3:00 pm 
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Excellent progress! The big issue with welding open elements is that sometimes it doesn't last.

But at least you can use it for now while completing the repairs to the chassis, and being on the lookout for a good CRT to replace that one. If it holds, that's more than enough emission to get a watchable picture in a dimly lit room. I've seen some as low as 100 microamps that are very watchable.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Sat 01, 2019 11:23 pm 
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Well, I have made some progress since my last update.

I spend quite a bit of time tracing the entire television circuit to make sure that I didn't make a wiring mistake or overlook anything. I found several out of tolerance resistors that I either overlooked previously or that had drifted in the couple of years since I first worked on this set. More significantly, I found that I had made a wiring error in the set.

The .1uF capacitor that couples the video signal from the video output tube to the picture tube grid had been connected to a terminal strip and the excess capacitor lead was used to jumper two of the terminals. When I replaced this capacitor, I connected it to the correct terminal but did not add a jumper to the terminal next to it. So the video signal was not being passed to the picture tube.

Additionally, I believe the video detector diode failed as it appears to measure as an open circuit. I have some 1N34A diodes on order if it is truly defective.

In the mean time, I connected some leads to the chassis to input a composite video and audio signal and see if I could get a picture on the screen. Well, it turns out I was able to exactly that. After correcting my above error, I powered up the set and after tweaking some of the picture controls, I was able to get an image.

The set still has some major issues, but I'm making progress. The linearity is all over the place, but I haven't really played with that yet. More of an issue is that the vertical hold is unstable, it is jittery and will roll periodically if the vertical hold control is not adjusted. The biggest issue seems to be that I am losing high voltage during operation. After about five minutes of operation, the HV will drop, the picture will lose width and focus and grow dark. I tried replacing the 1B3 with a know good tube and this improved the length of time before this occurs, but it will still occur.

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Hey Bluto, That's not Bad.jpg
Hey Bluto, That's not Bad.jpg [ 161.39 KiB | Viewed 391 times ]


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Bad Linearity.jpg
Bad Linearity.jpg [ 153.98 KiB | Viewed 391 times ]


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There Goes the HV.jpg
There Goes the HV.jpg [ 176.58 KiB | Viewed 391 times ]


At this point I still had no sound, which is odd because the circuit is not that complex and everything seemed to check out. Interestingly I found that the plate voltage was high on the 6AV6, like the tube was not drawing any current. Investigation further found that there was a tube pin stuck in the socket of the 6AV6 keeping the prongs for the plate pin spread open so it wasn't making contact with the tube in the socket. Removing this pin and crimping the tube socket tabs resolved this issue and the sound came back strong as ever.

I would be interested if anyone has suggestions on where to check for the HV problem, I presume that there is some component changing value as it heats up, although I don't know why changing the 1B3 would make a partial difference.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Sun 02, 2019 4:48 pm 
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A few things to check:

1. Adjust the Hor Linearity coil for MINIMUM CURRENT through the Hor OP tube (6DQ6?). If it has a cathode resistor, measure the voltage across that. Do it with the set DEAD COLD.
2. Once you have done that, see if that solves the problem.
3. If not, check the flyback windings for heating.
4. I have fixed a flyback with a "honeycomb" HV winding by soaking it while hot with CRC 2.26 spray. That was nearly 2 years ago, TV is used daily by a collector and is still OK.
6. If no heating, I assume you have tried a new 6DQ6? And you did replace the 6DQ6's G1 coupler and the B+Boost caps, didn't you?

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Mon 03, 2019 3:12 am 
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I have added a section of the schematic below for clarification below, it contains the HV and Horizontal sections.

Answering your suggestions in the order presented.

1. Have not tried this yet, will do so as soon as possible. By dead cold, I assume this should be done with as soon as possible after the set has come to full voltage and has a raster.

2. Will check as possible.

3. What is the suggested method for checking the flyback in operation for heating? I certainly do not want to touch it in operation. I do not see any melted wax on the outer housing but I can check it with a laser thermometer if that is applicable.

4. I am not familiar with the "honeycomb" nomenclature when referring to flyback transformers, is this an example of that type?

Attachment:
SAM_5613 (640x480).jpg
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5. Visible in the schematic section, the TV uses a 6BQ6 Hor. output tube. I have three available to me, one that is in the set currently, one that works fine in another television but gives no raster with all else the same in this TV and one that gives the same results as the first. I do have some 6DQ6 tubes, and I would temporarily substitute one in place of the 6BQ6 but this TV does not allow for the base size of the 6DQ6.

By the B+ boost caps, I presume you are referring to those like C175 and C189. These were both paper capacitors and were replaced with 1.6KV film capacitors. I presume the G1 coupler you are referring to is C169 that couples the Hor. sweep output to the 6BQ6 grid. This was a molded paper capacitor and was replaced with a 680pF 1000V mica capacitor.

Attachment:
Schematic- Horiz & HV (800x364).jpg
Schematic- Horiz & HV (800x364).jpg [ 158.9 KiB | Viewed 367 times ]

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Mon 03, 2019 4:56 am 
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In attempting to adjust the horizontal linearity coil for minimum current though the 6BQ6, I was monitoring the voltage drop across the 6BQ6 cathode resistor and found that the voltage drop was falling faster than I could adjust the horizontal linearity coil once the TV had come to full voltage. There is definitely something wrong here, as this would indicate that the 6BQ6 is drawing less current as the set heats up. This is the opposite of the horizontal output issue I am familiar with where the tube draws excess current, normally caused by the loss of the horizontal drive signal.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Mon 03, 2019 1:14 pm 
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Change the 6AX4 and report back, all the current for the 6BQ6 plate circuit in this set has to flow through the damper.

Make sure the filaments on the 6BQ6 and 6AX4 are not losing power.

Check to see that the screen resistor on the 6BQ6 is good, if G2 voltage drops, you lose width.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Mon 03, 2019 4:58 pm 
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OK, we are getting somewhere.

The flyack winding is not a "honeycomb" in this case and looks pretty sound - this type of transformer is known to be reliable. Still, switch off and feel for temp rise with a finger. It should never get to an uncomfortable heat. Dripping a small amount of wax from this type of flyback is normal.

The fact that the voltage drops is a good clue. Measure the G1 voltage and see if it goes more negative as it warms up. Might be that 1m resistor is high or open circuit. You will know for sure if connecting the probe fixes or improves the fault.

The 6DQ6 was an uprated replacement for the 6BQ6. Back in the day we didn't bother carrying 6BQ6s in service van stock. Yes the base is wider diameter, I never saw a TV it didn't fit though.

Oh, the 6AX4 also had an uprated replacement in the 6AU4. Same story there, we always used the newer tube.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Mon 03, 2019 9:22 pm 
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I went ahead and connected three meters to the set and monitored its performance in operation. No other changes have been done since last update. I noticed that it did take much longer (about 1/2 hour) for the HV to drop as before.

On turn on, after the voltages had stabilized they measured as such as referenced to chassis ground:

6BQ6 cathode: 13.6V

6BQ6 G1: -20.0V

6BQ6 G2: 185V

It took about 1/2 hour for the voltages to begin to change and the screen started to shrink and grow dark, indicating a loss of HV. At this point I measured the following before shutting the set off:

6BQ6 cathode: 9.25V

6BQ6 G1: -24.5V

6BQ6 G2: 149V

I also measured the flyback transformer temperature, it did not travel above 118 degrees Fahrenheit during operation, when I turned the set off it was only slightly warm to the touch.

I did also notice that the 6BQ6 screen grid resistor in my set (R196) is not the same as called out on the schematic. It is an 8.2K resistor and measures 8.21K ohms. The wire dress is of good workmanship so I cannot tell whether it is a replacement component.

At this point, based on the feedback you have all generously provided, I am leaning towards the 1 Meg. resistor off of G1 of the 6BQ6 being suspect. It measures within tolerance but it may be drifting in operation.

I am going to wait until the set cools off fully, then try the same test above but with a different damper tube, assuming the same results are seen, I will replace the 1 Meg. resistor and then report back.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Mon 03, 2019 11:53 pm 
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I replaced the damper tube and successfully watched a full movie on the set for an hour and a half. I am going to still replace that suspect 1 Meg. resistor, but it appears the HV problem has been licked.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Tue 04, 2019 3:18 am 
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In reference to the incompatibility of the 6DQ6 referenced above, I took this photo of the Horizontal Output socket.

The socket has this metal retaining collar riveted to the chassis, when a 6BQ6 is in this socket it retains the base and requires you to push down on the collar to pull the tube. A 6DQ6 has a wider base and the collar pushes up on this, it will not fit down into the socket. You could drill out the rivets and remove the collar, however as 6BQ6 tubes are nether expensive or rare, this seems unnecessary.

Attachment:
SAM_5616 (640x480).jpg
SAM_5616 (640x480).jpg [ 227.3 KiB | Viewed 326 times ]

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Tue 04, 2019 4:41 am 
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That chassis will work just fine with a good 6BQ6, Hunter. Yes the later models with a very similar chassis did use the 6DQ6 which may be capable of a few more watts of power, but it's not needed here.

I think the damper tube may have been the main issue, but you'll find out for sure after watching it for a few more days. Couldn't hurt to replace the resistor on G1.

I am troubled though by two things. First the drastic drop in G2 voltage as the HV drops, because based upon the cathode voltage and cathode current dropping you would expect that G2 was drawing less current at the same time, therefore its voltage ought to stay the same if not slightly increase. Second is the cathode voltage at 13.6 volts at the starting point, which would indicate a cathode current of 136 ma, way too high for a 6BQ6 and not good for the flyback. Tube manual shows max cathode current should be 110 ma.

With 10 volts across the 100Ω resistor in the cathode circuit, that would be 100 ma and about right.

But the fact that the flyback doesn't get overly warm is a good sign it's probably OK.

Just out of curiosity what is the B+ voltage in the set at the point marked +273 volts?

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Tue 04, 2019 5:00 am 
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On those voltage readings: The voltage reading on the cathode indicates that the current through the horizontal output tube is going down. I would expect that to cause the voltage on G2 to go up, but instead it went down. Now if the voltage available to the plate is going down due to a bad damper, could that cause G2 to pull more current? If you do replace the G1 resistor, perhaps you should also consider replacing the G2 resistor.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Tue 04, 2019 6:18 am 
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Dennis-

The B+ in the set measures 271-272V once the set has fully warmed up.

I am glad you brought up the cathode resistor on the 6BQ6 tube, on this set it has drifted out of tolerance and is measuring at about 120 ohms. I did not have a suitable replacement on hand of sufficient wattage and have been operating the set with this out of tolerance part while waiting for its replacement.

Assuming 120 ohms instead of 100, with a voltage drop of 13.6, this would work out to about 113mA, a little high for the current rating of the tube, but with the cathode voltage elevated by the drifty resistor, G1 would be comparatively more negative when referenced to the cathode causing the tube to draw less current, no? When I receive the replacement (hopefully soon), I will remeasure and I suspect the cathode voltage will fall back into spec.

With regards to the G2 voltage dropping, I have read that one should not operate a television with the plate cap removed from the horizontal output tube because it will cause the screen grid to draw more current and potentially damage the tube. In a situation where the damper is failing and the plate of the horizontal output is losing voltage, would this not create a similar situation and cause the screen grid to draw more current as a result? Or, perhaps C173 is leaky and causing the screen current to be loaded down, although I am not sure why that would correspond simultaneously to the loss of current through the tube if that were the case.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 06, 2019 1:00 am 
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Replaced the questionable resistors around the 6BQ6 and the cathode voltage fell to spec at 10.48V with a slight tweak to the Horiz. Linearity coil. Performance is as before, I decided to reconnect the RF and IF sections and see if we could get anything from the RF output on my VCR or converter box. I still don't have my 1N34As so I didn't expect any video, however I don't have any audio either, will need to investigate the tuner and IF further.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 06, 2019 2:18 am 
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I decided to dig out the oscilloscope and trace the signal path back from the audio amplifier. While I had it out I also took a look at the horizontal waveform. It looked a bit distorted at first:

Attachment:
SAM_5617 (640x480).jpg
SAM_5617 (640x480).jpg [ 113.28 KiB | Viewed 290 times ]


But a quick adjustment to the horizontal coil (L121) brought this about as close to textbook as one could get:

Attachment:
SAM_5618 (640x480).jpg
SAM_5618 (640x480).jpg [ 110.83 KiB | Viewed 290 times ]

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 06, 2019 2:53 am 
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Hcompton79 wrote:
I still don't have my 1N34As so I didn't expect any video, however I don't have any audio either, will need to investigate the tuner and IF further.

You need that diode for audio too. If you have any of 1N914, 1N916 or 1N4146, those should work OK. The 1N34 will theoretically start conducting at a slightly lower voltage, but the spec sheets show a 1 volt forward voltage drop for both the 1N34 and the 1N4146.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 06, 2019 3:02 am 
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Tom Schulz wrote:
Hcompton79 wrote:
I still don't have my 1N34As so I didn't expect any video, however I don't have any audio either, will need to investigate the tuner and IF further.

You need that diode for audio too. If you have any of 1N914, 1N916 or 1N4146, those should work OK. The 1N34 will theoretically start conducting at a slightly lower voltage, but there should be enough voltage available to make the difference unimportant.


Ugh, Face-palm. You are absolutely right. If I would have looked at the schematic, I would have noticed that the sound take off occurs on the plate of the 6X8 1st video amplifier.

It'll have to wait then, I have no fast recovery small signal diodes on hand, just rectifier types.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 13, 2019 1:49 am 
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So, I have done some more work on the set. I replaced a couple of rough looking wires along with the HV anode lead as it was looking a bit tired.

I also replaced the video detector diode with a 1N34A, however this did not solve my lack of video or audio. I began to dig further into the IF and video circuits.

If I inject a video signal into the grid of the 6X8, I have a good picture and naturally see a normal composite video waveform at that point when I check with my oscilloscope. If I check at that point with the output from the IF strip, I have a very different waveform that looks like this (apologies for poor photo, camera won't focus on CRT of oscilloscope):

Attachment:
SAM_5626 (800x600).jpg
SAM_5626 (800x600).jpg [ 110.89 KiB | Viewed 232 times ]


I don't think that the new video detector diode is at fault as I made sure to install it correctly and heat sink the leads when soldering it in place. With that in mind I took a closer look at the IF section and checked the grids of the three IF amplifier tubes and found no waveform at any of these three points that looked similar or close to the IF waveform shown in the service documentation.

Now, checking some voltages in the area, the one major discrepancy I have found is that the grid of the 1st IF tube (6CF6) is much more negative than is called for on the schematic. With a strong signal it should be around -4.7V and with no signal it should be 0.1V, however I am seeing -46V at this point.

The tube I have in this position tested good for emissions and shorts, can anyone suggest what may be causing this condition, I see nothing nearby on the schematic that would result in such a negative voltage at this point were something miswired and I am at a loss to explain this. Clearly with the grid this negative, the tube would be biased into cutoff and pass no signal.

Here are some relevant portions of the schematic:

Attachment:
IF Strip.jpg
IF Strip.jpg [ 116.17 KiB | Viewed 232 times ]


Attachment:
Tuner (800x433).jpg
Tuner (800x433).jpg [ 246.08 KiB | Viewed 232 times ]


Attachment:
Video circuit.jpg
Video circuit.jpg [ 234.43 KiB | Viewed 232 times ]

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 13, 2019 2:26 am 
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That negative voltage would be coming from the AGC circuit. I am not sure if a mis-adjusted AGC control could result in that much voltage, but you could try adjusting it and see what happens. Otherwise check everything around that circuit.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 13, 2019 4:24 am 
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Not sure if even an oscillating 1st IF tube could drive the grid that far negative.

Note well that the 1st and 2nd IF tubes in this set are a stacked pair with the plate of the 1st IF getting its voltage from the cathode of the 2nd IF, thereby allowing the AGC voltage applied to the 1st to also control the 2nd. I never thought much of that design but it was fairly common for a while.

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